Helvetostrea, Koppka, Jens, 2015

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 36

Helvetostrea new genus

Type species. Ostrea sequana Thurmann & Etallon, 1862 ; lectotype designation herein (see under species); Upper Oxfordian, Switzerland (Canton Jura).

Diagnosis. Inequivalve, left valve moderately inflated, outline irregularly ovate, rectangular to slenderspatulate; umbo truncated owing to large attachment area. Outer surface with thick (squamous) irregular growth lamellae. Ligament area becoming very high (similar to Saccostrea ), growing essentially ventrally in adult; resilifer deep and flanked by equally broad bourrelets. Umbonal cavity (LV) deep; adductor scar round to high-oval, posterocentral to posterodorsal. Left valve with many large structural chambers, hollow or filled with sparry cement or sediment; internal structures of RV unknown.

Etymology. Word combination derived from Helvetia (= neo-Latin for Switzerland) and the Recent oyster genus Ostrea . The gender is female.

Remarks. Species of Helvetostrea differ from Liostrea , Pernostrea , Africogryphaea , and Deltoideum by developing a large Saccostrea -like ligament area, a deep umbonal cavity and many large shell chambers, the latter eventually leading to considerable growth in thickness. They differ from most Praeexogyra species by being highly chambered, except for “P. ” hebridica (see discussion under Praeexogyra ). Helvetostrea is distinguished from P. hebridica and from Crassostrea species by having an essentially round and posterodorsal adductor scar and from Saccostrea by lacking chomata.

The genus differs from Ostrea (and actually the entire subfamily Ostreinae ) in its well-developed umbonal cavity and its adductor outline and position. In addition there is no evidence to assume brooding for any Jurassic, and in fact any Mesozoic oyster species, as is characteristic of Recent Ostreinae ( Malchus 1995; Malchus & Sartori, 2013; own data herein). The Middle Jurassic (Bajocian, Bathonian) Helvetostrea n. gen. explanata (Goldfuss, 1833) (b) may be the oldest representative of the genus currently known.