DOES THE ITERATIVE EVOLUTION OF ECOMORPHS REFLECT CLIMATIC CYCLES?

Iterative patterns in North American Cenozoic mammalian evolution suggests a direct connection between climate and morphology. These patterns are expressed in animals that share such fundamental similarity that we are compelled to assign them similar behaviors and ecological positions. If only a few taxa were involved and their temporal distribution stochastic, these convergent patterns might be accounted for by accidental processes. However, they exist in many co-occurring taxa and, historically, have helped form the basis of biostratigraphic units. The extinction of any one of these adaptive types (ecomorphs) would not be surprising, as extinction is the normal fate of any lineage. What is exceptional is the rapid replacement of ecomorphs by organisms that may or may not be closely related. For example, the felid sabertooth Smilodon replaced the nimravid Barbourofelis, and over the same period, the horse Equus replaced Hipparion. This demonstrates a number of theoretically important points. The conditions that originally resulted in extinction ceased to be active, and not long after the extinction, similar conditions returned to those that had originally favored the development of particular ecomorphs. It is difficult to envision any mechanism besides climate that would simultaneously affect the extinction and re-evolution of disparate ecomorphs. If this is true, then extinction episodes represent periods of change in climatic parameters that were eventually reversed (at some scale), bringing back similar conditions that had created the former community structure. In other words, a cycling of climate exists. The ecomorphs, themselves, indicate that mammals respond in a very limited way to selection pressures in connection with their ecological positions and that the power of natural selection to create similarities often overwhelms phylogenetic history. Examples are drawn from carnivores, ungulates, and rodents.