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Pseudocriboconcha prinsi Sánchez de Posada sp. nov.
Derivation of the name - The species is named in honour of Dr. Cor Winkler Prins for his contribution to Carboniferous geology.
Type material – Holotype: a left valve, DGO 18001 (Pl. 1, figs. 1, 2). Paratypes: 24 left valves, DGO 18002 (Pl. 1, fig. 4); DGO 18003-18007, 18008, 18008a, b (Pl. 1, fig. 9), 18009, 13 right valves; DGO 18010 (Pl. 1, fig. 3); DGO 18011 (Pl. 1, fig. 5); DGO 18012-18016; and a carapace, DGO 18017 (Pl. 1, figs. 6-8). The material is housed with the palaeontological collections of the Department of Geology of the University of Oviedo, Spain (DGO).
Type locality – Abandoned quarry 2 km northeast of Entrago (Teverga, Asturias, Spain). Sheet number 52 (Proaza) of the Geological Map of Spain, scale 1:50.000.
Type horizon - Black limestones at the base of the San Adrián Member of the Alba Formation, Serpukhovian, late Mississippian (early Carboniferous).
Diagnosis - A species of Pseudocriboconcha with distinct pits covering a more or less extensive area of the lateral surface. Posterior ridge ending in dorsal and ventral spines. Each valve with a rounded rim subparallel to the borders. This rim bears a spine directed dorsally or anterodorsally and located slightly above the anterior end.
Description - Medium sized ostracods (maximum length of the largest valve about 1.5 mm) with suboval lateral outline. Dorsal border convex, devoid of dorsal angulation; ventral border also convex, less arched than the dorsal margin. Posterior margin convex, shorter than the more broadly convex anterior margin. Maximum height at the central third of the valve. Left valve larger than right valve and overlapping it. Bow-shaped projection at the posterior part of the ventral border. The lateral outline of left and right valves are very different. The left valves are ovoid in shape, with distinctly and regularly arched dorsal border. The right valves, in contrast, possess less curved, usually straight dorsal and ventral borders; the dorsal margin is provided with a dorsal angulation at the anterior end. On the whole, the lateral outline of right valves is more polygonal in shape than the outline of left valves (compare Pl. 1 figs. 1, 4 with figs. 3, 5). The left valves have a smaller L/H than the right ones (see Fig. 3).
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Fig. 3. Height/length diagram for 39 left and right valves of Pseudocriboconcha prinsi Sánchez de Posada gen. et sp. nov. |
A straight or nearly straight dorsoventral ridge similar to that of Healdia Roundy and Criboconcha Cooper exists in the posterior part of both valves. It extends from near the dorsal border to near the ventral border, delimiting a depressed posterior area. Two spines of variable length exist at the dorsal and ventral ends of the ridge. Sometimes the length of the spines exceed one third of the length of the valve. Judging from the specimens on hand with long, well preserved spines, the ventral spine is usually longer than the dorsal one. Nevertheless, the opposite is true in some specimens. Uncommonly, the spines are reduced to tubercles.
A more or less extensive area of the lateral surface anterior to the ridge is sculpted with rather big pits similar to those characteristic of Criboconcha. In some specimens (Pl. 1, fig. 3, for instance) the pitted area reaches the anterior fourth of the valve while in others it does not exceed the posterior third. On the whole, pits are more closely packed towards the posterior part of the carapace. In examples where the pitted area covers the middle region of the valve, pits are interrupted in the area of adductor muscle scars.
Each valve with a rounded rim subparallel to the borders. These rims are easily noticeable along the dorsal, anterior and posterior borders of isolated valves. At the posteroventral border the rim deflects from the border to the ventral surface of the valve and becomes smaller anteriorly. The rim of the left valve is wider and more rounded than that of right valve. At the borders the rim of the left valve lies beside the contact margin and the hinge. A narrow depressed area exists between the rim and the border of right valve (that of the dorsal area can be, in fact, an accommodation groove). Several valves show a spine at the anterior part of the marginal rim. Its absence in other valves is probably a preservational artifact.
As a consequence of silicification, the hinge can not be studied in detail, but some relevant traits are visible. A straight and wide furrow ventrally bounded by a continuous list is present on the left valve and, in its ventral position, a shallow sulcus and a discontinuous list seem to exist. The ventral furrow appears to continue in short anterodorsal and posteroventral depressions (hinge sockets?, sensu Adamczak, 1976) giving the hinge a tripartite condition, but this is uncertain.
Measurements (in mm) of figured specimens
Holotype DGO 18001: L=1.06; H=0.72.
Paratype DGO 18002: L=0.8; H=0.56.
Paratype DGO 18008b: L=1.0; H=0.6
Paratype DGO 18010: L=1.27; H=0.79.
Paratype DGO 18011: L=0.8; H=0.44.
Paratype DGO 18017: L=0.43; H=0.32.
Remarks – The ornamentation of the valves and, to some extent, the lateral outline of the studied material are quite similar to those of Criboconcha Cooper, 1941. However, our specimens differ from that genus (and also from other Healdiidae) in several important features that, in turn, point to bairdiocypridoidean affinities. In fact, although the hinge cannot be studied in all the details, the differences and similarities in the hinge line of the Spanish material, and that of Healdiidae and Bairdiocypridoida, respectively, are remarkable, especially in the lateral right view of the carapaces. Compare, for instance, DGO 18017 (Pl. 1, fig. 6) with both the specimens of Bairdiocypris shown in Adamczak, 1976 (especially with B. lamellaris Adamczak, 1976, and B. marginata Adamczak, 1976), on one hand, and the specimens of Criboconcha figured by Cooper (1941) on the other. Besides the apparent tripartite character of the hinge, the hinge line of Pseudocriboconcha extends along the whole dorsal border (and not just along the posterodorsal margin), with a roughly anteroposterior direction and not inclined towards the posterior as in Healdiidae. Moreover, a bow-shaped projection is present at the ventral border and the closure of the valves is far from the holosolenic contact characteristic of the Healdiidae.
Additional differences concern the shape. In the Spanish material, the right and left valves are rather unlike in lateral outline. In particular the right one, with a straight dorsal border, is quite different from Criboconcha.
The new genus differ from all known Bairdiocyipridoidea in its distinctive ornamentation. The asymmetrical, wide carapace of Pseudocriboconcha show affinities with Pachydomellidae. The spinose pachydomellids studied by Becker (2000) neither show a posterior ridge nor a pitted lateral surface.