A NEARLY COMPLETE SKULL OF THE BEAVER, CASTOR CANADENSIS, FROM THE IRVINGTONIAN BADLANDS OF GOLFO DE SANTA CLARA, SONORA, MEXICO

Early to middle Pleistocene Colorado River Delta deposits exposed in the upper Gulf of California, northwestern Sonora, México are host to a diverse paleo-fauna and –flora, the El Golfo local paleobiota, which dates to the Irvingtonian North American Land Mammal Age. The fossiliferous exposures are found in badlands developed in fluvio-deltaic sediments that have been mildly deformed during late Pleistocene doming along the Cerro Prieto Fault. For the past two decades, the El Golfo Project has been part of a resource inventory for the Upper Gulf of California and Colorado River Delta Biosphere Reserve and has recovered over 7,000 fossils through joint efforts by Arizona Western College, the George C. Page Museum, and the Biosphere Reserve. The preserved paleobiota, now numbering over 120 species, suggests an Irvingtonian tropical to semitropical climate and the existence of four ecologic communities: freshwater aquatic, riparian gallery forest, shrub and brush woodland, and savannah-like grassland.

In 2011, a near-complete skull of Castor canadensis, was recovered as part of the ongoing El Golfo Project. The specimen lacks only the left second molar. To our knowledge, this is the most complete skull found at any Irvingtonian site in North America. The family Castoridae reached its highest diversity in the Oligocene and Miocene, but by the beginning of the Pleistocene only two genera are known and only one survives today, the genus Castor. There are two living species of Castor, the Eurasian C. fiber and the North American C. canadensis. Two North American fossil species are recognized as conspecific with Castor canadensis.