The Multituberculates as Living Animals: New Insights into the Ecomorphology, Behavior, and Life History of One of the Most Successful Mammalian Lineages

Abstract

Mammals from the Mesozoic and early Cenozoic exploited a diversity of ecological niches, approaching the complexity and specialization of extant therian mammals. Among the most evolutionarily diverse and numerically abundant of these early mammals were the Multituberculata, an extinct group of mostly small-bodied (< 1 kg) herbivores and omnivores whose fossil record spans the Middle Jurassic (ca. 170 Ma) through the end of the Eocene (ca. 35 Ma). Despite their predominance in early mammalian fossil assemblages, the paleobiology of multituberculates remains relatively obscure, due in part to their lack of modern descendants, and in part to their highly specialized, non-analogous dental and skeletal morphology. In this dissertation, I use a combination of linear morphometrics, comparative anatomy, functional morphology, sedimentology, taphonomy, and bone histology to shed further light on the ecomorphology, behavior, and life history of multituberculates. In particular, this dissertation focuses on the fossil record of the multituberculate subgroup Cimolodonta from the Late Cretaceous through early Paleogene of the Western Interior of North America. In Chapter 2, a co-author and I examine how the shape of the blade-like lower premolars (p4s) of cimolodontans changed from the mid-Cretaceous through early Paleogene, and we find that the cimolodontan chewing cycle may have functionally constrained p4 shape and that their specialized blade-like dentitions may have limited their ability to exploit a broader array of herbivorous niches. In Chapter 3, co-authors and I describe exceptionally preserved skeletons of a cimolodontan from the Late Cretaceous Egg Mountain locality of Montana; therein, we describe a new genus, Filikomys, and infer that it was capable of burrowing. We then examine how the skeletons were preserved and hypothesize that these animals were preserved in burrows and that they were aggregating in life, thereby exhibiting the earliest evidence of mammalian social behavior. In Chapter 4, co-authors and I investigate the bone tissue microstructure (bone histology) of extant marsupial and placental mammals and discover that the bone histology of those groups are distinct and likely reflect their divergent reproductive strategies. We then apply our histological correlate of reproductive strategies to a sample of multituberculate femora from the Late Cretaceous and Paleocene of Montana and find that their bone histology is nearly identical to that of placentals, indicating that they likely had placental-like life histories. Those findings shed new light on the marsupial-placental dichotomy and challenge the hypothesis that Cenozoic rise of placental mammals was driven by unique reproductive innovations. In sum, this dissertation suggests that at least some multituberculates approached the level of behavioral and reproductive complexity of extant small-bodied placental mammals, and that their highly-specialized dentitions facilitated their Mesozoic success, but also their Cenozoic demise.