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The earliest Ancistrolepis (Gastropoda: Buccinidae) and its geologic implications


The buccinid gastropod Ancistrolepis carolineae Squires, 1984 is the earliest known and shallowestmarine species belonging to the extant genus Ancistrolepis Dall, 1895, which is endemic to the NorthPacific region. This species is of late early Eocene (Ypresian Stage) age and is about 7 million years older than the previously oldest known record (late Eocene) of the genus. This rare species occursat several localities in a 1-m thick bed (“Stewart bed”) of fossiliferous shallow-marine sandstone within the Llajas Formation, on the north side of Simi Valley, Ventura County, southern California.The “Stewart bed” contains a rich fauna of subtropical mollusks and other invertebrates, which lived just below effective wave base, at the distal edge of a braid delta, immediately adjacent to an upper bathyal prodelta/slope environment, where a rich microfauna of calcareous nannofossils and benthic foraminifers lived. Dispersal of A. carolineae would have been either via drifting of buoyant “pouches” containing its non-planktonic eggs or by rafting of its adults in buoyant plant material debris. The dispersal of A. carolineae northward coincided with the cooling of the NorthPacific waters during the late Eocene, thereby providing the opportunity for Ancistrolepis to adapt to living in bathyal waters and also to living in association, in some cases, with chemosynthetic (methane) cold seeps.

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