Revision of the Late Cenozoic Hyotissa (Bivalvia: Pycnodonteinae) from Western North America

Abstract

-Some Neogene Pycnodonte species, related to the ancient Gulf of California and coastal California, are morphologically similar to Hyotissa hyotis (Linnaeus), the living honeycomb oyster. Morphological variation in these species or in H. hyotis has never been studied. In order to document variation and use the results to detect a possible taxonomic overspliting, the present authors described the morphology and morphometry of fossil shells of H. hyotis from the early Pleistocene Loreto Basin, Baja California Sur, Mexico. It was compared with type and non-type specimens from the Early Miocene Vaqueros sandstone (coastal California), Late Miocene rocks of Isla Tiburón (Gulf of California), Pliocene Imperial Formation (southern California) and Pleistocene rocks in Loreto area (Gulf of California). The specimens from coastal California and Isla Tiburón were re-assigned to a new genus, and to Hyotissa cf. H. hyotis; and synonymized P. (Pycnodonte) heermanni (Conrad) from southern California with H. hyotis. With this new information, the fossil record of Hyotissa in western America ranges from Early Miocene to Recent; H. hyotis lived through the whole ancient Gulf of California during the Pliocene, contrary to the current distribution in the modern gulf. Finally, Hyotissa species exhibit a little variable morphology, which is contrary to traditional views, complicating Pycnodonteinae taxonomy and classifi cation.