652
JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS
FIGURE 5. Climatic body-size gradients in modern and early Barstovian canids. Spearman’s rho (Ρ) and p-value are indicated for each relationship.
TABLE 3. Comparison of temperature and body mass between early Barstovian and modern genera. NALMA subdivision, sample size, Spearman’s rho (Ρ), and probability that a significant relationship exists between temperature and body size are shown for each sample. Shaded cells indicate samples for which significant relationships exist.
Genus
Odocoileus Hypohippus
Archaeohippus Desmatippus Merychippus Canis
Microtomarctus Age
Recent Ba1 Ba1 Ba1 Ba1
n 5
14 4
12 93
Recent 77 Ba1
4 Ρ
−0.45 −0.35 −0.26 −0.13
p
0.45 0.15 0.74 0.69
−0.35 0.00069 −0.21 0.26
0.067 0.74
latitudinal gradients. It is also the only taxon to show a significant negative relationship with temperature; the absence of such relationships in all other genera examined further under- scores the disjunction between climate change and the strength and frequency of body-size gradients.
Sampling and Taphonomic Bias The results obtained in this study have thus
far been considered to represent genuine ecological signals, but as is always the case in paleontology, taphonomic bias and analytical limitations must be considered. One potential confounding factor in this study is the scarcity of paleoclimatic data from the southern end of the transect (particularly California and Mexico). Floras from which paleoclimate can be reconstructed are scarce south of Nevada, and paleoclimatic reconstructions based on
paleopedological or isotopic proxies are non- existent, even for extremely productive and well-studied localities and faunas (the most striking example being the Barstow Fauna of Southern California, a fauna that has been so well studied that it has lent its name to a NALMA but has never been the subject of a rigorous, quantitative paleoclimatic analysis). This dearth of climatic data was the rationale for using latitude as a proxy for temperature during most intervals. Climatic data from the U.S. National Oceanic and Atmospheric Administration for sites yielding the extant material included in this analysis show that temperature and latitude are tightly correlated (R2=0.67, p<0.0001) in modern ecosystems, but the relationship is not perfect. In particular, it is worth considering local variations in climate and whether these variations and their effects are obscured by using latitude as the sole proxy for climate. However, the compar- ison of specimens from coastal and inland sites failed to show consistent differences in size. This suggests that small-scale climatic differ- ences are not being obscured by large-scale latitudinal patterns and that latitude is an appropriate proxy for temperature when no direct measurement is available. Similarly, it is possible that the evolutionary
response of body size to climatic change occurs on a short timescale that is obscured by the relatively large temporal bins used here. While several of the biostratigraphic units into which specimens have been sorted represent long periods of time (5Ma in the case of the late
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192
