Sphenope, Vörös, 2013

Vörös, A., 2013, Sphenope, a new genus of Pygopidae (Terebratulida, Brachiopoda) from the Mediterranean Late Jurassic and Early Cretaceous, Fragmenta Palaeontologica Hungarica 30, pp. 1-14 : 4-6

Sphenope gen. n.

Type species – Terebratula misilmerensis Gemmellaro, 1871 .

Diagnosis – Large terebratulids with subtriangular or bilobate outline; strongly biconvex, maximum convexity at mid-length. Beak moderately high; foramen large, mesothyrid. Marked beak ridges, deep planareas. Lateral commissures arched or run obliquely; anterior commissure nearly straight or ligate. Opposite sulci may be present. Shell smooth. Cardinal process massive, crenulated. Adductor muscle scars short, slightly divergent. Hinge plates reduced, crural processes crescentic. Loop narrow, short; transverse band faintly arched.

Etymology – Wedge-shaped; sphen, σφήν (greek) = wedge.

Species – S. misilmerensis ( Gemmellaro, 1871) , S. bifida sp. n.

Discussion – By its external appearance (large, smooth, laterally expanded) and simple internal morphology, Sphenope fits well to the family Pygopidae . It is imperforate, therefore its best place is in the subfamily Triangopinae , in the present classification.

Sphenope can be distinguished clearly from the other two genera of Triangopinae ( Figs 1–12 ).

There are several differences from the contemporary ( Late Jurassic to Early Cretaceous) Triangope . In outline, Triangope shows cardinal protrusions (wider hinge margin), and nearly straight anterior margin, therefore it tends to be subpentagonal, while Sphenope has narrow hinge margin and straight or concave lateral margins, i.e. a more wedge-shaped outline. In lateral view, Triangope has straight lateral commissures in contrast to the oblique and ventrally arched lateral commissures of Sphenope . In cross section (at about the posterior one-third of the shells), Triangope does not show planareas, but the lateral commissures run in a deep furrow bordered by rounded swells, while Sphenope has deep and smooth planareas, delimited by marked beak ridges; the lateral commissures approach the ventral beak ridges. In anterior view, the commissure of Triangope shows a widely arched and mostly deep sinus, while the anterior commissure of Sphenope is straight to gently uniplicate, or tends to be ligate.

The differences between Sphenope and the Pliensbachian Securithyris are much fewer, though they are rather distant in time. In lateral view, Securithyris has straight commissures, in contrast to the oblique and ventrally arched lateral commissures of Sphenope .In cross section, the planareas of Securithyris are less deep and shorter and its beak ridges are weaker than those of Sphenope . Interestingly, the opposite sulci and the ligate anterior margin occur also in Securithyris , at the species S. filosa (Canavari, 1880) (see VÖRÖS 2009: pls XXIII–XXIV).

The above discussion emphasises the taxonomic significance of the features of the lateral regions of brachiopod shells. It is advised to give more importance to these features in the classification of brachiopod groups of simple morphology as for example the pygopides.

Distribution – Tithonian and Berriasian; Sicily ( Italy), Bakony and Gerecse ( Hungary).

Figures

Figs 1–12. Diagnostic differences between Sphenope gen. n. and the other two genera of the subfamily Triangopinae.Drawings of dorsal (1, 5, 9), lateral (2, 6, 10), anterior (3, 7, 11) views and cross sections (4, 8, 12) are shown for comparison. Arrows on the dorsal views mark the locations of the cross sections (not to scale)