Hokkaidoconcha Kaim, Jenkins, and Warén, 2008

Kiel, Steffen, Campbell, Kathleen A., Elder, William P. & Little, Crispin T. S., 2008, Jurassic and Cretaceous gastropods from hydrocarbon seeps in forearc basin and accretionary prism settings, California, Acta Palaeontologica Polonica 53 (4), pp. 679-703 : 684

Genus Hokkaidoconcha Kaim, Jenkins, and Warén, 2008

Type species: Hokkaidoconcha tanabei Kaim, Jenkins, and Warén, 2008 , Cenomanian ( Upper Cretaceous ), Yezo Group, Kanajirisawa Creek, Hokkaido, Japan .

Discussion.—Small, elongate gastropods with mainly axial ornament like those described here as Hokkaidoconcha are common in late Mesozoic shallow water deposits and they belong to different taxonomic groups including zygopleurids, cerithiopsoideans, and cerithoideans. Gastropods with similar adult shells found at ancient vents and seeps were either placed in these groups ( Stanton 1895; Little et al. 1999; Gill et al. 2005), or were considered as provannids ( Little and Vrijenhoek 2003; Campbell 2006), a gastropod group restricted to reducing deep−sea habitats with adult shells similar to those of zygopleurids or cerithoideans. However, placing these gastropods in any of these families without knowledge of their larval shells is guesswork at best. Recently, Kaim et al. (2008) showed that Cretaceous seep−restricted gastropods with cerithioid teleoconch shape have provannid or provannid−like (i.e., hokkaidoconchid) larval shells, which nicely documented that provannids and other abyssochrysoids have a history at seeps that goes back at least into the Late Cretaceous.

Hokkaidoconcha occidentalis ( Stanton, 1895) , H. bilirata sp. nov., H. morenoensis sp. nov., and H. tehamaensis sp. nov. are placed here in Hokkaidoconcha based on their similarity in teleoconch shape to H. tanabei , but evidence from larval shells is lacking. These four species share the more−or−less opisthocline ribs that are tuberculate to various degrees below the suture and tend to fade on the lower half of the whorl; and the increased strength of the spiral sculpture at the basal margin. The main differences are in the convexity or straightness of the whorls’ flanks and the development of the spiral sculpture. In the Californian species, their intra−specific variability made it somewhat difficult to distinguish among these species, and they are here referred to as the Hokkaidoconcha occidentalis − group. The principal species was originally placed in the zygopleurid genus Hypsipleura Koken, 1892 with some hesitation ( Stanton 1895). However, Hypsipleura has higher whorls with a steep and angulated shoulder ( Kittl 1891; Wenz 1938 –44).

The zygopleurid genus Ampezzopleura is very similar regarding shell shape and sculpture, especially as spiral sculpture was documented for the type species ( Nützel 1998). Very similar opisthocline ribs can, for example, be seen in Ampezzopleura hybridopsis Nützel, 1998 . Nützel (1998) introduced the new genus Striazyga for species that are similar to Ampezzopleura but also have spiral sculpture. However, in that same paper he described and figured “weak spiral sculpture” for the type species of Ampezzopleura , A. tenuis , which sheds doubt on the validity of the genus Striazyga .

Similar shell characters can also be found in several procerithiid or cryptaulaxid species, although usually the axial ribs are straight in these groups. For example, the juvenile whorls of holotype of Procerithium quinquegranosum Cossmann, 1902 , type species of Procerithium , figured by Gründel (1997: pl. 5: 1, 2) are very similar to those of Hokkaidoconcha occidentalis . Procerithium russiense d’Orbigny, 1945 as figured by Guzhov (2004: pl. 5: 4–10), has similar axial ribs with strong knobs on the anterior side of its very early whorls. However, that species develops the typical cancellate procerithiid sculpture on its later whorls, which is not seen in Hokkaidoconcha occidentalis . Also some Cryptaulax Tate, 1869 species have similar straight−sided or slightly convex whorls with strong axial ribs, but intersections with spiral cords are usually spiny ( Gründel 1999), as in most procerithiids and cryptaulaxids.

Other groups with slender turriculate shells and subsutural nodes or spines like Hokkaidoconcha occidentalis are certain campanilids or possibly related taxa. Campanile , although usually much larger, has similar subsutural nodes, and also has similar opisthocline growth lines ( Houbrick 1981; Houbrick 1989; Kiel et al. 2000). The Early Cretaceous Diatrypesis kurushini Kaim, 2004 , which has a campaniloid protoconch (cf. Kiel 2006a), has subsutural nodes and growth lines that are opisthocyrt at the base.