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Morphological evolution of the mammalian jaw adductor complex

Lautenschlager, Stephan; Gill, Pamela; Luo, Zhe-Xi; Fagan, Michael J.; Rayfield, Emily J.


Stephan Lautenschlager

Pamela Gill

Zhe-Xi Luo

Michael J. Fagan

Emily J. Rayfield


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.


Lautenschlager, S., Gill, P., Luo, Z., Fagan, M. J., & Rayfield, E. J. (2017). Morphological evolution of the mammalian jaw adductor complex. Biological Reviews, 92(4), 1910-1940.

Journal Article Type Article
Acceptance Date Oct 17, 2016
Online Publication Date Nov 23, 2016
Publication Date 2017-11
Deposit Date Nov 30, 2016
Publicly Available Date Nov 30, 2016
Journal Biological reviews
Print ISSN 1464-7931
Electronic ISSN 1469-185X
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 92
Issue 4
Pages 1910-1940
Keywords Mammalian evolution, Cynodontia, Digital reconstruction, Musculoskeletal evolution, Vertebrate palaeontology, Mesozoic
Public URL
Publisher URL
Additional Information Copy of article first published in: Biological reviews, 2017, v.92, issue 4.


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