Camptochlamys (Camptochlamys) clathratus

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.