Hokkaidoconcha hikidai, Kaim & Jenkins & Warén, 2008

Kaim, Andrzej, Jenkins, Robert G. & Warén, Anders, 2008, Provannid and provannid-like gastropods from the Late Cretaceous cold seeps of Hokkaido (Japan) and the fossil record of the Provannidae (Gastropoda: Abyssochrysoidea), Zoological Journal of the Linnean Society 154 (3), pp. 421-436 : 428-429

HOKKAIDOCONCHA HIKIDAI SP. NOV. ( FIG. 3K–Q)

Derivation of name: In honour of Dr Yoshinori Hikida, for work on chemoauthotrophy-based associations in the Nakagawa area.

Type material: UMUT MM29391 (holotype), adult or adolescent shell, and five paratypes , UMUT MM29386–29390 .

Type locality: Yasukawa locality, Nakagawa area, Hokkaido, Japan .

Type horizon: Campanian (Late Cretaceous) carbonate seep deposits.

Description: Shell ( Fig. 3P, Q) of tall, cylindrical cerithioid shape. Protoconch unknown. Shell of the best preserved specimen ( UMUT MM 29391, holotype) consists of eight strongly elongated whorls, still embedded in carbonate rock matrix ( Fig. 3P, Q). Most juvenile part poorly preserved with strong axial ribs and weaker spiral ridges. Whorls are weakly convex; ornamentation changes during ontogeny; basally only 1–2 spiral ridges remain just above suture. Eleven to 13 sturdy opisthocyrtic axial ribs sometimes more, sometimes less co-labral; start at the most apical part of whorl and continue to spiral rib(s) just above suture and disappear. Sutural rib(s) form first one(s) of a series of 9–10 ridges ornamenting base of last whorl. Axial ribs ornamented by blunt nodes in their most apical and basal portions, apparently remnants of intersections with weak spiral ridges on early teleoconch. The juvenile UMUT MM 29389 is also of tall, cylindrical cerithioid shape and ornamented by axial ribs and spiral ridges, later spiral ridges at medial part of the lateral flank weaken and finally disappear ( Fig. 3N). Apical spiral ridge strongest, produces weak nodes at intersections with axial ornamentation; also visible on some other juvenile specimens (e.g. UMUT MM 29387; Fig. 3L) although these are laterally flattened. Aperture is not fully preserved, but seemingly the shell had no siphonal channel or notch ( Fig. 3P).

Dimensions: Hototype ( UMUT MM 29391): maximum shell height 20.36 mm, diameter 4.54 mm.

Remarks: Shells of H. hikidai are relatively common in the Yasukawa methane seep, but they are incompletely preserved. The specimens from mudstones are usually flattened and fragmented while the specimens from the seep-carbonate underwent re-crystallization that commonly distorts the outer surface and the apex of the shells. Therefore, we could not find any protoconch of this gastropod and it is also difficult to find specimens with a well-preserved teleoconch. The teleoconch of juvenile shells strongly resembles some of the Mesozoic cerithioids, e.g. Cryptaulax and Cirsocerithium (compare Kaim, 2004 and Kiel, 2006a, respectively) and without the adolescent portion of the shell and a complete aperture, it is difficult to classify such a gastropod appropriately. H. hikidai differs from H. tanabei in having two strong spiral ribs on the abapical part of the shell flank and these ribs are not crossed by axial ribs. The spiral ribs of juveniles are narrow with wide interspaces in H. hikidai whereas they are widely blunt with very narrow interspaces in H. tanabei .

UNNAMED HOKKAIDOCONCHID SPECIES

Kanajirisawa hokkaidoconchid sp. indet. ( Fig. 2F, J, R): This species is represented by a tiny specimen ( UMUT MM 29361) consisting of two protoconch and two teleoconch whorls. The whole shell is 1.65 mm high and 0.77 mm broad. The protoconch is 0.63 mm high and 0.59 mm broad. The whorls of the protoconch are more convex than any provannid protoconch described thus far. The axial ribs are sturdy and sigmoid and stronger than those of H. tanabei . The spiral ribs on the protoconch follow the pattern known from the provannids and hokkaidoconchids. The teleoconch is ornamented exclusively by slightly opisthocyrtic, sturdy and shouldered ribs, strongly protruding in their apical quarter. No spiral ribs are present.