The fossils reported herein are from the Panaca Formtion, which is generally considered to be fine-grained basin fill that is exposed in basins of eastern Nevada (Phoenix 1948;
Ekren et al. 1977;
Pederson et al. 2000). The Panaca small mammal fauna is derived from 12 sites in the vicinity of Panaca, near the northern margin of Meadow Valley. These sites were placed in five stratigraphic sections (Figure 2) arranged according to the paleomagnetic and stage-of-evolution correlations described in
Lindsay et al. (2002,
figure 7). These sites are correlated within the limits of chrons C3n.2r through C3n.3r of the Geomagnetic Polarity Time Scale (GPTS), which places them between 4.60-4.96 Ma, based on the calibration of
Berggren et al. (1995). We were unable to locate and place one other site, Rodent Quarry #2 of Galusha and Emry. We equate Rodent Quarry #1 of Galusha and Emry with our locality UALP 9504, the Fejfar site. One of our small mammal sites, UALP 9502, is identical with the AMNH Double Butte Quarry of Hazen and Gentry.
As noted by Lindsay et al. (2002) the stratigraphic-chronologic framework for the Panaca Fm. is constrained by an ash bed that overlies our highest small mammal site and has been identified as the Healdsburg Tephra by A. Sarna-Wojcicki of the United States Geological Survey (USGS, Menlo Park). That tephra has been dated 4.69 Ma, using the 27.84 Ma monitor for the Fish Canyon sanidine interlaboratory standard. The Healdsburg Tephra is shown in the upper part (marked as XXXX in
Figure 2) of the Rodent Hill section.
All of the small mammal sites currently known from the Panaca Fm. are considered early Blancan NALMA whereas the large mammals reported from the Panaca Fm. by
Stock (1921) are considered Hemphillian NALMA. Based on the location given by
Stock (1921), we believe the fossils collected and reported by Stock were from stratigraphic levels below our lowest small mammal site (e.g., in magnetozone B+) in the Little Hogback section (Figure 2). Note that the rate of sediment accumulation in the Little Hogback section is lower than in the other sections as it is farther from the edge of the basin where most of the fossil sites are located. We searched the lower exposed strata in the area of the Little Hogback section very thoroughly but were unable to find any identifiable fossils, or any sediment that would yield small mammals from that area. See
Lindsay et al. (2002) for more details on the geology and chronology of the Panaca l.f.