648


JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS


of inland sites. However, due to the richness of the fossil record in the region, it is possible to test whether or not such differences affected mammal body size. For genera found in both coastal and inland localities at comparable latitudes, mean body size and 95% confidence intervals for those means were calculated to test for significant differences in mass between the coast and the continental interior. While the bulk of this analysis uses latitude


as a proxy for mean annual temperature, extensive research on Oregon paleosols allows climate for most NALMA intervals to be reconstructed there (Retallack 2007), and early Barstovian floras in central Nevada and California’s San Joaquin valley have been used to estimate temperature in those regions (Yang et al. 2011; Table 1). This allows body mass to be compared directly to temperature in early Barstovian genera present in Oregon, Nevada, or the San Joaquin valley. For both sets of correlations, the probability that a significant relationship existed between tem- perature or latitude and body size was calculated. Because it posits a positive relation- ship with latitude and a negative relationship with temperature, only significant relation- ships that meet this criterion were taken to support Bergmann’s rule. In order to test the second hypothesis, that


body-size gradients should be more prevalent during cooler intervals such as those that characterize Recent ecosystems, the data set was expanded to include body masses of extant taxa from along the same transect (which was extended to include available data from British Columbian and Alaskan speci- mens). As with the fossil data, Recent data were compared with both latitude and mean annual temperature for the site at which they


were collected using a Spearman rank correla- tion test. This was preferable to using existing studies of Bergmann’s rule as a basis, because it provided a higher degree of control, both analytically (both modern and fossil trends could be observed at the genus level, while most modern studies of body-size focus on species-level trends) and geographically (both modern and fossil trends could be observed along a West Coast transect rather than extrapolating from continent-wide patterns). Data were gathered from the online databases of the National Museum of Natural History, University of Alaska Museum of the North, University of California Museum of Vertebrate Zoology, University of New Mexico Museum of Southwest Biology, and University of Washington Burke Museum. The taxon chosen as a comparison for canids was the genus Canis.No truly wild equids are currently extant in North America, so the cervid Odocoileus was used as a proxy. Deer are more common in most collections than other potentially ana- logous taxa, such as Antilocapra and Bison. While deer are browsers and thus ecologically not comparable to living horses, they are good analogues for Oligo-Miocene equids, many of which likely retained a much higher percen- tage of browse in their diet than modern taxa (MacFadden et al. 1999; Janis et al. 2000).


Results The first series of tests was to determine


whether or not latitudinal or climatic body-size gradients were present in Oligo-Miocene mammals. Of the 17 genera for which latitudinal trends could be analyzed, only four were found to have a significant (p<0.05) relationship between latitude and body size (Fig. 2 and 3,


TABLE 1. Sources of early Barstovian paleoclimatic data and estimated mean annual temperatures (MAT) in degrees Celsius.


Locality 49 Camp


Buffalo Canyon Eastgate


Fingerrock Goldyke


Mascall Ranch Middlegate Temblor


Proxy


Paleobotanical Paleobotanical Paleobotanical Paleobotanical Paleobotanical Paleopedological Paleobotanical Paleobotanical


Region


Central Nevada Central Nevada Central Nevada Central Nevada Central Nevada Columbia Plateau Central Nevada


San Joaquin Valley


MAT 9.4


7.5 9


8.6 8.7


14 8.9 17.3 Source


Yang et al. 2011 Yang et al. 2011 Yang et al. 2011 Yang et al. 2011 Yang et al. 2011 Retallack 2007 Yang et al. 2011 Yang et al. 2011


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