Discussion

Ochotona was recovered in 6 of the 26 sites known in Porcupine Cave. Each excavated area presents its own set of chronological problems (see review in Bell and Barnosky, 2000). The age of most deposits in Porcupine Cave appears to be Irvingtonian, although some sediments within Mark's Sink might actually be Blancan (based on the presence of Hypo-lagus, Ophiomysparvus, Mictomys vetus, and Pliolemmus antiquus; C.J. Bell, pers. comm.). The majority of pika remains come from the DMNH Velvet Room excavation (n = 50 specimens), with equal numbers coming from the Pit (n = 34) and Mark's Sink (n = 34) (appendix 14.1). Probably the best chronological control is that for the excavations in the Pit. Table 14.2 demonstrates that the number of pika specimens increases as one progresses higher in the deposit; the greatest number (19) are from the surface. One speculation is that pikas were becoming increasingly more numerous in the local environment during the time of this late deposition. However, in all well-

table 14.2 Numbers of Specimens of Ochotona from the Stratigraphie Units in the Pit table 14.3

Locations within Porcupine Cave for the Unusual Form of Pika Here Termed Ochotona sp. near Trout Cave Form

Location

(Grid, Level) Taxon n

G1L1 Ochotona sp. 19

G1L2 Ochotona sp. 4

G1 L3 Ochotona sp. near Trout Cave form 1

G1 L5 Ochotona sp. 4

G2 L10 Ochotona cf. princeps 1

G3 L4 Ochotona sp. near Trout Cave form 1

G3 L6 Ochotona sp. near Trout Cave form 2

G5/6 L1-3 Ochotona sp. 1

G6 L1-3 Ochotona sp. 1

Location

(Grid, Level) Assigned Age in Years Based on Stratigraphy*

The Pit

Gl L3

Within the range 600-850 Ka

G3 L4

Within the range 750-900 Ka

G3 L6

Within the range 750-900 Ka

DMNH Velvet Room excavation

DMNH Velvet Room excavation

Below horizon C >780 Ka aSee text and chapters 6 and 7.

sampled levels in the Pit (levels 1-7), pikas occur in about equal relative abundance in comparison with the total sample of small mammals (see figure 23.4).

We have not been able to readily discern distinct species and do not see clear-cut distinctions between the various fossils. We have divided the pika specimens into four categories: (1) Ochotona cf. princeps, (2) Ochotona sp. near Trout Cave form, (3) Ochotona sp. small morph A, and (4) Ochotona sp. What all this means is that the pikas of North America (other than O. spanglei and the large O. whartoni) are not readily changing in dental form and size, compared with other groups such as arvicoline rodents (Bell and Barnosky, 2000). The p3s from the Pit and the Velvet Room (table 14.3) show some promise for understanding possible subtle changes in small Ochotona of North America. It may be that with a better understanding of Ochotona from the Pliocene, the specimens from Trout Cave and some of the p3s from the Pit and the Velvet Room will be described as a new taxon.

Whether represented by different species or as one species (Ochotona princeps) with morphologically variable populations, pikas seem to have been established in the Rocky Mountains as far south as central Colorado by the middle Irving-tonian. Ochotona is recorded no lower than level 10 (G2 L10) in the Pit. Levels 4-8 seem to date to somewhere between 900 and 780 Ka (chapter 7). The higher stratigraphic levels of the Pit (levels 1-3, where most of the Ochotona are found) probably are no younger than about 600 Ka and could be as old as about 800 Ka, although Bell and Barnosky (2000) regarded them as dating to somewhere between 750 and 252 Ka. Ochotona is recorded from the Velvet Room excavation from as low as horizon M, which would be older than 780 Ka and possibly approaching 1 Ma. The conservative estimation would place Ochotona in this excavation area by at least 780 Ka.

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