MORPHOLOGICAL SPECIALIZATION AS A MACROEVOLUTIONARY RATCHET IN CARNIVORES (Invited Presentation)

The history of carnivorous mammals is marked by the repeated evolution of similar forms, some of which are quite extreme such as sabertooth and bone-cracking species, respectively. Perhaps the most common of these to evolve are large-bodied hypercarnivores, species specialized for killing large prey. Despite the fact that selection seems to favor their evolution, large hypercarnivores often appear late in a clade’s history and are associated with subsequent declines to extinction. Previous work on carnivoran evolution suggested that the morphological and ecological specializations necessary for hypercarnivory act as a macroevolutionary ratchet by limiting subsequent evolution and also increasing vulnerability to extinction. For example, a 2004 paper found that large hypercarnivorous Oligocene and Miocene canids of North America appear to have had shorter species durations than more generalized species, and their appearance in the record marks the onset of decline of their subfamilies. However, this study was based on a somewhat small sample size (42 species) and uncertain species duration data. To further explore the dynamics of canid diversification and the drivers of clade decline, we undertook two investigations, one data-based and the other simulation-based. In the first we expanded and refined our fossil canid database to include over 100 species as well as data on locality coverage for each species, and calculated sampling-adjusted species durations. Although locality coverage had no impact, we confirmed the association between shorter species duration and hypercarnivory, and also revealed a parallel pattern among specialized species on the opposite end of the dietary spectrum, hypocarnivores. This reinforces the notion that specialization can negatively impact subsequent diversification. In the second study, we use simulations to explore the relationship between trait evolution, extinction, and clade decline under macroevolutionary ratchet-type scenarios. Preliminary results suggest that ratchet type scenarios often produce asymmetrical clade histories dependent on extinction strength, and suggest that we may be able to recover signatures of the macroevolutionary ratchet that can be applied to empirical data.