Conchocele kiritachiensis, Hryniewicz & Amano & Jenkins & Kiel, 2017

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

Conchocele kiritachiensis sp. nov.

Figs. 8 , 9 .

Etymology: Named for the type locality (Kiritachi).

Type material: Holotype: JUE 16036 ; an articulated and undeformed specimen with most of the shell preserved. Other figured specimens designated as paratypes ( JUE 16037-1 to 16037-7 ), all from the type locality.

Type locality:A hydrocarbon seep deposit at Kiritachi , Tomamae Town, Hokkaido, Japan .

Type horizon: Sakasagawa Formation , upper Eocene .

Material.— Type material and 10 unnumbered specimens from the type locality, all stored in JUE paleontological collections .

Dimensions.— The holotype is 45.5 mm long and 41.1 mm high.

Diagnosis. — Medium-size species of Conchocele with umbones displaced strongly towards anterior, straight anterior shell margin, pointed beaks and obtuse anteroventral angle. Posterior sinus deep and obtuse. Posterior fold broad but short.

Description. — Shell medium-sized, up to 62.1 mm long, equivalve, moderately inflated, rhomboidal in outline. External ornament composed of densely spaced, low commarginal growth lines. Umbones terminal, umbonal angle around 95°, beaks prosogyrous, not incurved, anteroventral angle around 120°. Lunule absent, but anterior area demarcated with crude ridge running from umbo toward anteroventral angle. Ventral margin nearly straight close to anteroventral angle, well rounded in median and posterior part, with deepest curvature located posterior to mid-line. Posterior angle around 95°, posterior sinus deep, forming an obtuse angle. Posterior sulcus dorsally convex, sharp. Posterior dorsal margin rounded. Anterior adductor muscle scar well-impressed, striated, elongate ovate. Posterior adductor muscle scar rather small, weakly impressed, visible in both side of posterior sulcus. Ligament sunken but hinge structure unknown.

Remarks. — This species belongs to a group of Conchocele species with rhomboidal shells with pointed beaks and straight anterior shell margin. A similar and coeval species from Eastern Pacific is Conchocele bathyaulax Hickman, 2015 from non-seep deposits of the upper Eocene–lower Oligocene Keasey Formation, Oregon, which has narrower and longer posterior fold than C. kiritachiensis . Conchocele cf. bathyaulax from upper Eocene–Oligocene seeps in Washington State is also similar to C. kiritachiensis , but differs by having a more inflated and shorter shell and an overall larger size.

ventral margin

There are several similar species known in the Cenozoic of the Northwestern Pacific. Yabe and Nomura (1925: pl. 23: 8, 10a) figured “ Thyasira ” bisecta from the Neogene of Sakhalin. Similar specimens from the upper Pliocene of south-western Sakhalin were later figured and designated by Kristofovich (1936: 29, pl. 1: 3, 4) as “ Thyasira ” bisecta var. alta . In the following work, Kristofovich (1964: 213, pl. 53: 5) mentions “ Thyasira ” disjuncta var. alta to occur in the upper Miocene of Sakhalin, Kamchatka and Pliocene of Sado Island. The age of the Sakhalin occurrence was later found by Khudik (1989) to be Miocene–Pliocene. This species is more elongated, has a smaller apical angle, and narrower posterior fold than C. kiritachiensis , but is otherwise similar and belongs to the same species group. Some specimens described by Kristofovich (1936) as new species “ Thyasira ” pervulgata ( Kristofovich 1936: 45, pl. 6: 4, 6) from the Eocene? and “ Thyasira ” wajampolkana ( Kristofovich 1936: 44, pl. 6: 3, 3a) from the Miocene of western Kamchatka could also belong to the same species group as Conchocele kiritachiensis . However, both species are not sufficiently well described and figured to allow a more detailed comparison.

Stratigraphic and geographic range. — Type locality and horizon only.

Figures

Fig. 8. Schematic illustration of the external shell of the thyasirid bivalve Conchocele kiritachiensis sp. nov. highlighting its main morphological features.

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.