Conclusions

The combination of magnetostratigraphic, biochronologic, biostratigraphic, and amino acid racemization techniques that has been applied to Porcupine Cave sediments suggests that the Pit sequence has an age near 780 Ka at its top and is as old as 900 Ka or 1 Ma at its bottom. The DMNH Velvet Room appears younger than the uppermost Pit sediments at its top (levels A-C), with levels D and E potentially coeval with Pit levels 1-3, level F in the vicinity of Pit levels 3 or 4, and levels G-I potentially as old as Pit levels 9 or 10. The CM Velvet Room sediments appear at least as young, and probably younger, than level 1 in the Pit. The Badger Room seems coeval with the interval represented by levels 8-4 in the Pit. The correlations between the Pit, Velvet Room, and Badger Room localities are based primarily on a combination of LSDk-HSDk zones, relative abundance zones, and assemblage zones defined for arvi-coline rodents in the Pit, which are the only component of the fauna that has been well enough identified from all the pertinent localities and levels for meaningful biostratigraphic application. These zones presently are applicable only within Porcupine Cave and possibly in the immediate vicinity; they should not be assumed to apply over broader geographic regions. The correlations are supported by biostratigraphic evi dence from squirrels and rabbits, internally consistent paleo-magnetic data, and the amino acid racemization results.

Nevertheless, important caveats should be kept in mind. The stratigraphy in the cave is complex and unconformities probably abound, as illustrated in the Pit and probably in the DMNH Velvet Room. Especially in the DMNH Velvet Room, the assignment of specimens from a particular excavation bag to a definite stratigraphic interval is sometimes difficult. Moreover, the arvicoline biochronology is correlated to an independent time scale primarily from sites at much lower elevations than Porcupine Cave. Thus the role of biogeographic differences in influencing interpretations of temporal distributions has been little studied; in fact, the Porcupine Cave data are among the most applicable in this regard. Finally, many additional specimens remain to be identified from both of the Velvet Room excavations, as well as from most of the other localities in the cave. Much of the material has not even been processed out of the matrix.

As the sample size from the Velvet Room and other localities in Porcupine Cave grows in the coming years, additional analyses and comparisons to other important sites—such as those at nearby Hansen Bluff and SAM Cave (Rogers et al., 1985, 1992, 2000; Rogers and Wang, 2002)—should provide valuable tests of the chronology proposed here. These further tests are particularly crucial because, as the most diverse sample of high-elevation Irvingtonian fauna located in the heart of the Rocky Mountains, Porcupine Cave provides a unique perspective on faunal dynamics, faunal provinciality in the West, and attendant biochronologic questions.

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