Pectinida, Gray, 1854

Hryniewicz, Krzysztof, Little, Crispin T. S. & Nakrem, Hans Arne, 2014, Bivalves from the latest Jurassic-earliest Cretaceous hydrocarbon seep carbonates from central Spitsbergen, Svalbard, Zootaxa 3859 (1), pp. 1-66 : 22

Pectinida gen. et sp. indet.

( 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).

Figures

FIGURE 10. (A–B) Pectinida gen. et. sp. indet. (A) Cross section of an articulated specimen. (B) Internal mould of the upper valve. Note possible adductor muscle scar and a line close to the dorsal margin, representing a possible dorsal furrow. (C–G) Oxytoma octavia (d’Orbigny, 1850). (C) Lateral view of a small left valve internal mould with fragments of the shell preserved showing weak radial ornament. (D) Enlarged view of anterodorsal portion area showing weak byssal notch. (E) Lateral view of a large left valve showing well developed radial ornament composed of primary and secondary ribs. (F) Lateral view of a right valve; lack of ornament is caused by shell delamination. Note very weak posterior sulcus and flattening of the valve. Probable shape of auricle is shown with dashed line (evidence from other partial specimens). (G) Plasticine cast of right valve external mould. Note radial ornament composed of primary and secondary ribs. (H–K) Camptonectes (Costicamptonectes) aff. milnelandensis Fürsich, 1982. (H–I) Right valve internal mould with shell partially preserved showing some divaricating ornament preserved close to the posterior shell margin. (J–K) External mould of right valve of a specimen from the Dorsoplanites bed, showing radial ribs close to the anterior shell margin and divaricating ornament preserved close to the posterior shell margin. (L–O) Camptonectes (Camptochlamys) clathratus (Roemer, 1836). (L–M) Lateral view of poorly preserved left valve showing cancellate external ornament in ventral margin. (N–O) Lateral view of a disarticulated and compressed shell from the Dorsoplanites bed showing cancellate ornament. Note distinct anterior auricle in the right valve. (P–R) Pseudolimea arctica (Zakharov, 1966). (P) Lateral view of left valve internal mould with some shell preserved around the ventral margin, with V-shaped ribs on the outer shell surface and corresponding rounded ribs on the internal mould. (Q) Lateral view of right valve, with anterior and posterior auricles and lack of secondary ribs caused by shell delamination. (R) Silicone rubber cast of small area of shell surface showing detail of external ornament, with large primary ribs and smaller secondary riblets in the interrib spaces. (A) PMO 226.609, (B) PMO 226.610, (C–D) PMO 217.562, (E) PMO 217.555, (F) PMO 217.214, (G) PMO 217.570, (H–I) PMO 226.606, (J–K) PMO 226.604, (L–M) PMO 226.605, (N–O) PMO 226.603, (P) PMO 217.194, (Q) PMO 217.353, (R) PMO 217.370. Scale bars 5 mm (A–I), 10 mm (J–R).