Dacromya chetaensis Sanin, 1976

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 : 15-16

Dacromya chetaensis Sanin, 1976

( Figure 5 O–Q )

1976 Dacromya chetaensis nov. sp. —Sanin, p. 26, pl. 6, figs. 10–11; pl. 7, figs. 1–6.

Material examined. Three specimens; articulated internal moulds with some preserved shell material. See Appendix 1 for the list of specimens.

Dimensions. 7–11.5 mm in length, 5.1–7.25 mm in height, 3.1–5.25 mm in width. See Appendix 2C for details.

Description. Shell small, with tear-drop shaped outline. Preserved shell fragments thin, with fine, densely spaced commarginal growth lines. Moderately to strongly inflated, with greatest width medially. Shell height around two-thirds of length, reaching a maximum medially and gradually diminishing towards anterior and posterior extremities. Anterior part of shell longer than posterior, arcuate, gently tapering. Anterodorsal margin weakly convex, more inclined than anteroventral margin. Ventral margin very gently rounded. Posterior shell part shorter than anterior, rostrate, rounded, and curved dorsally. Posterodorsal margin concave, hosting deep escutcheon. Beaks opisthogyrate, lunule elongated, sunken close to beaks, emerging towards anterior. Dentition taxodont with more than 10 teeth in anterior row and 7 to 10 teeth in posterior row. Cardinal area and ligament not preserved. Anterior and posterior adductor muscle scars circular, small, adhering to pallial line and positioned close to shell extremities on inside of pallial line. Posterior adductor muscle scar connected to beak with grooves, most likely formed during descending muscle growth. Pallial line entire.

Remarks. We include this species into Dacromya chetaensis Sanin, 1976 , based on its shell proportions, gently arched anterior margin and external ornament. It differs from Dacromya venusta ( Sauvage, 1871) from the Kimmeridgian of England ( Clausen & Wignall 1990) in having a more rounded anterior margin, less sloping anterodorsal margin and an external ornament of commarginal growth lines. Dacromya chetaensis is less elongated, has more pointed umbones and shorter rostrum than D. gigantea Zakharov & Shurigin, 1974 , from the Aalenian of the Taimyr Peninsula ( Zakharov & Shurigin 1974; 1978).

Occurrence. Seep 12 (Upper Ryazanian), Slottsmøya Member, Svalbard ( Tab. 1 ). Known also from the Volgian–Valanginian of the Khatanga Depression, Northern Siberia ( Sanin 1976).

Palaeoecology. Dacromya chetaensis was probably a mobile shallow burrowing deposit feeder without a siphon, as indicated by the lack of a pallial sinus. The streamlined shell with a blunt, weakly pointed anterior and short, rostrate posterior shows that it was probably an efficient burrower. In Siberia it is associated with sandy mudstones that were deposited in a low energy, shallow water setting ( Sanin 1976). This mode of life is typical for Recent nuculids, which are shallow burrowers using palps to collect sediment, which they process for organic material ( Stanley 1970).

Figures

FIGURE 5. (A–N) Nucinella svalbardensis sp. nov. (A–D) Lateral views of holotype showing shape of the articulated internal mould, anterior adductor muscle scar and possible anterior pedal retractor scar, rounded posterior margin and fine radial striations on internal mould surface. (E) Dorsal view showing weakly opisthogyrate umbo and deep ligament pit on the posterodorsal margin. (F) Anterior view showing anterior adductor muscle scar projecting weakly beyond pallial line. (G) Lateral view showing commarginal ornament on external shell surface. (H–I) Dorsal view of partially articulated specimen showing length of the cardinal teeth and relation of the possible ligament to the cardinal dentition. (J–K) Lateral view of the cardinal part of the hinge plate showing arrangement and shape of the cardinal teeth. (L) Ventral view of silicone rubber cast of the dorsal margin of articulated internal mould showing sabre-shaped lateral teeth and interlocked cardinal teeth. (M–N) Silicone rubber casts of right valve internal moulds showing two lateral teeth. (O–Q) Dacromya chetaensis Sanin, 1976. (O–P) Lateral and dorsal views showing shape of internal mould, taxodont dentition, anterior and posterior adductor muscle scars. (Q) Posterodorsal shell margin showing weak growth lines. (R–T) Mesosaccella rogovi sp. nov. (R) Lateral view of holotype left valve shell surface showing characteristic growth-line deflections around the posterior margin and carina subparallel to the dorsal margin. (S–T) Lateral view of left valve internal mould showing anterior adductor muscle scar, weak posterior adductor muscle scar and pallial line. (A–F) PMO 217.171, (G) PMO 224.978, (H–I) PMO 225.042, (J–K) PMO 224.981, (L–M) PMO 217.217, (N) PMO 225.020, (O–P) PMO 217.581, (Q) PMO 226.600, (R) PMO 224.971, (S–T) PMO 217.229. Scale bars 5 mm.