Rolling of the jaw is essential for mammalian chewing and tribosphenic molar function

Autor: Elizabeth L. Brainerd, Juri A. Miyamae, Armita R. Manafzadeh, Alfred W. Crompton, Bhart-Anjan S. Bhullar, Catherine Musinsky, Eva A. Hoffman
Rok vydání: 2019
Předmět:
Zdroj: Nature. 566:528-532
ISSN: 1476-4687
0028-0836
Popis: Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations1-3. Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin3-5. 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 hemimandible6,7. Following the appearance of precise dental occlusion near the origin of the mammalian crown8,9, 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 cusps10-12. 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 groundplan13.
Databáze: OpenAIRE
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