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