Article: Cretaceous opine bivalves from the Pacific slope of North America and palaeobiogeography of subfamily Opinae Chavan, 1969

Opines constitute a small subfamily of Mesozoic astartid bivalves that lived mostly in the Tethys Sea region. They first appeared in western Europe during the Middle Triassic, became most widespread during the Jurassic and had their first undoubted appearance in the New World during the Late Jurassic. Their Pacific slope of North America record is studied in detail for the first time. The earliest of the Pacific slope opines is Opis californica Stanton of Early Cretaceous (Hauterivian or Barremian) age. The other six opine species in the study area are of Late Cretaceous age (collectively Middle Turonian – early Late Maastrichtian), have hinges bearing two strong cardinal teeth in each valve, and are placed in the new subgenus Hesperopsis, which is the first astartid to have a partially internal ligament. These six species comprise two morphological lineages inferred to have evolved from O. (H.) popenoei sp. nov. The ‘holzana’ lineage includes O. (H.) holzana sp. nov. and O. (H.) rosarioensis Anderson and Hanna. The ‘anae’ lineage includes O. (H.) anae sp. nov., O. (H.) vancouverensis Whiteaves and O. (H.) triangulata (Cooper). Hesperopis lived in warm temperate waters in offshore shelfal areas. It had a vertical commissure and probably lived as an edgewise recliner. Through time, the species of both lineages became larger and either broader or more elongate. Neither Opis shastalis Anderson, of Aptian age, nor Opis virginalis Waring, of late Palaeocene age, are opines. A junior secondary homonym of Opis trigonalis (Sowerby) was detected and is renamed here as Opis tamurai nom nov.