Neotherium mirum

Neotherium mirum is an extinct species of basal walrus that lived during the Middle Miocene along the North Pacific coast, with fossils known from California and from Shimane Prefecture in Japan. It was smaller than the living walrus and did not have long tusks. Males were larger than females, so the family already showed marked differences in body size between the sexes about 15 million years ago. Stable isotope evidence indicates that in the eastern North Pacific Neotherium had foraging habits intermediate between the nearshore specialist Pithanotaria and the offshore specialist Allodesmus, which places it between shore and open water in how it used the sea.