Data from: Latitudinal body size trends in Oligo-Miocene mammals

Paleecological data allow not only the study of trends along deep-time chronological transects but can also be used to reconstruct ecological gradients through time, which can help identify causal factors that may be strongly correlated in modern ecosystems. We have applied such an analysis to Bergmann’s rule, which posits a causal relationship between temperature and body size in mammals. Bergmann’s rule predicts that latitudinal gradients should exist during any interval of time, with larger taxa toward the poles and smaller taxa toward the equator. It also predicts that the strength of these gradients should vary with time, becoming weaker during warmer periods and stronger during colder conditions. We tested these predictions by reconstructing body-mass trends within canid and equid genera at different intervals of the Oligo-Miocene along the West Coast of North America. To allow for comparisons with modern taxa, body mass was reconstructed along the same transect for modern Canis and Odocoileus. Of the 17 fossil genera analyzed, only two showed the expected positive relationship with latitude, nor was there consistent evidence for a relationship between paleotemperature and body mass. Likewise, the strength of body-size gradients does not change predictably with climate through time. The evidence for clear gradients is ambiguous even in the modern genera analyzed. These results suggest that, counter to Bergmann’s rule, temperature alone is not a primary driver of body size and underscore the importance of regional-scale paleoecological analyses in identifying such drivers.

古生态学数据（Paleoecological data）不仅可用于探究深时年代样带的演化趋势，还可用于重建不同地质时段的生态梯度，助力识别现代生态系统中相关性极强的因果因子。我们将此类分析应用于伯格曼法则（Bergmann’s Rule）——该法则提出哺乳动物的体温与体型之间存在因果关系。伯格曼法则预测，在任意地质时段均存在纬度梯度：极地类群体型更大，赤道类群体型更小；同时该梯度的强度随时间变化，温暖时期梯度减弱，寒冷时期梯度增强。我们通过重建北美西海岸渐新世-中新世（Oligo-Miocene）不同时段犬科与马科属级类群的体型体重趋势，对上述预测进行了检验。为便于与现生类群对比，我们以相同的样带标准重建了现代犬属（Canis）与空齿鹿属（Odocoileus）的体型体重。在本次分析的17个化石属中，仅2个类群呈现出预期的与纬度的正相关关系，且未发现古温度与体型体重间存在稳定关联的证据。同样，体型梯度的强度也未随气候波动呈现可预测的时间变化。即使在本次分析的现生类群中，清晰梯度的相关证据也较为模糊。上述结果表明，与伯格曼法则相悖，仅温度并非体型演化的主要驱动因子，同时凸显了区域尺度古生态学分析在识别此类驱动因子时的重要性。