Cantrainea omagariensis, Kaim & Jenkins & Hikida, 2009

Kaim, Andrzej, Jenkins, Robert G. & Hikida, Yoshinori, 2009, Gastropods from Late Cretaceous Omagari and Yasukawa hydrocarbon seep deposits in the Nakagawa area, Hokkaido, Japan, Acta Palaeontologica Polonica 54 (3), pp. 463-490 : 474-476

publication ID	https://doi.org/ 10.4202/app.2009.0042
persistent identifier	https://treatment.plazi.org/id/03FBF625-FFD8-2132-7D2E-90F3FE222647
treatment provided by	Felipe
scientific name	Cantrainea omagariensis
status	sp. nov.

Treatment

Cantrainea omagariensis sp. nov.

Fig. 7A–F View Fig .

2008 ataphrid gastropod; Kaim et al. 2008b: 100, fig. 3h.

Etymology: After the type locality.

Holotype: UMUT MM30166 View Materials , Fig. 7B View Fig , moderately preserved shell with no protoconch.

Type locality: Omagari site, Nakagawa area , Northern Hokkaido, Japan. Coordinates 44 ° 39’26’’ N, 144 ° 2’25’’ E GoogleMaps .

Type horizon: Fossil hydrocarbon seep deposits of Omagari Formation, Campanian, Upper Cretaceous.

Material.—310 moderately preserved specimens without protoconchs: 225 at UMUT and 85 at NMM, all from the Omagari site.

Dimensions.—The holotype is 4 mm high and 4.66 mm wide.

Diagnosis.—Shell low turbiniform, moderately inflated. Surface smooth apart from narrow subsutural ramp. Growth lines strongly prosocline. C. omagariensis is slightly larger than C. yasukawensis but smaller than any other species of Cantrainea . C. omagariensis differs also from C. yasukawensis in having a lower ultimate whorl in comparison to its width. C. omagariensis is smaller than C. nuda Okutani, 2001 and differs also in having a subsutural ramp rather than a pleated cord and no knob on the inner lip.

Description.—The protoconch is unknown. The shell is turbiniform, smooth apart from a narrow subsutural ramp. The transition between lateral flank and the base is rounded with no demarcation. The shell wall is relatively thick and bi−layered; the inner layer is nacreous and the outer layer is prismatic. The ultimate whorl is characterized by a slightly dipping suture line. The generating curve is circular, aperture tangential, and peristome uninterrupted. The outer lip is smooth. The inner lip is poorly preserved in all available specimens but it seems to be narrow. The umbilicus is absent although in some specimens a poorly developed umbilical chink is visible. The latter feature may have resulted, however, from erosion of the inner lip.

Discussion.— C. omagariensis is very similar to C. yasukawensis . The latter species, however, is smaller and much more variable morphologically, especially in respect to its spiral ornamentation. C. omagariensis has a much more stable morphology and is represented only by smooth shells. A similar species is known from a single specimen found at “a small tributary of the Abesinai−gawa” (Abeshinai River) in the Nakagawa area from rocks of Senonian age ( Nagao 1939: 215); Ataphrus tesioensis Nagao, 1939 differs from C. omagariensis in lacking subsutural ramp and in having “...prominent bundles of lines of growth near the suture, with numerous longitudinal impressed lines...” ( Nagao 1939: 214), a feature not observed on our specimens. Further studies are needed to substantiate relation between these two species. C. omagariensis is very common at the Omagari seep site. A similar although poorly preserved shell has been reported from a plesiosaur fall association by Kaim et al. (2008b). Another similar shell has been reported by Kiel et al. (2009) as “low−spired skeneiform gastropod” from the Upper Cretaceous Akita Creek wood−fall association ( Kiel et al. 2009: 77).

Stratigraphic and geographic range.—Omagari hydrocarbon seep in Nakagawa area of northern Hokkaido, Japan. Campanian, Upper Cretaceous.