TAXONOMIC REPLACEMENT AND BODY SIZE EVOLUTION IN OLIGO-MIOCENE CANIDS

The study of geographic and temporal body mass trends in both extant and extinct mammals has led to the formulation of several models for body size evolution (e.g. Bergmann's rule and James' rule). Generally, these rules hold that changes in climactic or biotic factors, such as temperature or primary productivity, drive ecosystem-wide changes in body mass. However, different taxa often show varied responses to the same ecological pressures, and within some taxa these responses are strongly influenced by phylogeny. Such trends are, for example, clearly visible in Oligo-Miocene canids from the northwest United States (Oregon, Washington, and northern Nevada). At the family level, mean canid body mass does not vary greatly during the relevant interval. However, finer-scale taxonomic analysis reveals a succession of canid subfamilies, with large-bodied Oligocene hesperocyonines being replaced by borophagines in the early Miocene, which are in turn largely replaced by canines in the late Miocene. Similar trends are also visible at the ecosystem level; groups of closely-related taxa form clusters of similarly-sized individuals within faunas. This does not imply that climatic and biotic variables play no role in driving canid body mass evolution, as large-scale taxonomic replacement requires an ecological mechanism. Rather, it suggests that for at least some mammalian taxa, phylogeny plays a major role in determining a taxon's response to ecological change and underscores the notion that paleoecological analyses should always be conducted within an appropriate taxonomic framework.