Xylodiscula okutanii, Kiel & Goedert, 2007

Kiel, Steffen & Goedert, James L., 2007, New mollusks associated with biogenic substrates in Cenozoic deep-water sediments of Washington State, Acta Palaeontologica Polonica 52 (1), pp. 41-52 : 45-46

Xylodiscula okutanii sp. nov.

Fig. 4 .

2006 Hyalogyrina n. sp.; Kiel and Goedert 2006a: 2626, fig. 2j, l.

Derivation of the name: After Takashi Okutani, Kanagawa.

Holotype: USNM 531406 . Well−preserved specimen with lower half of aperture and base concealed by matrix.

Type locality: USGS loc. 26897−A, Merrick's Bay , Clallam County, Washington State, USA .

Type horizon: Uppermost Oligocene to lowermost Miocene, Pysht Formation.

Material.— Holotype only.

Diagnosis.—A Xylodiscula with almost three whorls, protoconch made of one and one−fifth whorls, 211 ̊m wide, initial part with fine polygonal pattern; teleoconch made of one and three−quarter well−rounded whorls, growth lines sinuous and well−visible near suture.

Description.—Embryonic part of protoconch heterostrophic, one−half whorl, 102 ̊m wide; larval shell smooth except for occasional broad, sinuous wrinkles. Teleoconch skeneiform, one and three−quarter whorls, smooth, growth lines sinuous near suture, otherwise prosocline. Shell composed of an inner and an outer layer of simple prismatic structure, and an intersected crossed−acicular layer in between. Diameter 1.03 mm. Discussion.—We assigned this species earlier to the genus Hyalogyrina Marshall, 1988 because it resembles some low−spired species of this genus, like the Japanese H. depressa Hasegawa, 1997 . But we are now convinced that this species rather belongs to Xylodiscula , because Hyalogyrina has a different type of protoconch, which usually consists of half or three−quarters of a whorl only, and has a larger whorl expansion rate than the protoconchs of Xylodiscula . Most Recent species of Xylodiscula differ from X. okutanii by having a more−or−less well developed shoulder. This includes the type species which has a strong shoulder. An additional difference to X. okutanii is the more pronounced transition from protoconch to teleoconch. X. eximia Marshall, 1988 is somewhat shouldered, although much weaker than X. vitrea , and the shoulder may be indicated by a spiral ridge only. Another species with slightly angular whorls is the vent−inhabiting X. analoga Warén and Bouchet, 2001 . The shell microstructure of X. okutanii is very similar to that of X. analoga ( Kiel 2004) .

A somewhat similar heterobranch from the uppermost Eocene of western Washington, but with a planispiral shell is Cyclostremella (?) chimaera Hickman, 1980. That species has an entirely sinistrally coiled protoconch ( Hickman 1980b), whereas in Xylodiscula okutanii only the embryonic shell is sinistral.

Stratigraphic and geographic range.—Uppermost Oligocene to lowermost Miocene, upper part of the Pysht Formation, Washington State, USA.

Figures

Fig. 4. Xylodisculid allogastropod Xylodiscula okutanii sp. nov., from a uppermost Oligocene or lowermost Miocene wood−fall in Merrick's Bay, holotype USNM 531406 from USGS loc. 26897−A. A. Apical view. B. Oblique view. C. Shell microstructure. D. Close−up of embryonic shell. E. Close−up of protoconch, arrow indicates transition from protoconch to teleoconch.