title: Morphological evolution of the mammalian jaw adductor complex.
abstract: The evolution of the mammalian jaw during the transition from non-mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more-efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont-mammaliaform transition have been debated for nearly a century. We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three-dimensional models of the jaw adductor musculature in a number of representative non-mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution. As an emerging consensus, current evidence suggests that the mammal-like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral ('reptilian') to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol, BS8 1TQ, U.K.|School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, U.K.
authors.forename: Stephan
authors.initials: S
authors.lastname: Lautenschlager
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol, BS8 1TQ, U.K.|Earth Science Department, The Natural History Museum, London, SW7 5BD, U.K.
authors.forename: Pamela
authors.initials: P
authors.lastname: Gill
authors.affiliation: Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL, 60637, U.S.A.
authors.forename: Zhe-Xi
authors.initials: ZX
authors.lastname: Luo
authors.affiliation: School of Engineering, University of Hull, Hull, HU6 7RX, U.K.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Fagan
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol, BS8 1TQ, U.K.
authors.forename: Emily J
authors.identifier: http://orcid.org/0000-0002-2618-750X
authors.initials: EJ
authors.lastname: Rayfield
mesh_terms: D000818:Animals; D005075:Biological Evolution; D007568:Jaw / Q000033:anatomy & histology*; D008322:Mammals / Q000033:anatomy & histology* / Q000235:genetics*; D008410:Masticatory Muscles / Q000033:anatomy & histology* / Q000254:growth & development
keywords: Cynodontia; Mesozoic; digital reconstruction; mammalian evolution; musculoskeletal evolution; vertebrate palaeontology