Thalassoleon

Thalassoleon is an extinct genus of large fur seal. Thalassoleon inhabited the Northern Pacific ocean in latest Miocene and early Pliocene time. Fossils of T. mexicanus are known from Baja California and southern California. T. macnallyae is known from central California, and T. inouei (which may be a synonym of T. macnallyae) is known from Japan. Thalassoleon could be the ancestor of the modern northern fur seal.

T. mexicanus was comparable in size to the largest fur seals, with an estimated weight of 295-318 kg (650-700 lb). T. macnallyae, based on size of the mandible, may have grown much larger, similar in size to a walrus.