Bhullar, Bhart-Anjan S.Manafzadeh, Armita R.Miyamae, Juri A.Hoffman, Eva A.Brainerd, Elizabeth L.Musinsky, CatherineCrompton, Alfred2020-07-102019-02Bhullar, Bhart-Anjan & Manafzadeh, Armita & Miyamae, Juri & Hoffman, Eva & Brainerd, Elizabeth & Musinsky, Catherine & Crompton, Alfred. (2019). Rolling of the jaw is essential for mammalian chewing and tribosphenic molar function. Nature. 566. 10.1038/s41586-019-0940-x.0028-08361476-4687https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37363147Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations. Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin. The flexible mandibular joint and the unfused symphysis of ancestral mammals-in combination with transformations of the adductor musculature and palate-are thought to have permitted greater mobility of each lower jaw, or hemimandible. Following the appearance of precise dental occlusion near the origin of the mammalian crown, therians evolved a tribosphenic molar with a craggy topography that is presumed to have been used to catch, cut and crush food. Here we describe the ancestral tribosphenic therian chewing stroke, as conserved in the short-tailed opossum Monodelphis domestica: it is a simple symmetrical sequence of lower tooth-row eversion and inversion during jaw opening and closing, respectively, enacted by hemimandibular long-axis rotation. This sequence is coupled with an eversion-inversion rotational grinding stroke. We infer that the ancestral therian chewing stroke relied heavily on long-axis rotation, including symmetrical eversion and inversion (inherited from the first mammaliaforms) as well as a mortar-and-pestle rotational grinding stroke that was inherited from stem therians along with the tribosphenic molar. The yaw-dominated masticatory cycle of primates, ungulates and other bunodont therians is derived; it is necessitated by a secondarily fused jaw symphysis, and permitted by the reduction of high, interlocking cusps. The development of an efficient masticatory system-culminating in the tribosphenic apparatus-allowed early mammals to begin the process of digestion by shearing and crushing food into small boli instead of swallowing larger pieces in the reptilian manner, which necessitates a long, slow and wholly chemical breakdown. The vast diversity of mammalian teeth has emerged from the basic tribosphenic groundplan.en-USMultidisciplinaryJournal Article2020-07-1010.1038/s41586-019-0940-x