Cretaceous Acila (Truncacila) (Bivalvia: Nuculidae) from the Pacific Slope of North America

The Cretaceous record of the nuculid bivalve Acila (Truncacila) Grant & Gale, 1931, is established for the first time in the region extending from the Queen Charlotte Islands, British Columbia, southward to Baja California, Mexico. Its record is represented by three previously named species, three new species, and one possible new species. The previously named species are reviewed and refined. The cumulative geologic range of all these species is Early Cretaceous (late Aptian) to Late Cretaceous (early late Maastrichtian), with the highest diversity (four species) occurring in the latest Campanian to early Maastrichtian. Acila (T.) allisoni, sp. nov., known only from upper Aptian strata of northern Baja California, Mexico, is one of the earliest confirmed records of this subgenus. ‘‘Aptian’’ reports of Truncacila in Tunisia, Morocco, and possibly eastern Venzeula need confirmation. Specimens of the study area Acila are most abundant in sandy, shallow-marine deposits that accumulated under warmwater conditions. Possible deeper water occurrences need critical evaluation.