Caspiconcha sp.

Jenkins, Robert G., Kaim, Andrzej, Little, Crispin T. S., Iba, Yasuhiro, Tanabe, Kazushige & Campbell, Kathleen A., 2013, Worldwide distribution of the modiomorphid bivalve genus Caspiconcha in late Mesozoic hydrocarbon seeps, Acta Palaeontologica Polonica 58 (2), pp. 357-382 : 371-372

Caspiconcha sp.

Fig. 11.

Material.—NMNS PM25523 ( Fig. 11A): One incomplete adult shell with two articulated valves lacking the posterior part and much of the ventral part of the right valve; NMNS PM25524 ( Fig. 11B) partially preserved anterior part of the left valve of large specimen; NMNS PM25525 ( Fig. 11C) juvenile mould of a shell with two articulated valves in butterfly position preserving some shell material in the umbonal areas of both valves. All specimens from the Utagoesawa Creek locality, Yubari City, Hokkaido, seep carbonate in Main Part Formation, Upper Albian, Lower Cretaceous.

Description.—The shell is large (the largest specimen NMNS PM25523 is 247 mm long) and elongated, subtrapezoidal in lateral aspect, with strong inflation in the dorsal part of the central flank running obliquely towards posterior part of the ventral margin. The valves are equivalve and strongly inequilateral. The umbones are close to the anterior end, situated at, or slightly below, the hinge line. The shell is thick, especially in the anterior part where the growth lines become more densely packed. A triangular depression runs from the umbonal area widening towards the mid flank. The dorsal margin is slightly curved around its whole extent. The posterior margin in the adult shells is not preserved, while in the juvenile it is rounded, but strongly curved at the junction with ventral margin which, in turn, becomes slightly rounded starting from this point. The exterior ornament consists of commarginal growth lines, more pronounced on the shell anterior.

The shell interior is well preserved only in NMNS PM25524 and represented by a generally smooth surface with a large and elongated anterior adductor muscle scar bordered posteriorly by a stout myophoric buttress. Oblique grooves are visible on the mould surface of the juvenile NMNS PM25525 corresponding to the oblique ridge of the shell. The hinge is stout with very long ligamental groove. The other features of the hinge are obscured by diagenesis.

Remarks.—The specimens are partially silicified including some parts of adjoining sediment what hampered appropriate preparation. We could not observe the internal surface of the most completely preserved specimen (NMNS PM25523); thus, we decided to leave this species in open nomenclature. The species is similar to C. whithami in having a very thick shell, subtrapezoidal shape, and elongated anterior adductor muscle scar, but differs in having a curved hinge line and strong inflation of the lateral flanks. It differs from C. major in having very thick subtrapezoidal shell, with strongly inflated lateral flanks and elongated anterior adductor muscle scar. The Utagoesawa Caspiconcha is probably a new species, but we feel this needs confirmation from better preserved material.