Biochronology

Figures 7.1-7.3 show stratigraphic ranges of Pit taxa. Arvico-line rodents currently provide the most feasible link from biostratigraphic to biochronologic time scales (Repenning, 1987; Fejfar and Repenning, 1992; Bell, 2000; Bell et al., in press). At least 10 species of arvicolines range through the Pit (figure 7.2). Biochronologically informative species fall into two groups. One group, including Phenacomys gryci, Mimomys vir-ginianus, and AHophaiomys pliocaenicus, is known elsewhere, primarily from sites older than 800 Ka. The second group is composed of species that elsewhere first appear 900-800 Ka ago: Microtusparoperarius, Lemmiscus curtatus (primitive four-triangle morphotype), and Microtus meadensis (see figures 7.1-7.3 and tables 10.1-10.13 for common names). The sym-patric occurrence of all these species at Porcupine Cave suggests that levels 8-4 were deposited sometime between 900 and 800 Ka ago. Bell and Barnosky (2000) provided details of the species ranges for the relevant taxa and for species identifications. Bell and Barnosky (2000) believed that levels 8-4 dated to between 850 and 750 Ka. This slightly younger estimate assumed that the Brunhes-Matuyama boundary was captured in the sequence somewhere above level 8. However, based on the absence of viable magnetostratigraphic information above level 8 and the biostratigraphic correlations to the DMNH Velvet Room sequence explained later in this chapter, an estimate of 750 Ka for level 4 could be too young.

Stratigraphically within the Pit, A. pliocaenicus, M. virgini-anus, and P. gryci disappear at the level 4/3 transition. This probably does not reflect sampling bias because levels 4-1 produce the most specimens of any level. Given that these species disappear by 800 Ka ago elsewhere (by circa 1.5 Ma ago in the case of P. gryci; Repenning et al., 1995), it seems probable that levels 4 and below date to at least 800 Ka. The taxa remaining in levels 1 and 2 may have survived elsewhere to about 250 Ka in the case of M. paroperarius, M. meadensis, and Mictomys kansasensis/meltoni, and to the present for L. curtatus. However, most localities with Mictomys kansasensis/meltoni date to between 2 Ma and 600 Ka, with the most consistently reliable dates on M. meltoni being from 600 to 700 Ka. In view of this observation, plus the fact that levels 1-3 probably rep resent an interglacial that lasted no more than 100,000 years, it seems unlikely that level 1 would be younger than about 600 Ka. These age estimates based on biochronology are consistent with the independent magnetostratigraphic interpretations. Note, however, that the estimate is substantially older than that posited by most earlier papers on Porcupine Cave (Barnosky and Rasmussen, 1988; Wood and Barnosky, 1994; Barnosky et al., 1996). Dating the top of the sequence is still problematic. Bell and Barnosky (2000) estimated that Pit levels 1-3 were deposited sometime between 252 and 750 Ka ago. Based on the information presented in this chapter, we revise Bell and Barnosky's (2000) estimate of the upper age of the Pit to be greater than 600 and very probably closer to 800 Ka.

Levels 9 and below contain two additional taxa that, like P. gryci, became extinct elsewhere by early Irvingtonian time (circa 1.5 Ma ago). These are the rabbit Hypolagus sp. and the ground squirrel Spermophilus (Otospermophilus) sp.; the latter was not named as a species but has morphological affinities to early Blancan species in Kansas (Rexroad and Fox Canyon faunas) (Goodwin, chapter 17). Unlike P. gryci, both disappear from the Porcupine Cave record by level 9 (figure 7.1). Sampling issues might explain their absence in level 9, where fossils are scarce, but not above that level because specimens become more abundant, and congeners are well represented in higher levels. Hypolagus and the Rexroad / Fox Canyon-like morph of Spermophilus (Otospermophilus) sp. co-occur with L. curtatus, but the other two arvicolines that suggest ages younger than 900 Ka do not appear until higher in the strati-graphic section (figure 7.2). This finding indicates either significant temporal range extensions for Hypolagus and the Rexroad / Fox Canyon-like morph of Spermophilus (Otosper-mophilus) sp., as was apparently also the case for P. gryci (Bell and Barnosky, 2000), or an even earlier first appearance for Lemmiscus curtatus than has previously been recognized. Based on its co-occurrence with P. gryci, M. virginianus, and A. pliocaenicus, Bell and Barnosky (2000) agreed with Repenning (1992) in extending the range of L. curtatus from circa 300 to 800 Ka, a decision that has been further substantiated by the association of L. curtatus with magnetically reversed sediments in SAM Cave, New Mexico (Rogers et al., 2000). In light of this, it seems more parsimonious to extend the range of one species (L. curtatus) downward rather than those of two species (Hypolagus and the primitive, unnamed Spermophilus [Otospermophilus] sp.) upward. Therefore, it would not be surprising if levels 10 and below in the Pit were substantially older than 800 Ka, even in view of the presence of L. curtatus. On these grounds the bottom of the Pit deposit is considered younger than 1 Ma.

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