Hokkaidoconcha novacula, Kiel & Campbell & Gaillard, 2010

Kiel, Steffen, Campbell, Kathleen A. & Gaillard, Christian, 2010, New and little known mollusks from ancient chemosynthetic environments, Zootaxa 2390 (1), pp. 26-48 : 32-34

Hokkaidoconcha novacula sp. nov.

( Fig 4 )

Diagnosis: Hokkaidoconchid shell with nearly straight-sided whorls; sculpture of ten sickle-like axial ribs per whorl, often axially aligned over entire shell, tuberculate to spiny below suture; inner lip of aperture reflected and convex.

Holotype: FSL 286 452, H = 11 mm.

Paratypes: FSL 286 453, H = 16 mm; FSL 286 454, H = 12 mm; FSL 286 455, specimen with wellpreserved aperture, W = 4.5 mm.

Type locality and strata: Seep deposits at Beauvoisin, Drôme, France, 44°18’N, 5°12’E; Late Jurassic, Oxfordian .

Description: Slender, high-spired shell, at least ten whorls, very finely incised suture, protoconch unknown; whorls straight-sided or minimally convex, sculptured mainly by ten axial ribs per whorl; ribs sharp-crested, straight or slightly opisthocyrt to opisthocline, one tubercle or short spine just below suture, producing small, subsutural shoulder; ribs usually axially aligned over entire shell, but at least on earlier whorls; axial ribs crossed by six to nine very fine spiral threads, strongest thread at position of subsutural spines, second strongest at basal margin, next to the lower suture; threads finer and more densely spaced in middle of whorl than towards suture; at least two additional threads on base; aperture oval and apparently no siphonal notch, inner lip reflected and convex, no plates or folds. Maximum height 17 mm.

Remarks: The description is based on six rubber casts of external molds. The fine spiral sculpture can only be seen on well-preserved specimens; slightly worn species, in which the axial ribs are rounded rather than sharp crested, do not show these fine spirals. However, this is clearly an artifact of preservation. The Californian seep-related Hokkaidoconcha occidentalis (Stanton, 1895) is similar with regard to shell shape and sculpture, but its ribs and subsutural row of tubercles are rounded and blunt, and not sharp-crested and pointed. This smoother sculpture in the Californian species is real, and not an artifact of preservation, unlike in Hokkaidoconcha novacula . In the type species the whorls are more rounded than in H. novacula , and it lacks the spiny subsutural cord of H. novacula .

Very similar in general shape and sculpture is Cerithium subscalariforme d’Orbigny var. spinicostata Hudleston, 1877 , which seems to have slightly stronger and certainly more numerous spiral threads (Hudleston 1887, p 151, pl. 8 fig. 8). It also has a well-developed siphonal notch in the aperture, unlike Hokkaidoconcha novacula . Also similar is Cerithium subglabrum Hudleston, 1887 , which has the same number of axial ribs per whorl as Hokkaidoconcha novacula but appears to have no spiral threads on the whorl’s periphery (Hudleston 1877: pl. 8 fig. 13). Although Hudleston did not figure the aperture, the siphonal projection on the base strongly suggests the presence of a corresponding siphonal notch.

Distribution: Type locality only.

Etymology: Novacula, Latin for sharp knife, referring to the very sharp-crested axial ribs of this species.

Figures

FIGURE 4. Hokkaidoconcha novacula n. sp. from Oxfordian (Late Jurassic) seep carbonates at Beauvoisin, southeastern France; all figured specimens are rubber casts. A. Holotype showing the aperture (FSL 286 452), B. Paratype with slightly rounded (weathered) ribs (FSL 286 453). C. Paratype showing the sharp-crested axial ribs (FSL 286 454). D. Paratype showing spiral cords on the base and the reflected inner lip of the aperture (FSL 286 455).