The harpid neogastropod genus Oniscidia Mörch, 1852, which has not been recognized before in the northeast Pacific fossil record, is represented there by rare specimens of Oniscidia plectata (Waring, 1917) n. comb., of late early Paleocene age, in a region extending from southern California, USA to Baja California, Mexico. This species is the earliest unequivocal record of Oniscidia and its only known Paleocene record. It apparently lived in silty, inner- to middle-shelf depths, which were inherently cooler than adjacent shallower marine depths. Its habitat was subject to the influx of shallow-marine shells, especially turritellas, contained in turbidity currents emanating from nearshore depths.

The global paleogeography of Oniscidia, which is presented here for the first time, has been overlooked previously because this genus has a long and complicated history of taxonomic confusion with the harpid genus Morum Röding, 1798. Oniscidia questionably originated during the Late Cretaceous (Campanian) in southern India and apparently dispersed westward through the Tethys Seaway into the New World. Paleocene and early Eocene occurrences of this genus are rare, and middle Eocene occurrences are uncommon. During the cool times of the Oligocene and into the early Miocene, it was most widespread. Its range became restricted during the middle Miocene and continued to be so during the Pliocene, Pleistocene, and modern day, with occurrences only in the Caribbean Sea region, Florida, and the western Pacific. Its distribution through warm and cool times was most likely controlled by its habitat preference for relatively deep cool waters.