The evolution of anteriorly directed molar occlusion in mammals

Abstract

In non-mammalian synapsids and early mammals, evolutionary transformations in the feeding and hearing apparatuses are posited to have been prerequisites for the radiation of extant mammals. Unlike most vertebrates, including many early synapsids, mammals have precise dental occlusion, a lower jaw composed of one bone, and middle ear ossicles derived from ancestral jaw bones. We illuminate a related functional transition: therian mammals (eutherians and metatherians) evolved anteriorly directed chewing strokes, which are absent in other synapsid lineages. Anteriorly directed jaw movement during occlusion necessitates anteriorly directed muscle force vectors, and we posit that a shift in muscle orientation is reflected in the fossil record by the evolutionary appearance of a posteriorly positioned angular process in cladotherians (therians and their close kin). Anteriorly directed occlusion might have been absent in earlier synapsids because of the presence of attached middle ear elements in the posterior region of the jaw that prohibited the posterior insertion of jaw musculature. These changes to the masticatory apparatus in cladotherians are likely to have permitted the evolution of novel masticatory movements, including grinding in both the anterior and medial directions (e.g. rodents and ungulates, respectively). Thus, this evolutionary transition might have been a crucial prerequisite for the dietary diversification of therians.

Ontogenetic growth in the crania ofExaeretodon argentinus(Synapsida: Cynodontia) captures a dietary shift

“…Herein, we find that the anisotropic patterns of a small individual (MCZ VPRA-4470) are oriented apicobasally, nearly perpendicular to the orientation of scratches evidenced in larger individuals of E. argentinus ( Kubo, Yamada & Kubo, 2017 ). If the patterns of anisotropy in MCZ VPRA-4470 are similarly reflective of jaw motion, mastication would be primarily orthal, suggesting an m. temporalis driven bite (see Grossnickle et al, 2022 and references therein), contra to the inferred palinal movements in larger Exaeretodon ( Kubo, Yamada & Kubo, 2017 ). Interpreting ecological differentiation based on microwear alone assumes all dental modifications are feeding-related but finding consistent patterns in the ontogeny of masticatory muscle sizes and sutural complexity all together support younger individuals of E. argentinus having a dietary ecology distinct from older conspecifics.…”

Derived faunivores are the forerunners of major synapsid radiations

“…Relative to contemporary clades in the Mesozoic Era, however, therians could be considered very derived and specialized. For instance, they (or their close relatives) evolved a tribosphenic molar that permitted more complex masticatory functions than most coeval faunivores Martin 2014, Jäger et al 2020), novel masticatory movements via asynchronous contractions of jaw muscles (Williams et al 2011, Grossnickle 2017, Bhullar et al 2019, Grossnickle et al 2022, and improved auditory and olfactory senses via evolutionary changes to the cochlea (Luo and Manley 2020) and face (Higashiyama et al 2021), respectively. A similar pattern applies to the other synapsid radiations, with as many as 55 novel synapomorphies differentiating early therapsids from pelycosaurs Hopson 1998, Kemp 2006) and 27 synapomorphies separating cynodonts from other therapsids (Hopson and Kitching 2001).…”

Evolution and development of the mammalian jaw joint: Making a novel structure

“…The evolution of the mammalian TMJ is closely related to changes in the dentition and food processing that occurred early in synapsid (mammal lineage) evolution (Bhullar et al, 2019 ; DeMar & Barghusen, 1972 ; Gill et al, 2014 ; Grossnickle, 2017 ; Grossnickle et al, 2021 ; Sidor, 2003 ; Zhou et al, 2013 ). As well as having a novel jaw joint, mammals have teeth that differ from those of most other jawed vertebrates.…”

“…These differences include a single replacement, rather than continual loss and replacement of teeth (diphydonty rather than polyphydonty), heterodonty (varied tooth shapes in different parts of the jaw) rather than homodonty (uniform tooth shapes throughout the jaw), and most importantly for the evolution of the TMJ, occlusion of the upper and lower dentition. Occlusion is facilitated by complimentary cusp patterns in the upper and lower dentition for shearing and crushing of food items (Crompton, 1981 ; Grossnickle et al, 2021 ; Jäger et al, 2020 ). The changes in the dentition would have put more strain on the primary jaw joint, leading to the added involvement of additional bones to buttress these effects and support the jaw during movement (Crompton & Hylander, 1986 ).…”