BODY-MASS TRENDS


History, John Day Fossil Beds National Monument, Natural History Museum of Los Angeles County, Raymond Alf Museum, San Bernardino County Museum, Sierra College Museum of Natural History, San Diego Nat- ural History Museum, South Dakota School of Mines and Technology, Universidad Nacional Autónoma de Mexico, University of California Museum of Paleontology, University of Oregon Condon Fossil Collection, and Univer- sity of Washington Burke Museum. Body-mass data for fossil taxa were ana-


lyzed within genera. Most studies of body-size evolution in extant animals have focused on trends at the species level (Ashton et al. 2000), but a higher taxonomic level was used in this study for two reasons. Bergmann’s rule, as it was originally formulated, was meant to explain genus-level trends (Watt et al. 2010). Bergmann (1847) found the strongest body- size gradients within genera, with large species toward the poles and small species toward the equator. As such, any test of Bergmann’s rule sensu stricto should be conducted at the generic level. In addition, an examination of species-level patterns in body size is necessa- rily short in duration; the average mammalian species duration is approximately two million years (Foote and Raup 1996; Alroy 2000; Vrba and DeGusta 2004) and would not permit a long-term analysis of size evolution. The Oligo-Miocene fossil record makes species- level analyses impractical. While the Arikareean–Hemphillian record is outstanding in its quality, it is not complete, and at the lower the taxonomic level, fewer specimens are available. Several genera are represented by sufficient numbers of individuals to make robust analyses possible, but few species are present in large enough numbers or over a large enough range to make them suitable subjects for body-mass research. Besides this, few groups of Oligo-Miocene mammals have been the subject of intensive, large-scale taxo- nomic studies (though canids are an exception to this rule; Wang 1994; Wang et al. 1999; Tedford et al. 2009), and as such the diversity of named species may not reflect a taxon’s true species diversity. Only in the case of the equid genus Merychippus was body size examined at other taxonomic levels. Merychippus is a


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paraphyletic genus that includes species of basal equines, hipparionins, and equins (MacFadden 1992). In the interest of including onlymonophyletic taxa, equin and hipparionin Merychippus were considered as two separate genera; only the latter was present along a large enough portion of the transect to be included here. Due to the nature of the data sampled here,


which in many cases are dominated by speci- mens from a small number of localities, a nonparametric test of the relationship between latitude and body size is more appropriate than least-squares regression. We performed a Spearman rank correlation test on body size and paleolatitude binned into NALMA sub- divisions (Tedford et al. 2004). Biostratigraphic units were used instead of million-year inter- vals due to the imprecision of dating for many West Coast sites, the vast majority of which are dated using relative rather than absolute methods (Carrasco et al. 2005). For allNALMA subdivisions for which data were available, body mass was regressed against latitude, a proxy for temperature during intervals in which no paleoclimatic estimates exist. Faunas from the relatively static North American Plate were deposited at paleolatitudes roughly com- parable to the modern latitude at which they have been uncovered. However, a number of sites in Southern California lie on the Pacific Plate and havemoved northward significantly since the Oligocene. For these localities, paleo- latitude was calculated using the rates of plate movement estimated by Atwater and Stock (1998). While there is a long tradition in the study of Bergmann’s rule of using latitude as a proxy for temperature, it is reasonable to expect that temperature might vary between sites at similar latitudes but at differing distances from the coast. This is particularly a concern in California and Nevada, where sites from the Great Basin and Mojave Desert sit at the same latitude as sites from the San Francisco Bay Area and the Los Angeles Basin, respectively. Without more paleoclimatic work, it is impossible to quantitatively assess the magnitude of the difference in temperature between these regions, but it is likely that, just as today, coastal temperatures were mediated by the ocean and were likely lower than those


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