Meeting Abstract
The lower jaws of major mammaliaform clades of the Mesozoic Era (252-66 million years ago) have shown disparate morphologies. It is expected that such major differences would be correlated with differences in biomechanical functions in the mandible and middle ear evolution. Here, I present a geometric morphometric analysis of jaw morphology to examine early jaw adaptations within Mesozoic mammals. Utilizing fossil jaw images belonging to 105 premamammalian cynodont and mammaliaform taxa, shapes of the angular process and the coronoid process were analyzed, and the elevation of the mandibular condyle was measured. For both the angular process and coronoid process, shapes were quantified and compared using semilandmark outlines subjected to two-dimensional geometric morphometric techniques. To assess broad evolutionary trends, average shapes of the jaw processes for mammalian groups were considered in a phylogenetic context. Results indicate convergent jaw changes within three long-lived groups: cimolodontan multituberculates, the stem lineage of monotremes, and the clade that includes therians (placentals and marsupials) and their close relatives. These groups develop an elevated condyle, a posteriorly-positioned angular process, and a lower and more inclined coronoid process. The jaw changes arose only after the evolutionary detachment of postdentary bones and Meckel’s cartilage within these lineages, which may have freed the jaw for musculoskeletal remodeling. In addition, the jaw changes may have co-evolved with increased grinding function of the molars, allowing for a diet consisting of increased plant matter. Therefore, the adaptations of the jaw and molars appear to be correlated for masticatory efficiency in omnivorous/herbivorous clades, contributing to the long-term survivals of these clades.
