Taxonomic Diversification Rates and Ecological Niches of Neoplagiaulacid and Ptilodontid Multituberculate Mammals in North America's Terrestrial Paleocene Recovery

Study of enamel microwear on the teeth of Mesodma thompsoni, a multituberculate mammal species which survived the end-Cretaceous extinction, indicates that diets of Cretaceous individuals had different physical properties than the diets of earliest Paleocene individuals in the same depositional basin. This supports the hypothesis that M. thompsoni was a dietary generalist, rather than a specialist lucky in the survival of a particular food source. It also supports the hypothesis that M. thompsoni occupied a somewhat different niche in the Paleocene than it did in the Cretaceous.

Mesodma is a relatively primitive member of the family Neoplagiaulacidae. Phylogenies of multituberculates suffer from missing data and too many taxa relative to the number of reliable characters, and all have low support indices. Some plausible hypotheses suggest that Neoplagiaulacidae as currently defined is a paraphyletic group from which the family Ptilodontidae originates. Within this hypothesized clade, there is directional change in molar tooth morphology that is mechanically consistent with the dietary change indicated by dental microwear in Mesodma. Moreover, this clade becomes taxonomically diverse by the end of the Paleocene. The rate at which this diversification occurs is consistent with the diversification rates of some marine groups, and may indicate very general similarities. Without further study, however, questions remain whether the Paleocene diversification of this group indicates expansion into new niches that would not necessarily by indicated by the physical properties of diet (for example, transition to arboreal habitus, which would involve the postcranial skeleton) or narrower partitioning of ecospace.

Immigration seems to have played a large role in the restructuring of North American terrestrial ecologies, and the Neoplagiaulacidae are evolutionarily interesting as an autochthonous radiation. In particular, the rate of new first appearances within the clade is unlikely to be skewed by immigration, and to better reflect evolutionary origination rates.