Sphenope bifida, 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 : 9-13

Sphenope bifida sp. n.

( Figs 26–37)

Pygope janitor ( Pictet, 1867) – FŐZY et al. 1994: 159 (pars), pl. 2, fig. 17 (non fig. 18). Gerecse Triangope ? aff. misilmerensis ( Gemmellaro, 1871) – VÖRÖS 2013: 403, pl. 5, figs 3–5. Gerecse

Holotype – HNHM (Budapest), inventory number: PAL 2013.27.1.

Type locality – Borzavár (Szilas Ravine), Bakony Mountains, Hungary. Collected by József Konda in 1982.

Type layer – Bed 41, Pink, micritic limestone, Berriasian.

Paratype – HNHM (Budapest), inventory number: PAL 2013.28.1 .

Etymology – Bifidus (latin) = bifurcate, split.

Diagnosis – Large, triangular Sphenope with opposite sulci and ligate anterior part. Cardinal process well-developed. Hinge plates reduced, form coherent plate with inner socket ridge. Loop narrow; transverse band very low, projects anteriorly.

Material – 37 specimens; Gerecse, Bakony, Sicily, Tithonian and Berriasian ( Table 1) .

Measurements – L = 40.8; W = 44.5; T = 23.0 (holotype, Figs 26–28).

Description – External characters: This is a medium-sized Sphenope with rounded subtriangular, ligate outline. The lateral margins are usually straight or gently concave; the anterior margin is bilobed. The maximum width is attained at around the anterior third of the length. The valves are moderately ventribiconvex. The maximum convexity lies near mid-length. The beak is rather low, erect to slightly incurved. The beak ridges are very strong. The foramen is poorly seen; it seems to be mesothyrid. The delthyrium is poorly preserved, low. True planareas are deep, wide and long. The lateral commissures run diagonally and ventrally arched; they approach the ventral beak ridges; consequently, the planarea occupies mostly the dorsal valve. The anterior commissure is rectimarginate. A well-developed, deep sulcus starts near the posterior part of the dorsal valve and runs medially to the anterior margin. The ventral valve bears a similar medial sulcus. Accordingly, a ligate anterior part develops, which is deeply incised in typical cases. The surface of the shells is almost smooth, except fine, irregularly spaced growth lines.

Internal characters ( Figs 36–37): There is a well-developed pedicle collar. The delthyrial cavity is rounded subcircular to subtrapezoidal in cross section showing the traces of muscle scars enhanced by thickenings of secondary shell material. The hinge teeth are very massive, strong; denticula are well-developed. There are thin, fused deltidial plates. The cardinal process is wide and forms an elevated platform with a markedly crenulated surface. The outer socket ridges are low and wide. The inner socket ridges bend considerably over the sockets. The hinge plates are strongly reduced; in some specimens they are somewhat masked by secondary shell material (callus). They are attached to the medial part of the inner socket ridges; their ventral surface forms a coherent plate with the ventral part of the inner socket ridges. The crural bases are narrow blade-like in cross section. The crural processes are subvertical and subparallel or crescentic in cross section. The loop is narrow and short; lateral flanges are not present. The transverse band is very low and forms the most distal part of the loop.

Remarks – The very remarkable bifid appearance of this species somewhat resembles the early (juvenile) stage of some species of Pygope , first of all P. janitor ( Pictet, 1867) . But S. bifida is significantly different, because its lobes are narrowly spaced and the incision between the lobes is rather sharp. Moreover, their anterior commissure is straight, in contrast to the sulcate commissure of P. janitor . For this reason, the P. janitor specimens figured by FŐZY et al. (1994) now are partly included to S. bifida .

Further important features of our specimens are seen on their lateral parts: they have marked, long beak ridge, and wide, flat planarea (or at least the division of the planarea which falls to the dorsal valve). It is also seen that the lateral commissure runs diagonally and seem to approach the ventral side. These features are shared with S. misilmerensis ( Gemmellaro, 1871) , described above, therefore S. bifida is believed to have the same generic position. The bifidate appearance clearly separates it from the securiform S. misilmerensis .

A morphological variant of this species shows a pronounced dorsal sulcus but it is only incipiently bilobate (its anterior part is only faintly incised) and has a characteristic bell-shaped outline ( Figs 34, 35). Only disarticulated valves of this form are available what is not sufficient to introduce a separate species. G. Vigh labelled one of the specimens figured here ( Fig. 34) as “ Pygope triangulus excavatus ” but this name has never been published.

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