Understanding the functional evolution of the mammalian middle ear and jaw joint across the cynodont-mammaliaform transition

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The origin and evolution of mammals is a key event in vertebrate evolutionary history, and a textbook example of an evolutionary transition. From around 230 million years ago, the fossil record documents an uncharacteristically well-preserved sequence of transitional fossils evolving key mammalian features such as deciduous and permanent teeth, a large brain, strong skull and the unique mammalian middle ear. Rather than a single middle ear bone, mammals have a more finely tuned middle ear comprising three small bones, or ossicles, the malleus, incus and the stapes. Along with a coiled cochlea, this structure enables high frequency sound detection. Combined evidence from the fossil record, embryology and development reveal a remarkable example of transformation in structure and function: bones forming the jaw joint of mammalian ancestors transform into the minute middle ear structures of mammals. We know that as the tooth-bearing bone, the dentary, increases in size, the jaw joint bones become smaller and loosely attached. Eventually the dentary contacts the squamosal part of the skull forming a true mammalian 'dentary-squamosal' (temperomandibular) hinge. We even know that at one point in mammalian evolution, animals existed with two jaw hinges with a dual feeding and auditory function. A long-standing point of debate is how the bones of the ancestral jaw hinge were able to reduce in size, whilst at the same time still functioning as a viable jaw joint. Additionally puzzling, is that during this transition, the skull is supposed to be strengthening, as the jaw-closing musculature reorganises to become a more efficient force generating system. The jaw joint should become stronger, not weaker and degenerate. Perhaps most startling, is that this transition has happened more than once.Theoretical models proposed in the 1970s and 80s suggested that reorganization of the jaw musculature lead to reduced loading at the jaw joint in the ancestors of mammals, allowing the ancestral hinge to become smaller and detect sound whilst the new mammalian hinge took over. These predictions are central to how the mammalian jaw and ear evolved, yet they have never been tested. This is largely because we have not had the means, until recently, to go beyond theory. We are now able to bring new computational biomechanical techniques, that we as a team have pioneered, to address the question of how the definitive mammalian middle ear and jaw joint were able to evolve yet remain functionally viable. We have obtained CT scans of five key transitional taxa. Through detailed study of fossil specimens we will reconstruct the patterns of musculoskeletal evolution across the origin of mammals, particularly in light of new fossil discoveries and suggestions of reversal back to ancestral forms. Using 3D muscle reconstructions and multibody dynamics analysis, we will determine how the ancestral, dual jaw joint and true mammalian jaw joint function during feeding behaviour. We will test if there is a transfer of function from ancestral to modern mammals with the evolution of the dual jaw joint as predicted. For example, do the component parts of the dual joint bear load, and can they function without joint disarticulation; and how is load transferred from the ancestral to modern hinge during this transition. Using finite element models we will test how the bones of the jaw hinge withstand load and strains during feeding. We will test if skulls do become stronger across the transition, as predicted, and how this relates to predicted bite forces. Comparative anatomists, biomechanists, evolutionary and developmental biologists, palaeontologists and biomedical engineers will benefit from this work. Benefits to UK science include multidisciplinary training of a young scientist and overseas collaboration. The visual aspect of this work and the focus on mammals is likely to appeal to the general public, offering engagement opportunities and media interest.