Among mammals’ most diagnostic features are a dentary-squamosal jaw joint and a middle ear made up of three ossicles. The fossil record documents the origin of the mammalian middle ear from the postdentary bones of non-mammaliaform cynodonts and the acquisition of the dentary-squamosal joint in mammaliaforms. The cynodont-mammaliaform transition has been studied for decades, but a wealth of new fossils, many of which occupy key phylogenetic positions, and developmental data from living mammals have been made available in recent years. Despite their potential for shedding light on the factors driving mammalian morphological evolution, much of these data have yet to be examined using modern digital techniques. In this thesis I integrate novel data from fossils and neonatal mammals to test how development and function drove the morphological evolution of the mammalian jaw joint and middle ear. I use micro-CT scanning to describe changes to the jaw joint and postdentary bones across the cynodont-mammaliaform transition and review the factors governing their morphological evolution using newly discovered fossils and revised phylogenetic hypotheses. Initially focusing on Triassic derived probainognathians, I demonstrate that the dentary-squamosal contact evolved independently in ictidosaurian cynodonts 17 million years before it appeared in mammaliaforms. Secondly, I use a unique sample of neonatal echidnas to demonstrate how the poorly understood development of monotremes can provide insight into the drivers underpinning the separation of the middle ear from the jaw in extinct crown mammals. I find that the cellular processes involved in the separation of the middle ear are shared between monotremes and therian mammals, demonstrating that the underlying developmental mechanisms are ancestral to crown mammals. Finally, I use finite element analysis to test hypotheses regarding the function of the dentary-squamosal joint, demonstrating that the acquisition of this feature was pivotal to reducing stresses in the cynodont middle ear.
The evolution, development and function of the mammalian jaw and middle ear
Rawson, J. (Author). 9 Dec 2025
Student thesis: Doctoral Thesis › Doctor of Philosophy (PhD)