Camptonectes (Costicamptonectes) aff. milnelandensis Fürsich, 1982

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 : 27

Camptonectes (Costicamptonectes) aff. milnelandensis Fürsich, 1982

( Figure 10 H–K )

1982 aff. Camptonectes (Costicamptonectes) milnelandensis sp. nov. —Fürsich, p. 50, figs. 23 C–D, F, G.

Material examined. Two poorly preserved right valves. In addition a single external mould with silicone rubber cast from the Dorsoplanites bed ( PMO 226.604). See Appendix 1 for the list of specimens.

Dimensions. 15–30.5 mm in length, 15–28.2 mm in height. See Appendix 2G for details.

Description. Shell small to medium sized, very thin, appears to get shorter at later growth stages. Right valve suborbicular, moderately inflated, with small posterior and larger anterior auricle, ventrally delineated by a deep byssal notch. Ctenolium well developed. Antero- and posterodorsal margins subequal, anterodorsal margin slightly concave. Anterior margin slightly more convex than posterior margin, ventral margin evenly rounded. Preserved ornament consists of weak divaricating radial ribs, intersected by weak but dense growth lines. Anterodorsal margin ornamented by ca. 6 subparallel radial ribs. Left valve, musculature and dentition unknown.

Remarks. We compare our specimens with C. (Co.) milnelandensis because of its ornament of commarginal growth lines accompanied by divaricate ribs and with additional radial ribs close to the anterodorsal margin. The Svalbard specimens have a slightly higher umbonal angle than comparatively sized specimens of C. (Co.) milnelandensis from Greenland ( Fürsich 1982). However, no specimens larger than 18 mm were reported by Fürsich (1982) so we are unable to compare the ontogenetic changes of umbonal angle of specimens from both localities. In addition, the Svalbard material is composed of right valves only. Until more is known about C. (Co.) milnelandensis from both Greenland and Svalbard, we prefer to leave our species in open nomenclature.

Occurrence. Camptonectes (Co.) milnelandensis : Upper Callovian–Upper Oxfordian of Milne Land, East Greenland ( Fürsich 1982). Camptonectes (Co.) aff. milnelandensis: Seep 8 (Upper Volgian), Slottsmøya Member ( Tab. 1 ); also Dorsoplanites bed (Middle Volgian), Svalbard.

Palaeoecology. We assume that C. (Co.) aff. milnelandensis was a byssally attached suspension feeder, as indicated by the presence of a well-developed ctenolium. Byssal attachment is a characteristic feature for the genus Camptonectes , although larger species in the subgenus McLearnia Crickmay, 1930 , probably lost a functional byssal attachment in maturity (e.g. Fürsich 1982). An expanded anterior auricle in pectinids is an adaptation for byssal attachment with the commissure non-perpendicular to the substrate ( Stanley 1970). In such a position the auricle and anterior shell margin acts as a fulcrum and prevents the shell overturning from its set position. As in most of the pectinids, C. (Co.) aff. milnelandensis probably had the ability to swim to escape predation, although the umbonal angle is too acute to assume this was habitual ( Stanley 1970).

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).