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DESCRIPTION AND SIGNIFICANCE OF THE MICROTUS (RODENTIA: ARVICOLINAE) FROM THE TYPE IRVINGTON FAUNA, ALAMEDA COUNTY, CALIFORNIA

Abstract The arvicoline rodent Microtus is represented in the Irvington fauna by 72 specimens. The majority of these are isolated molars. Lower first molars are diagnostic and clearly identifiable as Microtus specimens with five closed triangles. No other arvicolines are present in the fauna. Morphological variation in the sample is present, but not extensive. Morphological features permitting species-level identification are lacking, and no species assignation can be supported with the available data. Changing perspectives on Pleistocene biochronology and reinterpretation of several purported early records of Microtus indicate that the Irvington specimens of Microtus with five closed triangles are among the oldest recorded in North America. Chronological control is limited to paleomagnetic data that reveal that the Irvington fauna dates to somewhere between 0.78 Ma and somewhat older than 1.21 Ma. The uncertainty cannot be reduced based on current understanding of the taxonomic affinity and known temporal extent of other mammals from Irvington.