Environmental Interpretation

The Pit sequence may document at least two glacial-interglacial cycles, with most study to date having focused on one putative glacial-interglacial couplet (levels 4-5 and 1-3, respectively) (Barnosky et al., 1996). Sedimentary and faunal variation is consistent with a model of wet glacial-dry interglacial environmental fluctuation for this upper couplet (Barnosky et al., 1996). Environmental conditions at other sites in Porcupine Cave have not been investigated adequately.

Substantial variation among sites and strata in relative abundance of most common sciurid taxa (exclusive of Marmota, which has not been studied other than in the Pit) probably reflects broader environmental history (figure 17.3B), although further work is needed to rule out taphonomic biases. Striking fluctuations in relative frequency of Marmota (determined only for the Pit sequence; calculated as percent total sciurid MNI) are also evident (figure 17.3B). Tamiasciurus hudsonicus is present but never dominant through the well-sampled parts of the Pit sequence (see figure 23.4), the clearest sciurid indicator of persistent woodland habitat within sampling range of the cave. It decreases in relative abundance upsection from level 6 and is rare above level 3 (MNI = 2 of 153 exclusive of Marmota), consistent with the hypothesis of interglacial drying (Barnosky et al., 1996). Absence from other well-sampled sites—especially Mark's Sink, which has some constituents that appear to overlap in time with portions of the Pit sequence (figure 17.6)—may reflect sampling bias. As of this writing, I have not examined all fragmentary material from Mark's Sink, and all specimens of Tamiasciurus known from the Pit are isolated teeth.

T. minimus and S. lateralis are often associated with woodland but do not strictly require this habitat. When grouped with Tamiasciurus as potential "woodland" indicators, a substantial decrease in relative abundance upsection from level 4 (figure 17.3B) may indicate a decrease in woodland-shrubland habitat and, by extension, increasing aridity. However, variation within the presumed glacial interval (levels 4-5) complicates simple modeling. Perhaps most significantly, potential "woodland" indicators show sharp contrast in the VR-DMNH sequence—constituting more than 20% of the sciurid assemblage (exclusive of Marmota) in composite horizon C-F (deep in VR-DMNH), but less than 2% in composite horizon A-B (completely absent from the well-sampled horizon B). Fau-nally (at least in terms of the sciurid assemblage), horizon B of VR-DMNH reflects the least wooded (and, by extension, perhaps the driest) environment of any well-sampled site within the cave.

"Grassland" indicators in the sciurid assemblage include S. cf. S. elegans, Cynomys, and (perhaps) ?C. andersoni. S. cf. S. elegans is present and abundant at all well-sampled sites, but prairie dogs show marked variation in relative abundance. Stratigraphic variation in the Pit sequence is complicated by taxonomic turnover. However, relatively low frequencies of Cynomys in presumed glacial deposits (levels 4-5) contrast with sharply higher frequencies in levels 1-3 and with peak abundance (circa 35% of the non-Marmota sciurid assemblage) at level 2. This is, again, consistent with regional drying and local expansion of the relatively open habitat favorable for Cynomys. Not surprisingly, prairie dogs are relatively abundant in the upper horizons of the VR-DMNH. The numerical dominance of ?C. andersoni in two old sites (GD, BR) may partially be an artifact of the small sample sizes from these sites (NISP = 41 and 24, respectively).

Dramatic fluctuations in the relative abundance of Marmota sp. have been noted elsewhere (Wood and Barnosky, 1994; Barnosky et al., 1996) and interpreted as consistent with the glacial-interglacial hypothesis: Marmota is numerically dominant in levels 4-5 (glacial) but much less abundant higher in the sequence (interglacial) (figure 17.3B). Further work is needed to clarify the variation in frequency of marmots among other sites in Porcupine Cave.

0 0

Post a comment