Pseudotrapezium aff. groenlandicum Spath, 1936

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 : 41-43

Pseudotrapezium aff. groenlandicum Spath, 1936

( Figure 16 H–P )

1936 aff. Pseudotrapezium groenlandicum sp. nov. —Spath, p. 125, pl. 49, fig. 7a–c.

? 1982 aff. Hartwellia (Hartwellia) groenlandica (Spath) —Fürsich, p. 89.

2011 Arcticid—Hammer et al., fig. 7m, tab. 2.

Material examined. 820 specimens, mostly articulated or semi-articulated shells and internal moulds. See Appendix 1 for list of specimens.

Dimensions. 12–15 mm in length, 9.8–11 mm in height, 8–10 mm in width. See Figure 18 A–D and Appendix 2L for details.

Description. Small, moderately inflated with very thin shell. Beaks prosogyrate, very strongly incurved. Lunule moderately deep. Anterior margin projecting, arcuate, passing into broadly arched ventral margin. Posterior area with two carinae, posteroventral extremity slightly rostrate where intersected by first carina, passing into truncated oblique posterior margin. Second carina weak, parallel to the posterodorsal margin. Posterodorsal margin straight. External ornament composed of very weak commarginal growth lines. Hinge plate large and thick; lateral tooth AI developed, separated from 1; 3a absent. 1 and 3b separated, diverging from umbo; 1 strong and vertical; 3b thick, anterodorsally inclined. Angle between 1 and 3b ca. 70°. Left valve dentition unknown. Anterior adductor muscle scar weak, elongated along pallial line, around twice as long as wide, not detached. Posterior adductor muscle scar weak, rounded. Pallial line weak, entire.

Remarks. We compare our specimens with Pseudotrapezium groenlandicum Spath, 1936 , from Milne Land, East Greenland ( Spath 1936) due to their thick hinge plate, similar cardinal dentition and external shape. However, the Svalbard specimens are much smaller and more thin shelled than the Greenland material, so we are not entirely sure they represent the same species and leave them in open nomenclature. Fürsich (1982) moved P. groenlandicum into the genus Hartwellia Kitchin, 1926 , and redescribed it as Hartwellia (H.) groenlandica . Hartwellia was considered by Cox (1944) to be a synonym of Pronoella Fischer, 1887 . Including P. groenlandicum into Pronoella is not justified by the hinge of our Svalbard specimens. Both Pronoella and Pseudotrapezium have relatively thick hinge plates and a similar external shape (e.g. Casey 1952), but AI and 1 are connected in Pronoella into a single elongate denticle and separated in Pseudotrapezium ( Benecke 1905) , showing the genera are not the same.

Occurrence. P s eudotrapezium groenlandicum : Lower Volgian–Lower Ryazanian of Milne Land, East Greenland ( Spath 1936; Fürsich 1982). Pseudotrapezium aff. groenlandicum : seeps 2, 3, 5, 8, 9 and 12 (Upper Volgian–uppermost Ryazanian), Slottsmøya Member, Svalbard ( Tab. 1 ).

Palaeoecology. We assume that P. aff. groenlandicum was a burrower, feeding on organic-rich sediment layer while resting in the shallow subsurface. The lack of a pallial sinus suggests that P. aff. groenlandicum possessed very short siphons, similar to its extant relative Arctica islandica ( Linnaeus, 1767). Arctica islandica has very short siphons and positions its posterior extremity at the sediment-water interface for feeding on organic matter from the sediment surface ( Morton 2011). As it does so it remains relatively motionless, circulating water only by means of ciliary movement ( Brand & Taylor 1974). It can rebury itself into deeper sediment layers, where it remains isolated from seawater for up to seven days when it is not feeding, and respires anaerobically ( Taylor 1976). The clustering of very large numbers of P. aff. groenlandicum specimens in seep 9 is difficult to explain. No association between arcticids and chemosymbiotic bacteria has been noted to date, so we do not envisage any specific trophic link between P. aff. groenlandicum and the seep environment.

Subclass Anomalodesmata Dall, 1889

Order Pholadomyoida Newell, 1965

Figures

FIGURE 16. (A–G) Cretaxinus hurumi gen. et sp. nov. (A) Lateral view of left valve of articulated internal mould, showing a distinct, triangular shape. (B–C) Anterior views showing shape and size of lunule and anterior pedal retractor scar. (D) Dorsal view showing shape of the posterodorsal margin. (E–F) Cardinal and posterodorsal portion of left valve internal mould shell showing posterior adductor muscle scar with distinct notch on anteroventral margin and posterior pedal retractor scar. (G) Silicone rubber cast of (E–F) showing small and elongated groove, possibly ligament groove. (H–P) Pseudotrapezium aff. groenlandicum (Spath, 1936). (H) Lateral view of the shell showing the outline and carina. (I–J) Anterior and posterior views of articulated specimen with most of the shell preserved. (K) Dorsal view showing flattened posterior area, two carinae and very thin shell. (L–M) Anterior view of articulated specimen with some preserved shell, showing deep and small lunule and rounded anterior adductor muscle scar. (N–O) Oblique posterior view showing weak pallial line and weakly impressed posterior adductor muscle scar. (P) Right valve dentition and large hinge plate. (A–D) PMO 217.540, (E–G) PMO 217.175, (H–K) PMO 226.632, (L–M) PMO 226.633, (N–O) PMO 226.635, (P) JUE no. 15926. Scale bars 10 mm (A–G), 5 mm (H–P).

FIGURE 18. Measurements of Pseudotrapezium aff. groenlandicum Spath, 1936 (in mm). (A) Height to length, (B) Width to length, (C) Width to height, (D) Length of the posterior to total length.