Cornwallius is an extinct herbivorous marine mammal of the family Desmostylidae. Cornwallius lived along the North American Pacific Coast from the Early Oligocene (Chattian) through the Oligocene (28.4 mya—20.6 Mya) and existing for approximately . The type locality is the Chattian Sooke Formation, Vancouver Island, British Columbia, Canada (, paleocoordinates ). Cornwallius was named by . Its type is Desmostylus sookensis, named by and recombined to Cornwallius sookensis by . Fossils have been discovered from Baja California Peninsula, Oregon and Washington coasts, and Unalaska Island. Notes References Desmostylians Oligocene mammals Prehistoric placental genera Fossil taxa described in 1923 Oligocene mammals of North America