Scripta Geologica, 139 (November 2009)
The evolution of the brain in Canidae (Mammalia: Carnivora)
George A. Lyras
Keywords: palaeoneurology, cerebral cortex, cortical folding, craniodental adaptations.
Canid brain evolution followed three independent, yet convergent paths. Each of the three canid subfamilies (Hesperocyoninae, Borophaginae and Caninae) started with a simple brain, which gradually became more complicated as the cerebral cortex became larger and more fissured, the cerebellar hemispheres became larger and the vermis more twisted. The extent to which these evolutionary changes took place differs between the three canid subfamilies. Caninae, the living group, has the most advanced external brain anatomy. This is related to the general tendency of the carnivore brain to become more convoluted through geological time. A parallel development of similar sulcal patterns took place in independent lineages within the Canidae. As a result, some sulci appeared independently several times during canid evolution. The cruciate sulcus appeared four times; the Sylvian, endolateral, ectolateral and ectosylvian sulci appeared three times. The skulls of species with short rostra have a more posteriorly placed posterior border of the palate than those of their close relatives with long rostra. This arrangement affects the position of the cribriform plate (posterior to which the olfactory bulbs are housed), which is also moved backwards and, as a result, the frontal lobes appear more massive. Some canid lineages evolved large size and certain craniodental characters (e.g., deep jaws and large canines), which allowed them to prey on large-bodied animals. In these cases a stasis is noted in brain evolution. This phenomenon might be related to energetic constraints.
Palaeoneurology is the study of the nervous system of fossil animals. Palaeoneurology is as old as palaeontology itself: Cuvier (1804) realised that casts of the brain cavity of fossil vertebrates could be informative concerning the external anatomy of the brain (Edinger, 1962). To prove this, he peeled away the dorsal surface of the neurocranium of a fossil mammal to reveal a natural stone cast of the brain, created by the sedimentary rock that had filled the neurocranium after the animal’s death.
Subsequently, the brain has received considerable attention from vertebrate palaeontologists (for a review of the literature, see Edinger, 1977), but it was Edinger who reformed the scope of palaeoneurology from the mere description of incidental findings of fossil brains into the study of the evolutionary history of the brain, thus founding modern palaeoneurology (Buchholtz & Seyfarth, 1999). Edinger understood that by applying methods of comparative neuro-anatomy to endocranial casts of animal groups, whose evolutionary history is well established through independent phylogenetic and stratigraphic work, one could reveal the actual evolutionary transformations of brain.
Palaeoneurological data are the only direct evidence available about the brains of the past. Using only living species to reveal the evolutionary history of the brain has its limitations, due to the danger of so-called Scalae Naturae (pseudo-evolutionary sequences) in which a living species is regarded as representative of the ancestral stage of another species. In such a way, misleading phylogenetic schemes can be constructed, such as for carnivores (England, 1973; see Preuss, 2000, for a further discussion of this problem). To avoid the effects of parallelisms, the contribution of the fossil record is crucial (Radinsky, 1971; Lyras & Van der Geer, 2003). While comparative neurology tries to unravel evolution by comparing the brains of living animals only, palaeoneurology does so by observations on fossil brains. As Edinger (1962) noted, “paleoneurology introduced to comparative neurology a fourth dimension: time.”
This palaeoneurological approach is an important contribution to comparative neuro-anatomy, because it helps us to resolve several misconceptions concerning brain evolution (Edinger, 1962; Dermitzakis, 2002). These misconceptions result from the way in which the problem of brain evolution was originally approached. In early theories, which were based mainly on the study of living species and a few fossil specimens, it was argued that brains evolve in a linear manner from fish to man (for a critique, see Hodos & Campbell, 1969), and that brain size is correlated with intelligence and, therefore, with species success (for a critique, see Radinsky, 1982). These theories influenced, and in many cases still influence, neuro-anatomical studies (Striedter, 1998). Additionally, despite the great diversity of living animals, neuro-anatomy is founded on studies of very few model organisms, mainly rats, cats, pigeons and macaque monkeys, an approach that further restricts our view of brain evolution (Preuss, 2000).
The present study is a contribution to the study of the evolution of the brain of the Canidae. This family includes wolves, jackals, foxes and many other less familiar species. In total there are 35 living species of wild canids (Wozencraft, 2005), that exhibit a wide range of morphological and ecological diversity, from the insectivorous Otocyon megalotis to the almost exclusively meat-eating Lycaon pictus. Fossil canids are even more spectacular. They belong to the oldest carnivore group still extant today and their past morphological diversity was even greater than currently, as they were not only filling ecological niches that today are occupied by canids, but also those occupied by hyenas, raccoons (Wang et al., 2004) and, to a certain extent, cats (Van Valkenburgh, 1991).
Materials and methods
Endocranial casts ― Soft tissues usually do not fossilize, yet there is good fossil evidence about brain morphology. This is because the brain is protected within the neurocranium, a hard bony structure that is often fossilized, and which receives much attention during palaeontological excavations and subsequent conservation. Particularly in the case of placental mammals, the brain almost entirely fills the neurocranium, so that casts of the brain cavity (endocranial casts or endocasts) reveal most of the features of external brain anatomy. The only exceptions are the large-brained mammals such as dolphins and great apes, in which only the general shape of the brain is preserved. In all the other mammals, the reproduction of the morphology of the external cerebral morphology is so perfect, that it is better to study endocranial casts than the actual brains, which have the tendency to deform once they are out of the skull (Edinger, 1948). Cerebellar morphology is usually blurred, and only the outlines of the vermis and main lobes are visible.
In the present study, two types of endocranial casts are used. The first type is artificial latex casts that are made after removing the sediment from the brain cavity and by using the neurocranium as a mould to prepare the cast (for descriptions of this technique, see Radinsky, 1968; Hildebrand, 1968; Murrill & Wallace, 1971). The second type is natural stone casts of fossilized sediment that fill the neurocranium. These casts were further prepared either by removing part of the bone of the neurocranium (usually one half of the dorsal surface) or were created naturally by erosion.
Most of the endocranial casts used in this study were prepared by Radinsky and are now in the collection of FMNH (for institutional abbreviations, see below). Additional casts were prepared from AMNH, AMPG, GIN, NNHM, NNM, PIN, SASN specimens and are now in the collections of NNM or in the institution of their origin (see Appendices 1 and 2 for a list of endocasts).
Computed Tomography (CT scan) ― A very rare canid skull (Eucyon davisi; F:AM 97057) was analyzed by high resolution computed tomography (CT) at the University of Texas X-ray CT facility. The original CT images (500 slices) were recorded in the coronal plane. They were further processed by the same laboratory in order to segment the endocast from the neurocranium and to construct an endocranial representation.
Two additional Nyctereutes specimens (N. tingi; AUT-MEL-1 and N. megamastoides; AUT-DFN-20) were scanned with a medical tomoscan at the Second Radiological Clinic of the University of Athens. The CT images (40 slices for each neurocranium) were acquired with a General Electric Hispeed scan and were further processed at AMPG.
Quantitative analysis of endocranial casts ― Most quantitative studies deal with relative brain size or encephalization index. As this subject has been covered in previous works (e.g., Jerison 1973; Finarelli & Flynn, 2007; Finarelli, 2008), it has been left out of the present study. Instead, the quantitative analysis herein is based on the relative length of the superficially exposed gyri and sulci. Their length is correlated with the gyrification index (the ratio between the entire and the outer cortical surface) and, therefore, the evolutionary history of cortical folding can be investigated with these simple length measurements (Zilles et al., 1989).
The length of the gyri was measured directly from the endocasts with the use of a curvimeter (map-reading wheel). For the specimens that were CT-scanned, a stereological method was applied, in which the total length of the superficially exposed gyri (L) was calculated as the product of the total number of convexities (P) and the average thickness of the slices (h): L=P*h. For details on both methods, see Elias & Schwartz (1971).
When a natural endocast or plaster copy of a latex endocast was available, its volume was determined from its water displacement. When a plaster copy of the endocast was not available, the volume was measured with mustard seed poured directly from the original skull. In cases in which parts of one hemisphere or the olfactory bulbs were missing, the missing parts were restored before measuring the volume based on the preserved parts of the same endocast or on endocasts of other individuals of the same species. These cases are indicated with an asterisk in Appendix 3.
The volume of the endocasts that were investigated with computed tomography was calculated as the product of the average thickness of the slices (h) and the sum of the surface area (Sn) of each section: V=h*Sum(S1+S2+…..Sn). The Gyral Length Index (GLI) of an endocast is the ratio between the total length of the superficially exposed gyri (GL) and the cubic root of the endocast volume (V): GLI=GL/(V^3).
Craniodental adaptations and dietary specialization ― Most carnivores prey on other vertebrates. During the killing and eating of the prey, the heavy loads applied on the predator’s skull can cause dental loss or damage (Biknevicius & Van Valkenburgh, 1996). In order to avoid this, predatory mammals have developed specific craniodental adaptations and killing techniques. These adaptations (such as short and broad maxilla, thick mandible and strong incisors) are particularly evident in highly carnivorous mammals, which include a very large percentage of meat in their diet and often prey on animals larger than themselves. Since specialization towards a highly carnivorous diet is associated morphologically with certain changes in the skull, mandible and dentition, we can rely on measurements of these changes in order to investigate the dietary specialization.
The estimation of certain cranial, mandibular and dental proportions was based on ratios that were developed by Van Valkenburgh & Koepfli (1993) in a study of predatory adaptations of the living Canidae. The values for living Canidae were taken from Van Valkenburgh & Koepfli (1993) and for most Borophaginae from Van Valkenburgh et al. (2003). For the rest of the Borophaginae, Hesperocyoninae and some other carnivore groups, the ratios were calculated from linear measurements (Fig. 1; Table 1) that were taken from AMNH specimens according to the method of Van Valkenburgh & Koepfli (1993). For a list of specimens, see Appendix 4. The ratios were analysed using principal component analysis in JMP (SAS).
Phylogenetic, taxonomic and stratigraphic framework ― Phylogenetic and stratigraphic data are crucial for the outcome of this project. In order to formulate a complete picture of canid brain evolution, it is necessary to place the descriptions of the endocasts within a phylogenetic and chronologic framework.
Phylogenetic relationships follow Wang (1994) on Hesperocyoninae, Wang et al. (1999) on Borophaginae, Tedford et al. (1995) for the living Caninae, Wang et al. (2004, 2008) for the fossil North American Caninae and Berta (1987, 1988) for the fossil South American Caninae. Additional data were taken from the works of Nowak (1979), Tedford & Qiu (1991, 1996), Rook (1992, 1993, 1994), Sotnikova (2001, 2006), Lyras et al. (2006) and Wang et al. (2008). The stratigraphic framework (Fig. 2) follows the geochronological time scale of Gradstein et al. (2004), and the definitions of the land mammal ages in Tedford et al. (1987, 1996) for North America, Flynn & Swisher (1995) for South America, Steininger et al. (1996) for Europe and Flynn et al. (1995) for China.
Fig. 1. Views (top) of the mandible and skull of a canid illustrating the measurements used in some of the Van Valkenburcgh & Koepfli’s (1993) ratios (bottom) that were applied to quantify ecologic specialization. Key: RBL= relative blade length of the lower first molar (m1); RGLA= relative lower molar grinding area; IxP4= relative resistance of dentary to bending in the parasagittal plane (jaw rigity); C1= relative size of the upper canine (C); I3 = relative size of the upper third incisor (I3); C1C1 = relative rostral breadth. (a) Maximum length of the m1 trigonid. (b) Maximum length of m1 talonid. (c) Maximum breadth of m1 talonid (d): maximum breadth of the lower second molar (m2). (e) Maximum length of m2. (f) Anteroposteral length of the m1. (g) Maximum dentary height at the p3-p4 interdental gap. (k) The distance between the posterior margin of the mandibular condyle and the anterior margin of the lower canine. (l) Maximum dentary width at the inter-dental gap between the lower third and fourth molars (p3-p4). (m) Maximum breadth of C. (n) Maximum length of C. (o) Maximum breadth of I3. (p) Maximum length of I3. (q) Mediolateral breadth between lateral margins of the upper canines. (r) Skull (condylobasal) length.
Table 1. Variables (ratios) used in the analysis and their definitions (according to Van Valkenburgh et al., 2003).
Fig. 2. Simplified marine-continental chronologic correlation of the Cenozoic terrestrial faunas used in the text (see materials and methods for further information).
Fig. 3. Sulci and gyri of the brain of a domestic dog (A-C) and the Oligocene Leptocyon gregorii (D). (A) Anterodorsolateral view. (B) Dorsal view of the frontal pole of the right hemisphere. (C) Lateral view of the frontal pole of the left hemisphere. (D) Dorsal view. The drawings of A, B and C are addapted from Lyras & Van der Geer (2003). 1. Sylvian sulcus (sulcus Sylvii, sulcus pseudosylvius); 2. ectosylvian sulcus (sulcus ectosylvius) ; 3. suprasylvian sulcus (sulcus suprasylvius); 4. lateral sulcus (sulcus lateralis); 5. ansate sulcus (sulcus ansatus); 6. coronal sulcus (sulcus coronalis); 7. radial process (processus acominis); 8. endolateral sulcus (sulcus endolateralis); 9. longitudinal fissure (fissura longitudinalis cerebri); 10. ectolateral sulcus (sulcus ectolateralis); 11. cruciate sulcus (sulcus cruciatus); 12. praecruciate sulcus (sulcus praecruciatus); 13. postcruciate sulcus (sulcus postcruciatus); 14. presylvian sulcus (sulcus praesylvius); 15. proreal sulcus (sulcus proreus); 16. intraorbital sulcus (sulcus interprorealis, sulcus intraorbitalis); 17. sigmoid gyrus: a. anterior part (pars anterior gyri sigmoidei), b. posterior part (pars posterior gyri sigmoidei); 18. proreal gyrus (gyrus proreus); 19. orbital gyrus (gyrus orbitalis); 20. rhinal sulcus (sulcus rhinalis), a. anterior part ( pars anterior sulcus rhinalis), b. posterior part (pars posterior sulcus rhinalis); 21. lateral gyrus (gyrus lateralis); 22. coronolateral sulcus (the coronal and lateral sulci together); cer. cerebellum; ol. olfactory bulb.
Anatomical nomenclature and homologies ― The anatomical nomenclature of the skull and soft tissues (including the brain) follows that of Evans & Christensen (1993). The interpretation of homologous gyri and sulci on the cerebral cortex of extinct and living carnivores is done on the basis of macroscopic morphological observations, with the use of sulcal and gyral maps from previous works on this subject. The problem of homologies has been stressed in the past by several authors (including Haller Von Hallerstein, 1934; Kreiner, 1968; Campbell & Hodos, 1970; Uylings & Van Eden, 1990) and several criteria have been suggested. However, as in fossils, the actual brain is not available and only the macroscopic morphology can be used in this study. In evolutionary lineages for which a good representation of fossil endocasts is available, it is possible to track the evolutionary changes and identify the gyri of fossil specimens, simply by noting the sequence of gyral appearance.
The anatomical nomenclature of the cortical convolutions of canids follows Filimonov (1928; redrawn by Adrianov & Mering, 1959). The nomenclature of groups that have no living representatives and in which the sulcal pattern differs substantially from that of the living forms (e.g., Creodonta, Miacidae and Nimravidae) follows the nomenclature and identifying criteria summarized by Welker (1990).
History of research
Extant canids ―Although there is an extensive literature on the anatomy of the nervous system of the domestic dog (Fox, 1971; Beitz & Fletcher, 1993; and references cited therein), there are few publications on the brain anatomy of the wild canids. Early studies on the external brain anatomy of wild canids were published by Gervais (1870), Mivart (1885) and Klatt (1928). More recent studies are those of England (1973) on the cerebrum of Canis latrans, C. lupus, Urocyon cinereoargenteus, Vulpes vulpes and the dingo, and Atkins (1978) on Canis latrans and C. rufus. A more extensive study was that of Radinsky (1973), who described and figured twelve species of wild canids (Vulpes vulpes, V. zerda, Urocyon cinereoargenterus, Otocyon megalotis, C. lupus, C. adustus, C. simensis, Lycaon pictus, Cerdocyon thous, Lycalopex sechurae, Speothos venaticus and Nyctereutes procyonoides) and gave brief comments on a few other members. Radinsky (1978a) figured Vulpes vulpes and Canis mesomelas. Lyras & Van der Geer (2003) described and figured the external anatomy of the cerebrum of 29 living wild canids (Canis lupus, C. adustus, C. aureus, C. mesomelas, C. sinensis, Lycaon pictus, Cuon alpinus, Atelocynus microtis, Speothos venaticus, Lycalopex culpaeus, L. griseus, L. sechurae, L. gymnocercys, L. vetulus, Dusicyon australis, Cerdocyon thous, Nyctereutes procyonoides, Urocyon cinereoargenteus, Otocyon megalotis, Vulpes lagopus, V. vulpes, V. zerda, V. bengalensis, V. corsac, V. rueppelli and V. chama) and demonstrated the usefulness external brain anatomy for phylogenetic interpretations (Fig. 4).
The above mentioned papers were focused on the description of individual specimens and did not attempt to investigate the evolutionary history of the brain in the entire family. This was first done by Radinsky (1969), who gave an outline of canid brain evolution with the description of endocasts of four genera (Hesperocyon, Mesocyon, Paratomarctus and Canis). Subsequently, Radinsky (1973) described endocranial casts from 18 fossil canid genera (13 for the first time) and figured endocasts from several living species (see above). With the use of this endocranial material and with phylogenetic data from an unpublished study by Tedford & Taylor, Radinsky presented the most rigorous work on canid brain evolution up to that time (Table 2).
Fig. 4. (A) Different types of outlines that are created by the coronal and ansate sulci on the dorsal surface of the cerebrum (from Lyras & Van der Geer, 2003). (B) Probable phylogenetic relationships within the living Caninae and occurrence of different types of sulcal pattern (from Lyras & Van der Geer, 2003). Outline patterns: a, pentagonal; b, heart-shaped; c, parenthesis-like; d, orthogonal. Caninae phylogenetic relationships: 1, Tedford et al. (1995) morphological cladogram; 2, Cladogram based on Wayne et al. (1997) molecular parsimony cladogram; 3, Tedford’s and Wayne’s combined cladogram by Wayne et al. (1997).
Fossil canids ― The earliest description of a fossil canid brain (Scott, 1895) was based on a natural endocast of Cynodesmus. Subsequently, brains of fossil canids have been described by several authors. Of particular interest are Moodie (1922), who described and figured an artificial endocast of Canis dirus; Pei (1934), who figured a natural endocast of Nyctereutes sp.; Tilney (1931) and Scott & Jepsen (1936), who gave a detailed description of a natural endocast of Hesperocyon gregarius; Jakway & Clement (1967), who figured and described a natural endocast of Paratomarctus (Tomarctus in their work); and Czyżewska (1981), who described two natural endocasts (Nyctereutes and “Canis”) from Poland.
Table 2. List of fossil canid endocasts that were figured by Radinsky (1973).
Some of the most important conclusions of Radinsky’s work include the brain of the first Canidae had fewer sulci than the brains of living species and even sulci that are almost ubiquitous in modern carnivores were absent in the early forms. During evolution, lobule VII of the cerebellar vermis became enlarged and twisted, the neocortex expanded and gradually more cortical folds appeared (by 25 Ma the ectosylvian, ectolateral and presylvian sulci appeared, and at about 12 Ma the sylvian sulcus). Expansion of the cortex in the early stages of canid evolution took place at the posterior and temporal lobe, and in later stages (particularly after the Late Miocene) at the anterior lobe. The most important difference among brains of the living canids is the relative size and shape of the proreal gyrus (long and bilaterally constricted in dogs, short and broad in foxes).
More recently, Lyras (2001) and Lyras et al. (2001) investigated some aspects of the evolutionary radiation of the Caninae brain that took place in the Mio-Pliocene. They noted the different sulcal patterns in Vulpini (foxes) and Canini (dogs), and that these appeared during the course of the evolutionary radiation. Lyras & Van der Geer (2003) presented more endocasts of fossil and living Caninae (see above) that gave additional evidence for the evolutionary radiation of their sulcal pattern. Finally, Dermitzakis et al. (2004) presented a paper on the evolutionary history of the brain of Nyctereutes (Fig. 5).
Fig. 5. Nyctereutes as part of the evolutionary radiation of the Caninae brain (from Dermitzakis et al., 2004).
Outline of canid evolution ― Simpson (1945, p. 222), in his classification of mammals, noted that “fossil canids are very abundant, and this richness is very embarrassing since it suffices to complicate rather than to elucidate...” This might have been true for his time, but during the last fifteen years a series of palaeontological and neontological studies (Berta, 1988; Rook, 1992, 1993, 1994; Wang, 1994; Tedford et al., 1995; Wang et al., 1999; Ivanoff, 2000, 2001; Sotnikova, 2001; Martìnez-Navarro & Rook, 2003), and molecular and karyological studies (Wayne et al., 1987a, b, 1997), as well as ecomorphology and functional morphology (Munthe, 1989; Van Valkenburgh, 1985, 1989, 1991; Van Valkenburgh & Koepfli, 1993; Van Valkenburgh et al., 2003; Andersson, 2003, 2005), have shed new light on canid evolution. The family is now one of the best known carnivore groups.
Fig. 6. Phyletic relationship of the major divisions of Canidae and their temporal ranges (redrawn from Wang et al., 1999).
Canid evolution is mainly a North American story. Most of their evolutionary history took place in North America, as canids were restricted to that continent from the Late Eocene until the Late Miocene. Contrary to the early studies (Matthew, 1930; Simpson, 1945; MacDonald, 1963), that agreed on a bipartite division of the family, recent works follow the suggestion of Tedford (1978), according to whom canids are divided into three subfamilies (Fig. 6): the Hesperocyoninae, which radiated mainly during the Oligocene; the Borophaginae, which radiated during the Oligocene and Miocene; and the Caninae, which radiated during the Plio-Pleistocene. Although their evolutionary radiations were sequential, their start was more or less contemporaneus: Late Eocene for the Hesperocyoninae, Oligocene for the Caninae and Borophaginae (Wang, 1994; Wang et al., 1999).
The first canids were small, fox-sized carnivores belonging to the genera Prohesperocyon (a transitional form between Miacidae and Canidae; Wang, 1994) and Hesperocyon (the first true canid and ancestor of all later forms). Hesperocyon appeared during the Duchesnean (Late Eocene) (Bryant, 1992), but it became abundant somewhat later, during the Chadronian (latest Eocene). The Oligocene was a critical period for the Canidae, since at that time many new forms appeared and canids reached their maximum species richness. This is because they entered several empty niches that originated from a faunal reorganization (Woodburne & Swisher, 1995) and environmental changes due to a long episode of Antarctic glaciation (Tedford et al., 1996).
The first subfamily to radiate was that of the Hesperocyoninae (Fig. 7). With the exception of one genus (Osbornodon), all Hesperocyoninae were small in size and had a clear tendency towards hypercarnivorous dental adaptations (such as reduced last molars and a trenchant talonid of the m1). These characters led Van Valkenburgh (1991) to suggest that many Hesperocyoninae were filling cat-like niches, which were empty at the beginning of the Miocene in North America. Wang (1994) recognised four main evolutionary lineages: Paraenhydrocyon, Mesocyon - Enhydrocyon, Osbornodon and Ectopocynus. Hesperocyoninae became extinct in the Middle Miocene.
The first Borophaginae were, like the Hesperocyoninae, small in size. Their dental adaptations tended towards hypocarnivory (an omnivorous diet with dental adaptations opposite to those of hypercarnivory, with an increased grinding area of the molars and reduced cutting section of P4/m1). Later Borophaginae included a wide spectrum of adaptations, and occupied several ecological niches that are today filled by raccoons, canids and hyenas. Wang et al. (1999) published a phylogenetic analysis of the subfamily in which four major clades were recognized (Fig. 7). Some of these lineages showed a clear tendency towards hypercarnivory. There were also species with dental adaptations comparable to those of the living hyenas (hence their common name “hyena-dogs”). Borophaginae became extinct during the Pliocene.
The earliest member of the Caninae is the genus Leptocyon. This genus was adapted to mesocarnivory (moderate carnivorous adaptations intermediate between hypo- and hyper-carnivory). The genus is not only the first, but also the only representative of the family for most of its history, ranging from Early Oligocene until Late Miocene. The radiation of Caninae started when several ecological niches became available again, due to the extinction of all small Borophaginae (Wang et al., 2004). During the early stages of this radiation all major clades of living Canidae appeared (Tedford et al., 2001).
Fig. 7. Phylogenetic relationships of canid taxa. Temporal ranges are indicated by individual bars. The thick bars indicate species from which endocasts were available. These are discussed further in this study. Phylogenetic relationships and temporal ranges taken from Berta (1988), Wang (1994) and Wang et al. (1999, 2004, 2008).
At the end of the Miocene Caninae migrated into the Old World (Fig. 8). Their appearance in Eurasia is associated with the extinction of the small canid-like hyenas at the Miocene-Pliocene boundary (Werdelin & Turner, 1996). During the Plio-Pleistocene, several Caninae species appeared in the Old World, either as immigrants from North America or as new species. Some of those new canids returned to North America (Fig. 8). The migration of Caninae into South America began in the Pliocene after the establishment of the Panama Isthmus (Berta, 1988). Apart from the immigrants from the north, South America turned out to be an important place for diversification and many new endemic species, adapted to local conditions, evolved there (Hunt, 1996).
Fig. 8. Caninae dispersal through time (the dispersal routes are plotted on the present-day geography). (1) Evolution of Caninae in North America: An evolutionary radiation at the end of the Miocene (Clarendonian and Hemphillian) resulted in the appearance of early members of all the major Caninae clades: Vulpes, Urocyon (Wang et al., 2004), Eucyon/Canis (Tedford & Qiu, 1996), Cerdocyon (Torres & Ferrusquia-Villafranca, 1981; McKenna & Bell, 1997) and Chrysocyon (McKenna & Bell, 1997). (2) Dispersal and evolution of Caninae in Eurasia: In the latest Late Miocene- Early Pliocene, Caninae underwent an extensive radiation in Eurasia, resulting in the appearance of new species of Eucyon (Rook, 1992; Tedford & Qiu, 1996) and Vulpes (Qiu & Tedford, 1990), the appearance of Nyctereutes (Tedford & Qiu, 1991) and some new, still undescribed genera (Tedford & Qiu, 1996; Ivanoff, pers. comm. 2003; Sotnikova, 2006). The figure suggests that Caninae entered first the Far East (2a) and then Europe (2b). However, the fossil evidence suggests the opposite, as the first European fossils (later Late Miocene: Tourolian: MN12) are older than the Chinese ones (Early Pliocene). If the East-West-East immigration actually did take place, Caninae must have followed a route without leaving (or not yet found) fossil evidence (De Bonis et al., 2007, suggested an African route), came to Europe and then moved to Asia (Zhang, pers. comm. 2004). (3) At the Late Miocene Caninae entered Africa. They were represented by Vulpes (De Bonis et al., 2007) and Eucyon (Morales et al., 2005). The genus Canis is definitely present in Afrika at 3.5 Ma (Werdelin & Lewis, 2000) and probably at Laetoli slightly earlier (Weredlin & Dehghani, in press). Nyctereutes-like were present in Afrika in the Pliocene (Barry, 1987; Wedelin & Lewis, 2005). (4) During the Late Pliocene-Pleistocene, North America continued to be a centre of canid evolution, with the appearance of several new Canis species (Wang et al., 2004). (5) Caninae entered South America after the establishment of the Panama Isthmus (around 3 Ma): The oldest canid record in South America is from the Late Pliocene (Berman, 1994). During the Early Pleistocene (Ensenadan), canid diversity increased with species of Lycalopex, Chrysocyon, Theriodictis, Protocyon and Canis (Berta 1988; Berman 1994; Prevosti et al., 2005). At the end of the Pleistocene, all large canids, with the exception of Chrysocyon, became extinct in South America (Berta, 1987). (6) Evolution of Caninae in Eurasia: During the Late Pliocene Nyctereutes was distributed throughout Eurasia. The genus became extinct in Europe before the beginning of the Pleistocene, while in Asia it survived to the present (Soria & Aguirre 1976; Tedford & Qiu, 1991). During the Plio-Pleistocene, new Canis-like species appeared in Eurasia. These belong to the genera Canis, Cuon and Xenocyon (Flynn et al., 1991; Rook, 1993; Sotnikova, 2001). Canis appeared in Asia during the Pliocene (6a). The earliest record of Canis in Europe (6b) is from latest Pliocene (Rook & Tore, 1996), but it became abundant only after the Plio-Pleistocene boundary. Some Canis-like lineages (Canis, Cuon and Xenocyon) became a specialization towards hypercarnivory. Lycaon seems to be related to one of these lineages (Rook, 1994; Martínez-Navarro & Rook, 2003). During the Pleistocene, several Eurasian Caninae (Vulpes, Canis,
Cuon and Xenocyon) entered North America (Rook, 1993; Wang et al. 2004). (7) Evolution of Caninae in Africa: During the Late Pliocene, Nyctereutes was present in Africa (Geraads, 1997). During the Pleistocene the jackals and Lycaon appeared (Werdelin & Lewis, 2005).
The first canids and the Hesperocyoninae
The first canids, Prohesperocyon and Hesperocyon, appeared in North America in the Late Eocene. During the Oligocene, Hesperocyon gave rise to several canid clades, some of which were grouped by Wang (1994) within the subfamily Hesperocyoninae. This subfamily reached its maximum diversity during the Late Oligocene and went extinct in the Middle Miocene.
Fig. 9. Prohesperocyon wilsoni AMNH 127172 (plaster cast of TMM 40504-126). (A) Dorsal, (B) left lateral and (C) posterior views. cl: coronolateral sulcus, ss: suprasylvian sulcus, cv: cerebellar vermis, ch: cerebellar hemishere.
Late Eocene: Prohesperocyon
Phylogenetic position ― Prohesperocyon is a transitional form between the Miacidae and Canidae (Wang, 1994). The dentition of Prohesperocyon wilsoni, the only known species in the genus, shows similarities to Miacis and Procynodictis. However, the anatomical features of its basicranium (fully ossified bulla, medially expanded petrosal and extrabullar position of the internal carotid artery) indicate that it is a canid (Wang & Tedford, 1994).
Material ― Prohesperocyon wilsoni AMNH 127172 (plaster cast of TMM 40504-126) - Chadronian (Late Eocene) from the Airstrip Local Fauna, southern Sierra Vieja area, Presidio County, Texas, U.S.A.; skull with partially exposed natural endocast (Fig. 9).
Description ― Natural erosion of the neurocranial wall of the only known skull of this species has exposed the dorsolateral part of its endocast. Although the exposure is not complete, it is sufficient to give a good picture of the brain of Prohesperocyon.
The cerebral hemispheres are pear-shaped, posteriorly broad, anteriorly tapering and decreasing in vertical as much as in transverse diameter. The temporal and, particularly, the frontal lobes are clearly smaller than those of any living canid. There are only two neocortical sulci: the coronolateral, which runs almost parallel to the longitutional fissure; and the suprasylvian, which forms a wide vertical arch on the lateral side of the brain. The overlap of the cerebellum by the cerebrum is small, leaving almost the entire vermis exposed. In dorsal view, the fissura prima lie half way between the rostral and occipital lobes of the vermis. Although the erosion of the endocast has removed much of the detail of the cerebellar hemispheres, it is clear that they are relatively narrow, as they do not extend further than the lateral margins of the foramen magnum.
Late Eocene - Early Oligocene: Hesperocyon
Phylogenetic position ― Hesperocyon gave rise to several lineages within the initial radiation of the Canidae (Wang, 1994; Munthe, 1998). Its earliest certain appearance is the Chadronian (Late Eocene), though there is dental evidence that extends its range into the Duchesnean (Bryant, 1992). These were small animals, with an elongated body, long tail and subdigitigrade forefeet with probably retractile claws (Wang, 1993). Hesperocyon is the most common fossil carnivore in the badlands of the northern Great Plains (reviewed by Wang, 1994) and several natural endocasts of this genus are preserved.
Material ― Hesperocyon gregarius F:AM 50277 - Chadronian to Orellan (Late Eocene- Early Oligocene), upper end of Shack Draw, Wyoming, U.S.A. A nearly complete skull with a dorsally and laterally exposed natural endocast. The dorsal part of the cerebellum is hidden under the parietal bone and part of the cerebrum is covered by the field label. The olfactory bulbs are not exposed (Fig. 10 C, D).
Hesperocyon gregarius F:AM 76518 - Orellan (Early Oligocene), Herb Geike Ranch, Sioux County, Nebraska, U.S.A. Skull and mandible with partially exposed natural endocranial cast of the cerebrum and cerebellum. The rostral part of the cerebrum and the olfactory bulbs are hidden within the skull. The endocast is slightly distorted by lateral compression (Fig. 10A, B).
Hesperocyon gregarius F:AM 76520 - Orellan (Early Oligocene), Herb Geike Ranch, Sioux County, Nebraska, U.S.A. Partial skull with exposed cerebrum and part of the cerebellum. A fragment of the parietal bone intervenes between the cerebrum and right cerebellar surface. The left cerebellar hemisphere is covered by the field note (Fig. 10H).
Fig. 10. Natural endocranial casts from Eocene and Oligocene Hesperocyon. Hesperocyon gregarius F:AM 76518, (A) dorsal and (B) left lateral views. H. gregarius F:AM 50277, (C) dorsal and (D) right lateral views; H. gregarius AMNH 1408, (E) dorsal and (F) left lateral views; Hesperocyon sp. AMNH-uncataloged (field number: Doug. 10-354), (G) dorsal view; H. gregarius F:AM 76520, (H) dorsal view. cl: coronolateral sulcus, ss: suprasylvian sulcus, rs: rhinal sulcus.
Hesperocyon gregarius AMNH 1408 - Early Oligocene, Oreodon Beds of Big Badlands, South Dakota, U.S.A. Natural endocast with partial skull, left ramus, left tarsus and metatarsus. The endocast is nearly complete with only the olfactory bulbs being hidden within the cast of the nasal cavities. Its ventral surface is poorly preserved (Fig. 10E, F).
Fig. 11. Natural endocranial casts from Oligocene Hesperocyon. Hesperocyon gregarius AMNH 85888, (A) dorsal view; Hesperocyon sp. AMNH 129176, (B) dorsal view; Hesperocyon sp. AMNH 39476 (C) dorsal, (D) right lateral and (E) ventral views; H. gregarius F:AM 76166, (F) dorsal and (G) lateral views; Hesperocyon sp. AMNH 9766, (H) anterior, (I) posterior, (J) dorsal, (K) left lateral and (L) ventral views. cl: corono-lateral sulcus, ss: suprasylvian sulcus, ol: olfactory bulb.
Hesperocyon sp. AMNH - uncatalogued (field number: Doug. 10-354) - Orellan (Early Oligocene), middle part of nodule zone, Wolff Ranch, southeast of Douglas area, Converse County, Wyoming, U.S.A. Natural endocast party covered by matrix. The olfactory bulbs and the occipital part of the cerebellum are missing (Fig. 10G).
Hesperocyon gregarius AMNH 85888 - Orellan-Whitneyan (Early Oligocene), “Middle” Brule Formation, Robert’s Draw, Nebraska, U.S.A. Partial skull and natural endocast of the anterior cerebrum (Fig. 11A).
Hesperocyon gregarius F:AM 76166 - Orellan (Early Oligocene), upper part of nodular layer, Reno Ranch, Converse County, Wyoming, U.S.A. Endocranial cast of the cerebrum and part of the cerebellum of a nearly complete skull with mandible. The right cerebellar hemisphere is covered by the field label (Fig. 11F, G).
Hesperocyon gregarius YPM 10067 - Early Oligocene, South Dakota, U.S.A. Natural endocast that was described and figured by Scott & Jepsen (1936) under the name Pseudocynodictis (Radinsky, 1969, 1973; Edinger, 1977).
Hesperocyon sp. AMNH 129176 - Orellan (Early Oligocene), west of Ant Hill Locality, Brule Formation, Wyoming, U.S.A. A nearly complete natural endocast. The olfactory bulbs are missing and parts of the cerebellum are covered by bone (Fig. 11B).
Hesperocyon sp. AMNH 9766 - Early Oligocene, Oreodon Beds, Indian Creek, Big Badlands, South Dakota, U.S.A. An excellently preserved natural endocast (Fig. 11H-L). The specimen was figured and described in detail by Tilney (1931) under the name Cynodictis (Radinsky, 1969, 1973; Edinger, 1977).
Hesperocyon sp. AMNH 39476 - Early Oligocene, south of Scenic, South Dakota, U.S.A. Natural endocast from which only the rostral part of the olfactory bulbs is missing (Fig. 11C, D, E). Radinsky (1973) used it, together with YPM-10067 and YPM-PU 12732, for a composite drawing and description of the Hesperocyon brain.
Description and comparisons ― The brain of Hesperocyon is slightly more globular than that of Prohesperocyon, but is otherwise similar in proportions and important features. The rhinal sulcus is relatively high, at about 25 % of the brain’s total height. The olfactory bulbs are large and not covered by the cerebral hemispheres. The frontal lobe is short and narrow. There are only two cortical sulci, the coronolateral and the suprasylvian. In some specimens the coronolateral sulcus approaches to the dorsal midline at the level of frontoparietal suture and then bows out slightly. The extent of this is variable and is at a maximum in F:AM 76518 (Fig. 11A) in which the coronolateral sulcus forms a clear bend at the level of the frontopariental suture that distinguishes the rostral (the coronal) from the occipital (the lateral) part.
The more globular shape of the Hesprerocyon brain might be a result of differences in the general cranial proportions of the two genera. Prohesperocyon has a more elongated skull than Hesperocyon and differences in skull proportions are known to affect the general shape of the brain (Weidenreich, 1941; Seiferle, 1966; Radinsky, 1973).
Tilney (1931), Scott & Jepsen (1936) and Radinsky (1969, 1971, 1973, 1978a) gave detailed descriptions of the Hesperocyon brain based on some of the specimens mentioned above. The descriptions of Tilney (1931) and Scott & Jepsen (1936) are quite accurate, although as they were trying to locate the sulci of living carnivores in Hesperocyon, they incorrectly interpreted a small depression at the temporal lobe as the sylvian sulcus (see below). In addition, Scott & Jepsen (1936) suggested that the endocasts do not reproduce all sulci present on the living brains, because the cruciate sulcus, which is present in all living carnivore families, is absent in Hesperocyon. Their problem was rather obvious; since all living carnivore families share the same sulci and the fossil canids do not have them, these sulci must have appeared either by parallel evolution or the fossils are misleading. Scott & Jepsen (1936) chose the second option, but Radinsky (1971) showed that most of the homologous sulci of the carnivores are actually the result of parallel evolution. According to Radinsky (1971), sulci with a universal distribution among living carnivores, like the cruciate sulcus, are absent from their early representatives and appeared independently at least five times. In addition, he indicated that the reproduction of the external brain anatomy of Hesperocyon by the endocasts is as accurate as in the similar-sized exant canids (Radinsky, 1973).
Fig. 12. Endocranial casts from Mesocyon. Mesocyon sp. AMNH 6946, (A) dorsal, (B) right lateral, (C) left lateral and (D) ventral views; Mesocyon coryphaeus AMNH 6920, (E) dorsal, (F) lateral and (G) ventral views. cl: coronolateral sulcus, ss: suprasylvian sulcus, es: ectosylvian sulcus, el: ectolateral sulcus, rs: rhinal sulcus. cl: coronolateral sulcus, ss: suprasylvian sulcus, es: ectosylvian sulcus, rs, rhinal sulcus, ol: olfactory bulb.
Oligocene - Early Miocene: Mesocyon, Cynodesmus, Enhydrocyon
Phylogenetic position ― This is the most diverse lineage of hespero-cyonines, beginning with Mesocyon, and leading to Enhydrocyon through the transitional forms Cynodesmus, Sunkahetanka and Philotrox. The lineage is characterized by an increased trend towards hypercarnivory. Endocranial material is known from three genera: Mesocyon, Cynodesmus and Enhydrocyon.
Fig. 13. Endocranial casts from Cynodesmus and Enhydrocyon. Cynodesmus thooides YPM-PU 10412, (A) dorsal, (B) left lateral and (C) right lateral views; Cynodesmus thooides F:AM 63373 (D) dorsal and (E) lateral views. Enhydrocyon stenocephalus FMNH PM 59036 (plaster cast of USNM 7745), (F) dorsal view.
Material ― Mesocyon coryphaeus AMNH 6920 - Oligocene of Camp Creek, John Day Basin, Oregon, U.S.A. Partial skull and a plaster copy of its nearly complete endocranial cast; the skull roof is restored and, therefore, the original endocast is presently unavailable. The olfactory bulbs and part of the cerebrum are hidden within the cast of the nasal cavities (Fig. 12E-G). The specimen was used by Radinsky (1973), together with AMNH 6946 and F:AM 63373 (a Cynodesmus, see below) for a composite description of the Mesocyon brain.
Mesocyon sp. AMNH 6946 - Early Arikareean (Late Oligocene), Turtle Cove of John Day Formation, Oregon (no locality data), U.S.A. A complete and excellently preserved natural endocast (Fig. 12A-D). It was figured by Edinger (1956) under the name Daphoenus, and described and figured by Radinsky (1969, 1973, 1978a).
Cynodesmus thooides YPM-PU 10412 - Early Arikareean (Late Oligocene) of Smith River and Spring Creek, Meagher County, Montana, U.S.A. Partial skull and mandible. The roof and lateral walls of the neurocranium are eroded, exposing a natural endocast in which the dorsal and left lateral side are excellently preserved (Fig. 13A-C). The specimen was described by Scott (1895).
Cynodesmus thooides F:AM 63373 - Whitneyan to Early Arikareean (Late Oligocene) of Wounded Knee area, Brule Formation, Washabaugh County, South Dakota, U.S.A. Partly exposed natural endocast. The ventral and posterior part of the endocast is covered by bone. The anterior part of the cerebrum and the olfactory bulbs are missing (Fig. 13D, E).
Enhydrocyon stenocephalus FMNH PM 59036 (plaster cast of USNM 7745) - Early Arikareean (Late Oligocene), Turtle Cove Member of John Day Formation, Grant County, Oregon, U.S.A. Partial skull with a nearly complete natural endocast (Fig. 13F). The specimen was figured and described by Radinsky (1973, fig. 3).
Description and comparisons ― Scott (1895) described the brain of Cynodesmus YPM-PU 10412, and Radinsky (1973) gave more extensive composite descriptions of Mesocyon and Cynodesmus brains based on YPM-PU 10412, AMNH 6946, F:AM 63373 and F:AM 61300. However, after the extensive work on canid phylogeny of recent years, many specimens have been taxonomically re-assigned. As a consequence, F:AM 63373 is no longer considered Mesocyon, but Cynodesmus (according to Wang, 1994). On the other hand, F:AM 61300 is not Cynodesmus, and certainly not “Cynodesmus petersoni,” which is a nomen nudum (Wang, 1994), but the borophagine Desmocyon matthewi (Wang et al., 1999). As a result, it has become evident that several anatomical features were attributed by Radinsky (1973, p. 174) either to the wrong taxon or were characterised as “differences [of] evolutionary significance or [reflecting] individual variation.” Therefore, in order to avoid confusion, the brains of Mesocyon and Cynodesmus are redescribed herein.
The brain of Mesocyon is almost twice as large as that of Hesperocyon. The general appearance of the brains of the two genera is similar. The frontal and temporal lobes are small, the rhinal sulcus of Mesocyon is somewhat lower than that of Hesperocyon, the overlap of the cerebellum is small and the posterior cerebellar vermis is straight. An advance in Mesocyon over Hesperocyon is the appearance of two more sulci; in addition to the coronolateral and suprasylvian sulci, there are also an ectolateral and an ectosylvian sulci. The ectolateral sulcus is short, but well defined. The ectosylvian sulcus is well defined on one side, and shallower and broken in two parts on the other (Fig. 12), a condition that is seen in both specimens. In AMNH 6946, the coronolateral sulci diverge posterolaterally significantly more than in AMNH 6920.
Cynodesmus thooides has a brain that is of similar size and external anatomy to that of Mesocyon. The cerebral cortex of Cynodesmus seems somewhat less expanded than in Mesocyon. It has two main sulci, the coronolateral and suprasylvian, but lacks the ectolateral sulcus, while the ectosylvian sulcus is represented by a series of variable depressions (Fig. 13). An advance over Mesocyon is the presence of a small presylvian sulcus.
The brain of Enhydrocyon stenocephalus is characterised by high and wide frontal lobes, resulting in a rectangular outline in dorsal view in contrast to the more tapering brains of all the other canids of the Late Eocene-Early Miocene. Its brain has been described by Radinsky (1973, p. 176), who noted about its sulcal ... pattern that there is “a well defined ectolateral sulcus and an incomplete ectosylvian sulcus the left suprasylvian sulcus is broken with the two pieces slightly overlapped. A short presylvian sulcus delimits a narrow orbital gyrus.” Further, the rhinal sulcus is as high as in Mesocyon and the overlap of the cerebellum, which has a straight cerebellar vermis, is small.
Fig. 14. Dorsal and lateral views of skulls and dorsal views of endocranial casts of Cynodesmus (left) and Enhydrocyon (right). The skull are Cynodesmus thooides AMNH 129531 (plaster cast of Black Hills Institute 1965) and Enhydrocyon
stenocephalus F:AM 6901. Endocast drawings are based on YPM-PU 10412 (for Cynodesmus) and USNM 7745 (for Enhydrocyon).
An interesting point in the evolutionary history of these three canids is their tendency towards hypercarnivory. Enhydrocyon is not only bigger than Cynodesmus and Mesocyon, but also has stronger and broader zygomatic arches, shorter and broader maxillae, a thick mandible and rather modified teeth. Conversely, the changes in the brain are minimal (Fig. 14). The only important change is that of its general shape, which is anterodorsally more compressed in Enhydrocyon. However, this is mainly a side effect of cranial evolution. As the snout became shorter, the palatine bone and the ethmoid plate were pushed backwards, and, therefore, the frontal lobes of the brain became anteroposteriorly compressed. In other respects - sulcal pattern, overlap of the cerebellum, position of the rhinal sulcus and torsion of the cerebellar vermis - the brain of Enhydrocyon is comparable to those of Cynodesmus and Mesocyon.
Early Miocene: Osbornodon
Phylogenetic position ― This genus, in contrast to the other hesperocyonines, is characterized by mesocarnivorous dental characters. It also has some other craniodental features that resemble those of Caninae. However, Wang (1994) considered Osbornodon as a member of the Hesperocyoninae based on the morphology of the m1 talonid. The stratigraphic range of Osbornodon is from the Orellan (Early Oligocene) to the Early Barstovian (Middle Miocene). From the seven described species of Osbornodon (see Wang, 2003), only one endocast is known, belonging to Osbornodon fricki.
Material ― Osbornodon fricki - F:AM 67098 - Late Hemingfordian (Early Miocene) of Sioux County, Nebraska, U.S.A. A complete skull from which a latex endocast (FMNH PM 58964) has been prepared (Fig. 15). The endocast was figured and described by Radinsky (1973, fig. 4) as “unnamed new genus.”
Fig. 15. Endocranial cast from Osbornodon fricki F:AM 67098. (A) Dorsal and (B) left lateral views. cla: anterior part of coronolateral gyrus; clp: posterior part of coronolateral gyrus; sg: sigmoid gyrus.
Description and comparisons ― The brain of Osbornodon fricki is twice the size of that of Mesocyon and six times the size of that of Hesperocyon. Its overall morphology has neither the rectangular outline of Enhydrocyon nor the tapering brains of the other Eocene-Oligocene hesperocyonines. Its frontal lobes are more developed that those of Hesperocyon and Mesocyon, the overlap of the cerebellum is larger, the olfactory bulbs are rostrally expanded and the cerebellar vermis is slightly twisted. The most important features of the O. fricki cerebral cortex were described by Radinsky (1973), who noted, in addition to the sulci seen on Cynodesmus, a large orbital gyrus, a small sylvian sulcus, a complete ectosylvian sulcus and large cruciate depression. This depression is at the centre of the anterior part of the two coronolateral sulci, which come closer to the dorsal midline at the level of the frontoparietal suture and then bow out laterally. This was considered by Radinsky (1973, p. 178) as “the first appearance of a distinct sigmoid gyrus.” This tendency also can be seen in Mesocyon and Cynodesmus, and even in a couple of Hesperocyon specimens. However, it is in Osbornodon fricki that it becomes evident.
Summary and conclusions concerning the Hesperocyoninae brain
Remarks ― Prohesperocyon, which is the transitional form between the Miacidae and the Canidae, has a brain with pear-shaped cerebral hemispheres and undeveloped temporal and frontal lobes. Its cortex has only two neocortical sulci (the coronolateral and suprasylvian). The cerebellum is widely exposed and has a straight cerebellar vermis and small cerebellar hemispheres. Of similar morphology is the brain of the first hesperocyonine, Hesperocyon. The major difference between the brains of the two genera is that the brain of Hesperocyon is more globular, something that is related to differences in cranial proportions.
The brains of the derived Oligocene hesperocyonines, Mesocyon, Cynodesmus and Enhydrocyon, have a more expanded cerebral cortex. The brain of Mesocyon is slightly more advanced than that of Cynodesmus. In Mesocyon there is an ectolateral and an ectosylvian sulcus, while in Cynodesmus there is no ectolateral sulcus and only a series of variable depressions in the place of the ectosylvian sulcus. However, Cynodesmus does have a presylvian sulcus. The brain of Enhydrocyon has four neocortical sulci (coronolateral, suprasylvian, ectosylvian and presylvian). The most important difference between Enhydrocyon and its ancestor (Cynodesmus) is that its brain has very high frontal lobes, a feature that is related to its cranial morphology (short rostrum).
The cerebral cortex of the Early Miocene hesperocyonine, Osbornodon fricki, is far more advanced than that of its Oligocene relatives. In addition to the sulci seen in Cynodesmus, it has a large orbital gyrus, a small sylvian sulcus, a complete ectosylvian sulcus and a large cruciate depression. The rostral parts of the coronolateral sulci bow out laterally, indicating a separation between the coronal and lateral parts, and the first appearance of the sigmoid gyri. Although such a cortical pattern appears for the first time in Osbornodon fricki, early signs of such an arrangement can be seen also in Mesocyon and Cynodesmus, and even in a couple of Hesperocyon specimens.
This group of canids is often called hyena-dogs, because some of them show a convergent evolution towards hyenas. During their evolutionary history, borophagines acquired a wide range of morphologies and filled several ecological niches (Wang et al., 1999).
Oligocene: Archaeocyon, Otarocyon, Rhizocyon
Phylogenetic position ― During the Oligocene many new canid species appeared. Most of them had numerous derived characters, and were classified by Tedford (1978) and Wang (1994) within the subfamily Hesperocyoninae. Other canids, however, do not present any clear distinguishing characters and placing them in any of the three canid subfamilies is problematic. Although there is a lack of cladistic resolution, Wang et al. (1999) considered them as early members of the Borophaginae (Archaeocyon, Otarocyon, Oxetocyon and Rhizocyon). The same authors also suggested an alternative scenario, in which these taxa may be independent clades of an evolutionary radiation. Four endocasts of these canids, belonging to three genera, were studied, namely Archaeocyon, Otarocyon and Rhizocyon.
Fig. 16. Endocranial casts of Oligocene Borophaginae. Archaeocyon leptodus F:AM 49033, (A) dorsal, (B) right lateral and (C) detail of the posterodorsolateral views. Archaeocyon leptodus F:AM 49031 (D) dorsal and (E) posterior views. Otarocyon cooki F:AM 49043, (F) dorsal, (G) lateral and (H) ventral view; Rhizocyon oregonensis AMNH 6879, (I) dorsal and (J) lateral views.
Material ― Archaeocyon leptodus F:AM 49031 - Medial Arikareean (Late Oligocene) of Muddy Creek, Niobrana and Platte Counties, Wyoming, U.S.A. Incomplete skull from which the left part of the roof and sidewall of the neurocranium have been removed in order to expose a natural endocast (Fig. 16A-C). The specimen has been figured and briefly described by Radinsky (1973, fig. 3) under the name Nothocyon.
Archaeocyon leptodus F:AM 49033 - Same locality and stratigraphic position as the previous specimen. Partial skull with a dorsally exposed natural endocast (Fig. 16D, E).
Otarocyon cooki F:AM 49043 - Early Arikareean (Late Oligocene) of Little Muddy Creek, Niobrana County, Wyoming, U.S.A. Left ramus and posterior part of skull from which a latex endocast (FMNH PM 58987) has been prepared (Fig. 19F-K). The endocast was briefly described by Radinsky (1973) under the name Nothocyon.
Rhizocyon oregonensis AMNH 6879 - Early Arikareean (Late Oligocene) of John Day Basin, Oregon, U.S.A. Skull and left ramus. Parts of the dorsal and lateral walls of the neurocranium are missing exposing a partial natural endocast (Fig. 16L, M).
Description and comparisons ― Some aspects of the brains of Archaeocyon and Otarocyon were discussed by Radinsky (1973), who used two of the above listed specimens (F:AM 49031 and F:AM 49043) for the description of Nothocyon. Today, this genus is considered to be an arctoid (Wang & Tedford, 1992), and the two specimens are attributed to Otarocyon (F:AM 49043) and Archaeocyon (F:AM 49031). Radinsky (1973, p. 174) described F:AM 49043 as being “similar to Hesperocyon, except for showing a short presylvian sulcus on one side” and F:AM 49031 as “10 % longer, [having] a slightly more posteriorly extended coronolateral sulcus and a short but distinct ectolateral sulcus.”
Actually, the two endocasts are quite different from each other, which is explained by the fact that they belong to different genera. The brain of Otarocyon is about the same size as that of Hesperocyon. Its general shape is dorsoventrally more compressed, due to the proportions of its skull, which is short-snouted, and its neurocranium is laterally expanded. The most characteristic feature of Otarocyon brain is the small expansion of its neocortex; the rhinal sulcus is very high, almost a third of the total height of the brain, and the coronolateral and suprasylvian sulci are short, even shorter than those of Hesperocyon. As Radinsky (1973) noted, an advance over Hesperocyon is the presence of a short presylvian sulcus. In total, Otarocyon is the canid with the least derived brain in terms of external brain anatomy. The simplicity of its brain comes ito sharp contrast with its rather derived cranial anatomy (Fig. 17). The skull of Otarocyon has many derived characters and is similar morphologically to the living fennec fox of Africa and Arabia (Vulpes zerda) (Wang et al., 1999). Its brain, on the other hand, is much simpler.
In contrast to Otarocyon, the brain of Archaeocyon has more similarities with that of Hesperocyon. As Radinsky (1973) noted, its only differences are its size (the brain of Archaeocyon is a bit larger), the more caudally extended coronolateral sulcus and the presence of a short ectolateral sulcus (Fig. 16).
The brain of Rhizocyon is known only from a partly exposed specimen (Fig. 16K, L). From what is exposed, its brain has the general morphology similar to that of Archaeocyon, although there is no trace of an ectolateral sulcus and the coronolateral sulcus does not extend quite as far caudally.
Fig. 17. Dorsolateral views of skulls and endocranial casts from Otarocyon cooki and Vulpes zerda. (A) Otarocyon cooki F:AM 49020; (B) V. zerda FMNH 91604 (reversed from the right); (C) O. cooki F:AM 49043 (endocranial cast FMNH PM 58987); (D) V. zerda NNM RMNH cat ost c.
Oligocene - Early Miocene: Phlaocyon
Phylogenetic position ― Phlaocyon is one of two genera of the tribe Phlaocyonini. Members of this group converge on procyonids dentally (Munthe, 1998) and therefore it is believed that they occupied raccoon-like ecological niches (Wang et al., 2004).
Material ― Phlaocyon marslandensis UNSM 26153 - Early Hemingfordian (Early Miocene) of Runningwater Formation, Box Butte County, Nebraska, U.S.A. Partial skull from which a latex endocast (FMNH PM 58977) has been prepared (Fig. 18A). The cerebellum and the occipital part of the right cerebral hemisphere are missing. The specimen was figured and described by Radinsky (1973, fig. 4).
Description and comparisons ― The brain of Phlaocyon has three main sulci, the coronolateral, ectolateral and ectosylvian. The rostral part of the coronolateral sulcus bows out lightly laterally and, in this way, the coronolateral gyrus becomes broader. In the medial part of this widening there is a depression in contact with the dorsal midline, which according to Radinsky (1973, p. 176) represents an incipient cruciate sulcus. According to the same author this sulcus appears to be “the extension onto the dorsal cerebral surface of a continuation of the splenial sulcus [...] rather than the beginning of a new sulcus.” He also noted that the rostral end of the coronolateral sulcus is forked; a feature unique among canids.
Early Miocene: Desmocyon
Phylogenetic position ― Desmocyon is a transitional form. It gave rise to all the later borophagines (Wang et al., 1999).
Material ― Desmocyon matthewi F:AM 61300 - Early Hemingfordian (Early Miocene) of Runningwater Formation, Dawes County, Nebraska, U.S.A. Partial skull from which a latex endocast (FMNH PM 58983) has been prepared (Fig. 18B, C). The endocast is almost complete, lacking only the orbital gyri and a part of the left hemisphere. It was used by Radinsky (1973), together with a Cynodesmus endocast, for a composite description of the brain of Cynodesmus.
Fig. 18. Endocranial casts from Phlaocyon, Desmocyon and Paracynarctus. Phlaocyon marslandensis UNSM 26153 (endocranial cast FMNH PM 58977), (A) dorsal view; Desmocyon matthewi F:AM 61300 (endocranial cast FMNH PM 58983), (B) dorsal and (C) lateral views; Paracynarctus sinclairi F:AM 61009 (endocranial cast FMNH PM 58973), (D) dorsal and (E) lateral views. crd: cortical depresion at the position of cruciate sulcus; cl: coronolateral sulcus; ss: suplasylvian sulcus; es: ectosylvian sulcus; el: ectolateral sulcus.
Description and comparisons ― The rhinal sulcus is lower than in Phlaocyon, and there is an angle between its occipital and rostral parts. The cerebrum has three sulci, the coronolateral, suprasylvian and ectolateral. The coronolateral sulcus forms a clear bend at the level of frontopariental suture and bows out anteriorly, forming a wider corono-lateral gyrus. The suprasylvian sulcus extends further posteroventrally than in the earlier borophagines. The ectolateral sulcus is small, but well defined, and the suprasylvian sulcus form a wide vertical arch on the lateral side of the cortex. The cerebellum is widely exposed. The cerebellar hemispheres are well developed, with the paramedian and ansiform lobules twisted as in living canids. The cerebellar vermis is straight.
An interesting detail in the brain of Desmocyon is that there is no indication of a cruciate sulcus. The coronolateral sulcus is broader in the rostral part, just as in Phlaocyon, but Desmocyon lacks a depression on the cortex in contact with the dorsal midline of the brain (see Fig. 18A, B). Since later borophagines have a cruciate sulcus (see descriptions below) and since they all evolved from Desmocyon (see phylogenetic analyses by Wang et al., 1999), that sulcus must have appeared independently twice within the borophagines.
Phylogenetic position ― During the Miocene, borophagines reached their maximum diversity. Wang et al. (1999) recognized several clades, including the Cynarctina, which includes Cynarctus and Paracynarctus. Its members were adapted to hypocarnivory.
Material ― Paracynarctus sinclairi F:AM 61009 - Early Barstovian (Middle Miocene) of the Olcott Formation, Sioux County, Nebraska, U.S.A. Complete skull from which a latex endocast (FMNH PM 58973) has been prepared (Fig. 18D, E). The specimen was figured and described by Radinsky (1973, fig. 4) as an unnamed new genus of a Phlaocyon-like canid.
Description and comparisons ― The brain of Paracynarctus sinclairi is about 20 % larger than that of Desmocyon. Radinsky (1973) noted that it has four sulci, the coronolateral, ectolateral, suprasylvian and ectosylvian; that there is no indication of a cruciate sulcus, that there is a well developed orbital gyrus; and that the cerebral vermis is slightly twisted. He also suggested that a sylvian sulcus might be present.
The outline of the brain of Paracynarctus is more comparable to that of Desmocyon than to Phlaocyon. This is to be expected, as phylogenetically Paracynarctus is more distantly related to Phlaocyon, something that also explains the absence of a cruciate depression in Paracynarctus, although it is present in the stratigraphically earlier Phlaocyon.
Miocene: Tomarctus, Microtomarctus, Aelurodon
Phylogenetic position ― Tomarctus and Aelurodon form a Borophaginae clade that originated in the Barstovian with Tomarctus hippophagus and ended in the Clarendonian with Aelurodon taxoides (Wang et al., 1999). The lineage is characterized by increased hypercarnivory, a tendency that is particularly evident in A. ferox and A. taxoides (Baskin, 1980). Microtomarctus is phylogenetically related to Tomarctus (Wang et al., 1999). Seven endocasts from this group are used.
Material ― Tomarctus hippophagus F:AM 61179A - Early Barstovian (Middle Miocene) of the Olcott Formation, Sioux County, Nebraska, U.S.A. Part of a skull from which a latex endocast (NNM RGM 195225) has been prepared. The complete left hemisphere, the cerebellum and part of the right hemisphere are preserved (Fig. 19A, B).
Tomarctus hippophagus F:AM 61179B - Posterior part of skull from the same locality and stratigraphic position as the previous specimen. A latex endocast (FMNH PM 58959) has been prepared (Fig. 19C-E). The endocast preserves the part that is caudal to the frontoparietal suture.
Tomarctus brevirostris F:AM 61179C - Posterior part of a skull from the same locality and stratigraphic position as the previous specimens. A latex endocast (FMNH PM 58957) has been prepared (Fig. 19F, G), which, as in the previous specimen, preserves only the part of the brain that is caudal to the frontopariental suture.
Microtomarctus confertus F:AM 27534 - Early Barstovian (Middle Miocene), ‘‘First Division,’’ Barstow Formation, San Bernardino County, California, U.S.A. Partial skull from which an artificial endocast (FMNH PM 58967) was made (Fig. 20D). The endocast preserves only a small part of the left hemisphere.
Aelurodon ferox F:AM 61746 - Late Barstovian (Middle Miocene), Devil’s Gulch Member, Valentine Formation, Brown, Cherry, and Keyapaha Counties, Nebraska, U.S.A. Skull with left ramus and partial skeleton. From the skull, an almost complete endocast (FMNH PM 58965) was prepared (Fig. 20A-C). The specimen is excellently preserved, being incomplete only in the right hemisphere where a hole was opened in the neurocranial wall during the preparation of the endocast. The endocast was figured and described by Radinsky (1973, fig. 5) together with F:AM 25230 as Aelurodon platyrhinus, a synonym of A. ferox.
Fig. 19. Endocranial casts from Tomarctus. Tomarctus hippophagus F:AM 61179A (endocranial cast NNM RGM 195225), (A) dorsal and (B) lateral views. T. hippophagus F:AM 61179B (endocranial cast FMNH PM 58959), (C) dorsal, (D) posterior and (E) left lateral views; T. brevirostis F:AM 61179C (endocranial cast FMNH PM 58957), (F) dorsal and (G) left lateral views. pl: pyriform lobe; ra: anterior part of the rhinal sulcus; el: ectolateral sulcus; rp: posterior part of the rhinal sulcus.
Fig. 20. Endocranial casts of Aelurodon and Microtomarctus. Aelurodon ferox F:AM 61746 (endocranial cast FMNH PM 58965), (A) dorsal, (B) lateral and (C) posterior views; Microtomarctus confertus F:AM 27534 (endocranial cast FMNH PM 58967), (D) lateral view; A. taxoides UNSM 25933 (endocranial cast FMNH PM 58969), (E) dorsal and (F) lateral views. sg: sigmoid gyrus; s: Sylvian sulcus; el: ectolateral sulcus; l: lateral sulcus
Fig. 21. Endocranial casts of Aelurodon. Aelurodon ferox F:AM 25230 (endocranial cast FMNH PM 58951), (A) left lateral and (B) dorsal views; A. taxoides F:AM 67015 (endocranial cast FMNH PM 58971), (C) dorsal, (D) lateral and (E) posterior views. cr: cruciate sulcus; pcr: postcruciate sulcus; cs: coronal sulcus; ss: suprasylvian sulcus; el: ectolateral sulcus; l: lateral sulcus.
Aelurodon ferox F:AM 25230 - Skull with mandible from the same location and stratigraphic position as the previous specimen. A complete endocast (FMNH PM 58951) was prepared (Fig. 21A, B), which was used by Radinsky (1973) for a composite description of the brain of synonymous Aelurodon platyrhinus.
Aelurodon taxoides UNSM 25933 - Clarendonian (Middle Miocene), lower part of the Ash Hollow Formation, Banner County, Nebraska, U.S.A. Complete skull from which a latex endocast (FMNH PM 58969) has been prepared. The endocast is complete, although the reproduction of part of its dorsal surface is not good (Fig 20E, F).
Aelurodon taxoides F:AM 67015 - Clarendonian (Middle Miocene), Ash Hollow Formation, Sheridan, Brown, Cherry and Hitchcock Counties, Nebraska, U.S.A. Partial skull, from which a latex endocast (FMNH PM 58971) has been made (Fig. 21C-E). The endocast preserves most of the cerebellum, the right hemisphere and part of the left hemisphere. The olfactory bulbs and the brainstem are missing. It was mentioned by Radinsky (1973) as “unidentified species of Aelurodon.”
Description and comparisons ― Before the publication of the phylogenetic analysis of Borophaginae by Wang et al. (1999), Tomarctus was one of the most confusing canid taxa and several species of that genus were erected. Previous palaeoneurological studies were based on the assumption that these species (later assigned to Tomarctus, Microtomarctus or Paratomarctus) all belonged to Tomarctus. Therefore, in order to clear out this confusion, the brains of these genera are described again (Tomarctus in this section and Paratomarctus below).
The brain of Tomarctus is about the same size as that of Paracynarctus. The cerebellar vermis is straight. The posterior part of the rhinal sulcus forms a sharp angle with its rostral part and, in this way, the posterior part of the pyriform lobe is more covered by the neocortex. The lateral sulcus steadily diverges caudally. The coronal sulci swell out only slightly, just a bit laterally to the lateral sulcus. The caudal part of the coronal sulcus is situated more laterally than the rostral part. In the centre of the cortex area between the coronal sulci there is a depression, which seems to represent an incipient cruciate sulcus. There is a short Sylvian sulcus. There are no ansate and endolateral sulci.
The available material from Microtomarctus is rather limited (Fig. 20D). The coronal sulci bow out laterally and, just as in Tomarctus, the caudal part of the coronal sulcus is more laterally expanded than the anterior. There is a very short cruciate and a Sylvian sulcus.
The brain of Aelurodon is about 30 % longer than that of Tomarctus. The posterior cerebellar vermis is straight or slightly twisted. The lateral sulcus extends straight back rather than diverging caudally (Radinsky, 1973) as in Tomarctus. The Sylvian sulcus is better developed.
In an early form of Aelurodon, A. ferox, there is no clearly formed cruciate sulcus and, just like in Tomarctus, there are no endolateral and ansate sulci. In a later species, A. taxoides, the sigmoid gyri are more expanded and there are clearly formed cruciate and postcruciate sulci (compare Figs. 20 with 21).
Miocene-Pliocene: Paratomarctus, Carpocyon, Epicyon, Borophagus
Phylogenetic position ― The last Borophaginae group appeared at the Middle Miocene. It originated with Paratomarctus, which at the end of Miocene gave rise to Carpocyon and to Epicyon-Borophagus. The species of Carpocyon were jackal-sized canids with meso-hypocarnivorous dentitions (Wang et al., 1999). The species of Epicyon and Borophagus were large (E. haydeni is the largest canid ever lived) and had craniodental adaptations that parallel those of the Old World hyenids (Werdelin, 1989; Baskin, 1998; Van Valkenburgh et al., 2003). In the Pliocene, Borophagus was the only surviving member of the subfamily.
Fig. 22. Endocranial casts from Paratomarctus. Paratomarctus temerarius F:AM 61070 (endocranial cast FMNH PM 58958), (A) dorsal, (B) left lateral, (C) right lateral and (D) anterior views; Paratomarctus sp. CSCLA 207, (E) dorsal, (F) lateral and (G) ventral views. cr: cruciate sulcus; pcr: postcruciate sulcus; s: Sylvian sulcus; sg: sigmoid gyrus; pl: pyriform lobe; ra: anterior part of the rhinal sulcus; rp: posterior part of the rhinal sulcus.
Material ― Paratomarctus temerarius F:AM 61070 - Late Barstovian (Middle Miocene) from Mouth of Snake River, Crookston Bridge Member, Valentine Formation, Cherry and Brown Counties, Nebraska, U.S.A. Skull with partial ramus from which a latex endocast (FMNH PM 58958) has been prepared (Fig. 22A-D). The specimen was described and figured by Radinsky (1969) as Tomarctus and later (Radinsky, 1973, 1978a) as Tomarctus cf. T. euthos. The specimen number was misprinted in Radinsky’s papers as F:AM 61074.
Fig. 23. Endocranial casts of Carpocyon and Epicyon. Carpocyon webbi F:AM 61328 (endocranial cast FMNH PM 58964), (A) dorsal and (B) lateral views; E. saevus F:AM 67331 (endocranial cast FMNH PM 58970), (C) dorsal, (D) right lateral and (E) left lateral views. og: orbital gyrus; ss: suprasylvian sulcus; el: ectolateral sulcus; l: lateral sulcus.
Paratomarctus sp. CSCLA 207 - Late Barstovian (Middle Miocene) from San Bernardino County, California, U.S.A. Isolated natural endocast. The specimen is almost complete, lacking only the olfactory bulbs (Fig. 22E-G). It was figured and described by Jakway & Clement (1967) as Tomarctus sp. CSCLA 207 is a natural endocast without cranial material; the attribution to Paratomarctus is based on the similarity of the external brain anatomy between F:AM 61070 and CSCLA 207 (see description below and Fig. 22).
Carpocyon webbi F:AM 61328 - Late Late Barstovian (Middle Miocene) from Midway Quarry, Burge Member, Valentine Formation, Cherry County, Nebraska, U.S.A. Skull from which a latex endocast (FMNH PM 58964) has been prepared (Fig. 23A, B). The endocast is complete, and was figured and described by Radinsky (1973, fig. 5) as “unnamed new genus close to Osteoborus.”
Fig. 24. Endocranial of from Epicyon and Borophagus. Epicyon haydeni F:AM 61474 (endocranial cast FMNH PM 58976), (A) dorsal view; E. haydeni F:AM 61476A (endocranial cast FMNH PM 58968), (B) dorsal and (C) lateral views; B. secundus F:AM 61649 (endocranial cast FMNH PM 58954), (D) lateral and (E) dorsal views. cr: cruciate sulcus.
Epicyon saevus F:AM 67331 - Late Clarendonian (Middle Miocene) from the Ash Hollow Formation, Brown and Cherry Counties, Mensinger Quarry, Nebraska, U.S.A. Partial skull with I1-P1 alveoli and P2-M2 from which a latex endocast (FMNH PM 58970) was made. The endocast is almost complete, lacking only the rostral part of the olfactory bulbs (Fig. 23C-E). It was mentioned by Radinsky (1973) as Aelurodon cf. salvus.
Epicyon haydeni F:AM 61476A - Early Hemphillian (Late Miocene) from Jack Swayze Quarry, Ogallala Group, Clark County, Kansas, U.S.A. Posterior part of a skull from which a latex endocast (FMNH PM 58968) has been prepared. The specimen lacks the anterior part of the brain (olfactory bulbs, orbital and anterior sigmoid gyri) and part of the left temporal lobe (Fig. 24B, C). It was used by Radinsky (1973), together with F:AM 61474, for a composite description of the Epicyon brain (under the name “unnamed new species of Aelurodon”).
Epicyon haydeni F:AM 61474 – Early Hemphillian (Late Miocene) from F. Sebastian Place, Ogallala Group, Decatur County, Kansas, U.S.A. Anterior half of skull from which a latex endocast (FMNH PM 58976) has been prepared. The specimen preserves only the olfactory bulbs, and the orbital and sigmoid gyri (Fig. 24A). Radinsky (1973) used it, together with F:AM 61476A, for a composite description of the Epicyon brain (see previous specimen).
Borophagus secundus F:AM 61649 - Late Hemphillian (Late Miocene) from Edson Quarry, Ogallala Group, Marshall Ranch, Sherman County, Kansas, U.S.A. Part of a skull from which a latex endocast (FMNH PM 58954) has been prepared. The specimen is incomplete as the olfactory bulbs and a small part of the brain stem are missing. It was mentioned by Radinsky (1973) under the name Osteoborus cyonoides (synonym of Borophagus secundus).
Borophagus dudleyi MCZ 3688 - Late Late Hemphillian (Late Miocene) from the phosphate pits near Mulberry, Bone Valley Formation, Polk County, Florida, U.S.A. Complete skull from which a latex endocast (FMNH PM 58963) has been prepared (Fig. 25). The endocast is of excellent preservation. It was figured and described by Radinsky (1973, fig. 5).
Description and comparisons ― The brain of Paratomarctus is quite similar in size and general morphology to that of Tomarctus (see description above). There are, however, some distinguishing features: the sigmoid gyri bow out more; there are clearly formed cruciate and post-cruciate sulci; there is an endolateral sulcus; and the posterior part of the pyriform lobe is exposed. Additionally, the brain of Paratomarctus has posterior sigmoid gyri that bow out more laterally than the anterior sigmmoid gyri, there is no ansate sulcus and there is a complete ectosylvian sulcus.
The brain of Paratomarcus is far more derived than that of Tomarctus and, despite its smaller size, is comparable to that of Aelurodon taxoides. Carpocyon webbi has a brain that is twice the size of that of Paratomarctus. The most characteristic features are the long orbital gyrus and that the olfactory bulbs are more pedunculate (rostrally extended) (Fig. 23A). The morphology of the lateral gyrus is similar to that of Paratomarctus. There are endolateral and postcruciate sulci. The cruciate sulcus is in the form of a depression. The posterior cerebellar vermis is slightly more twisted than that of Paratomarctus.
The brain of Epicyon saevus is in many ways similar to that of Aelurodon ferox. It is of about the same size and shares some features of the cortical pattern; the species was considered by Radinsky (1973) as Aelurodon. Just like Aelurodon, and unlike Paratomarctus and Carpocyon, it has no endolateral sulcus and the lateral sulcus extends straight back. However, E. saevus has many features that distinguish it from Aelurodon: the cerebellum is more overlapped; the vermis is slightly more twisted; the pyriform lobe is less exposed; the sigmoid gyri are more expanded; and there are clearly formed cruciate and postcruciate sulci. In addition, the Sylvian sulcus is more pronounced.
Epicyon haydeni has a much larger brain than all previous mentioned canids as is to be expected of the largest canid that ever lived. Its brain is similar to that of E. saevus. The cruciate sulcus is longer (Fig. 24A) and the frontal lobes are more massive.
The brain anatomy of Borophagus is known in two species. The coronal sulci bow out more in B. secundus than in the older borophagines. The cortex between the coronal sulci is even more expanded in the later species, B. dudleyi. Further, that species has a longer Sylvian sulcus and the most massive frontal lobes of any Borophaginae (Fig. 25).
Fig. 25. Endocranial cast of Borophagus dudleyi MCZ 3688 (endocranial cast FMNH PM 58963), (A) lateral and (B) dorsal view.
Summary and conclusions concerning the Borophaginae brain
Remarks ― The first borophagines (Archaeocyon, Rhizocyon, Otarocyon) have brains similar in size, lobation and fissuration to that of Hesperocyon. The differences between them and Hesperocyon are small: Archaeocyon has a more posteriorly extended corono-lateral sulcus and has a short ectolateral sulcus. On the other hand, Rhizocyon has no ectolateral sulcus and the coronolateral sulcus is shorter. Otarocyon has a neocortex with a very small expansion (the rhinal sulcus is very high, and the coronolateral and suprasylvian sulci are very short), yet it has a short presylvian sulcus, which is not present in the other forms.
The differences between Hesperocyon and Archaeocyon indicate that some small changes took place in the brain from one genus to the other. The presence of a presylvian sulcus in Otarocyon suggests that, at least concerning brain evolution, this genus followed a different path from the other early borophagines.
Further advances can be seen in Phlaocyon and Desmocyon. The brain of Phlaocyon has three main sulci (the coronolateral, ectolateral and ectosylvian). Additionally, there is a widening of the rostral part of the coronolateral gyrus and, in the middle of this area, an incipital cruciate sulcus. The rostral end of the coronolateral sulcus is forked, a feature that is unique among canids. This feature and the presence of an incipient cruciate sulcus distinguish Phlaocyon from the rest of the canids. The presence of a cruciate sulcus is not, of course, a unique feature. However, in this particular case, it is unrelated to the cruciate sulcus of the other borophagines. This can be inferred from an examination of the external brain anatomy of Desmocyon (the genus from which all later borophagines evolved); it has a brain with similar fissuration (three main sulci) and gyrification (a wider anterior part of the coronolateral gyrus) as Phlaocyon, yet its general shape is very different and it has no indication of a cruciate sulcus. Since later borophagines have a cruciate sulcus, this structure must have appeared independently twice within the subfamily.
Paracynarctus is a later borophagine. The outline of its brain is comparable to that of Desmocyon. Its cortex has four sulci (the coronolateral, ectolateral, suprasylvian and ectosylvian), perhaps a very small Sylvian sulcus and a well developed orbital gyrus. The posterior cerebellar vermis is slightly twisted.
The evolutionary history of the Middle and Late Miocene borophagines followed two major paths, the Tomarctus-Aelurodon and Paratomarctus-Borophagus clades. Both clades show a tendency to increase brain (and body) size, expansion of the occipital and temporal lobes, and expansion of the cortex medial to the coronal sulci. The expansion of the cortex of the occipital lobe is expressed in the larger overlap of the cerebellum and the expansion of the temporal lobe by the elongation of the Sylvian sulcus. The cortex medial to the coronal sulci expanded by bowing out of the coronal sulci, and by developing the cruciate and postcruciate sulci.
The cortical expansion was not the same in both groups. The Paratomarctus-Borophagus group began at a more advanced stage of gyral complexity and carried its development much further than did the Tomarctus-Aelurodon group. In both groups the posterior cerebellar vermis remained straight or (in some later forms) was only slightly twisted.
The coronal sulci of Tomartus bow out only slightly and there is an incident cruciate sulcus in the form of a depression. A similar condition is seen in Aelurodon ferox, but in a later species, A. taxoides, the sigmoid gyri are more expanded, and there are clearly formed cruciate and postcruciate sulci.
In Paratomarcus the sigmoid gyri bow out more, and there are clearly formed cruciate and postcruciate sulci. The sigmoid gyri are further expanded in Epicyon saevus, and the cruciate and postcruciate sulci are clearly formed. An even longer cruciate sulcus can be seen in a later species of Epicyon (E. haydeni) and in Borophagus.
The development of the sigmoid gyri took place independently in Paracynarctus, Paratomarctus and Tomarctus. In general, this area expanded independently several times in carnivore evolution. There are also some morphological differences between the sulcal patterns of these genera that support the hypothesis of parallel evolution. It is, however, somewhat implausible to suggest that out of the three available specimens, all three are cases of parallel evolution. A more reasonable explanation is that the development of the sigmoid gyri started with Desmocyon matthewi and then an evolutionary radiation in the brain took place. Such a case of radiation in the sulcal pattern around the sigmoid gyri took place during the major evolutionary radiation of the Caninae at the latest Miocene and earliest Pliocene (Lyras & Van der Geer, 2003). It is possible that the same evolutionary process took place in the Middle Miocene during the evolution of the Borophaginae. However, the analysis of Caninae was based on many more specimens, while in this case, due to the limited material, this idea should only be considered a working hypothesis.
An interesting detail is the form of lateral gyrus in the early and later forms of the two clades. The lateral sulcus diverges caudally in Tomarctus and Paratomarctus, while in Aelurodon, Epicyon and Borophagus it extends straight back. In addition, there is an endolateral sulcus in Paratomarctus, but not in Epicyon and Borophagus. Finally, Carpocyon webbi has a long orbital gyrus and the olfactory bulbs are more pedunculate (rostrally extended), yet in contrast Epicyon haydeni and Borophagus dudleyi have more compressed and massive frontal lobes than their close relatives.
Otarocyon has a rather simple brain, which, although comparable to that of contemporareous canids, contrasts with its rather derived ear anatomy and general cranial morphology (which can be compared with that of the living Vulpes zerda).
Remarks ― The Caninae first appeared in the Early Oligocene. Their major radiation occurred much later, during the latest Miocene - Early Pliocene, and is probably still ongoing (Laidlaw et al., 2007). For the description of the evolution of the Caninae brain, not only endocasts from fossils will be used, but also from extant taxa.
Oligocene - Miocene: Leptocyon
Phylogenetic position ― Leptocyon is the earliest representative of the Caninae (Tedford, 1978). During the course of its evolution the mesocarnivorous Leptocyon remained small and inconspicuous (Wang et al., 2004).
Material ― Leptocyon gregorii F:AM 49063 - Late Arikareean (Early Miocene) of Keeline, Niobrana County, Wyoming, U.S.A. A nearly complete skull from which a latex endocast (FMNH PM 58961) has been prepared (Fig. 26A). The specimen was figured and described by Radinsky (1973, fig. 6) as Leptocyon sp.
Fig. 26. Endocranial casts from Leptocyon and Vulpes. (A) Dorsal view of Leptocyon gregorii F:AM 49063 (endocranial cast FMNH PM 58961); (B) dorsal and (C) lateral views of Leptocyon sp. F:AM 49433 (endocranial cast NNM RGM 195222); (D) dorsal view of Vulpes stenognathus F:AM 49284 (from Lyras, 2001).
Leptocyon sp. F:AM 49433 - Clarendonian (Late Miocene) of Hollow Horn Bear Quarry, South Dakota, U.S.A. An isolated brain case from which a latex endocast has been prepared (NNM RGM 195222 and FMNH PM 58959). The endocast is incomplete and lacks the olfactory bulbs, orbital gyrus and part of the left hemisphere (Fig. 26B, C). The specimen was figured and described by Radinsky (1973), Lyras (2001) and Lyras & Van der Geer (2003).
Description and comparisons ― The earliest available Leptocyon endocast (Fig. 26) is from the Early Miocene, which is much later than the appearance of Leptocyon itself. Its morphology is comparable to that of the early members of the other two subfamilies (such as Hesperocyon and Archaeocyon). According to Radinsky (1973), who described the specimen, there is no ectolateral sulcus (except for a dimple), the coronolateral sulcus does not extend very far caudally and there is a small presylvian sulcus. Another interesting feature of the Leprocyon gregorii brain is the shape of the coronolateral sulcus; its rostral and occipital parts are separated by a slight approach of the sulcus to the cerebrum midline at the level of the frontoparietal suture.
The brain of the Clarendonian Leptocyon has a more overlapped cerebellum and more twisted cerebellar vermis than any canid described above. Its sulcal pattern is similar to that of modern Caninae: the Sylvian sulcus is present and concentrically wrapped around with the ectosylvian; suprasylvian and ectolateral sulci are arranged as a series of vertical arches; and the lateral and endolateral sulci on the dorsal part of the cerebral cortex. The region medial to the coronal sulci is narrow. This is particularly evident in the shape of the coronal sulcus, which is the outer lower border of the sigmoid gyrus. This sulcus does not bow out laterally and is almost in line with the lateral sulcus.
Miocene - Holocene: Vulpes, Urocyon and Otocyon
Phylogenetic position ― The first foxes appeared in North America in the latest Miocene. They were primitive species of Vulpes, Urocyon and another, still undescribed, genus (Tedford, pers. comm. 2005). The evolutionary history of Urocyon is restricted to the New World, where it continues to the present. Vulpes migrated to the Old World and, during the Pliocene, became widespread in Eurasia (Qiu & Tedford, 1990). Today, Vulpes is a rather complex genus with several species that together occupy a wide geographic range. During the Pliocene, Otocyon (or a closely related form) appeared in Africa (Werdelin, pers comm., 2008). Otocyon is also known from the Pleistocene of Africa (Werdelin & Lewis, 2005) and still occurs there (O. megalotis, the bat-eared fox).
Material ― Vulpes stenognathus F:AM 49284 - Late Hemphillian (Late Miocene) of Spring Valley, Lincoln County, South Nevada, U.S.A. The specimen was figured and described by Radinsky (1973), Lyras (2001), Lyras et al. (2001) and Lyras & Van der Geer (2003).
Living species of the genera Vulpes, Urocyon and Otocyon: Vulpes vulpes (NNM RMNH cat. no. a, NNM RMNH cat. no. b. NNM RMNH reg. no. 932, NNMH-Z 6419, FMNH 67413, AMNH(M) 21847), Vulpes bengalensis (NNM RMNH cat. no. b. and NNM RMNH cat. no. c), Vulpes rueppelli (NNM RMNH cat. no. b), Vulpes corsac (NNHM-Z 11616/527/4, NNHM-Z 11614/383/2), Vulpes chama (NNM RMNH reg. no. 25900), Vulpes zerda (NNM RMNH cat. no. c, FMNH 90369, FMNH 91748, FMNH 91743), Vulpes lagopus (NNM RMNH cat. no. d, NNM RMNH cat. no. h, FMNH 74070, FMNH 74068, FMNH 9871), Urocyon cinereoargenteus (NNM RMNH reg. no. 626, AMNH(M) 68527) and Otocyon megalotis (NNM RMNH cat. no. a, FMNH 38419, AMNH(M) 169093, AMNH(M) 169099). All are complete or nearly complete endocasts, taken from skulls of Recent foxes (see Appendix 2).
Description and comparisons ― The brains of foxes are more advanced than any of the canid brains described so far. Their cerebrum has all the main sulci and gyri found on any living canid, as is evident from their earliest known representative, Vulpes stenognathus (F:AM 49284). The brain of this species has, in addition to sulci seen in Leptocyon, ansate and postcruciate sulci, a more expanded sigmoid gyrus and a longer cruciate sulcus. The overlap of the cerebellum is larger (but slightly smaller than in most modern foxes) and the external anatomy of the cerebellum is the same as that of the living species of Vulpes. The proreal gyrus is not preserved in F:AM 49284, but it is likely that its development was limited, since in all living Vulpes species this gyrus is very small.
There are some small differences in the size and general shape of the brains of the living Vulpes species checked, yet no reliable distinguishing feature can be noted, as these differences are related mainly to cranial proportions. Somewhat different from the other foxes is the arctic fox, V. lagopus, the frontal lobes of which are very high and the cerebellum more overlapped. The proreal gyrus of V. lagopus is more developed than in any other fox (Lyras & Van der Geer, 2003). Further, the ansate and postcruciate sulci are very well developed, creating a sulcal pattern at the region medial to the coronal sulci that is very distinct from the rest Vulpes species, although it also approaches a pentagonal outline (Fig. 27).
Fig. 27. Endocranial casts from living foxes (from Lyras & Van der Geer, 2003).
The coronal sulci of Urocyon cinereoargenteus bow out laterally, thus creating a heart-shaped outline. There are two sulci on the orbital gyrus. The proreal gyrus is very small and similar to the one seen in Vulpes.
Otocyon has the least overlapped cerebellum of all foxes. The frontal lobes are low and wide, and the length of the cruciate sulcus is variable. According to Radinsky (1973), the length and bilateral constriction of the proreal gyrus in Otocyon are also variable, and it is thought that they are affected by the size of the brain.
Miocene - Pliocene: Eucyon
Phylogenetic position ― Eucyon is a transitional form between the Vulpini and the Canini (the Canis-like and South American canids). Its exact phylogenetic position is still a matter of discussion. Tedford & Qiu (1996), who first recognised the genus, considered it as the sister group to the South American canids and the Canis-like species. Wang et al. (2004, fig. 2.1) placed Eucyon closer to the South American canids. More recently, Wang et al. (2008) placed Eucyon closer the Canis-like species.
In North America, Eucyon is represented by E. davisi (late Middle Miocene - Late Miocene, also Early Pliocene of Eurasia). At the end of the Miocene, Eucyon migrated to Eurasia, where it underwent a radiation that gave rise to several new forms. Formerly, finds of these canids were considered to belong to either Canis, Vulpes or Nyctereutes (for example, Teilhard de Chardin & Piveteau, 1930; Odintzov, 1967; Martin, 1973). In recent years, researchers have recognized that these fossils belong to either Eucyon (E. davisi, E. monticinensis, E. minor, E. zhoui) or new, Eucyon-like, genera (Nurocyon chonokhariensis, “Canis” adoxus, “Vulpes” odessana) (Rook, 1992, 1993; Tedford & Qiu, 1996; Koufos, 1997; Wang et al., 2004; Sotnikova, 2006; Spassov & Rook, 2006). Some of these canids have several derived characters, and represent either transitional forms between Eucyon and true Canis or examples of convergent evolution towards Canis or Nyctereutes (Sotnikova, 2006).
Material ― Eucyon davisi F:AM 63005 - Late Hemphillian (Late Miocene) of Bird Bone Quarry, Arizona, U.S.A. Maxilla and dorsal part of skull from which a partial latex endocast (NNM RGM 195215) has been made. The endocast preserves only the dorsal part of the cerebrum (Fig. 28). It was figured by Radinsky (1973, fig. 6) under the name Canis cf. C. latrans, and described and figured by Lyras (2001), Lyras et al. (2001) and Lyras & Van der Geer (2003).
Fig. 28. Dorsal view of endocranial cast of Eucyon davisi F:AM 63005 (endocranial cast NNM RGM 195215) (from Lyras & Van der Geer, 2003).
Eucyon davisi F:AM 97057 - Early Pliocene of Hsia Kou, Shansi, China. Complete skull from which a digital endocast (Fig. 29) was created at the University of Texas.
Description and comparisons ― The brain of Eucyon davisi is very similar in size, proportions and general arrangement of the sulcal pattern to that of small jackals such as Canis adustus. However, there are some very important differences in the sulcal pattern which distinguish the brain of Eucyon from that of Canis. The proreal gyrus is bilaterally constricted in Eucyon, but it is shorter than that of similar-sized living Canis. The expansion of the sigmoid gyri of Eucyon is comparable to that of Vulpes. The cruciate and postcruciate sulci are better formed and there is a short ansate sulcus. The pyriform lobe is more exposed than in jackals and the ectolateral sulcus is not connected to the suprasylvian sulcus. In F:AM 63005 there is no clear endolateral sulcus, while in F:AM 97057, which is slightly younger, there is a very shallow endolateral sulcus. Overall, the brain of Eucyon davisi is more primitive than that of Canis and in many aspects comparable to that of Vulpes.
Fig. 29. Digital representation of endocranial cast of Eucyon davisi F:AM 97057. (A) Dorsal, (B) lateral, (C) posterior, (D) posterolateral, (E) anterolateral and (F) anterior views.
Miocene - Holocene: Cerdocyon and Nyctereutes
Phylogenetic position ― Molecular studies find no direct relationship between the living representatives of Nyctereutes and Cerdocyon (Wayne et al., 1997; Lindblad-Toh et al., 2005). Morphological (Tedford et al., 1995), palaeontological (Berta, 1988) and palaeoneurological evidence (Lyras & van der Geer, 2003; Dermitzakis et al., 2004), on the other hand, support a very close relationship between them.
Based on the fossil record, their evolutionary history started in North America with the appearance of Cerdocyon at the end of the Miocene (McKenna & Bell, 1997). Soon after its appearance it dispersed to the Old World, where it rapidly evolved into Nyctereutes. During the Early Pliocene, there were two species of Nyctereutes in Asia, N. tingi and N. sinensis. The first, N. tingi, might be a transitional form between Cerdocyon and the more derived Nyctereutes sinensis. In Europe, Nyctereutes is represented by N. tingi and N. donnezani. During the Late Pliocene there were two species of Nyctereutes in Eurasia, N. sinensis in Asia and N. megamastoides in Europe. However, the distinction between the European and Asian forms is often based on geographic criteria, and it is possible that N. sinensis - N. megamastoides and N. tingi - N. donnezani are variations of the same species (Tedford & Qiu, 1991).
In Europe, Nyctereures became extinct before the beginning of the Pleistocene (Martin, 1971), while in Asia it has persisted to the present day with the living species N. procyonoides. During the 20th Century, individuals were introduced for their fur into western Russia, and subsequently the genus has again spread through most of Eastern and Central Europe.
Unlike Nyctereutes, Cerdocyon has an exclusively New World history. During the Late Miocene-Early Pliocene, Cerdocyon was present in Central America (Torres & Ferrusquia-Villafranca, 1981). After the establishment of the Panama Isthmus about 3 million years ago, it dispersed to South America (Berta, 1988), where it continues to live today with the species Cerdocyon thous.
Material ― Nyctereutes tingi AUT MEL-1 - Early Pliocene of Megalo Emvolon, Macedonia, Greece. Complete skull from which CT scan images were acquired (Fig. 30).
Nyctereutes tingi F:AM 96757 - Early Pliocene of Nan Chuang Kou, Shansi, China. Partial skull from which a latex endocast (NNM RGM 195216) has been prepared. The endocast preserves the sigmoid gyri, the orbital and proreal gyri, and the anterior part of the right temporal lobe (Fig. 31A-C). It was figured and described by Lyras (2001), Lyras & Van der Geer (2003) and Dermitzakis et al. (2004).
Nyctereutes sinensis F:AM 96750 - Late Pliocene of Hsia Chwang, Shansi, China. Partial skull from which a latex endocast (NNM RGM 195223) has been prepared. The endocast preserves the left sigmoid, the orbital and proreal gyrus, and the anterior part of the left temporal lobe (Fig. 31D, E). It was figured and described by Lyras (2001), Lyras & Van der Geer (2003) and Dermitzakis et al. (2004).
Nyctereutes sinensis F:AM 96792 - Late Pliocene of Hsia Chwang, Shansi, China. Partial skull from which a latex endocast (NNM RGM 195218) was prepared. The endocast, which is laterally compressed, preserves the anterior and part of the left temporal lobe (Fig 31F, G). It has been figured and described by Lyras (2001), Lyras & Van der Geer (2003) and Dermitzakis et al. (2004).
Fig. 30. CT slices from Nyctereutes tingi AUT-MEL-1. The position and orientation of the slices is indicated in the drawing at the right corner.
Nyctereutes sinensis F:AM 22336 - Late Pliocene of Niu Wako, Shansi, China. Complete skull from which a partial natural endocast (NNM RGM 195219; Fig. 32A, B) has been prepared by removing part of the neurocranial roof. The endocast, which is dorsolaterally compressed, was figured and described by Radinsky (1973, as N. megamastoides), Lyras (2001), Lyras & Van der Geer (2003) and Dermitzakis et al. (2004).
Nyctereutes megamastoides GIN uncatalogued - Late Pliocene of Beregovaya, Russia; Nearly complete skull from which a latex endocast (NNM RGM 195220) has been prepared (Fig. 32D). From the endocast only a part of the brainstem is missing.
Fig. 31. Endocranial casts from Pliocene Nyctereutes. (A) Right lateral, (B) dorsal and (C) left lateral views of Nyctereutes tingi F:AM 96757 (endocranial cast NNM RGM 195216); (D) lateral (mirrored from the left) and (E) dorsal views of Nyctereutes sinensis F:AM 96750 (endocranial cast NNM RGM 195223); (F) dorsal and (G) left lateral views of Nyctereutes sinensis F:AM 96792 (endocranial cast NNM RGM 195218). From Lyras & Van der Geer (2003) (exept C).
Nyctereutes megamastoides AMPG(V)-Σ 384 - Late Pliocene of Sesclo, Thessaly, Greece. Part of skull from which a latex endocast has been prepared (endocranial cast NNM RGM 195221). The endocast preserves the entire brain exept of its brainstem (Fig. 32C).
Nyctereutes megamastoides AUT-DFN 17 - Late Pliocene of Dafnero, Macedonia, Greece. Complete skull from which CT scan images were acquired (Fig. 33).
Endocasts from skulls of living crab-eating foxes and raccoon dogs: Cerdocyon thous (NNM RMNH cat. ost. no. b., AMNH (M) 14853, FMNH 20205, FMNH 20436 and FMNH 68889), Nyctereutes procyonoides (NNM RMNH cat. ost. no. a., NNM RMNH cat. ost. no. d., NNM RMNH cat. ost. no. h., NNHM-P 6433, NNHM-P 6434 and FMNH 36002).
Description and comparisons ― The cerebral hemispheres of Cerdocyon thous widen out abruptly immediately behind the presylvian sulcus, thereby creating an angular profile in dorsal view (Fig. 34). The proreal gyrus is long and bilaterally constricted, and the coronal sulci create a heart-shaped outline on the dorsal aspect of the cerebrum.
In the living raccoon dog, Nyctereutes procyonoides, there is only a slight incurvation in the lateral profile at the level of the presylvian sulcus. The region rostral to the presylvian sulcus gives the impression of not being well developed. The relative length of the proreal gyrus approaches that of most Canini (Radinsky, 1973). However, it is broad and low, and with hardly any bilateral constriction, which makes it comparable to that of Vulpes lagopus. The coronal sulci create a heart-shaped outline on the dorsal aspect of the cerebrum (Fig. 34). The ansate sulcus is never present, while in the small foxes (some Vulpes spp. and Urocyon) it may or may not be present.
Fig. 32. Endocranial casts from Pliocene Nyctereutes. (A) Lateral and (B) dorsal views of Nyctereutes sinensis F:AM 22336 (endocranial cast NNM RGM 195219); (C) dorsal view of Nyctereutes megamastoides AMNH (Y)-Σ 384 (endocranial cast NNM RGM 195221); (D) dorsal view of Nyctereutes megamastoides GIN (endocranial cast NNM RGM 195220).
In fossil Nyctereutes, most parts of the cerebrum are like those of the living species, N. procyonoides, though the fossil forms differ from the living in that they have an ansate sulcus and more wavy cerebral gyri (Radinsky, 1973). This difference might be simply the result of allometry, as the degree of complexity of the gyri is a common difference between brains of different size. The outline of the sulcal pattern of N. tingi is not very clear, due to damage in the form of cracks on F:AM 96757 and the low resolution of the CT-scan of AUT-MEL-1. On the other hand, the sulcal pattern in N. megamastoides (AMPG(V)-Σ 384) and N. sinensis (F:AM 96750, F:AM 96792) is much better preserved, and is clearly heart-shaped. The specimen F:AM 22336 was compressed and damaged, and thus became asymmetrical. The resulting pattern is therefore less clear than in the other specimens.
Fig. 33. CT slices of Nyctereutes megamastoides AUT-DFN-17. The position and orientation of the slices are indicated in the drawing at the right corner.
The most important difference between living and fossil Nyctereutes has to do with the development of the cortex rostral to the presylvian sulcus (Lyras & Van der Geer, 2003). In N. tingi (F:AM 96757), N. sinensis (F:AM 96750, F:AM 96792, F:AM 22336) and N. megamastoides (GIN-uncatalogued), the proreal gyrus is long and bilaterally constricted, and the orbital gyrus is well developed. An exception is the N. megamastoides specimen from Sesclo (AMPG(V)-Σ 384), in which the proreal gyrus is slightly shorter and broader.
Fig. 34. Endocranial casts from living Cerdocyon and Nyctereutes (from Lyras & Van der Geer, 2003).
A key point towards a better understanding of the differences in the shape of the proreal gyrus between living and Pliocene Nyctereutes (with the exception of that from Sesclo) is the differences in their cranial anatomy. The Sesclo material differs from the rest of the Pliocene specimens, not only in the brain, but also in a number of craniodental characters, two of which are the relatively short and broad rostrum, and that the caudal border of the palatine lies at the end of the tooth row (Athanasiou, 1998). These differences are even more obvious when the living raccoon dog is taken into consideration; it has a shorter rostrum and the caudal palatine border is situated even further back than in Pliocene Nyctereutes (Fig. 35). These cranial differences seem to have an effect on the morphology of the proreal gyrus. All canids that have relatively short faces such as Lycaon, Enhydrocyon, Borophagus and Speothos), also have relatively high and massive frontal lobes (Radinsky, 1973) and a caudal palatine border that extends caudal to the tooth row (Fig. 36). The same seems to be the case in many domestic dog breeds that have short faces (Seiferle, 1966).
Fig. 35. Ventral view of skulls of (1) Nyctereutes procyonoides, (2) Nyctereutes sinensis and (3) Cerdocyon thous. All skulls are depicted to the same condylobasal length.
Thus, the broader proreal gyrus of the fossil Nyctereutes from Sesclo and, particularly, of living Nyctereutes procyonoides can be due to the same morpho-genetic factors that affect the shortening of the facial region. From observations on living and fossil canids, it appears that the caudal palatine border always lies below the cribriform plate that houses the olfactory bulbs of the brain. As the caudal palatine border moves backwards, the cribriform plate does the same, resulting in the anterior lobe of the brain becoming anterodorsally compressed. Since there is no direct functional relationship between the olfactory bulbs and the nasal pharynx, it is possible that this anatomical relationship is the result of ontogenetic restrictions. Evans & Christensen (1993) indicated that the basicranium follows a different ontogenetic pattern than the rest of the skull; therefore, it is possible that changes in that part of the brain are more constrained.
Fig. 36. Detail of the ventral view of four Canidae skulls. (A) Canis variabilis (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing); (B) Enhydrocyon stenocephalus AMNH 6901; (C) Speothos venaticus AMNH (M) 76806; (D) Lycaon pictus AMNH(M) 81853. The length of the white bar is 3 cm.
Miocene - Holocene: Chrysocyon, Lycalopex, Dusicyon, Atelocynus and Speothos
Phylogenetic position ― During the Pleistocene, several new canid species appeared in or migrated to South America, where they underwent radiation that gave rise to the divergent group of South American canids. Their exact phylogenetic history and even taxonomy is still a matter of discussion (see, for example, Lyras & van der Geer, 2003; Zrzavý & Řičánková, 2004; Wang et al., 2004). The evolutionary history of the canids mentioned in this section can be summarized as follows.
Chrysocyon appeared in North America at the end of the Miocene (Berta, 1987; Mc-Kenna & Bell, 1997). The extant species, Chrysocyon brachyurus, is known from the Lujanian (Middle and Late Pleistocene) - Holocene of Bolivia and Brazil (Berta, 1988).
A more diverse group of South American canids includes the genera Lycalopex and Dusicyon. According to Berta (1988), the “ancestral” stock that gave rise to them is an early member of the genus Lycalopex. This genus appeared in the Late Pliocene (“Dusicyon” cultridens; see Berman, 1994) and today is represented by six species (Lycalopex culpaeus, L. fulvipes, L. griseus, L. gymnocercus, L. sechurae and L. vetulus). Lycalopex is phylogenetically close to Dusicyon. This genus is known from two species, one as a fossil from mainland South America (Burmeister, 1866) and the other as the wolf of the Falkland Islands. This wolf, which is not phylogenetically related to the members of the genus Canis, was endemic to the Falklands, were it became extinct in 1876 (Nowak & Paradiso, 1983).
Atelocynus and Speothos are two rather peculiar canids of similar appearance (Sheldon, 1992). Phylogenetically, they are probably related to Cerdocyon and Nyctereutes (Berta, 1987, 1988; Tedford et al., 1995); see, however, Wayne & O’Brien (1987) and Wayne et al. (1997) for alternative assessments.
Material ― Lycalopex culpaeus AMNH 27904 - Middle Pleistocene of Ecuador. Skull with mandible. Part of the left wall of the neurocranium is missing. From the specimen a latex endocast has been prepared (NNM RGM 195227). It preserves the cerebellum and the greater part of the right cerebral hemisphere, with only the anterior lobe missing (Fig. 37).
Dusicyon australis NNM RMNH reg. no. 19111 and NNM RMNH reg. no. 19112: Endocasts from the recently extinct Falkland Islands wolf (Fig. 38).
Living species of the genera Chrysocyon, Atelocynus, Speothos and Lycalopex (Figs. 38, 39): Chrysocyon brachyurus (NNM RMNH cat. no. a., AMNH(M) 133940, USNM 361013), Atelocynus microtis (AMNH(M) 76579, FMNH 98081, FMNH 146277), Speothos venaticus (NNM RMNH reg. no. 3224), Lycalopex culpaeus (NNM RMNH reg. no. 620, AMNH(M) 205782), Lycalopex sechurae (NNM RMNH reg. no. 2381) and Lycalopex vetulus (NNM RMNH cat. ost. no. a., AMNH(M) 68527). Endocasts were taken from skulls of these Recent South American canids.
Fig. 37. Endocranial cast of Pleistocene Lycalopex culpaeus AMNH 27904 (endocranial cast NNM RGM 195227), (A) lateral and (B) dorsal views.
Description and comparisons ― Features of this group, including the general shape of the brain, are morphologically rather variable. Apart from the differences in the brain’s overall morphology, the most important variations are in the cortex medial to the coronal sulci and anterior to the presylvian sulcus. The coronal sulci of Lycalopex culpaeus form a heart-shaped outline on the dorsal side of the cerebrum, similar to that of Cerdocyon. In the remaining Lycalopex species and Dusicyon australis, the coronal sulci form a parenthesis-like outline. In the genera Atelocynus and Speothos, the sigmoid gyri expand very abruptly. This creates an orthogonal or oblong outline of the ansate and coronal sulci on the dorsal surface of the cerebrum. In Chrysocyon brachyurus the posterior sigmoid gyri expand laterally more than the anterior sigmoid gyri.
Fig. 38. Endocranial casts from living and recently extinct (D. australis) South American Caninae (from Lyras & Van der Geer, 2003).
Fig. 39. Endocranial casts from living South American Caninae (from Lyras & Van der Geer, 2003).
All specimens in which the anterior part of the brain is preserved have a proreal gyrus that is relatively long and bilaterally constricted compared to that of most foxes. Its exact proportions are variable. Lycalopex vetulus has the shortest proreal gyrus. Chrysocyon and the rest of the living Lycalopex spp. have a much longer and bilaterally constricted gyrus. In Atelocynus and Speothos the proreal gyrus is shorter and considerably thicker. The orbital gyri of Chrysocyon and Lycalopex have only one sulcus (the proreal sulcus) which separates the proreal and orbital gyri. The two specimens of Dusicyon australis examined as part of this study, have three sulci in that region, that is the proreal sulcus, the intraorbital sulcus and a third sulcus, not found in the domestic dog, which form the dorsal boundary of the anterior portion of the orbital gyrus (Lyras & Van der Geer, 2003).
Speothos venaticus has relatively high frontal lobes, a feature related to the short rostrum of this species. As noted in other canids whose muzzles became shorter during evolution (such as Enhydrocyon stenocephalus, Borophagus dudleyi, Nyctereutes procyonoides), the broader or higher rostral part of the brain is the result of a packing rearrangement of the skull due to the backward movement of the caudal palatine border and the cribriform plate.
Fig. 40. Endocranial casts from Pleistocene Canis. (A) Dorsal view of Canis lepophagus WT 760 (endocranial cast AMNH 104782) (from Lyras & Van der Geer, 2003); (B) dorsal view of Canis edwardii F:AM 63100; (C) lateral and (D) dorsal views of Xenocyon lycaonoides GIN 3722-144 (endocranial cast NNM RGM 195206) (from Lyras & Van der Geer, 2003); (E) dorsal view of Cynotherium sardous SASN CB 84-8022 (from Lyras, 2001).
Pliocene - Holocene: Canis, Xenocyon, Cynotherium, Lycaon and Cuon
Phylogenetic position ― This group includes wolves, jackals, the dhole, the Cape hunting dog and their fossil relatives. The earliest record of the genus Canis is from the late Miocene of North America (Rook, 1993; Miller & Carranza-Castañeda, 1998). During the Early Blancan (Early Pliocene) the diversity of the genus increased with the appearance a series of jackal-sized species (such as C. ferox, C. edwardii, C. lepophagus) (Nowak, 1979). Canis entered the Old World in the Early Pliocene (Flynn et al., 1991), where it underwent an extensive radiation (Hunt, 1996; Wang et al., 2004). During the mid Pliocene Canis entered Africa (Werdelin & Lewis, 2005) and, at the latest Pliocene-Early Pleistocene, Europe (Rook & Torre, 1996). The genus Canis is also known from the Pleistocene of South America (Berta, 1988).
The grey wolf (Canis lupus) appeared in the Late Pleistocene. The jackals (Canis aureus, C. mesomelas and C. adustus) are believed to be of African origin (Rook, 1993). Of North American origin are the coyote (Canis latrans), and the extinct Canis dirus and Canis armbrusteri.
The Cape hunting dog, Lycaon pictus, is a canid phylogenetically related to some hypercarnivorous species of Late Pliocene-Middle Pleistocene. Martínez-Navarro & Rook (2003) suggested Xenocyon lycaonoides as its possible ancestor. A canid related to Xenocyon is Cynotherium sardous, an insular mammal from the islands of Sardinia and Corsica (Lyras et al., 2006). Another hypercarnivorous member of this group is Cuon. Today this animal lives only in south and southeast Asia, but Cuon occupied large parts of Eurasia and North America during the Pleistocene.
Material ― Canis lepophagus AMNH 104782 (cast of WT 760) - Late Blancan (Late Pliocene) of N. Cita Canyon, Texas, U.S.A. Complete skull with mandible, from which a nearly complete endocast has been prepared (Fig. 40). The endocast was figured and described by Lyras et al. (2001) and Lyras & Van der Geer (2003).
Canis edwardii F:AM 63100 - Late Blancan (Late Pliocene) of Dry Mountain, Graham County, South Arizona, U.S.A. Nearly complete skull with mandible from which a partial latex endocranial cast has been prepared (F:AM 63100) (Fig. 40). The endocast does not preserve the cerebellum and the posterior part of the cerebrum.
Canis mosbachensis IQW 1982/18052 (mei 17572) - Middle Pleistocene of Untermassfeld, Thüringen, Germany. Partial skull. Due to the fragmentary nature of the specimen, it was not possible to prepare an endocast. Therefore, the anatomy of the brain was inferred simply by studing the impressions of the sulci on the internal side of the neurocranium (Fig. 41).
Xenocyon lycaonoides GIN 3722-144 - Middle Pleistocene of Kolyma, North Siberia, Russia. Complete skull from which a complete endocast (RGM.195206) has been prepared (Fig. 40C, D). The endocast was figured and described by Lyras et al. (2001) and Lyras & Van der Geer (2003).
Fig. 41. Skull of Canis mosbachensis IQW 1982/18052 (mei 17572). (A) Lateral view and (B) detail of the internal walls of the neurocranium. Imprints of the sulci on the neurocranium walls, 1: cruciate sulcus; 2: coronal sulcus; 3: ansate sulcus.
Canis armbrusteri F:AM 67286 - Irvingtonian (Pleistocene) of Mcleod Lime Rock Mine, Levy County, Florida, U.S.A. Nearly complete skull from which an almost complete endocast has been prepared (Fig. 42). Only the olfactory bulbs are missing from this specimen.
Canis dirus FMNH PM 394 - Rancholabrean (Late Pleistocene), Rancho la Brea, Los Angeles County, California, U.S.A. Isolated neurocranium that was split in two, in order to prepare a plaster endocast (Fig. 42). The endocast was figured and described by Moodie (1922).
Cynotherium sardous SASN-CB-84-8022 - Late Pleistocene, Corbeddu Cave, Lanito Valey, Nuoro, Sardinia, Italy. Nearly complete skull with mandible, from which a latex endocast has been prepared. The specimen was figured and described by Lyras (2001) and Lyras et al. (2006).
Fig. 42. Endocranial casts from Pleistocene Canis. (A) Dorsal, (B) lateral and (D) anterior views of Canis armbrusteri F:AM 67286; (C) dorsal, (E) lateral and (F) posterior views of Canis dirus FMNH PM 394.
Living species of the genera Canis, Lycaon and Cuon (Figs. 43, 44): Canis lupus (NNM RMNH cat. no. l. and FMNH 7619), Canis mesomelas (NNM RMNH reg. no. 25891, NNM RMNH cat. no. l, AMNH(M) 2288, FMNH 85508), Canis aureus (NNM RMNH cat. ost. b, NNHM-Z 11597/378/3), Canis latrans (NNM RMNH cat. no. c), Canis rufus (AMNH 112), Canis adustus (NNM RMNH cat. no. h, FMNH 97857), Canis sinensis (NNM RMNH cat. no. b, FMNH 146291), Lycaon pictus (NNM RMNH cat. no. c, AMNH(M) 24218) and Cuon alpinus (NNM RMNH reg. no. 1546, FMNH 91246). Endocasts from the skulls of those Recent canids were taken.
Description and comparisons ― In all Canis-like species, the cerebral hemispheres widen out abruptly immediately posterior to the presylvian sulci, and the proreal gyrus is very long and bilaterally constricted, more than in any other member of the family Canidae. Additionally, the sigmoid gyri expand very abruptly in these species. This creates an orthogonal or oblong outline of the ansate and coronal sulci on the dorsal surface of the cerebrum (the only exception is Canis edwardii, see below).
The earliest Canis species available for study is C. lepophagus (WT 760, cast, AMNH 104782). Its proreal gyrus is not completely preserved, yet it clearly has a development similar to that of the living Canis species. The sigmoid gyri are more expanded than in Eucyon davisi, but not as much as in Canis latrans (the coyote), that once was thought to be its descendant (e.g., Kurtén, 1974).
A later member of this group is Canis edwardii (F:AM 63100). Its brain is unlike all the other Canis-like species studied herein. The coronal sulci form a parenthesis outline on the dorsal surface of the cerebrum similar to that of most South American canids and Nyctereutes. It should be noted that in two living jackals, C. aureus and C. adustus, the oblong pattern is also not very clear (Lyras & Van der Geer, 2003), yet it is more distinct than in C. edwardii. Further, its brain is rather elongated for Canis, but that might be simply the result of its cranial anatomy.
The Middle Pleistocene Canis mosbachensis and Xenocyon lycaonoides have an endocranial anatomy similar to that of modern, similarly-sized Canis-like species. The endocasts of the Late Pleistocene, large C. armbrusteri and C. dirus are of similar morphology. Canis dirus, in particular, has the most fissured brain of all canids studied, a feature that was already noted by Moodie (1922).
From all the living Canis species, C. lupus, C. rufus and C. simensis have relatively the narrowest proreal gyrus (Lyras & Van der Geer, 2003). Cuon alpinus and Lycaon pictus have a considerably broader proreal gyrus. Some additional details of the sulcal pattern of the living species have been described by Radinsky (1973) and Lyras & Van der Geer (2003). According to them, Canis lupus, Cuon alpinus and Lycaon pictus have a dimple in the middle of the coronal gyrus. Cuon alpinus and Lycaon pictus have a short sulcus that divides the anterior sigmoid gyrus into two parts (Radinsky, 1973). The orbital gyri of C. aureus, C. adustus and C. mesomelas have only one sulcus (the proreal sulcus), which separates the proreal and orbital gyri. Canis latrans, Cuon alpinus and Lycaon pictus have an additional sulcus, the intraorbital sulcus, which extends parallel to the lower section of the presylvian sulcus. Canis simensis, C. lupus and C. rufus have three sulci in that region, the proreal, intraorbital and a third sulci, not found in the domestic dog, that forms the dorsal boundary of the anterior part of the orbital gyrus (Lyras & Van der Geer, 2003).
Fig. 43. Endocranial casts from living species of Canis (from Lyras & Van der Geer, 2003).
Fig. 44. Endocranial casts from living species of Canis, Cuon and Lycaon (from Lyras & Van der Geer, 2003).
Cynotherium was an island species, endemic to Sardinia and Corsica, and differs from all the other canids included in this study with the exeption of the recently extinct Dusicyon australis. The external brain anatomy of Cynotherium is typical of the members of the Canis-like species (Lyras et al., 2006). A more interesting point concerning Cynotherium is its relative brain size. As noted in the introduction, the subject of relative brain size has been covered extensively by previous authors and, therefore, this aspect of the brain is beyond the scope of the present study. However, in contrast to the impressive literature on the brain size of mainland species, only a few papers have been published on the brain of insular mammals (Dechaseaux, 1961, and Köhler & Moyà-Solà, 2004, on Myotragus; Accordi & Palombo, 1971, and Palombo & Giovinazzo, 2005, on Elephas falconeri; Willemsen, 1980, on Lutrogale cretensis; Palombo & Giovinazzo, 2004, on Cynotherium, Falk et al., 2005, on Homo floresiensis). Dechaseaux (1961) noted that Myotragus had an extensive neocortex and a large frontal lobe, while Köhler & Moyà-Solà (2004) stated that it underwent a significant reduction in relative brain size. Falk et al. (2005) reported that the brain size of Homo floresiensis is considerably smaller than that of late Homo spp. The opposite result, a relativly larger brain, has been reported in the case of the dwarf elephant of Sicily, E. falconeri (Accordi & Palombo, 1971; Palombo & Giovinazzo, 2005). Palombo & Giovinazzo (2004) suggested that the brain of Cynotherium from Dragonara Cave was not relatively larger than that of Canis lupus or Canis mesomelas. So, in contrast to the well established patterns of relative brain size evolution in mainland mammals, only a few and contradictory results have been published about the brain of insular forms.
The relatively large neurocranium of Cynotherium (Malatesta, 1970; Lyras et al., 2006; Lyras & Van der Geer, 2006) raises the question as to whether its brain was equally large. In order to answer this question I followed Radinsky’s (1967) method of comparing the brain size with the area of the foramen magnum. The volume of the endocast of Cynotherium SASN -CB-84-8022 is 84 cm3. The width of the foramen magnum is 1.270 cm and its height is 1.804 cm. Using these values in Radinsky’s (1973) equation – log (endocranial volume) = 1.54 + 1.48 log (foramen magnum area) – we see that the endocranial volume of Cynotherium (log 84 = 1.924) is very close to the expected endocranial volume (1.54 + 1.48 log 1.804 = 1.919). In other words, Cynotherium has the brain size of living mainland canids of comparable body mass.
Summary and conclusions concerning the brain of Caninae
Remarks ― The Caninae appeared in the Oligocene with the genus Leptocyon, but the earliest available endocast is much younger, from the Early Miocene. The external brain anatomy of this early Leptocyon is comparable to that of Hesperocyon and Archaeocyon: the frontal lobes are very low; the overlap of the cerebellum is small; and there are only two main sulci, the coronolateral, which runs almost parallel to the longitudinal fissure, and the suprasylvian, which forms a wide vertical arch on the lateral side of the brain. This early Leptocyon has a small presylvian sulcus and a small rostral part of the coronolateral sulcus, which is clearly separated from the occipital part by a small inward turn of the sulcus.
The next Caninae endocast is from Late Miocene Leptocyon. Its brain has a more overlapped cerebellum and a more twisted cerebellar vermis than any canid of its time or earlier. Its sulcal pattern is, in general, similar to that of modern Caninae. The most important distinction from later Caninae is that the region medial to the coronal sulci is very narrow.
During the Mio-Pliocene Caninae radiation, the evolution of the cerebrum took place mainly in the region medial to the coronal sulci, in the form of different sulcal patterns, and in the region rostral to presylvian sulcus, in the shape and relative size of the proreal gyrus. During the radiation, several sulcal patterns and types of proreal gyri evolved.
One early outcome of this radiation was the genus Vulpes. Its earliest known species, Vulpes stenognathus, has a brain comparable to that of modern Vulpes. In all living Vulpes species, the coronal sulci form a pentagonal outline on the dorsal surface of the cerebrum and the proreal gyrus is short. The arctic fox, Vulpes lagopus, differs somewhat from the other foxes in that the frontal lobes are higher and the proreal gyrus is better developed than in any other fox. The coronal sulci of two other foxes, Urocyon cinereoargenteus and Otocyon megalotis, form a heart-shaped outline. The proreal gyrus in Urocyon species is small and similar to the one seen in Vulpes. In Otocyon the length of the cruciate sulcus, and the length and the bilateral constriction of the proreal gyrus, are variable.
Another outcome of this radiation was the dog-like species (Canini). The brain of their earliest representative, Eucyon davisi, is more primitive than that of Canis and in some aspects comparable to that of Vulpes. The proreal gyrus is bilaterally constricted, but it is shorter than the one in similar-sized living Canis. The expansion of the sigmoid gyri is comparable to that of Vulpes, the pyriform lobe is more exposed than in jackals, the ectolateral sulcus is not connected with the suprasylvian sulcus and there is no or only a very shallow endolateral sulcus.
The genus Nyctereutes appeared in Eurasia during the Pliocene. The endocranial casts of Pliocene Nyctereutes have a long and narrow proreal gyrus, and the coronal sulci form a heart-shaped outline on the dorsal surface of the cerebrum. These two features of Nyctereutes resemble Cerdocyon and give further support to the phylogenetic schemes that consider the two taxa to be related.
During the Pleistocene the evolutionary history of Nyctereutes was characterized by a reduction in size. However, living Nyctereutes differs not only in size from its Pliocene ancestors, but also in certain cranial and neural features. The most important distinction in its external brain anatomy is the form of the proreal gyrus, which is broader and lower in the living species than in the Pliocene forms. Also, the skull of the living raccoon dogs has a shorter rostrum and the caudal palatine border extends further caudally than in its Pliocene relatives. Since similar changes occur in the brain of other canids with short rostra (including Lycaon, Enhydrocyon, Borophagus and Speothos), it is possible that these changes in the form of the proreal gyrus of Nyctereutes are due to the shortening of their rostrum.
There is a considerable variation in the general shape of the brain of the South American Caninae. This is due to the general shape of the skulls, which are equally variable. Apart from these differences, all South American canids have well developed proreal gyri (although the exact shape and size shows a considerable variation) and three basic outlines of the sulcal patterns of the coronal and ansate sulsi: heart-shaped (Lycalopex culpaeus), parenthesis-like outline (the rest of the living Lycalopex species and Dusicyon australis) or orthogonal (Atelocynus and Speothos). In Chrysocyon brachyurus, the posterior sigmoid gyrus extents laterally more than the anterior sigmoid gyrus.
The cerebral hemispheres of all Canis-like species (members of Canis, Cuon, Cynotherium, Lycaon and Xenocyon) expand abruptly behind the presylvian sulcus. The proreal gyrus is long and bilaterally constricted, more so than in any other canid group. Additionally, in these species the sigmoid gyri expand very abruptly (with the exception of Canis edwardii). This creates an orthogonal or oblong outline of the ansate and coronal sulci on the dorsal surface of the cerebrum. This expansion is smaller in the Pliocene Canis lepophagus, while the Early and Middle Pleistocene Canis had brains with the modern morphology.
The external brain anatomy of the dwarf Cynotherium sardous (the only known fossil insular canid) is comparable to that of its mainland relatives. So, despite the fact that this canid evolved in an isolated island environment for more than 700,000 years (Lyras & Van der Geer, 2006), its brain did not change as the rest of its body did (whose size and relative proportions were considerably modified).
Brain evolution and craniodental specialization
Brain and skull specialization ― There is a common belief that species with larger or more complex brains are generally more intelligent and thus more successful. This idea was promoted by early authors (such as Marsh, 1879) and, despite negative criticisms (Edinger, 1962; Radinsky, 1982), it is still a widespread idea (Jerison, 1973; Colbert & Morales, 1991; Hone & Benton, 2005). There are many studies that show that species with larger brains perform better on intelligence tests (for example, Glickman & Sroges, 1966; Ehrlich et al., 1976; Riddell & Corl, 1977). However, there is no work that clearly indicates that increased intelligence leads to increased fitness.
The present study compares the specialization of the brain (as it is revealed in the endocasts) with the craniodental specialization of canids. The purpose of this comparison is to determine if there is any correlation between the rates of brain and craniodental evolution. Of course, it is not only the skull and teeth that evolve. Important modifications also take place in the rest of the body. However, due to the lack of a good fossil record of postcranial elements, it is necessary to focus on the skull. Therefore, the modifications of the skull and teeth should be considered as the minimum adaptations that can be measured, as in many cases the addition of postcranial adaptations would have increased the calculated specialization.
Since the main goal of this comparison is to investigate evolutionary processes, it will focus on a few genera that represent actual phylogenetic lineages. This approach considerably limits the amount of the available material since it is restricted not only to species from which complete endocasts are available (of which there are not many), but also to species that belong to the same evolving lineages. I have followed the common approach of comparing heterogeneous groups formed by animals that lived in successive geochronological time intervals. That increases the amount of available material; however, successive assemblages do not necessarily sample the same evolving lineages (Radinsky, 1975).
Principal component analysis of craniodental characters in canids ― Van Valkenburgh & Koepfli (1993) took 30 craniodental measurements on 27 species of modern canids and used them to produce a set of ratios that reflect three aspects of craniodental function: relative tooth size or shape; jaw muscle leverage; and relative rigidity of the dentary. A discriminant analysis of these ratios easily separated hypercarnivores as having relatively deep jaws, large canine and incisor teeth, reduced molar grinding areas and longer shearing blades on their lower carnassials (Van Valkenburgh & Koepfli, 1993). Later, Van Valkenburgh et al. (2003) compared the craniodental morphology of living canines, living hyaenids and borophagines. Their comparison was based on the results of a principal component analysis that was performed on a combination of the ratio data from the above-mentioned paper and new ones produced in a similar way from the two other groups. In a more recent paper, Van Valkenburgh et al. (2004) undertook a similar analysis in Borophaginae and Hesperocyoninae, but, due to the fragmentary and incomplete character of most fossil specimens, they focused on only three indices that were more simplified.
In the present study, a principal components analysis was performed with 13 out of the 16 ratios originally developed by Van Valkenburgh & Koepfli (1993) (see Table 1). The other three ratios were not used because complete specimens are rare in the fossil record. The results of the analysis are similar to that of Van Valkenburgh et al. (2003), although there are slight diferences in the importance of the factor scores and variances, mainly due to the addition of species (Hesperocyoninae, some Borophaginae and fossil Caninae).
The first principal component (PC1) represents 58 % of the total variance. The most significant variables of the first component are relative incisor size (I3), relative rostral breath (C1C1), jaw rigidity (IxP4) and the length of the m1 trigonid (M1BS). The second axis (PC2) accounts for another 14 % of the total variance, with the most important variables being the relative upper molar grinding area (RUGA) and the relative size of m2 (M2S).
The first principal component (PC1) can be used to infer hypercarnivorous adaptation. Species with relatively larger incisors, broader muzzles, deeper jaws and longer m1 trigonid have high positive scores on this component, whereas those with less hypercarnivorous features have low negative scores on this axis and positive scores on the second (Fig. 45).
Fig. 45. Scatter diagram of the first two principal components of canids (for further details see Appendix 4.2).
Brain and skull evolution in canids ― The first canid group that evolved towards hypercarnivory was the Hesperocyoninae. The trend towards hypercarnivory can be best seen in a hypothesized lineage from Mesocyon to Cynodesmus and then to Enhydrocyon (for more details, see the phylogenetic analysis of Wang, 1994). Of these genera, endocranial material is known from Mesocyon coryphaeus, Cynodesmus thooides and Enhydrocyon stenocephalus. Unfortunately, there is no complete mandible known of E. stenocephalus, so it cannot be included in the principal component analysis. However, considering its overall morphology (Fig. 14), it is obvious that Enhydrocyon was more adapted towards hypercarnivory than either Mesocyon or Cynodesmus.
Fig. 46. Simplified phylogenetic diagram of two Borophaginae clades (Aelurodontina and Borophagina), based of the analysis of Wang et al. (1999).
The brain of Enhydrocyon stenocephalus is not more fissurated than that of Mesocyon or Cynodesmus (see above). The sulcal length index of E. stenocephalus is practically the same as that of Mesocyon. There are some differences between the brains of Mesocyon and Enhydrocyon, but they have to do with general shape (the brain of E. stenocephalus is more compressed anteroposteriorly) and are related to packing problems due to shortening of the skull (Fig 14).
The second canid group is the Borophaginae, that is adequately represented by both endocasts and skulls, and therefore can be explored in more detail. A series of endocasts and skulls from Archaeocyon to Desmocyon, and then to Tomarctus-Aelurodon, Paratomarctus-Borophagus and Paracynarctus, illustrates the increase in the degree of cortical folding and the craniodental specialization in Borophaginae (Figs. 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 46). Desmocyon matthewi has the same sulci as Archaeocyon leptodus, yet they are longer and there is also a small lateral expansion of the rostral part of the coronolateral sulcus. As a result, the gyral length index of Desmocyon matthewi is almost twice that of in Archaeocyon leptodus.
The later Borophaginae are represented by Paracynarctus sinclairi, Tomarctus hippophagus and Paratomarctus temerarius. Their brains show greater development of the sigmoid gyri and an even greater lateral expansion of the coronal sulci. The somatic-sensory and motor-control cortical areas of living carnivores are located on the sigmoid gyri (Welker & Campos, 1963). Therefore, it is possible that the expansion of the sigmoid gyri is related to the development of better sensorimotor control abilities in those species (Radinsky, 1973). The development of this cortical area, which took place rather quickly, just after the end of Hemingfordian (16 Ma), was an important step towards the greater complexity of the Borophaginae brain. As a result of this expansion, the total sulcal length was increased and the later canids are located further to the right in Figure 47.
Fig. 47. Craniodental specialization (as it is expressed by the first principal component - PC1 - of the analysis) and gyral length index (the ratio between the total length of the superficially exposed gyri and the cubic root of the endocast volume) plotted against the species temporal distribution. The solid lines conect species which are phylogenticaly related, the dotted line indicates a more distant relationship. 1, Hesperocyon gregarius; 2, Archaeocyon leptodus; 3, Otarocyon cooki; 4, Mesocyon sp.; 5, Desmocyon matthewi; 6, Paracynarctus sinclairi; 7 Tomarctus hippophagus; 8, Paratomartus temerarius; 9, Aelurodon ferox, 10, Epicyon saevus; 11, Borophagus secundus; 12, Vulpes vulpes; Canis lupus; 14, Lycaon pictus. See Appendix 4.2 for data and specimens used for the PC1 and Appendix 3 for the gyral length index.
Tomarctus is phylogeneticaly related to Aelurodon, and Paratomarctus is phylogeneticaly related to Epicyon and Borophagus. These two lineages of large-sized hypercarnivorous canids have brains that are not more convoluted than their ancestors. Thus, although there was a leap forward from Desmocyon to the later borophagines, very few changes took place in the brain after that point.
The last canid group is Caninae, in which brain is relatively larger and more fissurated than that of the other two groups (Fig. 47). There is the even more lateral expansion of the sigmoid gyri and the development of the cortex anterior to the presylvian sulcus. The Caninae are represented by relatively unspecialized members. The only examples toward hypercarnivory are some large sized Canis spp., Lycaon, Xenocyon, Speothos and Ducicyon. Their brains are not more convoluted than their less specialized relatives.
The importance of brain in carnivore evolution ― Based on the data presented above, the degree of cortical folding evolved in a punctuated manner. Brains become more fissurated with the expansion of particular areas, such as the sigmoid gyri and the prefrontal cortex. According to the model of Uylings & Van Eden (1990), particular cortical fields rapidly became more developed and better separated (Fig. 48). It is reasonable to assume that the addition of new areas can be beneficial for the establishment and evolution of a species. However, it is not possible to say if such development of the brain is more crucial than any other development such as of the teeth. What can be demonstrated is that such brain development took place twice in canids, in Borophaginae during the Middle Miocene and in the Caninae during the Late Miocene – Pliocene. In both cases, it coincides with a successful establishment of the group as evidenced by their extensive radiation. It should be noted, however, that the more complex brains did not prevent the Borophaginae from going extinct, nor other groups, like the Nimravidae (which had a rather complex brain for their time, see Radinsky 1975).
Fig. 48. Schematic diagram of the evolution of cerebral cortex redrawn from Uylings & Van Eden (1990). Squares represent cortical fields. In the “primitive” mammals the cortical fields are smaller and have a considerable overlap. In the “higher” mammals the cortical fields are larger and are better separated. According to this model, during cortical evolution there is an expansion and, at the same time, a better separation of cortical fields.
An interesting case is the stasis in brain evolution during the development of hypercarnivorous dietary adaptations. Such cases are seen in the presumable transitions from Mesocyon to Enhydrocyon, from Paratomarctus to Borophagus, from Tomarctus to Aelurodon. A possible explanation for this phenomenon could be that this stasis is related to energetic constraints.
Among living terrestrial carnivores, half of species larger than about 21 kg prey on taxa as large or larger than themselves, whereas smaller carnivores can subsist on much smaller prey, such as invertebrates and rodents (Carbone et al., 1999). Further, during the transition to taking larger prey, carnivores evolve larger size and specific craniodental adaptations. These trends led Van Valkenburgh et al. (2004) to suggest that evolution towards hypercarnivory reflects energetic constraints imposed by tradeoffs between foraging effort and food acquired. Nervous tissue is one of the most energetically costly tissues in terms of both development and maintenance (Armstrong et al., 1995; Kaas, 2000; Köhler & Moyà-Solà, 2004). Because hunting large prey requires more energy (Carbone et al., 2007), it contradicts the development of more complicated and, therefore, energetically more expensive brains.
Conversely, larger or more complicated brains have many advantages, including greater efficiency of the sensory organs and better motor control. However, in the cases presented above, what was obviously more important was the development of craniodental adaptations rather than the development of the brain. Therefore, although there is a general selective pressure towards the common development of complex brains (Jerison, 1973, 1990, 1991; Radinsky, 1978b), in these examples development occurred primarily in the teeth and skulls.
Summary ― Evolution within some canid clades shows a clear tendency towards hypercarnivory, which means that they increased the percentage of meat in their diet. The representatives of these evolutionary lineages evolved craniodental characters - deep jaws, large canine and incisor teeth, reduced molar grinding areas - that were related to the general tendency of hypercarnivores to prey on large-bodied animals.
During the transition to hypercarnivory there was stasis in brain evolution in Canidae, from Mesocyon to Enhydrocyon, from Paratomarctus to Borophagus and from Tomarctus to Aelurodon. Previous work concluded that the development of hypercarnivorous craniodental characters is related to energetic constraints. Alternately, some studies indicate that the neural tissue is energetically one of the most expensive. Therefore, based on these conclusions and on the data of the present study, it might very well be possible that the stasis in brain evolution is due to energetic constraints. In other words, evolution favours the development of the skull and teeth (in order to facilitate the predation on larger animals) more than the development of brain.
In order to investigate the evolution of the canid brain, casts of the brain cavity (endocranial casts or endocasts) from Recent and fossil species were studied. The study of fossil brains is limited to their external anatomy. However, cortical folding, although just an external feature, can be used to infer internal differences between brains.
The examination of brain morphology is focused on cortical folding, which was studied descriptively (the sulcal pattern) and quantitatively (the length of the superficially exposed gyri). Further, the degree of cortical folding was compared with some particular craniodental parameters that are informative to infer dietary adaptations.
The evolution of general brain morphology and sulcal pattern ― Canid brain evolution followed three independent, yet convergent paths. This occurred because the evolutionary history of the family is characterized by an early tripartite separation, which gave rise to three independent evolutionary radiations. Each group began with a simple brain, which had pear-shaped cerebral hemispheres, small temporal and frontal lobes, a widely exposed cerebellum and a straight cerebellar vermis. The evolution of the three subfamilies led to the development of brains with similar morphology and sulcal pattern. The extent to which these evolutionary changes took place is different in the three subfamilies. The brain of the Hesperocyoninae (the first to radiate and also the first to go extinct) did not evolve so much as that of Borophaginae (the second group that radiated) and Borophaginae did not reach the evolutionary stage seen in Caninae today (Caninae started to radiate relatively recently).
Prohesperocyon ― The evolutionary history of canids began with Prohesperocyon, a form that was transitional from Miacidae to Canidae. The cerebral cortex of this genus had only two sulci (coronolateral and suprasylvian) and the cerebellum, which was widely exposed, had a straight cerebellar vermis and small cerebellar hemisphere.
Hesperocyoninae ― The oldest member of the subfamily is Hesperocyon. Its brain is similar to that of Prohesperocyon. The most important difference is that Hesperocyon had more globular cerebral hemispheres, something that is related to differences in their cranial proportions (Hesperocyon is more brachy-cephalic).
The cerebral hemispheres of the later Hesperocyoninae, Mesocyon, Cynodesmus and Enhydrocyon, had a more extensive cerebral cortex. Cynodesmus had one additional sulcus (the ectosylvian), Mesocyon had yet another (the ectolateral) and Enhydrocyon had four sulci (coronolateral, suprasylvian, ectosylvian and presylvian).
Osbornodon fricki was one of the last Hesperocyoninae. Its brain was far more advanced than that of any earlier member of the family, as it had a small sylvian sulcus and the rostral part of the coronolateral sulci bow out laterally, indicating a separation between the coronal and lateral parts. Early signs of such an arrangement can be seen in Mesocyon and Cynodesmus and even in a few Hesperocyon specimens; however, it is in O. fricki that it becomes distinct.
Borophaginae ― Early members of this subfamily (Archaeocyon, Otarocyon, Rhizocyon) had brains similar to that of Hesperocyon. An important change took place in two later Borophaginae, Phlaocyon and Desmocyon, which developed three major sulci (coronolateral, ectolateral and ectosylvian).
Desmocyon gave rise to all later Borophaginae, including Paracynarctus, Tomarctus and Paratomarctus. The genus Paracynarctus had five major sulci (the coronolateral, ectolateral, suprasylvian, ectosylvian and presylvian), a very well formed orbital gyrus and possibly a small Sylvian sulcus. Tomarctus and Paratomarcus had, in addition to the sulci mentioned above, a cruciate sulcus and a very well formed Sylvian sulcus. The cruciate sulcus, and commonly the cortex around that sulcus, was better developed in Paratomarctus than in Tomarctus. These two genera gave rise to two evolutionary lineages of hypercarnivorous canids of large size (such as Aelurodon, Epicyon and Borophagus). The evolution of the brain continued in these lineages without any radical innovations.
Caninae ― As in the case of the other two subfamilies, the earliest Caninae had a relatively simple brain. The major evolutionary radiation of the subfamily began in the Late Miocene-Pliocene, when the first representatives of the basic Caninae lineages appeared. During this period, the evolution of the cerebral hemispheres took place mainly in the cortex medial to the coronal sulci (sigmoid gyri) and rostral to the presylvian sulci (proreal and orbital gyri). The expansion of these gyri coincided with the beginning of the Caninae adaptive radiation. Thus, different sulcal patterns appeared in each major evolutionary line of the subfamily.
Vulpes has a short proreal gyrus and small expansion of the sigmoid gyri; the coronal sulci form a pentagonal outline on the dorsal surface of the cerebrum. Urocyon has a short proreal gyrus and the coronal sulci form a heart-shaped outline. The lateral expansion of the sigmoid gyri in Eucyon is also very small; however, the proreal gyrus is longer. The species of Canis, Cuon, Cynotherium, Lycaon and Xenocyon have a long and bilaterally constricted proreal gyrus, and the sigmoid gyri expanded very abruptly. In these genera the coronal and ansate sulci form an orthogonal outline on the dorsal surface of the cerebrum; the only exception is Canis edwardii, in which the coronal sulci form a parenthesis outline. All the South American Caninae have a relatively long proreal gyrus (although its exact size and form presents a considerable variation) and three basic patterns of the coronal sulcal outline, a parenthesis, heart-shaped and orthogonal. The Pliocene Nyctereutes have a long and bilaterally constricted proreal gyrus and the coronal sulci forming a heart-shaped outline on the dorsal surface of the cerebrum. The outline of the coronal sulci is the same in the living Nyctereutes; however, the proreal gyrus is broader and lower than that of its Pliocene ancestors. This is related with the shortening of its skull length during the last 2 million years.
Relation between brain shape and length of the rostrum ― The brains of species with short rostra have massive frontal lobes. In these species the posterior border of the palate extends more caudally than that of their close relatives with long rostra. The posterior position of the posterior palatine border affects the position of the cribriform plate, which also moves backwards. As a result, the olfactory bulbs (which are housed behind the cribriform plate) are pushed caudally, thereby causing the anterodorsal compression of the anterior lobes of the brain. In most cases, the shortening of the face is related to adaptation towards hypercarnivory in order to prey on larger animals (such as in Enhydrocyon, Borophagus and Speothos). In one case, that of Nyctereutes, the shortening of the rostrum took place without adaptation to a more carnivorous diet. However, in this case as well, the shortening of the rostrum had an effect on the shape of the anterior part of the brain.
Parallel evolution of the sulcal pattern ― The similar sulcal patterns seen in living carnivore families evolved independently in each family. A more extensive study of the evolutionary history of the brain in Canidae shows that the same phenomenon took place in independent lineages within the family. The cruciate sulcus appeared independently four times within Canidae; once in Hesperocyoninae (in Osbornodon), twice in Borophaginae (in Phlaocyon and Tomarctus) and once in Caninae (in Leptocyon). A similar parallel development occurred in the Sylvian, endolateral, ectolateral and ectosylvian sulci (three times within the family).
Cortical folding and craniodental specialisation ― Some canid lineages evolved large size and particular craniodental characters, such as deep jaws, large canine and incisor teeth and reduced molar grinding areas, which allowed them to prey on large-bodied animals. In these cases there is stasis in brain evolution. A possible explanation for this phenomenon may be that carnivores that prey on large animals expend relatively more energy during predation. Since neural tissue is energetically one of the most expensive tissues, the evolution of the brain may be too costly to occur. Therefore, despite the general trend towards larger and more complicated brains, and despite the obvious advantages of a more complex brain, in these particular cases the evolution of the teeth and skulls was of greater importance than that of the brain.
The idea to investigate the evolution of the canid brain was suggested to me by Michael Dermitzakis (AMPG), John de Vos (NNM) and the late Paul Sondaar. I am deeply indepted to them for their guidance and suport thoughout this study. I am most grateful to George Papadopoulos (Aristotelian University of Thessaloniki) for his guidance on various subjects of the neuroanatomy of the living canids during many stages of this work.
I further thank the following for sharing their knowledge with me on various aspects of carnivore evolution and palaeoneurology: Richard Tedford (AMNH), Marina Sotnikova (GIN), Blaire Van Valkenburgh (University of California), Tom Rothwell (AMNH), Lorenzo Rook (University of Firenze), Athanassios Athanassiou (Hellenic Ministry of Culture), Dimitri Ivanoff (NNHM), George Koufos (AUT), Harry Jerison (University of California) and John Finarelli (University of Michigan).
For their help with statistics, I thank Margarita Dimiza (University of Athens), Bas Van der Geer and Marieke de Loos (University of Eindhoven).
For their permission to study the collections in their care and for their assistance during my visits, I thank Chris Smeenk and Hein Van Grouw (NNM); John Flynn, Chris Norris, Susan Bell and Ivy Rutzky (AMNH); Bruce Patterson, Lawrence Heaney, Bill Simpson, Bill Stanley and Mery-Beth Prondzinski (FMNH); Ralf-Dietrich Kahlke and Lutz Maul (IQW); and Dmitry Ivanoff (NNHM). I thank Mathew Colbert and the staff of the X-ray CT facility of the University of Texas, and the staff of the 2nd Radiological Clinic of the University of Athens for analyzing three fossil canid skulls by means of computed tomography.
An earlier draft of this paper was reviewed by members of my dissertation committee, Michael Dermitzakis, Maria Triantaphylou, Konstantin Doukas, George Theodorou and Paris Pavlakis (University of Athens), George Papadopoulos and George Koufos (Aristotelian University of Thessaloniki), to whom I am most thankful. My sincere thanks are also due to Alexandra Van der Geer (AMPG) for correcting various draft versions of this paper, translating the German and Italian literature, and discussing problems concerning the ontogeny of the skull. I am also thankful to Alexandra Petal for correcting part of the original manuscript.
I am grateful to my reviewers, Dmitry Ivanoff and Lars Werdelin. Not only they critically reviewed the manuscript, correcting the numerous inconsistencies, spelling errors and misprints of the text, but they also pointed out and repaired an unintentional (but also unnecessary and harmful) teleology that was spread throughout the text. Additional critical changes and corrections were done by Lars Van den Hoek-Ostende, to whom I am thankful.
This study has been co-financed within Op. Education by the ESF (European Social Fund) and National Resources.
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Studied endocasts from fossil canids and the reference on which the attribution of the taxon was based.
Studied endocasts from living and recently extinct canids. See materials and methods section for institutional abbreviations.
Endocast measurements. Asterisks indicate estimates.
Factor scores of the first four principal components.
Values of the first two principal components used in the diagram of Figure 45.
Specimens used in the PCA and the values of their morphometric ratios.