To refer to this article use this url: http://www.scriptageologica.nl/07/nr135/a04
Tichosina inconstanta sp. nov.
Holotype – A conjoined pair, MGUH 28767 (Pl. 1, fig. 4a, b) from the Manchioneal Formation, Christmas River (J3), Jamaica.
Type locality and horizon – Christmas River, Manchioneal Formation (J3) (lower Pleistocene).
Material – About 400 shells of this species have been collected from localities on both Jamaica and Barbados.
Derivation of name – Name referring to the morphological variation seen in the species.
Diagnosis – Large, ventribiconvex Tichosina species of elongate oval to tear-drop shaped outline, variably uniplicate; pedicle foramen of moderate diameter.
Description – Large ventribiconvex valves, elongate oval to tear-drop shape outline. Maximum width over twice hinge width, occurring at about two-thirds valve length; cardinal extremities obtuse and rounded. Anterior commissure rectimarginate to sulcate; lateral commissures curved. Ventral valve almost nine-tenths as wide as long and about one-third as deep as long. Anterior profile uniformly convex; lateral profile with maximum curvature at umbo, elsewhere surface evenly curved except over anterior third where valve slopes steeply towards anterior commissure. Pedicle foramen variable, circular, oval or labiate, permesothyrid and of moderate diameter. Dorsal valve almost as wide as long and about one-fifth as deep as long. Anterior profile flatly convex with shallow, broad sulcus developing anteriorly from near mid-valve length; lateral profile convex over posterior half of valve, elsewhere surface slopes anteriorly. Ornament of variably accentuated, but subdued, concentric growth lines.
Ventral interior with small, elongate teeth and short pedicle collar. Dorsal interior with narrow loop; outer hinge plates, long and concave; flat-bladed crural bases form raised margin to outer hinge plates. Broad crura with wide transverse ribbon.
Statistics –
|
Discussion – The Jamaican material was originally compared (Harper, 1993) with Tichosina? lecta (Guppy, 1866), first described from the Eocene of Trinidad and subsequently re-described from the type area by Cooper (1979). This species is also reported from the Miocene and Pliocene of the Dominican Republic (Logan, 1987), although that material was relatively rare and poorly preserved. Based on data from Cooper (1979) and Logan (1987), a pooled sample of all three forms was investigated by PCA for the following variates: saglv, sagld, mwi and thickness (= dpv+dpd). Differentiation was achieved with respect of scores on the third eigenvector. The type specimens from Trinidad had higher scores (0.20, 0.42) on this eigenvector than the specimens from Jamaica and the Dominican Republic. The direction cosines of the third eigenvector (-0.69, -0.20, 0.60, 0.36) indicate the type T.? lecta to have relatively shorter, wider and more globose shells than those of the other two forms (Harper, 1993). However, larger samples of all three are required to confirm these apparent differences. More recent assessments of the Jamaican material (e.g., Harper et al., 1995) have compared it with T.? bartletti (Dall, 1920), a living species from the Caribbean with a more pronounced uniplicate anterior margin (Cooper, 1977). The large samples now available of the Pleistocene form together with details of its internal features allow, first a confident assignment to Tichosina and second the establishment of a new species. The cardinalia of the new species have the flat-bladed crural bases that are diagnostic for the genus. The large size, elongate oval shape and variably uniplicate anterior commissure associates the Pleistocene material with only T.? bartletti. However, that species has a small pedicle foramen and a more marked plication.
The large samples now available from both Jamaica and Barbados indicate extreme variation in the external morphology of this species. In fact, a large specimen of this species was originally ascribed to Gryphus? (Harper, 1993); information on the interior of that specimen is unfortunately lacking, but it now seems more probable, in view of the large and continuous sample variation now apparent (see statistics above), that the specimen should be included within the new species. It seems unlikely that populations of two or more species have been mixed; rather, the Pleistocene form of T. inconstanta had a fairly plastic external shape. It is also possible that the material described from Miocene and Pliocene of the Dominican Republic (Logan, 1987) may also belong within T. inconstanta, but more material of the latter is required to test this.
Although a large number of specimens of Tichosina are known from Barbados only two have yielded a complete set of measurements. These, together with a sample of 35 specimens from Jamaica, including the conjoined valves previous assigned to Gryphus? sp., were subjected to a PCA interrogation of the correlation matrix based on the following variates: saglv, sagld, mw, hinw, pmw, dpv and dpd. The first two eigenvectors account for 95% of the variation. The Barbados material could not be separated on these or indeed the next two eigenvectors that accounted together for further 2% of the sample variation. With regard to the variates analysed, both samples, including Gryphus? sp., are considered conspecific.
Occurrence – This species is comon on Jamaica at the type section of the Manchioneal Formation (J2) and Christmas River (J3), and occurs at Cruikshank Bay (J1) and Folly Point (J5); approximately 240 shells of this species are now known from the Jamaican localities. Additionally, eight specimens in the Lucas Barrett collection (BMNH) are from the ‘Tertiary and newer Pliocene’ of Jamaica (= Manchioneal Formation; see above). The localities on Barbados, Skeete’s Bay, Whitehaven (B1), Cluff’s Bay (B2) and Spring Bay, Ragged Point (B3) have yielded almost 160 shells; although a large number were recovered from the shell bed at Spring Bay, Ragged Point (B1), most are incomplete.
Superfamily Cancellothyridoidea Thomson, 1926
Family Cancellothyrididae Thomson, 1926
Subfamily Cancellothyridinae Thomson, 1926
Genus Terebratulina d’Orbigny, 1847
Type species – Anomia retusa Linné, 1767, by original designation; from the Recent of Norway.