Hokkaidoconcha, 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

GENUS HOKKAIDOCONCHA GEN. NOV.

Derivation of name: After Hokkaido, the northernmost island of Japan and latin concha, shell .

Type species: Hokkaidoconcha tanabei sp. nov.

Description: Protoconch multispiral and ornamented with numerous co-labral ribs and finer spiral threads. Teleoconch is of tall, cylindrical cerithioid shape; apical whorls ornamented with strong axial ribs and weak spiral cords. Later in ontogeny spiral ornamentation disappears except immediately above the suture. Basal surface demarcated by strong spiral rib.

Remarks: The protoconch of Hokkaidoconcha resembles the provannid multispiral protoconch, but in Provannidae the larval shell is always decollated when multispiral. Moreover, the teleoconch of provannid species is never as cylindrical or tall as it is in Hokkaidoconcha . This elongate shell shape is more characteristic for Abyssochrysos , but all known species of that genus have a lecithotrophic-type protoconch. Acanthostrophia Conti & Fischer, 1984 (Middle Jurassic), another genus of similar shell shape, until now within Protorculidae ( Nützel, 1998) , has a strongly elongate teleoconch but its teleoconch is ornamented by a row of nodes and axial ribs, more like Abyssochrysos . The larval shell of Acanthostrophia is imperfectly described and poorly preserved, hampering comparisons; however, based on similarities in the teleoconch we consider Abyssochrysidae to be a more likely home for it (see Abyssochrysidae ).

It is possible that some Cretaceous species from the US Pacific Coast identified as Hypsipleura and/or abyssochrysids ( Stanton, 1895; Campbell & Bottjer, 1993; Campbell, Carlson & Bottjer, 1993) may belong to Hokkadoconchidae, but they are too poorly preserved to classify.

HOKKAIDOCONCHA TANABEI SP. NOV.

( FIGS 2D, E, I, N, 3B)

Derivation of name: In honour of Professor Kazushige Tanabe, who discovered the carbonate seep deposits at Kanajirisawa.

Type material: UMUT MM29360 (holotype), juvenile shell with protoconch . UMUT MM29392 (paratype), adolescent shell with no protoconch .

Type locality: Kanajirisawa Creek , Tappu area, Hokkaido, Japan .

Type horizon: Middle Cenomanian (Late Cretaceous) carbonate seep deposits.

Description: Shell ( Fig. 2D, E) of tall, cylindrical cerithioid shape. Protoconch ( Fig. 2N) multispiral and ornamented with 30–32 axial ribs and 15–20 weaker spiral cords on its basal 1.5 whorls. Remaining about two apical whorls too corroded to provide details. Initial teleoconch whorls ornamented with 10–12 sturdy and blunt opisthocyrtic axial ribs and eight broad and flat spiral ridges, with very narrow interspaces. Later in ontogeny spiral ornamentation present only close to suture. Axial ribs sturdier on adult/adolescent shells. Peristome and siphonal canal (if any) not preserved.

Dimensions: Hototype ( UMUT MM29360 ): maximum shell height 3.31 mm, diameter 1.34 mm; protoconch: maximum height 1.08 mm, diameter 0.80 mm. Paratype ( UMUT MM29392 ): maximum shell height 9 mm, diameter 2.85 mm .

Remarks: The ornament of the protoconch of H. tanabei , consisting of the opisthocyrt axial ribs and weaker spiral ribs, resembles the ornamentation of provannid protoconchs (compare Warén & Bouchet, 1993: fig. 44C–E and Fig. 4A, B, E herein); the latter, however, always have a decollated tip to the protoconch (when multispiral) and orthocline axial ribs. The teleoconch of H. tanabei also differs from the ornamentation we know from Recent provannids.

Figures

Figure 4. Juvenile Recent Provannidae and Abyssochrysidae (A–D) and patterns of shell decolation and preservation (E–F). A, E, Desbruyeresia spinosa Warén & Bouchet, 1993 from North Fiji Basin (specimen illustrated also in Warén & Bouchet, 1993: fig. 44D). A, decollate protoconch; E, details of decollation; note no signs of shell abrasion on the protoconch edges B, decollated protoconch of Alviniconcha hessleri Okutani & Ohta, 1988 from north Fiji Basin (specimen illustrated also in Warén & Bouchet, 1993: fig. 44C). C, paucispiral protoconch of Provanna segonzaci Warén & Ponder, 1991 from Lau Basin. D, Abyssochrysos sp. from off New Caledonia; note axial ribs and spiral riblets at the beginning of paucispiral protoconch. F, decollation pattern of terrestrial gastropod Rumina decollata Linné, 1758. G, typical pattern of in-vivo apex erosion in Phymorhynchus sp. shell from the Lucky Strike site on the Mid-Atlantic Ridge; note shell surface abrasion but no sign of decollation.