(Figure 10 A–B)
2011 Ostreoidean—Hammer et al., tab. 2.
Material examined. Five specimens; fragmented and delaminated shells. See Appendix 1 for the list of specimens.
Description. Shell thin, roughly round in shape, only weakly inflated. Right valve thin, flat, visible only in cross-section. Left valve thin, weakly convex and larger than right valve; irregularly shaped, no sign of attachment found. Beak weakly incurved, accompanied by a shallow ventrally directed furrow. Weak muscle scar probably represent an adductor muscle scar. Shell ornamented by weak commarginal folds.
Remarks. The lack of a well demarcated external ligament, the thinness of the shells and the nature of cementation precludes our specimens belonging to the Ostreida Férrusac, 1822. Instead, the flat right valve facing the substrate and a larger covering left valve indicates that our specimens belong either to the Anomiidae Rafinesque, 1815, or the Dimyidae, Fischer, 1886 (e.g. Coan et al. 2000; Coan & Scott 2012). An anomiid origin is probably more likely because the dorsal furrow resembles the shell fusion scar characteristic for anomiids (Yonge 1977). However, the lack of well preserved right valves and adductor muscle scars means we cannot unequivocally include our specimens into either the Anomiidae and Dimyidae at present.
Palaeoecology. Our species was most likely a filter-feeder attached to hard substrates. Modern anomiids are shallow water forms, attached by a byssus protruding through a foramen in the lower valve (e.g. Yonge 1977). Recent Dimyidae is a cementing group found in deep water and cryptic habitats (e.g. Waller 2012). In the Mesozoic both groups were fairly common in shallow water habitats (e.g. Fürsich & Werner 1989; Hodges 1991; Todd & Palmer 2002).