BIOCHRONOLOGY

Based on radiometric dates and paleomagnetic data from other sites, Allophaiomys is thought to have existed from slightly less than 2.1 Ma to about 840 Ka; the five closed-triangle Microtus from either 1.4? Ma or 840 Ka to the present; M. paroperarius from about 840 Ka to 252 +/- 30 Ka and M. meadensis from 820 Ka to 252 +/- 30 Ka (Bell et al., 2004b). Allophaiomys is the defining taxon for the Irvingtonian I microtine stage east of the Rocky Mountains. Microtus meadenesis defines the Irvingtonian II and M. llanensis helps to characterize the stage (Bell et al., 2004b). The co-occurrence of Allophaiomys and M. meadensis suggests that the age of the Fiene l.f. is approximately about 840 to 820 Ka. However, a somewhat similar taxonomic assemblage from Cathedral Cave, Nevada, with external age control produced a significantly younger date (approximately 160 to 113 Ka; Jass, 2005). This information encourages the re-evaluation of the age of Fiene and similar sites as a test of the unique nature and provinciality of the Cathedral Cave assemblage, which may reflect the survival of these morphotypes in high altitude refugia (Jass, 2007).