BODY-MASS TRENDS
653
TABLE 4. Comparison of body-mass data from coastal and inland sites. Mean mass (in kg) and 95% confidence intervals are shown for each genus (samples in which n=1 do not include confidence intervals). Bay Area, San Joaquin valley, and transverse ranges biogeographic regions are considered coastal; western Nevada and Mojave Desert biogeographic regions are considered inland.
Taxon
Ba1 Microtomarctus Ba1 Hypohippus Ba1 Archaeohippus Ba1 Scaphohippus Ba1 Acritohippus Cl2 Hipparion Cl2 Pliohippus Cl3 Hipparion
Cl3 Neohipparion Cl3 Pliohippus
Bay Area
San Joaquin valley Transverse ranges Western Nevada Mojave Desert 15.44 ± 0.62
15.30
202.22 40.78
226.08 ± 107.57 409.23 ± 190.89 202.86 ± 40.65 126.46 ± 56.07
274.26 ± 40.70
46.26 ± 20.17 148.70 ± 24.71 169.73 ± 50.60
196.82 ± 36.53 273.69 ± 87.78 395.37 ± 136.50 492.17 ± 61.75
early Arikareean), the NALMA subdivisions that contained sufficientmaterial to reconstruct latitudinal gradients were all relatively short (Ar1, 2 Ma; Ba1, 1.2Ma; Ba2, 2.3 Ma; Cl3, 1Ma; Hh2, 0.7Ma; Hh4, 1 Ma; Tedford et al. 2004). These units are shorter in time than the subepochs used as bins in the studies by Smith et al. (2010) and Saarinen et al. (2014), both of which found a correlation between body size and temperature, and are comparable in dura- tion to the million year bins widely used in paleobiological analyses (e.g., Hunt and Roy 2006; Hopkins, 2007; Liow et al. 2008). As discussed above, the comparison between coastal and inland sites in California and Nevada suggests that the effects of small-scale environmental differences between localities are minimal compared with those of large- scale climatic trends. While having precise dates and paleoclimatic reconstructions for a larger number of localities would certainly allow for a more robust analysis, the length of the temporal bins used in this analysis should be sufficient for observing macroecological trends through time. A constant concern in paleontology is the
quality of the fossil record. Not only are whole ecosystems rarely preserved, but an already incomplete record is often further biased by differential preservation, collection (Behrensmeyer et al. 2000), and description (Davis and Pyenson 2007), creating tapho- nomic noise that can obscure true biological signals if insufficiently large samples are considered. This is especially a concern for taxa such as canids and other carnivores that
234.26 290.95
476.10 ± 355.57 404.70
are well sampled and extensively studied but are generally rare within ecosystems and for taxa such as rodents that are common within ecosystems but are either infrequently pre- served or undercollected. Sample size is demonstrably driving at least one signal in this study: late Clarendonian Epicyon shows a strong (though not significant) negative corre- lation with latitude, but this is almost certainly the result of an incomplete sample. Wang et al. (1999) note that two species of Epicyon are present at late Clarendonian sites throughout North America: the giant E. haydeni and the smaller E. saevus. Both are present in the Juntura Formation of Oregon (Orcutt and Hopkins, 2011), but only E. haydeni is repre- sented by dental material from the coeval Contra Costa Group of the San Francisco Bay Area.Were the sample size from this site larger, it would almost certainly include E. saevus, likely obscuring the see- mingly strong latitudinal gradient. The small sample size of many other canid taxa makes it possible, or even likely, that many of the patterns observed here do not reflect biological trends. However, sample size cannot be invoked to explain every body-size gradient—or lack thereof—observed in Oligo-Miocene canids. Mesocyon is both extre- mely common and extremely well sampled in Arikareean faunas (Wang 1994) and is present in large numbers in the latitudinally distant John Day (n=16) and Otay formations (n =9). While the Arikareean is one of the coldest intervals of the Oligo-Miocene, there is no evidence of a significant difference in Mesocyon
19.06 ± 7.25 207.42 46.90
174.85 ± 29.61 168.04 ± 58.98 319.81
336.29 ± 88.21 461.21
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