Paleoenvironmental Significance

Given the fact that so few species seem to inhabit the region surrounding Porcupine Cave today, the relatively low abundance of amphibian and reptile remains at the site is not unexpected. Most of the specimens recovered thus far are from test pit excavations, surface samples, and unstratified or mixed deposits. Consequently, the majority of the reptile and amphibian records from the cave simply confirm the presence of these taxa in South Park (an intermontane basin in the southern Rocky Mountains of central Colorado) during the time intervals represented by the discrete deposits.

The number of identified vertebrate specimens in Porcupine Cave exceeds 13,000, out of a total number of vertebrate specimens that, by conservative estimate, exceeds 45,000. The primary taphonomic vectors most likely responsible for the accumulation of bones in Porcupine Cave seem to be wood rats, raptorial birds, and mammalian carnivorans. This combination of vectors was shown by Hadly (1999) to be an excellent means of sampling extant vertebrate communities, and it is unlikely that the Porcupine Cave herpetofauna was subjected to a taphonomic filter that would artificially inflate or deflate their abundance in the deposits. For these reasons we are confident that the relative abundance of these taxa in the deposits is an accurate reflection of their relative abundance in the communities at the time their remains were being incorporated into the deposits.

If long-distance transport (by raptorial birds or mammalian predators) of vertebrate remains from lower elevations was an important factor in bringing materials into the cave, we would expect to find other vertebrate taxa typical of such lower elevations. The absence of small mammals typical of lower elevations (especially heteromyid rodents) argues against such longdistance transport. In the absence of data to the contrary, we consider the herpetofaunal remains of Porcupine Cave to have been derived from within relatively short distances of the cave, almost certainly within the confines of South Park.

We attempted special analysis of herpetofaunal remains recovered from the Pit and the DMNH Velvet Room excavations, the two localities within the cave that were excavated with stratigraphic control and had at least a reasonable sample size. Our goal was to determine if any environmental data could be derived from the amphibian and reptile record that might complement (or contradict) the patterns revealed through studies of the mammalian fauna. Our analysis at higher taxonomic levels does reveal some information that can help in interpretation of climate change and the consequent effects on biodiversity of amphibians and reptiles.

The DMNH Velvet Room excavation (locality 644) contains 19 snake specimens including only one vertebra of non-Natricine Colubridae with no stratigraphic control, and a single viperid vertebra from level 7c in grid 20. All other specimens are natricines or unidentified colubroids. No meaningful interpretations are made for this locality.

Of the localities excavated in the cave thus far, the stratified sediments in the Pit locality (CM 1925 / UCMP V93173) provide the best information available on the effects of middle Pleistocene climate change on the vertebrate faunas in South Park. Biochronologic and paleomagnetic data indicate that the sediments in levels 4-8 date to between approximately 750,000 and 850,000 yr BP (Bell and Barnosky, 2000), or perhaps slightly older (chapter 7). The stratigraphic data for the Pit locality were summarized by Barnosky et al. (1996) and are further elaborated in chapters 7 and 23. These sources document the fact that several sedimentological changes in the Pit sequence represent times of climatic transition. The transition that is best documented is that at the level 4/3 boundary (Barnosky et al., 1996). Lithology of the sediments and changes in the relative abundance of sciurid and arvicoline rodents indicate that the sediments in levels 4 and 5 represented a glacial interval and those in levels 1-3, an interglacial (Barnosky and Rasmussen, 1988; Wood and Barnosky, 1994; Barnosky et al., 1996; Bell and Barnosky, 2000). Although the fossils are not abundant, the stratigraphic change in taxonomic diversity of amphibian and reptile remains from the Pit locality supports this interpretation (table 11.1).

table 11.1 Stratigraphie Distribution of Amphibian and Reptile Remains from the Pit Locality

Level

Taxon

l-3

4-5

6

7

BA

Pelobatidae

l

Phrynosomatidae

l

Non-Natricine Colubridae

l

l

Natricinae

4

3

l

l

4

Viperidae

2

Indeterminate Colubroidea

5

2

l

3

notes: Levels 1-3 previously were interpreted to represent an interglacial deposit, and levels 4-5, a glacial deposit (Barnosky et al., 1996; Bell and Barnosky, 2000). Levels 6, 7, and 8A are interpreted in this book to represent an interglacial that was substantially cooler and moister than that represented by levels 1-3. Only specimens for which stratigraphic data are available (or can be inferred) are included. The single pelobatid specimen and the single phrynosomatid specimen most likely were deposited in levels 1-3, but a more precise determination cannot be made. See text for discussion.

notes: Levels 1-3 previously were interpreted to represent an interglacial deposit, and levels 4-5, a glacial deposit (Barnosky et al., 1996; Bell and Barnosky, 2000). Levels 6, 7, and 8A are interpreted in this book to represent an interglacial that was substantially cooler and moister than that represented by levels 1-3. Only specimens for which stratigraphic data are available (or can be inferred) are included. The single pelobatid specimen and the single phrynosomatid specimen most likely were deposited in levels 1-3, but a more precise determination cannot be made. See text for discussion.

Between levels 1-3 and the deeper levels of the Pit there is no indication of change in the taphonomic pathways responsible for bringing herpetofaunal remains into the deposit. The most abundant herpetofaunal taxon from the Pit locality is Natricinae, and all but one of the specimens recovered from the deeper levels of the Pit (a non-Natricine colubrid, represented by a single vertebra from level 7; table 11.1) are referred to this taxon or to undetermined Colubroidea. In the upper levels of the Pit (levels 1-3), the herpetofaunal remains include four natricine snake vertebrae, one vertebra of non-Natricine Colubridae, two vertebrae of a viperid snake, and five unidentified colubroids (table 11.1). In addition, a phrynosomatid lizard dentary and a sacrococcyx of a pelobatid frog are probably from levels 1-3 of the Pit. Precise strati-graphic data for these two specimens are not available, but the color and general preservation of the bones indicate that they were recovered from the upper levels of the Pit and not from deeper stratigraphic levels (Bell and Barnosky, 2000).

The change in herpetofaunal diversity from two taxa (Na-tricinae and non-Natricine Colubridae) in deeper levels of the Pit (8-4) to five taxa (Pelobatidae, Phrynosomatidae, Natricinae, non-Natricine Colubridae, Viperidae) is noteworthy. Although most paleoclimatic interpretations of fossil her-petofaunas are based on species-level identifications, it is clear that at least some tentative interpretations can be drawn from the material preserved in the Pit locality.

The pelobatid frog and viperid snake in the Pit fauna represent extralimital records in terms of both geography and elevation. No historical records of viperids or pelobatids are known from Park County (Hammerson, 1986, 1999). The only extant pelobatids in Colorado today (Scaphiopus and Spea) occur to the east and west of Park County, the closest records being from Jefferson, Douglas, El Paso, and Fremont counties (Hammerson, 1999). Two of these anurans (S. bombifrons and S. intermontana) currently range to above 2100 m elevation in Colorado, but no extant Colorado populations of pelobatids are found above 2440 m elevation (Hammerson, 1999).

Several North American viperid species have elevational ranges that approach or exceed the elevation of Porcupine Cave (e.g., Crotalus intermedius, C. lepidus, C. molossus, C. pricei, C. scutulatus, C. transversus, and Sistrurus ravus, all known from elevations above 2850 m, and C. triseriatus, which ranges as high as 4300 m; Lowe et al., 1986; Campbell and Lamar, 1989). Crotalus viridis is known from just outside Park County to the northeast and southeast, and it currently can be found as high as about 2896 m elevation (Hammerson, 1986, 1999).

The increased taxonomic diversity in the upper levels (1-3) of the Pit sequence supports previous interpretations that these levels represent an interglacial period and that at least levels 4 and 5 represent glacial intervals. In addition, the her-petofaunal diversity reflected in levels 1-3 in the Pit sequence exceeds that of the known extant diversity in the immediate vicinity of the cave (though not of Park County as a whole, which includes regions of considerably lower elevation than those represented in the immediate vicinity of the cave). This higher diversity suggests the possibility that the interglacial interval represented in the upper levels of the Pit was a warmer (though not necessarily a drier) interglacial than the one in which we currently live. This hypothesis is supported most strongly by the presence of a fossil pelobatid toad in the Pit. Extensive efforts to document the modern herpetofaunal communities in the vicinity of the cave are required to determine if the perceived temporal differences in overall diversity represent reality or are an artifact resulting from a lack of concerted effort to sample the local herpetofauna.

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