Circunula, 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 : 15-16

Genus Circunula new genus

Type species. Ostrea cotyledon Contejean, 1859 ; lectotype (see under species); Lower Kimmeridgian, France and Switzerland.

Diagnosis. Strongly inequivalve and thin-shelled. Left valve (LV) weak capacious, with upturned ventral margin, right valve (RV) usually dorsally convex and ventrally concave, margins upturned. Outline circular or suborbicular to oval, rarely subquadrate or subtriangular. Attachment area large (almost entire LV). Both valves smooth with fine concentric growth lines, RV with weak antimarginal riblets and occasionally with radial grooves and commarginal pustules. Antimarginal riblets or grooves restricted to attachment area of LV (rarely observable). Left valve lacks umbonal cavity. Posterior adductor scar posterodorsal, round (gryphaeate), deeply inserted. Juveniles with well-developed chomata on dorsal margin, lacking in later growth phases. Ontogenetic change of hinge morphology from triangular-ostreate to prosodetic twisted with a narrow, ventrally projected posterior bourrelet.

Microstructure of RV with outer shell layer of slightly reclined prisms, middle shell layer complex crossfoliated and thin low-angle (probably) regularly foliated structures; chambering absent; microstructure of LV not seen.

Etymology. Word combination derived from Latin circulus (= circle line), because of the characteristic round shape, and affix – nula, adopted from Catinula . The gender is female.

Remarks. The common oversight, especially in the older literature, of both chomata and coiling preferences of Jurassic oysters during early ontogenetic growth phases hampers comparisons with other taxa and evaluation of their taxon specificity, e.g., flexible coiling may be species-specific. However, none of the morphologically similar Jurassic genera as Liostrea (Liostreinae, Flemingostreidae), Catinula , Deltoideum and Pernostrea ( Gryphaeinae , Gryphaeidae ) develops chomata at any growth stage. This trait is considered generic, therefore, which warrants the separation of Circunula n. gen. from the taxa just mentioned. Furthermore, the other taxa do not share the preferred circular shape of Circunula . For example, Pernostrea has a spatulate outline , Deltoideum a strong pointed umbo associated with a concave posterodorsal margin, and Liostrea , which is also dorsally pointed, lacks radial riblets on its right valve. For species level comparisons see under C. cotyledon .

The phylogenetic position of the new genus is unclear, however. The presently tentative association with Pycnodonteinae is based on phenotypic similarities, including chomata, of Circunula n. gen. and small, largely attached pycnodonteine oysters (Phygraea Vialov, 1936, and Labrostrea Vialov, 1945 ) from the Upper Cretaceous ( Nestler 1965; Cooper 1992). There is presently no evidence of a vesicular shell structure in Circunula , however.