title: Resolving the evolution of the mammalian middle ear using Bayesian inference.
abstract: BACKGROUND
  The minute, finely-tuned ear ossicles of mammals arose through a spectacular evolutionary transformation from their origins as a load-bearing jaw joint. This involved detachment from the postdentary trough of the mandible, and final separation from the dentary through resorption of Meckel's cartilage. Recent parsimony analyses of modern and fossil mammals imply up to seven independent postdentary trough losses or even reversals, which is unexpected given the complexity of these transformations. Here we employ the first model-based, probabilistic analysis of the evolution of the definitive mammalian middle ear, supported by virtual 3D erosion simulations to assess for potential fossil preservation artifacts.
  
  
  RESULTS
  Our results support a simple, biologically plausible scenario without reversals. The middle ear bones detach from the postdentary trough only twice among mammals, once each in the ancestors of therians and monotremes. Disappearance of Meckel's cartilage occurred independently in numerous lineages from the Late Jurassic to the Late Cretaceous. This final separation is recapitulated during early development of extant mammals, while the earlier-occurring disappearance of a postdentary trough is not.
  
  
  CONCLUSIONS
  Our results therefore suggest a developmentally congruent and directional two-step scenario, in which the parallel uncoupling of the auditory and feeding systems in northern and southern hemisphere mammals underpinned further specialization in both lineages. Until ~168 Ma, all known mammals retained attached middle ear bones, yet all groups that diversified from ~163 Ma onwards had lost the postdentary trough, emphasizing the adaptive significance of this transformation.
authors: University of Queensland, School of Biological Sciences, Goddard Building 8 St. Lucia, Brisbane, QLD 4072 Australia.
authors: Héctor E
authors: HE
authors: Ramírez-Chaves
authors: University of Queensland, School of Biological Sciences, Goddard Building 8 St. Lucia, Brisbane, QLD 4072 Australia.
authors: Vera
authors: V
authors: Weisbecker
authors: Division of Zoology, School of Environmental and Rural Sciences, University of New England, Armidale, NSW 2351 Australia.
authors: Stephen
authors: S
authors: Wroe
authors: School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4000 Australia.
authors: Matthew J
authors: MJ
authors: Phillips
keywords: Australosphenida; Meckel’s groove; Middle ear detachment; Postdentary trough; Theria