Conchocele cf. bathyaulax Hickman, 2015

Hryniewicz, Krzysztof, Amano, Kazutaka, Jenkins, Robert G. & Kiel, Steffen, 2017, Thyasirid bivalves from Cretaceous and Paleogene cold seeps, Acta Palaeontologica Polonica 62 (4), pp. 705-728 : 714-717

Conchocele cf. bathyaulax Hickman, 2015

Fig. 10 .

Material.— Six specimens from Canyon River (upper Eocene–lower Oligocene, western Washington State, USA), including one figured ( ZPAL L.16/8; Fig. 10B ); and three specimens from East Twin River (upper Oligocene, northwestern Washington State, USA), including two figured ( ZPAL L.16/7, 9; Fig. 10A, C ) .

Description.—Shell large, up to 80 mm long, thin, equivalve, inflated, subquadrate to rhomboidal. External ornament composed of densely spaced, commarginal growth lines. Umbones terminal and prosogyrous, not incurved. Lunule absent, anterior of shell flattened with a weak anterior ridge running from the umbo towards anteroventral shell angle. Anterior margin straight, anteroventral angle obtuse. Ventral margin rounded, with deepest point located posterior to mid-line. Posterior shell extremity weakly pointed. Posterior sulcus straight to weakly dorsally convex. Posterior fold high, with weak ridge running amidst. Inner shell surface covered with thin radial striae. Anterior adductor muscle scar well impressed, elongated, striated, covering ca. 1/3 of shell length, separated from pallial line. Anterior pedal retractor muscle scar medium-sized, striated. Posterior adductor muscle scar weaker than anterior one, striated, restricted to posterior fold. Ligament unknown.

Remarks.— Conchocele bathyaulax and Conchocele cf. bathyaulax share a characteristic inflated shell with projecting umbo and straight anterior margin. They differ by the width of the posterior fold, which is narrower in C. bathyaulax compared to C. cf. bathyaulax , and C. bathyaulax is shorter than C. cf. bathyaulax . Due to the large intraspecific morphological variability exhibited by many thyasirids (e.g., Oliver and Killeen 2002; Oliver and Sellanes 2005; Rodrigues et al. 2008), we prefer to leave C. cf. bathyaulax species in open nomenclature and compare it to C. bathyaulax until material warranting a more coherent systematic statement is available. Another similar fossil species is Conchocele kiritachiensis sp. nov., which is less inflated than C. cf. bathyaulax and has a longer shell with more deeply incised posterior sulcus (cf. Fig. 9 ). All previously mentioned species are similar to the extant C. scarlatoi Ivanova and Moskaletz, 1984 , from shallow waters of the Sea of Japan ( Kharlamenko et al. 2016), with which they share shells with projecting umbones and straight anterior margin lacking a lunular incision. Concocele taylori from uppermost Eocene–Oligocene seep carbonates of paleo-Northern Pacific margin has more rounded shell, with more dorsally convex posterior fold and protruding anteroventral angle (cf. Fig. 7). The Miocene C. bisecta has a more rounded shell with a concave anterior margin, a lunular incision, and a rectangular anteroventral margin ( Moore 1963).

Figures

Fig. 10. Thyasirid bivalve Conchocele cf. bathyaulax Hickman, 2015. A. ZPAL L.16/7 from an upper Oligocene seep carbonate of the Pysht Formation, East Twin River, northwestern Washington State, USA; an articulated specimen in left (A1) and right (A2) views, showing characteristic rhomboidal shape with roughly parallel, convex dorsal and ventral margins, and roughly parallel anterior and posterior margins, asterisk shows a lateral aspect of a secondary ridge (A1); anterior view (A3), showing anterior pedal and adductor muscle scars, weakly incurved umbones and inflated shells, asterisk shows a frontal aspect of a secondary ridge shown in A1; dorsal view (A4), shows moderately deep posterior sulcus. B. ZPAL L.16/8 from an upper Eocene–lower Oligocene of the Canyon River, Lincoln Creek Formation, western Washington State, USA; a medium-sized specimen in left lateral view, showing incurved umbo. C. ZPAL L.16/9 from an upper Oligocene of the Pysht Formation, East Twin River, northwestern Washington State, USA; a large specimen in left (C1) and right (C2) lateral views with umbo more protruding than ZPAL L.16/7, 8 (A, B), asterisk shows a lateral aspect of a secondary ridge shown in C3; anterior view (C3) showing weak secondary ridge running from umbo towards the anteroventral angle (asterisked).

Fig. 9. Thyasirid bivalve Conchocele kiritachiensis sp. nov. from an upper Eocene seep carbonate of the Sakasagawa Formation, Kiritachi, Hokkaido, Japan. A. JUE 16036, holotype in right (A1) and left lateral (A2) views; dorsal view (A3) showing anterior flat area demarcated by rude ridge and a posterior fold. B. JUE 16037-3, paratype, young shell in right (B1) and left (B2) lateral and dorsal (B3) views. C. JUE 16037-2, paratype in right lateral view. D. JUE 16037-4, paratype in dorsal view, showing distinct anterior flat area demarcated by rude ridge and a distinct posterior sulcus. E. JUE 16037-7, inner side of paratype in left lateral view, showing both anterior and posterior adductor muscle scar. F. JUE 16037-6, paratype in left lateral view, the largest specimen of this species, having a pointed umbo and a wide posterior fold. G. JUE 16037-1, paratype, large specimen in left lateral view, showing a pointed umbo and a wide posterior fold. H. JUE 16037-5, paratype in right (H1) and left (H2) lateral views, showing narrow apical angle, instead posterior fold partly broken.