An unusual new gastropod from an Eocene hydrocarbon seep in Washington State

Abstract

Subduction-Related Hydrocarbon-Seep carbonates in Cenozoic deepwater sediments in western Washington State, USA, yield fossil invertebrate communities that are largely endemic to these localized microhabitats (Goedert and Squires, 1990; Peckmann et al., 2002; Goedert et al., 2003; Kiel, 2006). An unusually large species of the deep-sea gastropod Abyssochrysos Tomlin, 1927 has been described from the oldest of these seep carbonates, which occurs in the Humptulips Formation on the southern slopes of the Olympic Mountains (Goedert and Kaler, 1996). New material of this species was recently collected at this site and shows that this species has a deep notch and a selenizone at the base of the outer lip, a feature that is very unusual among living gastropods, and unknown from Abyssochrysos. This feature is also present on two gastropod specimens, found in the collection of the Burke Museum (UWBM) that are, according to the label, from the Humptulips area but without detailed locality information. Stable isotope analysis of the micritic matrix adhering to the specimens showed δ13C values as low as −37 to − 41‰ relative to PDB standard. Such negative values clearly indicate that the carbonate formed under the influence of anaerobic oxidation of biogenic methane (cf. Whiticar, 1999; Peckmann et al., 2002). The isotope analysis was carried out in the same lab using the same methods as described in Kiel (2006). The Humptulips area is currently covered with dense vegetation and attempts to locate the carbonate outcrop where these two specimens could have come from have been unsuccessful. The purpose of this note is to introduce a new genus for this unusual species, to discuss its possible place among the gastropods, and to speculate about the function of the two deep sinuses in the outer lip of the aperture.