Castor, the extant beaver, is an important member of North American and Eurasian communities, altering ecosystems and landscapes wherever it occurs. Castor has a fossil record extending back to the late Miocene in Europe, Asia, and North America. The origin and early evolution of this genus is unclear but it likely originated in Eurasia and subsequently dispersed to North America from Asia in the late Miocene. A new record of Castor from the Rattlesnake Formation of Oregon represents the earliest occurrence of the genus in North America. The age of this new find is inferred through radiometric and magnetostratigraphic dating to be between 7.05 and 7.3 Ma. This age is supported by the co-occurrence of another beaver, Dipoides stirtoni, which is characteristic of early Hemphillian deposits. This find indicates that Castor had dispersed to North America by the early Hemphillian, and helps resolve the timing of divergence between North American and Eurasian beavers. The Rattlesnake Formation specimens are not differentiable from other North American specimens of C. californicus in morphology or size. This suggests Castor changed little after dispersing to North America, whereas the endemic castoroidine beavers changed dramatically after its arrival.
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Vol. 85 • No. 5