Sphenope misilmerensis ( Gemmellaro, 1871 )

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 : 6-9

Sphenope misilmerensis ( Gemmellaro, 1871)

( Figs 13–25)

Terebratula Misilmerensis, Gemm. – GEMMELLARO (1871): 5, pl. I, figs 6–7. Sicilia.

Triangope? misilmerensis ( Gemmellaro, 1871) – VÖRÖS (2013): 401, pl. 5, figs 1–2. Gerecse ( Hungary).

Material – 14 specimens; Gerecse, Sicily, early Tithonian to early Berriasian ( Table 1) .

Measurements – L = 36.2; W = 32.2; T = 22.5 (specimen in Figs 13–16).

Description – External characters: This is a large pygopid with variable, elongated oval to subtriangular, wedge-shaped outline. In many cases the outline may be termed as “axiniform” (axe-shaped). The lateral margins are usually gently or deeply concave. The anterior margin forms an almost perfect half-circle. The outline can be even bell-shaped. The maximum width is usually attained at around the anterior quarter. The convexity of the valves varies from medium to strongly convex; the maximum convexity lies near mid-length in adult specimens, while it is in the posterior one-third in juveniles. The dorsal valve is markedly flattened as compared to the ventral one. The beak is moderately high, erect to incurved in convex adults. The foramen is mesothyrid. The delthyrium is barely seen. The hinge margins are short and they meet the lateral margins at an obtuse angle and elevated cardinal extremities are formed. There are wide and deep planareas, delimited by blunt but marked beak ridges. On these deeply concave planareas the lateral commissures run diagonally, arched ventrally and join the ventral beak ridges. Accordingly, the planarea lies mostly on the dorsal valve. The anterior commissure is rectimarginate to gently uniplicate. The surface of the shells is almost smooth, except fine, irregularly spaced growth lines or some stronger growth rugae. In decorticated specimens the surface of the inner mould shows a tributary pattern of simple mantle canals.

Internal characters ( Fig. 22): The delthyrial cavity is rounded in cross section. The hinge teeth are well developed, strong; denticula is present. The cardinal process forms a wide platform with a markedly crenulated surface. There are definite, narrow and short, slightly divergent adductor muscle scars. The outer socket ridges are rather massive and wide. The inner socket ridges bend considerably over the sockets. The hinge plates are very much reduced; they grow from the medial part of the inner socket ridges and inclined dorsally at around 30°. Their ventral surface forms a coherent plate with the ventral part of the inner socket ridges. The crural bases are indistinct, point-like in cross section. The crural processes are subvertical and subparallel and crescentic in cross section. The posterior part of the loop was not recorded.

Remarks – This is an almost forgotten species; after the original description by GEMMELLARO (1871) it was mentioned only by HOFMANN (1884) in a faunal list from the Gerecse, but this specimen was discovered in the collections of the Geological and Geophysical Institute of Hungary. More recently, “ T. misilmerensis ” was synonymised with Triangope triangulus (Valenciennes, 1819) by DIENI & MIDDLEMISS (1981: 40), but without any arguments. However, T. misilmerensis was correctly defined by GEMMELLARO (1871) who stressed the differences from T. triangulus , namely, T. misilmerensis is more convex, the lateral part is different and the median depression on the dorsal valve does not continue to the anterior margin, i.e. the anterior commissure has no sinus. Looking at Gemmellaro’s figures, one can see that the specimen has definite beak ridges (especially in the ventral valve), which delimit a long planarea, and the lateral commissure runs near the ventral beak ridge.

On the other hand, Gemmellaro’s figures are not the best in quality. The types of the species T. misilmerensis (now the type species of Sphenope ) were housed at the Museo Geologico G. G. Gemmellaro, Palermo. Dr Carolina D’Arpa, the curator of the palaeontological collections kindly informed the present author that one of Gemmellaro’s figured specimens seemed to be lost. Furthermore, she sent photographs of other specimens of T. misilmerensis from the locality Montagna Grande; some of these are shown in Figures 23–25. This documentation convincingly supports that the specimens from the Gerecse ( Figs 13–21) are rightly identified with S. misilmerensis .

As it was demonstrated above, in the definition of Sphenope gen. n., S. misilmerensis is clearly different from T. triangulus , which has straight lateral commissures, sulcate anterior commissure and no definite planarea. The outline of S. misilmerensis is securiform that is the anterior margin is semicircular, in contrast to the nearly straight or even retracted anterior margin of T. triangulus .

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