Discussion Regional Comparisons

Previous studies of late Pleistocene North American herpeto-faunas appear to indicate both taxonomic and geographic stability of reptile and amphibian taxa throughout the Rancho-labrean mammal age and into the Holocene (Fay, 1984, 1988; Brewer, 1985; Holman, 1991, 1992, 1995, 1999). The question

FIGURE 11.6. UCMP 399993; precloacal vertebra referred to non-Natricine Colubridae shown in anterior (top), left lateral (middle), and ventral (bottom) views. Scale bar = 4 mm.

of stability in early and middle Pleistocene faunas is not as well explored (see, e.g., Holman, 1995). Unfortunately, the material preserved in Porcupine Cave does not permit species-level identifications, and consequently we are limited in our ability to make definitive statements about fine-scale taxo-nomic and geographic stability. Nor is it possible for us to indicate in any specific way that climate changes might have affected amphibian and reptile species in central Colorado in the middle Pleistocene.

Comparisons of a more regional nature also are difficult. There are relatively few herpetofaunal assemblages from roughly contemporaneous deposits in the Rocky Mountain region (late Pleistocene and Holocene faunas far outnumber those of early and middle Pleistocene age in the western

United States generally). In addition, our somewhat more conservative approach to taxonomic identifications limits the kinds of informative comparisons that can be made. Nonetheless, some meaningful comparisons are possible with faunas from two localities south of Porcupine Cave: the Hansen Bluff faunal sequence in Colorado and the SAM Cave fauna in northern New Mexico.

The Hansen Bluff faunal sequence includes a diverse her-petofauna that was recovered from the Alamosa Formation in the San Luis Valley of south-central Colorado (16 km southeast of Alamosa, 2300 m elevation; Rogers et al., 1985, 1992). External chronological control was established by means of a paleomagnetic profile and the presence in the section of the Bishop Ash (the average age of which is now calculated at 758.9 ± 1.8 Ka; Sarna-Wojcicki et al., 2000). This lower-elevation fauna was reported to include Ambystoma, the hylid frog Pseudacris, the pelobatid Scaphiopus, two species of the frog Rana, two species of the toad Bufo, the xenodontine snake Heterodon, the colubrine Pituophis, two species of the natricine Thamnophis, the viperid Crotalus, and three phrynosomatid lizards (Holbrookia, Phrynosoma, and Sceloporus).

The SAM Cave fauna was recovered from a lava tube deposit located at 2737 m in Rio Arriba County, New Mexico (just north of San Antonio Mountain; Rogers et al., 2000). Six discrete faunas were recovered from the cave, the oldest of which have no external age control but contain the arvicoline rodents Allophaiomys, Mictomys kansasensis, Microtus, Clethrionomys, and Lemmiscus (with both four- and five-closed triangles). These rodents suggest an age roughly contemporaneous with at least some of the deposits in Porcupine Cave (e.g., the Pit locality; Bell and Barnosky, 2000). The preserved herpeto-fauna associated with these mammals was relatively depauperate, similar to that of Porcupine Cave, which is at a slightly higher elevation (2900 m). Reported taxa at SAM Cave include the salamander Ambystoma, the hylid frog Pseudacris, and the viperid Crotalus. Somewhat younger deposits in SAM Cave include the lizard Phrynosoma and the natricine Thamnophis as well.

These faunas suggest that the restricted taxonomic diversity of modern amphibians and reptiles at higher elevations in Colorado (Hammerson, 1986, 1999) is not unique to recent times. In overall herpetofaunal diversity, SAM Cave and Porcupine Cave are roughly comparable, and the relatively high elevations of these sites may be responsible for the low diversity and small sample sizes of amphibians and reptiles in the respective faunas. The Hansen Bluff fauna is approximately 600 m lower in elevation than that from Porcupine Cave, and Hansen Bluff contains a much more diverse anuran fauna, and possibly a slightly greater diversity of phrynosomatid lizards.

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