an unusual new gastropod from an eocene hydrocarbon seep in ...

J. Paleont., 82(1), 2008, pp. 188–191 Copyright 䉷 2008, The Paleontological Society 0022-3360/08/0082-188$03.00

AN UNUSUAL NEW GASTROPOD FROM AN EOCENE HYDROCARBON SEEP IN WASHINGTON STATE STEFFEN KIEL Earth Sciences, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom, and Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013-7012, USA, ⬍[email protected]⬎

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UBDUCTION-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 ␦13C 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. SYSTEMATIC PALEONTOLOGY

Class GASTROPODA Cuvier, 1797 Subclass CAENOGASTROPODA Cox, 1959 Family ABYSSOCHRYSIDAE? Tomlin, 1927 Genus HUMPTULIPSIA new genus Type species.⎯Abyssochrysos raui Goedert and Kaler, 1996; Eocene, Washington State, USA. Diagnosis.⎯Shell moderately large, turreted, suture fine and incised, whorls numerous, sculpture variable. Whorls with slight subsutural constriction, especially on later whorls; early whorls with orthocline, or opisthocyrt to opisthocline ribs, one moderately strong spiral cord near adapical suture, one near abapical suture, smaller spirals variably present between them (compare figs. 3, 6, and 9 of Goedert and Kaler, 1996); spiral cords tend to fade on later whorls. Aperture oval; outer lip with subsutural notch, peripheral projection, and deep notch near basal margin. Selenizone resulting from basal notch usually covered by succeeding whorl. Etymology.⎯Named after the Humptulips River and Formation in western Washington, where the type species occurs.

HUMPTULIPSIA

(Goedert and Kaler, 1996) Figure 1

RAUI

Abyssochrysos raui GOEDERT

AND

KALER, 1996, p. 68, figs. 3–9.

Emended diagnosis.⎯Large specimens develop broad sinus below adapical suture accompanied by constriction, deepest at abapical side of sinus; growth lines prosocyrt on periphery; at basal

margin aperture forms another deep sinus; growth lines strongly prosocyrt below, ending at aperture. Inner lip of aperture convex and callused. Material examined.⎯Twenty specimens from CSUN loc. 1583, including one figured (USNM 531408), four specimens from an unknown locality (Burke Museum locality number B6980) in the Humptulips Formation (UWBM 97853 and 97854), and one specimen from a small presumed coldseep carbonate block that was found on the banks of the Canyon River, Gray’s Harbor County, Washington State, in the late Oligocene part of the Lincoln Creek Formation (USNM 531409). This block also contained the bivalves Acharax dalli Clark, 1925, Conchocele sp., and Lucinoma sp., three genera that are known to harbor chemoautotrophic endosymbionts. Measurements.⎯UWBM 97853 (Fig. 1.1, 1.2): 68 mm; UWBM 97854 (Fig. 1.3, 1.4): height 54 mm; USNM 531408 (Fig. 1.6–1.8): height 74 mm; USNM 531409 (Fig. 1.9): 5.3 mm. The holotype of Goedert and Kaler (1996, figs. 3–5) is 48.2 mm high, their largest specimen 90 mm high. Occurrence.⎯In cold-seep carbonates, western Washington State, USA, Middle to Late Eocene, Humptulips Formation, and possibly Late Oligocene, Lincoln Creek Formation.

Discussion.⎯A well-preserved small specimen that resembles juvenile H. raui in every respect shows no sinuses in its growth line (Fig. 9). Assuming that it is conspecific with H. raui, the deep notch and the corresponding selenizone may develop only late in the ontogeny and are therefore not seen in this small specimen. Alternatively, this specimen could represents an as-yet undescribed species, most likely an abyssochrysid. DISCUSSION

Extant abyssochrysids usually do not exceed a few centimeters (Houbrick, 1979; Bouchet, 1991; Killeen and Oliver, 2000), and compared to these, H. raui is extremely large. Also, abyssochrysids are usually uncommon, whereas H. raui occurs in dense clusters at the two Eocene sites. These differences call for comparison to the supposed sister taxon of the abyssochrysids, the provannids (Ware´n and Ponder, 1991). Most provannids are small and graze on bacterial mats (Ware´n and Bouchet, 1986; Smith and Baco, 2003). At some hydrothermal vent sites in the West Pacific and Indian Ocean, however, the provannid genera Ifremeria Bouchet and Ware´n, 1991 and Alviniconcha Okutani and Ohta, 1988 (Van Dover et al., 2001) occur in dense clusters. These two taxa harbor chemoautotrophic endosymbionts, which allows them to reach sizes of 8 cm or more (Bouchet and Ware´n, 1991; Ware´n and Ponder, 1991). By analogy, I speculate that H. raui lived with chemotrophic symbionts because of its large size and occurrence in dense clusters at methane seeps. This reconstruction also provides a possible explanation for the sinuses in the outer lip of the aperture of H. raui. They could have been the place for inhalant and exhalant siphons used to provide the endosymbionts with a constant flow of methane- and/or sulfide-rich fluids. Humptulipsia with its apertural notch and selenizone near the base of the whorls is unusual, especially among high-spired caenogastropods. Several caenogastropod groups have shells with a selenizone or labral sinuses of some sort, but Humptulipsia does not appear to be related to any of these. The conspicuous lower notch is approximately in the position where Strombus Linnaeus, 1758 has the ‘stromboid notch’ (Abbott, 1960), but this notch does not leave a selenizone. Some turrids have a slit and a selenizone, but it is positioned in the apical or middle part of the whorl, not on the lower part (Wenz, 1938–1944; Hickman, 1976). In

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FIGURE 1—Humptulipsia raui (Goedert and Kaler, 1996) from seep carbonates in western Washington State. 1, Specimen from B6980, showing upper and lower sinuses, UWBM 97853, ⫻1.25; 2, Same specimen as 1, showing details of the lower apertural margin with selenizone, ⫻1.5. 3, 4, Specimen from B6980, showing upper and lower sinuses, and callused inner lip of the aperture, UWBM 97854, ⫻1.5. 5, Fragment consisting of the lower outer side of the aperture, attached to specimen UWBM 97853, ⫻1. 6–8, Newly collected specimen from the type locality (CSUN loc. 1583); 6, 7, Two views, USNM 531408, ⫻1; 8, close-up showing lower apertural notch. 9, Possibly a juvenile specimen of H. raui, from the late Oligocene of the Lincoln Creek Formation, USNM 531409, ⫻10.

addition, turrids have a straight inner lip of the aperture and a siphonal canal rather than a convex inner lip. Some turritellids have a deep sinus in their growth line, but as in turrids, not on the lower part of the whorl (Marwick, 1957). High-spired shells and a selenizone are characteristic for the Paleozoic Goniasma Tomlin, 1930 in which slit and selenizone are in the lower half of the whorl (Nu¨tzel and Bandel, 2000), but not at the basal margin, and Goniasma lacks the subsutural sinus of Humptulipsia. Another slit-bearing caenogastropod family is the Siliquariidae Anton, 1838 (e.g., Bandel and Kowalke, 1997). However, this sessile and usually uncoiled group of gastropods is unlikely to be related to Humptulipsia. Certain species of the pelagic Janthina Ro¨ding, 1798 have a rather deep sinus in a similar position as the lower one in Humptulipsia. However, Janthina has a low-trochiform shell and a very different mode of life, and a phylogenetic relationship between Janthina and Humptulipsia is unlikely. High-spired gastropods with a sometimes strong sinus in the upper part of the whorl are streptacids and donaldinids of the Heterobranchia Gray, 1840, but these species usually do not exceed a few millimeters in height and lack a slit on the basal margin (e.g., Bandel, 2005). Humptulipsia is here tentatively placed in the Abyssochrysidae because of the similarities of its juvenile whorls with those of Abyssochrysos, but its extraordinary apertural shape clearly distinguishes Humptulipsia from Abyssochrysos. Two high-spired

gastropod species from Cretaceous seep carbonates in California were considered abyssochrysids (Campbell, 2006, p. 382, fig. 6b, d). These specimens are currently under study and show no sinuses in their growth lines resembling those of Humptulipsia. Another seep-related Abyssochrysos species was described from the Eocene–Miocene Diapiric Me´lange in Barbados (Gill et al., 2005, p. 201, fig. 5d, e). This species shows a subsutural sinus and prosocyrt ribs similar to Humptulipsia raui but lacks the strong basal sinus. Abyssochrysos and the Abyssochrysidae, along with their supposed sister taxon, the Provannidae Ware´n and Ponder, 1991, have for a long time been placed in the Loxonematoidea Koken, 1889 (Houbrick, 1979; Killeen and Oliver, 2000; Ware´n and Bouchet, 2001), largely based on similarities in shell characters to Paleozoic and Mesozoic Pseudozygopleuridae Knight, 1930 and Zygopleuridae Wenz, 1938. However, as pointed out by Bandel and Kiel (2000), the name-giving type genus Loxonema Phillips, 1841 is only poorly known and most likely belongs to the extinct, highspired archaeogastropod group Stylogastropoda Fry´da and Bandel, 1997, and therefore a phylogenetic relation between loxonematoids and abyssochrysids is highly unlikely. Nu¨tzel (1998) emphasized similarities in teleoconch characters between zygopleurids and abyssochrysids, but pointed out that due to the large gap in the fossil record between the two groups (Jurassic to Eocene), a phylogenetic relationship between zygopleurids and

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abyssochrysids is unlikely. However, in his phylogenetic reconstruction of the Ptenoglossa Gray, 1853, abyssochrysids are hesitantly shown as descendants of zygopleurids and as a sister group to the janthinoids (Nu¨tzel, 1998). The discovery of dense communities thriving on geochemical energy sources in the deep sea (Corliss et al., 1979) significantly altered our view of the deep sea as a desertlike environment in which animals are few and far in between. On average one new species every two weeks has been described from such environments for the past 25 years (Van Dover et al., 2002). Recently, the discoveries of a vent gastropod with iron sulfide dermal sclerites (Ware´n et al., 2003), and of whale-bone specialists with symbiotic relationships previously unknown from the entire animal kingdom (Rouse et al., 2004), again highlighted our poor knowledge of deep-sea environments. Humptulipsia is the first gastropod genus from a Cenozoic coldseep that is unknown from modern vent or seep communities, with the possible exception of Thalassonerita Moroni, 1966. However, Taviani (1994) suggested that Thalassonerita is a synonym of extant Bathynerita Clarke, 1989. Humptulipsia occurs at a seep that is dominated by a lucinid bivalve, Cryptolucina Saul, Squires and Goedert, 1996, that also has no living congeners. This supports the view that seeps and presumably also vents have a dynamic history connected in yet unexplored ways to the evolution of the photosynthetic world (Little and Vrijenhoek, 2003; Campbell, 2006; Kiel and Little, 2006). ACKNOWLEDGMENTS

I would like to thank J. L. Goedert, Wauna, for his guidance during the field work and for drawing my attention to the two specimens from the Burke Museum, E. A. Nesbitt, Seattle, for making the Burke specimens available for study; M. Joachimski, Erlangen, for the isotope analyses; and K. Bandel, Hamburg, and A. Ware´n, Stockholm, for comments on this unusual gastropod. Constructive reviews were provided by K. Bandel and A. Nu¨tzel, Mu¨nchen, which is gratefully acknowledged. This work was financially supported by the Charles D. and Mary V. Walcott fellowship of the Smithsonian Institution, Washington DC, and by a Marie-Curie Intra-European fellowship of the European Commission. REFERENCES

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