Two new species of Retiskenea? (Gastropoda: Neomphalidae) from Early Cretaceous hydrocarbon seep-carbonates of California

Abstract

Two new Mesozoic gastropod species, provisionally attributed to the minute (height < 5 mm) coiled neomphalid genus Retiskenea?, are described from three geographically isolated, Early Cretaceous, hydrocarbon seep-carbonate sites at Wilbur Springs, Rice Valley, and Cold Fork of Cottonwood Creek, northern California (USA). A fourth paleo-seep locality at Paskenta, of probable Upper Jurassic age, also yielded a single specimen of a morphologically similar microgastropod that may be a neomphalid with affinities to the Lower Cretaceous specimens described herein. The limestone lenses are ∼2–260 m in length, ∼1–5 m in diameter, and surrounded by forearc siliciclastics of bathyal turbidites or sedimentary serpentinites in the Upper Jurassic to Lower Cretaceous (Tithonian–Albian) Great Valley Group and its equivalents. The Lower Cretaceous microgastropods are tentatively placed in Retiskenea? based on similar shell characters: size, globose shape, inflated reticulate protoconch, number and distinct inflation of the body whorls, and fine, prosocline sculpture of the final body whorl. The fossils occur in carbonate microbialites that formed in seafloor sediments during archaeal anaerobic oxidation of methane in the zone of bacterial sulfate reduction, associated with H2S- and CH4-rich fluid seepage. The California Retiskenea? fossils commonly are found in gregarious clusters, or closely affiliated with thin worm tubes or, in one case, a larger gastropod. These Mesozoic records increase the total known species attributable to this cold-seep endemic genus from two to four. Its spatial and temporal distribution thus may have spanned ∼9,000 km around the Pacific Rim from at least ∼133 m.y. to the present in 10 subduction-related seep sites from California (possibly Upper Jurassic–Lower Cretaceous), Washington (middle Eocene–Upper Oligocene), and modern offshore Oregon, the eastern Aleutians, and the Japan Trench. If the generic placement of these microgastropod fossils is correct, the California records are the oldest-known occurrences of Retiskenea, consistent with an estimated minimum Mesozoic origin for the ‘hot vent’ Neomphalidae, as inferred from molecular analyses published on other living members of the family.