Insights into the evolution of Late Cretaceous metatherian mammals of North America: interpreting feeding ecologies using quantitative analyses

Abstract

Metatherian mammals (the stem-based clade of extant marsupials and their closest relatives) were important members of North American communities during the Late Cretaceous: they were both taxonomically rich and numerically abundant. Previous studies have mostly focused on taxonomic diversity measures, which provide important information regarding ecosystem dynamics, but less attention has been devoted to understanding the ecology of this group of mammals. This dissertation seeks to add to our growing knowledge of both the anatomy and associated ecologies (i.e., ecomorphology) that North American metatherians possessed during the Late Cretaceous. I specifically focus on interpreting diet and feeding ecology because the fossil record of North American metatherians is mainly composed of teeth and tooth bearing elements. In the first study, I describe rare, new cranial fossils of the metatherian Alphadon halleyi that were discovered at the Egg Mountain locality (Montana, USA). These specimens represent some of the most complete cranial material for any North American metatherian. My co-authors and I use this new anatomical information to score previously unknown characters for A. halleyi and subsequently conduct a phylogenetic analysis in order to reassess the phylogenetic relationships among metatherians. Our results conflict with recent phylogenetic analyses and demonstrate that the place of origin of Marsupialia (crown marsupials) remains elusive. The remaining two studies of this dissertation concentrate on the ecomorphology of North American metatherians. My co-authored study of the evolution of durophagy (hard-object feeding) in stagodontid metatherians utilizes a relatively new method that quantifies biomechanical properties of the dentary (jaw). We apply beam theory to estimate bending force capabilities of dentaries of stagodontids and other metatherians. We find that the jaws of the two species of the stagodontid Eodelphis had different bending force profiles from each other and from the durophagous stagodontid Didelphodon. The jaw of E. browni was not adapted toward withstanding bending forces associated with durophagous habits, whereas the jaw of E. cutleri was adapted toward withstanding mediolateral bending forces associated with durophagy. However, the jaw of E. cutleri lacked other dorsoventral buttressing associated with exceptionally high bite forces of Didelphodon. Our results imply that Cretaceous metatherians had a wide range of feeding behaviors and some morphological changes associated with durophagy evolved twice within this clade, independently in E. cutleri and Didelphodon. Finally, my co-authored study on the dental ecomorphology of North American Late Cretaceous metatherians aims to take a more synoptic approach at examining the dental morphology of these animals. We use three-dimensional dental topographic analysis to predict the diets of metatherians to try to understand macroevolutionary patterns in dental morphology and dietary diversity. Although our results show that dental disparity and dietary diversity did not significantly change throughout the Late Cretaceous and that most metatherians were insectivorous, we also found that metatherians occupied a wide range of dietary niches. Metatherians were also arguably the most dietarily diverse of any mammalian clade of the Late Cretaceous. Our results also indicate that this ecological diversification was more correlated in time with the Cretaceous Terrestrial Revolution and the mid-Cretaceous taxonomic diversification of angiosperms. Overall, this dissertation serves to bolster our knowledge of ecological roles metatherians filled during the Late Cretaceous in North America. My results reinforce the concept that different proxies, when available, should be used to better infer ecology and further demonstrate that metatherians were more ecologically diverse than previously appreciated.