Cantrainea yasukawensis, 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

publication ID	https://doi.org/ 10.4202/app.2009.0042
persistent identifier	https://treatment.plazi.org/id/03FBF625-FFD8-2134-7E77-923FFBFB25A7
treatment provided by	Felipe
scientific name	Cantrainea yasukawensis
status	sp. nov.

Treatment

Cantrainea yasukawensis sp. nov.

Figs. 5 View Fig , 6 View Fig .

2007 Ataphridae View in CoL gen. et sp. indet.; Jenkins et al. 2007a: 8, fig. 5: 2.

2007 ataphrid gastropod; Jenkins et al. 2007b: 371, fig. 2c.

Etymology: After the type locality.

Holotype: UMUT MM30158 View Materials , Figs. 5F View Fig and 6H View Fig , well preserved shell without protoconch.

Type locality: Yasukawa site, Nakagawa area , Northern Hokkaido, Japan. Coordinates 44 ° 40’37’’ N, 142 ° 1’27’’ E GoogleMaps .

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

Material.—134 moderately to well preserved specimens without protoconchs.

Dimensions.—The holotype is 3.59 mm high and 3.9 mm wide.

Diagnosis.—Shell turbiniform with flat−topped or slightly concave ramp at the suture. The upper part of the lateral flank slightly concave. Growth lines strongly prosocline. Spiral ornamentation variable. C. yaskukawensis is much smaller than any living species of Cantrainea and also smaller that C. omagariensis described below. Moreover, it has a higher ultimate whorl in comparison to its width than C. omagariensis . C. yasukawensis is also much smaller than the similarly smooth−shelled Recent C. nuda Okutani, 2001 ; it differs also in having a subsutural ramp rather than a pleated cord and no knob on the inner lip of the aperture that is characteristic for C. nuda ( Fig. 7G View Fig ).

Description.—The protoconch is unknown. The shell is turbiniform with variable spiral ornamentation. The most typical example (see holotype, Figs. 5F View Fig and 6H View Fig ) has a wide concave subsutural ramp and the base demarcated by a weak and rather rounded angulation. Some other specimens have a pattern of delicate faint ribs ( Fig. 5B, C View Fig ) and the base demarcated by clear angulation at some specimens accompanied by a spiral rib ( Fig. 5E, H View Fig ). Some shells of C. yasukawensis lack any ornament apart from a narrow subsutural ramp ( Figs. 5K View Fig , 6A View Fig ). However, even on these smooth specimens there are some spiral undulations visible under higher magnification ( Fig. 5K View Fig 3 View Fig ). The shell wall is relatively thick and bi−layered. Inner nacreous layer is three times thicker than the outer prismatic layer. The ultimate whorl is characterized by slightly dipping suture line. The generating curve is circular, aperture tangential, and peristome uninterrupted. The outer lip is smooth. The inner lip has usually a narrow callus and there is no umbilicus. In some specimens, especially these smooth−shelled, there is a callosity extended over the umbilical area ( Fig. 5F View Fig ). Apertural elaborations are absent. The operculum is calcareous and concentric in the visible part ( Fig. 5J View Fig ).

Discussion.— C. yasukawensis is a species with highly variable shell ornamentation. Such plasticity in shell morphology is also observed in other gastropods inhabiting chemosynthesis−based communities and the most conspicuous examples are those of provannid gastropods: Provanna variabilis Warén and Bouchet, 1986 from Recent hydrothermal vents on East Pacific Rise and Provanna antiqua Squires, 1995 from Paleogene hydrocarbon seeps in Washington State ( Squires 1995). Paskentana paskentaensis (Stanton, 1895) from Early Cretaceous seep associations is another example of highly variable species (Kiel et al. 2008). C. yasukawensis is common in the proximity of methane influenced carbonate bodies and is also present in the peripheral areas of the hydrocarbon seep ( Jenkins et al. 2007a). This gastropod was preliminarily identified by Jenkins et al. (2007a, b) as an ataphrid and after a detailed examination it seems that Cantrainea is the best place for it although the latter genus includes mostly much larger collonins. C. yasukawensis is strikingly similar to the Recent C. nuda . The latter species differs from C. yasukawensis especially in having a distinctive knob on the inner lip ( Okutani 2001). The other possible place for C. yasukawensis is Homalopoma which includes small− and medium sized collonins. The species of Homalopoma , however, have usually evenly convex shells with no concave subsutural ramp.