The evolutionary origin of jaw yaw in mammals
David M. Grossnickle
Scientific Reports 7, Article number: 45094 (2017)
Received: 09 November 2016 Accepted: 17 February 2017 Published online: 21 March 2017
Theria comprises all but three living mammalian genera and is one of the most ecologically pervasive clades on Earth. Yet, the origin and early history of therians and their close relatives (i.e., cladotherians) remains surprisingly enigmatic. A critical biological function that can be compared among early mammal groups is mastication. Morphometrics and modeling analyses of the jaws of Mesozoic mammals indicate that cladotherians evolved musculoskeletal anatomies that increase mechanical advantage during jaw rotation around a dorsoventrally-oriented axis (i.e., yaw) while decreasing the mechanical advantage of jaw rotation around a mediolaterally-oriented axis (i.e., pitch). These changes parallel molar transformations in early cladotherians that indicate their chewing cycles included significant transverse movement, likely produced via yaw rotation. Thus, I hypothesize that cladotherian molar morphologies and musculoskeletal jaw anatomies evolved concurrently with increased yaw rotation of the jaw during chewing cycles. The increased transverse movement resulting from yaw rotation may have been a crucial evolutionary prerequisite for the functionally versatile tribosphenic molar morphology, which underlies the molars of all therians and is retained by many extant clades.
For constructive feedback, I thank Kenneth D. Angielczyk, Zhe-Xi Luo, Callum F. Ross, Allison K. Bormet, Julia A. Schultz, David A. Reed, Courtney P. Orsbon, Peter J. Makovicky, Michael I. Coates, Graham J. Slater, Ali Nabavizadeh, Nicholas J. Gidmark, Susumu Tomiya, Dallas Krentzel, P. David Polly, James S. Mellett, Bhart-Anjan S. Bhullar, and three anonymous reviewers. I thank P. David Polly and Aaron M. Olsen for assistance with geometric morphometrics. I am especially grateful to David A. Reed, Jose Iriarte-Diaz, and Callum F. Ross for discussions on jaw biomechanics, since many ideas presented in this study stemmed from those conversations. The Field Museum of Natural History (Brown Graduate Fellowship) and the University of Chicago (Committee on Evolutionary Biology) provided funding assistance.
University of Chicago, Committee on Evolutionary Biology, Chicago, 60637, USA
David M. Grossnickle
D.M.G. performed all work in this study.
The author declares no competing financial interests.
Correspondence to David M. Grossnickle.