Therian mammals experienced an immense evolutionary radiation and now represent one of the most ecologically diverse clades on Earth. However, therians and their closest relatives (i.e., cladotherians) were primarily small insectivores for the first half of their history, a period from approximately 165 to 80 million years ago, suggesting that this radiation was delayed or suppressed during this period. Although research has often examined phylogenetic relationships and morphological evolution of early cladotherians, little work has been devoted to analyzing their ecological diversity and biomechanical functions. Thus, questions remain about the adaptive nature of early evolutionary changes and the timing of the ecomorphological radiation of cladotherians. In this dissertation, I address these topics by examining jaw and molar morphologies, which are associated with diet and biomechanical function. I investigate two evolutionary events in particular. First, I analyze changes to jaw morphologies that accompanied the evolution of the tribosphenic molar (and its precursors). Biomechanical analyses of musculoskeletal configurations support the hypothesis that changes to jaw, molar, and ear morphologies in cladotherians are associated with increased transverse movement via yaw rotation during mastication. These changes may have been especially important prerequisites for the evolution of the tribosphenic molar morphology, an evolutionary innovation that likely assisted in the longterm survival and diversification of therians. In addition, the evolution of a posteriorly positioned angular process (and accompanying changes to masticatory muscles) may have been crucial for the development of yaw rotation, and the functional significance of the angular process is further supported by analyses that demonstrate that its size and position is a strong correlate of diet. Second, I use molar and jaw morphologies to examine the ecomorphological patterns of therians in the Cretaceous and early Cenozoic. I find evidence that the adaptive radiation of therians began in the latest Cretaceous, a period in which angiosperms, social insects, and non-therian mammals (e.g., multituberculates) are also diversifying. This suggests that the macroevolutionary patterns of mammals in the Cretaceous were an interplay of functional adaptations (e.g., evolution of tribosphenic molars) and evolutionary patterns in likely food sources (i.e., plants and insects). Further, the molar data indicates that mammals experienced a selective extinction at the Cretaceous-Paleogene (K-Pg) boundary, and jaw morphologies show evidence for a multi-step radiation of mammals between 80 and 50 million years ago. I conclude by proposing that the evolutionary adaptations of therians were not only important to their ecomorphological diversification, but they may have also been critical to their survival during periods of major environmental perturbations such as the K-Pg mass extinction event.