Site Correlation and Biochronology

Variation in taxonomic composition, morphology, and relative frequency of sciurids was used to generate an age corre

FIGURE 17.6 Preliminary age correlation hypothesis for Porcupine Cave sites based on the sciurid fauna. *, S. cf. S. elegans not measured; t, S. lateralis not present (but samples were small); **, single specimen may represent C. cf. C. leucurus, suggesting a mixed age.

FIGURE 17.6 Preliminary age correlation hypothesis for Porcupine Cave sites based on the sciurid fauna. *, S. cf. S. elegans not measured; t, S. lateralis not present (but samples were small); **, single specimen may represent C. cf. C. leucurus, suggesting a mixed age.

lation hypothesis for Porcupine Cave sites with sciurids (figure 17.6). Using the stratified Pit sequence as a reference, three temporal zones can be recognized based on the presence or absence and the abundance of two prairie dog species (figure 17.6): Pit levels 6 and deeper (?C. andersoni only), levels 4-5 (both species subequal in abundance), and levels 1-3 (almost exclusively the "old" morph of C. cf. C. leucurus; one of more than 330 Cynomys specimens was ?C. andersoni). Three additional sites (BR, GD, FFA) have ?C. andersoni but not C. cf. C. leucurus and may be roughly equivalent with deeper levels in the Pit; age relationships among these sites are not clarified by sciurid data. MS includes both prairie dogs, with confusing distribution: ?C. andersoni is found as high as level 13, and C. cf. C. leucurus as low as level 26. This site appears to be mixed, spanning several zones as documented in the Pit sequence (figure 17.6). All rare sciurids from Porcupine Cave (those documented by fewer than three specimens each) occur in "old" sites: Spermophilus possibly S. meadensis (GD), S. (?Otospermo-philus) sp. (deep in the Pit, levels 10 and 11), S. (Spermophilus) sp. (MS, BR), and ?Cynomys sp. (MS).

Two additional sciurid assemblages (VR-DMNH below horizon B, WH) are suggested as roughly age equivalent with upper levels of the Pit. The presence of C. cf. C. leucurus with deflected hypoconid on m3, and of narrow-toothed S. cf. S. elegans (figure 17.3A), clearly links WH with the upper section of the Pit. The other site, VR-DMNH (deep), exhibits S. cf. S. ele-gans with a relatively narrow p4 (figure 17.3A) but has little Cynomys material. The few specimens examined from these deeper levels suggest equivalence with the upper Pit.

The youngest Porcupine Cave fauna with sciurids appears to be the upper horizons (A-B) of VR-DMNH, postdating the entire Pit sequence (figure 17.6). Cynomys fossils from this sample are more derived (C. cf. C. leucurus with a less deflected hypoconid on m3); specimens of S. cf. S. elegans are more modern in aspect with significantly greater trigonid width of p4 (figure 17.3A); and S. lateralis is conspicuously absent in a large ground squirrel sample (125 specimens of S. cf. S. elegans) from horizon B of this site (figure 17.3B). S. lateralis is present in a small sample from horizon A of the VR-DMNH, indicating recolonization of the Porcupine Cave vicinity. One other site with an adequate sample (VR-CM) has not been studied morphometrically. One reasonably preserved m3 from the site seems to suggest "young" C. cf. C. leucurus.

Sciurids may provide clues for broader biochronologic interpretation. ?C. andersoni most closely resembles a suite of primitive prairie dogs and prairie dog-like taxa present in the latest Blancan and earliest Irvingtonian (?S. cragini, ?C. vetus, C. hibbardi). Two of the rare taxa found in "old" sites also show resemblance to Blancan taxa: Spermophilus possibly S. mead-ensis (possible link with late Blancan Borchers local fauna; Hibbard, 1941a), and S. (Otospermophilus) sp. (possible link with S. rexroadensis from early Blancan Fox Canyon and Rexroad local faunas; Hibbard, 1941c). Sciurids provide fewer constraints on younger Porcupine Cave sites.

0 0

Post a comment