Acharax mikasaensis, Kiel & Amano & Jenkins, 2008

Kiel, Steffen, Amano, Kazutaka & Jenkins, Robert G., 2008, Bivalves from Cretaceous cold-seep deposits on Hokkaido, Japan, Acta Palaeontologica Polonica 53 (3), pp. 525-537 : 526-527

Acharax mikasaensis sp. nov.

Fig. 2A, B .

1993 Solemya (Acharax) sp.; Kanie et al. 1993: 34, fig. 2.

1993 Calyptogena sp. ; Kanie et al. 1993: 34, fig. 2.

1997 Solemya sp. ; Kanie and Sakai 1997: 218, fig. 8.3 (but listed as Solemya cf. angusticaudata Nagao, 1932 in their table 2).

1997 Calyptogena sp. ; Kanie and Sakai 1997: 210, fig. 8.1.

2002 Solemya (Solemya) angusticaudata Nagao ; Kanie and Kuramochi 2002: 51, fig. 2.1.

Etymology: For Mikasa City, place of the type locality.

Type material: Holotype: UMUT MM 29524 ; an internal mold showing internal features, 41 mm long, 19 mm high; paratype: UMUT MM 29523 ; a large specimen showing external features, 55 mm long, 21 mm high.

Type locality: Seep carbonate on the banks of the Ponbetsu River in Mikasa City, Hokkaido, Japan .

Type horizon: Albian (Lower Cretaceous) of the Yezo Group .

Material.— 60 specimens and fragments from the type locality.

Diagnosis.—An elongate−oval Acharax with elongate and slightly concave posterior adductor scar, anterior adductor D−shaped with a close, almost vertically ascending band extending from its posteroventral corner.

Description.—Shell with elongate−oval outline, dorsal margin straight, anterior margin rounded, ventral margin slightly convex, posterior margin truncated and pointed. Sculpture of broad radial ribs, grooves between ribs ca. 1/3 of rib width, about 10 ribs on anterior part of shell, ribs weak to absent below umbo. Anterior lateral extension lacking; posterior adductor scar elongate, curved, tapering towards the umbo; posterior ligament external, distinct and strong. Anterior adductor scar broad D−shaped, from its posteroventral corner undulates an ascending band to the dorsal margin where it broadens out.

Discussion.—A very unusual feature of this species is the undulating band extending from the posteroventral corner of the anterior adductor scar: in most if not all solemyids this band ascends obliquely to the dorsal margin ( Pojeta 1988; Taylor et al. in press; and observations by SK on Recent solemyids in the USNM zoology collection), whereas in Acharax mikasaensis sp. nov. it is almost perpendicular to it. In his description of Solemya angusticaudata from the Yezo Group of Hokkaido Nagao (1932) does not mention the ligament but describes the posterodorsal margin as “sloping backward and downward towards the posterior end”, and indeed, his figure shows a specimen in which the posterodorsal margin is slightly concave, which is in contrast to Acharax mikasaensis where the posterodorsal margin is clearly convex. Acharax cretacea Kanie and Nishida, 2000 is more elongate than Acharax mikasaensis and appears to have a more angular anterior margin. The posterior adductor scar of Acharax mikasaensis is more elongate than in the Recent type species Acharax johnsoni but resembles that of Zesolemya parkinsoni Smith, 1874 from New Zealand (Taylor et al. in press: fig. 1D). Clearly distinct is the lower Miocene Solemya (Acharax) tokunagai Yokoyama, 1925 as figured by Kamada and Hayasaka (1959) from the Joban coal−field, because it has a more elongate posterior end than Acharax mikasaensis .

Stratigraphic and geographic range.— Only known from the Albian type locality on Hokkaido, Japan .

Figures

Fig. 2. Solemyid bivalves from Cretaceous cold seep deposits on Hokkaido, Japan. A, B. Acharax mikasaensis sp. nov., from the Albian Ponbetsu site in Mikasa City. A. Paratype (UMUT MM 29523) showing external sculpture, dorsal (A1) and lateral (A2) views. B. Holotype (UMUT MM 29524) showing features of the shell interior; arrow in B1 indicates posterior adductor muscle scar, B2 shows a dorsal view, white arrow in B3 indicates the anterior adductor muscle scar, black arrow indicates the narrow band that ascends from its posteroventral margin. C–E. Acharax cretacea Kanie and Nishida, 2000, from the Campanian Yasukawa site. C. Right valve of a slightly deformed, medium−sized specimen (UMUT MM 29525), length 34 mm.D. Small specimen (UMUT MM 29526) (length 8 mm) showing the rounded posterior shell margin, dorsal (D1) and lateral (D2) views. E. Cross section of shell showing the prismatic microstructure.