Atresius Gabb, 1869

Kiel, Steffen, Campbell, Kathleen A., Elder, William P. & Little, Crispin T. S., 2008, Jurassic and Cretaceous gastropods from hydrocarbon seeps in forearc basin and accretionary prism settings, California, Acta Palaeontologica Polonica 53 (4), pp. 679-703 : 691

Genus Atresius Gabb, 1869

Type species: Atresius liratus Gabb, 1869 ; Early Cretaceous , Great Valley Group , Wilbur Springs (site 5), California, USA .

Original diagnosis.—“Shell elongate, spire elevated; whorls rounded, aperture ovate, slightly produced in front, outer lip entire, thin; columella not incrusted, imperforate; surface marked by revolving ribs.” ( Gabb 1869: 168).

Emended diagnosis.—High−spired, littoriniform shell with five or more whorls and deeply incised suture; shell thin; whorls almost straight−sided, weakly convex, or with steep shoulder; sculpture of several spiral cords of subequal strength, interspaces mostly wider than spirals; basal margin rounded to strongly angular and keeled; spirals on base similar to those on whorl’s flank but finer and more narrowly spaced; umbilical slit may be present; growth lines opisthocyrt, fine, and numerous, occasionally clumped into ribs; aperture rhomboid, inner lip apparently smooth and without folds, short siphonal notch.

Discussion.— Gabb’s (1869) original diagnosis obviously applies to a broad range of gastropods, therefore an emended diagnosis is provided here, based on more material than was available to Gabb (1869). The taxonomic position of Atresius is doubtful. Suggestions include littorinids ( Gabb 1869; Stanton 1895), turbinids ( Stanton 1895), or trichotropids ( Cossmann 1906; Wenz 1938 –44). Gabb (1869: 169) suggested that Atresius should be placed near Tuba Lea, 1833 and Spironema Meek, 1876 , two genera that Wenz (1938 –44) placed in the heterobranch family Mathildidae Dall, 1889 . A family of cenogastropods that also builds similar shells is the Planaxidae Gray, 1850 , which today is largely found in the intertidal zone. This wide range of placements reflects the difficulties in classifying this genus if no additional information on shell microstructure and/or protoconch is available. We find it unlikely that Atresius belongs to the Turbinidae Rafinesque, 1815 because most members of this family are not as high−spired as Atresius , and have a round aperture, rather than a strongly angulate one like Atresius . Regarding sculpture and growth lines, Atresius also resembles Paskentana . Based on currently available evidence, we consider Atresius to be a cenogastropod which could have affinities to the littorinids, cerithioideans, provannids, and/or Paskentana .

Several species have been assigned to Atresius over the years, but none of them can be confirmed. Diller and Stanton (1894: 442) reported “ Atresius liratus Gabb (?)” from a carbonate block in shales in CFCC that also contained the seep−restricted “ Modiola ” major Gabb, 1868 . A year later Stanton (1895: 69) mentioned that “ Atresius liratus Gabb (?)” belonged “to a distinct, undescribed species”. The species in question was not figured and the material could not be tracked down, thus is remains uncertain whether this is A. liratus or a new species. Cossmann (1906) placed several European species in Atresius , including Cerithium cornuelianum d'Orbigny, 1842 , for which Wood (1910) pointed out that it bears little resemblance to the type of Atresius ; Cerithium lallierianum d’Orbigny, 1842 , which Kollmann (2005) convincingly classified as Cirsocerithium ; and “ Cerithium ” cenomanense d’Orbigny, 1843, which is the only species from Cossmann’s list that does indeed resemble Atresius liratus to some extent, and was re−illustrated and marked as “ species incertae sedis ” by Kollmann (2005: 171–172, pl. 18: 1a, b). Casey (1961: 557) reported “ Atresius fittoni (d’Orbigny) ” of Casey (1961) from the Sandgate Beds in Sussex, England, which is in fact Cerithium lallierianum (i.e., Kollmann, 2005: 160). Five further species which Cossmann (1906) considered as belonging to Atresius were reported by Holzapfel (1888) from the Vaals greensands but they resemble Cretaceous cerithiids, epitoniids, and nystiellids rather than Atresius . In sum, Atresius appears to be monospecific and restricted to Early Cretaceous seep carbonates in the Great Valley Group in California, USA.