The Load Paths in the Jaws of Extant Mammals and Fossil Mammaliaforms and Their Significance for Mammal Ear and Jaw Evolution

Abstract

The early evolution of mammals involved major transformations of their jaw and middle ear, with profound changes in feeding and hearing functions. The novel dentary and squamosal jaw joint in mammals evolved from a double joint of fossil mammaliaforms, formed by the dentary-squamosal articulation and ear bones fully attached to the dentary. Thus, the mandibular middle ear is structurally a part of the musculoskeletal system that resists forces of chewing. How could mammaliaforms maintain a delicate mandibular ear on the robust dentary while resisting high stresses during feeding? Here I developed a new biomechanical analysis, load path analysis, to shed light on how the mandibular middle ear was retained and could be functional for hearing in stem mammals. The mechanics of mandibular middle ear function was approached via four different aims: the development of a load path analysis for mandibles, the search for morphological markers of load paths in mammal jaws, using load paths to identify load bearing structures in the mammaliaform Morganucodon, and vibroacoustic analysis of the mandibular middle ear in the more stemward cynodont Thrinaxodon. The results of these aims demonstrate that load paths are found in the medial ridge of the dentary in mammaliaforms, a conserved osteological feature that insulated the mandibular middle ear from feeding stresses. All the while, the tympanum on the mandibular middle ear was a functional sound receiver. Thus, the medial ridge facilitated the transition of the postdentary bones from a jaw joint into a purely sensory organ system