Abstract
A nearly complete pinniped skeleton from the middle Miocene Okoppezawa Formation (ca 16.3-13.9 Ma), Hokkaido, northern Japan, is described as the holotype of Allodesmus uraiporensis sp. nov. The new species is distinguishable from other species of the genus by having the palatine fissure (incisive foramen) that is located anterior to the canine, an anteriorly located supraorbital process of the frontal, and by having the calcaneum with a developed peroneal tubercle. Our phylogenetic analysis suggests that the subfamily Allodesminae are represented by two genera, Atopotarus and Allodesmus, and the latter genus is represented by at least six species; Al. kernensis, Al. sinanoensis, Al. naorai, Al. packardi, Al. demerei and Al. uraiporensis sp. nov. Allodesmus uraiporensis sp. nov. is one of the oldest and the northernmost record of the genus in the western North Pacific, and it suggests that the diversification of the genus in the western North Pacific was synchronous to the time of their diversification in the eastern North Pacific.