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The Early to Late Oligocene  Propalaeocastor  is the earliest known beaver genus from Eurasia. Although many species of this genus have been described, these species are defined based on very fragmentary specimens.  Propalaeocastor irtyshensis  from the Early Oligocene Irtysh River Formation in northwestern Xinjiang, China is one of the earliest-known members of  Propalaeocastor . This species is defined on a single maxillary fragment. We revise the diagnosis of  P. irtyshensis  and the genus  Propalaeocastor , based on newly discovered specimens from the Irtysh River Formation. The dental morphology of  P. irtyshensis  is very similar to other early castorids. The caudal palatine foramen of  P. irtyshensis  is situated in the maxillary-palatine suture. This is a feature generally accept as diagnostic character for the castorids. On the other hand,  P. irtyshensis  has two upper premolars, a rudimentarily developed sciuromorph-like zygomatic plate, and a relatively large protrogomorph-like infraorbital foramen. Some previous researchers suggested that  Propalaeocastor  is a junior synonym of  Steneofiber , while other took it as a valid genus. Our morphological comparison and phylogenetic analysis suggest that  Propalaeocastor  differs from  Steneofiber  and is a valid genus. We also suggest that  Agnotocastor aubekerovi ,  A. coloradensis ,  A .  galushai ,  A. readingi ,  Oligotheriomys primus , and “ Steneofiber  aff.  dehmi ” should be referred to  Propalaeocastor .  Propalaeocastor  is the earliest and most basal beaver. The origin place of  Propalaeocastor  and castorids is uncertain. The Early Oligocene radiation of castorids probably is propelled by the global climate change during the Eocene-Oligocene transition.

Morphology of an Early Oligocene beaver Propalaeocastor irtyshensis and the status of the genus Propalaeocastor