Margarites sasakii, 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 : 476

publication ID	https://doi.org/ 10.4202/app.2009.0042
persistent identifier	https://treatment.plazi.org/id/03FBF625-FFDE-2132-7E77-9029FBFB2558
treatment provided by	Felipe
scientific name	Margarites sasakii
status	sp. nov.

Treatment

Margarites sasakii sp. nov.

Fig. 8 View Fig .

2003 Margarites sp. ; Hikida et al. 2003: 338, fig. 10: 1, 2.

Etymology: In honour of Dr. Takenori Sasaki.

Holotype: UMUT MM30174 View Materials , Fig.8D View Fig , moderately preserved shell without protoconch.

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

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

Material.—37 moderately to poorly preserved specimens from the Omagari site; 35 at UMUT and two at NMM already illustrated by Hikida et al. (2003).

Dimensions.—The holotype is 9.42 mm high and 12.86 mm wide; NMM 229 is 10.85 mm high and 12.7 mm wide. Note that both shells are incomplete .

Diagnosis.—Shell turbiniform, whorls inflated. Juvenile and adolescent whorls ornamented by spiral cords present both on the lateral flank and the base. Umbilicus absent. M. sasakii differs from other species of the genus by the absence of an umbilicus.

Description.—The protoconch is not preserved. The shell is turbiniform with a dark−brown amorphous outermost layer ( Fig. 8B View Fig 3 View Fig ). This might be a re−crystallized calcitic outer layer ( Kiel and Goedert 2007) or even fossilized periostracum. Juvenile whorls are ornamented by spiral cords. There are four ribs at the lateral flank, one on the demarcation with the base and at least four on the base. The ribs fade away at the terminus being only weak undulations crossed by strongly enhanced prosocline growth lines. Some of these structures are apparently growth interruptions. The generating curve is circular, aperture tangential, and peristome uninterrupted. Apertural elaborations are absent.

Discussion.—The shells under consideration are difficult to classify as they could be included in many different vetigastropod genera. We preliminarily classify them as Margarites pending better preserved material. M. sasakii differs from other species of Margarites by lacking an umbilicus and possessing spiral cords also on the base. However, the remaining gross morphology strongly recollects M. ryukyuensis Okutani, Sasaki, and Tsuchida, 2000 from a Recent chemosynthesis−based community in the Okinawa Trough ( Okutani et al. 2000). The other similar genus is the seguenzioid Cataegis McLean and Quinn, 1987 that lacks an umbilicus but usually possesses much stronger ornamentation (see e.g., McLean and Quinn 1987; Hickman and McLean 1990; Warén and Bouchet 1993; Fu and Sun 2006). Another similar genus is the trochid Pseudotalopia Habe, 1961 , which again possesses an umbilicus and much more delicate ornamentation (Okutani 2000). M. sasakii is the largest vetigastropod recovered so far from the Omagarii seep site. It is usually poorly preserved but easily identifiable due to its dark brown outer layer.

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