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Fossil vertebrates from the Alturas Formation in northern California have previously been considered important for defining the age of the boundary between the Hemphillian and Blancan North American Land Mammal Ages. Diatomaceous mudstone of the upper Alturas Formation contain fossil mammals including the arvicoline rodent Mimomys (Ogmodontomys) sawrockensis that is diagnostic of Blancan faunas. New paleomagnetic and geochemical data from the upper Alturas Formation constrain the age of the first stratigraphic occurrence of M. (O.) sawrockensis at Crowder Flat Road to between 4.5 and 4.6 Ma. This age is approximately 0.2-0.4 Ma younger than previously reported such that the oldest record of Mimomys in North America, south of 55oN, is from Panaca, Nevada, and is constrained geochronologically to be approximately 4.9 Ma. The Hemphillian – Blancan North American Land Mammal Age boundary probably occurs within magnetic polarity Chron C3n.3r at approximately 4.9 Ma.

Geochronology of the Upper Alturas Formation, northern California: Implications for the Hemphillian-Blancan North American Land Mammal Age boundary