Evolutionary History and Paleobiology of Early Cenozoic Multituberculata (Mammalia), with Emphasis on the Family Ptilodontidae. (Volumes I and II).

Abstract

Multituberculates are an extinct order of small, nontherian mammals that were among the most abundant and taxonomically diverse of early Cenozoic mammals. This study was undertaken with four principal objectives: (1) to document the phylogenetic history of North American Paleocene and early Eocene multituberculates, with emphasis on those in the family Ptilodontidae; (2) to provide a ptilodontid-based biostratigraphic zonation of the Paleocene; (3) to test the current working hypothesis explaining the extinction of multituberculates; and (4) to reconstruct various aspects of the paleobiology of multituberculates, particularly those pertaining to feeding and locomotion. Analysis of all known specimens of ptilodontid multituberculates reveals a much greater diversity than previously documented. The family Ptilodontidae includes four genera (one new) and 14 species (five new). Ptilodontids, like several other multituberculate taxa, became extinct near the Paleocene-Eocene boundary. Systematic revision of all multituberculate taxa of the Clarkforkian (latest Paleocene - earliest Eocene) and Wasatchian (early Eocene) L and -Mammal Ages documents diversities much lower than for the preceding Torrejonian and Tiffanian. The reduced diversity and abundance of North American multituberculates in Clarkforkian and later faunas may have been due to diffuse competition with rodents, which dispersed from Asia at the beginning of the Clarkforkian. Ptilodus, the most common Paleocene multituberculate, is represented by well preserved cranial and gnathic remains that permit an analysis of jaw movement and dental function. Ptilodus had two cycles of mastication (slicing-crushing, grinding) that followed different paths of movement (orthal, propalinal) and utilized different sets of teeth (premolars, molars). The type and distribution of dental wear striations and the small body size of many species suggests that some ptilodontoids, at least, were omnivorous. A recently discovered and nearly complete skeleton of Ptilodus exhibits specializations of the hind foot and tail for arboreal locomotion. A survey of all known postcranial remains of North American multituberculates reveals similar adaptations for climbing in other genera.