Camptochlamys (Camptochlamys) clathratus ( Roemer, 1836 )

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 : 27-28

Camptochlamys (Camptochlamys) clathratus ( Roemer, 1836)

( Figure 10 L–O )

1836 Pecten clathratus sp. nov. —Roemer, p. 212, pl. 13, fig. 9.

1984 Camptonectes (Camptochlamys) clathratus (Roemer) —Johnson, p. 143, pl. 4, figs. 23, 26, 27, pl. 5, figs. 1–3, 6, text-figs. 131–136 and references therein.

1984 Camptonectes (Camptochlamys) intertextus (Roemer) —Kelly, p. 75, pl. 6, figs. 16–18.

Material examined. Two poorly preserved specimens. In addition, a single disarticulated specimen from the Dorsoplanites bed ( PMO 226.603). See Appendix 1 for the list of specimens.

Dimensions. 15.8–23.8 mm in length, 15.8–23.2 mm in height. See Appendix 2H for details.

Description. Shell small to medium, thin. Both valves suborbicular, left valve moderately inflated, with anterior and posterior auricles subequal and not prominent. Umbonal angle ca. 100°; hinge line straight, umbones orthogyrate and not projecting. Curvature of right valve unknown. Anterodorsal margin straight to very weakly convex, posterodorsal margin straight. Anterior and posterior margins subequal, with anterior margin slightly more rounded. Ventral margin evenly rounded. Ornament composed of series of dense commarginal lamellae intersected by fine primary ribs and secondary radial riblets. Intersection of radial and commarginal ornament forms a cancellate pattern. Specimen measuring 23.2 mm in height has three growth lines per millimeter. Musculature and dentition unknown.

Remarks. Our specimens have an external ornament and shell morphology within the range of Camptonectes (Camptochlamys) clathratus ( Roemer, 1836) . Similarly shaped and ornamented specimens have been recorded by Kelly (1984) as C. (Cc.) intertextus ( Roemer, 1839) , which is the younger synonym of C. (Cc.) clathratus according to Johnson (1984).

Occurrence. The species is known from Bajocian to Kimmeridgian of Western and Central Europe ( Johnson 1984), and the Middle Volgian of Greenland ( Fürsich 1982). It has been recorded previously from the Jurassic of Svalbard as Pecten (Chlamys) pertextus var. densiradiatus var. nov. by Sokolov & Bodylevsky (1931, p. 54, pl. 3, figs. 6a–b). Seep 8 (Upper Volgian), Slottsmøya Member ( Tab. 1 ); also Dorsoplanites bed (Middle Volgian), Svalbard.

Palaeoecology. Camptonectes (Camptochlamys) clathratus was a byssally attached filter feeder ( Johnson 1984) and in the Svalbard seeps it probably attached onto exposed carbonate substrates. The umbonal angle increase during ontogenesis ( Johnson 1984) suggests that juveniles may have been attached to a firm substrate with a byssus, which was then lost in adults so they became recliners (e.g. Stanley 1970). In the Mid- to early Late Jurassic C. (Cc.) clathratus is often associated with shallow water corals, upon which juveniles were supposed to attach byssally, and oolitic facies, in which the adult forms reclined ( Johnson 1984). The Svalbard specimens ( Sokolov & Bodylevsky 1931; this study) represent both the youngest occurrence and the most offshore occurrence of the species, which suggests that it either had a broader ecological tolerance than previously suspected, or C. (Cc.) clathratus shifted its preference to more offshore environments around the latest Jurassic.

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).