A new <em>Calliovarica</em> species (Seguenzioidea: Chilodontidae) from the Eocene of Oregon, USA: Persistence of a relict Mesozoic gastropod group in a unique forearc tectonic setting

Abstract

A new, enigmatic chilodontid gastropod with distinctive periodic varices is described as Calliovarica oregonensis. It is based on ten specimens from slope deposits of the early late Eocene Nestucca Formation on the coastal Cascadia margin of present day Oregon, U.S.A. It is the last appearance of a Mesozoic group of epifaunal basal gastropods with periodic varices. It is the third species in a Cenozoic genus previously known only from the early Eocene Lodo Formation in California and the late Paleocene to early Eocene Red Bluff Tuff in New Zealand. The type species, C. eocensis, is refigured to clarify the nature of the axial varices as well as a terminal thickening and flaring of the apertural lip immediately following deposition of the final varix. Detailed preservation of microstructure in the nacreous layers of crushed and disintegrating shell fragments demonstrates the value of collecting material typically left behind in the field. Calliovarica oregonensis n. sp. is part of a poorly understood molluscan fauna that lived during an unusual paleoclimatic interval immediately prior to global cooling and extinctions in the late Eocene. It also thrived in an unusual volcano-sedimentary interval at the onset of subduction at the Cascadia margin. The tectonic setting provides a unique snapshot of a depositional environment receiving periodic influxes of ash from the young volcanic arc to the east and periodic basalt intrusions from the underlying asthenosphere into the forerarc over a hotspot or through slab window emplacement. Links between biofacies and lithofacies demonstrate the ability of paleontology and geology to provide reciprocal illumination, especially in dynamic settings with no modern counterparts. The link between local persistence of relict Mesozoic taxa and localized tectonic events merits further integrative investigation.