Plionarctos, a tremarctine bear (Ursidae : Carnivora) from western North America

The Pliocene Ringold Formation of eastern Washington has yielded important new materials of tremarctine bears of the anagenetic Plionarctos lineage. T he genus is reviewed in light of this new material and observations made on other described specimens. One of these was previously described from the medial Hemphillian Rattlesnake Formation of Oregon and is recognized as a t remarctine bear, thus extending the earliest record of the group into the e arly part of the late Miocene. The late Hemphillian P. edensis, the genotyp ic species, is rediagnosed, although no new material is added to its hypody m. A new species from the early Blancan White Bluffs sites, P. harroldorum, appears to have been derived from P. edensis. Referred Plionarctos sp. fro m the medial Blancan Taunton Locality has some dental features that are mor e derived and approach those of the Pleistocene Tremarctos floridanus, Plio narctos forms a paraphyletic stem-group for the Tremarctinae. Species of th is genus can be traced successively into the Pliocene where they form the s tock from which the Pleistocene and Recent species of Tremarctos, Arctodus, and Pararctotherium arose.