646
JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS Methods The first hypothesis was tested by recon-
structing body-size trends along a transect running along the West Coast of North America from Washington to Oaxaca (Fig. 1). Specimens from sites dating to the Arikareean– Hemphillian (Oligocene–Miocene, 30–5Ma; Tedford et al. 2004) North American Land Mammal Ages (NALMAs) were included. This interval and region was chosen because of the remarkably rich fossil record available (Carrasco et al. 2005). The West Coast of the United States (encompassing, for the purposes of this study, the states of Washington, Oregon, Idaho, Nevada, and California) has an exten- sively sampled fossil record that has been collected for well over a century. While Mexican faunas have been the subject of less
study historically, recent research has uncov- ered several diverse faunas, particularly from the states of Chihuahua, Guanajuato, and Oaxaca. Besides being extremely well sampled, the Arikareean–Hemphillian interval encom- passes several important climatic events (Zachos et al. 2001), making it an ideal natural laboratory in which to examine the influence of temperature on biotic variables. The late Oligocene is characterized by relatively cool temperatures, the onset of which was concurrent with the beginning of continental glaciation in Antarctica. The early Miocene was characterized by markedly warmer tempera- tures, which culminated in the mid-Miocene Climatic Optimum(MMCO; 16–14 Ma), a brief but significant warming spike representing the warmest period in Earth’s history since the Eocene. Climate cooled steadily in the late Miocene, approaching the cold global tempera- tures seen today by 5Ma. The huge size of the Oligo-Miocene fossil
FIGURE 1. Map of study area. Circles represent formation included in this study. Scale bar, 500km.
record in North America makes an analysis of body-size evolution in all mammals impracti- cal, so this study focuses on trends within two representative families: equids and canids. These two families are distinct from one another in body size, diet, and ecology, and both are well represented in the fossil record (Carrasco et al. 2005). Besides being common, equids and canids have historically been the focus of a great deal of research, and this extensive study has led to the construction of robust and well-resolved phylogenies for both (Mac Fadden 1992; Wang 1994; Wang et al. 1999; Tedford et al. 2009). Crucially for the aims of this project, robust approximations of body mass exist for each family. For canids, body mass is approximated using the length of the first lower molar (Van Valkenburgh 1990). Several dental proxies for mass exist for equids, including the lengths of all lower cheek teeth and the second upper molar (Janis 1990). The majority of the dental measurements used in this study were obtained from specimens in museum collections, though these were supplemented by some previously published measurements for faunas that were underrepresented in the collections visited. These collections were the American Museum of Natural History, Idaho Museum of Natural
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