Biomechanics of the intramandibular joint in Alligator mississippiensis

Tetrapod vertebrates possess skulls composed of variably articulating bones which they use to apprehend, process, and ingest food. Natural selection must therefore optimize craniomandibular sutures for load resistance, but sutural patency is required for normal craniofacial development to occur. While mammals seemingly escaped this constraint in their mandible by simplifying it into a single bony element (i.e., the dentary), sauropsids retain a composite mandible with a prominent, and occasionally flexible, intramandibular joint (IMJ) separating the rostral, dentigerous elements from the caudal elements onto which the jaw muscles insert. How sauropsids simultaneously construct a mandible robust enough for feeding that nevertheless maintains sutural patency for proper growth is a biomechanical paradox of keen interest to functional morphologists. Sauropsids may either passively reduce IMJ strain by expanding IMJ complexity or actively by using isometric contraction of specialized jaw muscles to resist excursion. American alligator (Alligator mississippiensis) mandibles possess a rather complex IMJ that must accommodate extreme magnitude and highly dynamic loads during feeding. Importantly, they also possess large m. intramandibularis (mIM) and m. pterygoideus ventralis (mPTv) muscles that may reduce IMJ strain during feeding, making them an ideal taxon to investigate the effect of joint morphology and muscle activity on IMJ and mandibular strain. We therefore constructed several 3D finite element models of Alligator mandibles with varyingly shaped IMJs to test the effect of IMJ orientation, complexity, and differential muscle activity on mandibular bending deformation and joint strain. Simple planar IMJs, regardless of orientation, reduce positive sagittal bending and medial wishboning deformation, and increase inversion of each hemimandible's dorsal margin. Changes in bending deformation during bilateral bites as the joint surface was reoriented from rostrally sloped to Vertical to caudally sloped are partially attributable to changes in joint surface area, though bending deformation is sensitive to both bite point location and joint orientation during unilateral bites. Increasing IMJ surface area reduces IMJ strain magnitudes, with a highly complex IMJ experiencing the most uniform and lowest magnitude joint ligament strains. Differential activation of mIM and mPTv do not significantly reduce IMJ strains but do affect mandibular bending deformation, suggesting that available joint surface area, and not isometric muscle contraction, is the greatest variable controlling IMJ strains in adult Alligator. Instead, mIM may significantly control bite point reaction forces due to its very long moment arm, whereas mPTv indirectly reduces medial wishboning by pulling the caudal elements against the pterygoid buttress, inducing a powerful, laterally directed reaction force on the caudal elements. However, while sauropsids appear susceptible to medial wishboning owing to the prominent medially directed pull of their jaw muscles, overall relationships between IMJ form and flexibility are unclear, as groups with complex intramandibular sutures may be akinetic (e.g., crocodylians) or kinetic (e.g., varanids). Further research will clarify IMJ morphological diversity and disparity among reptiles and divulge form-function relationships of this critical, but underappreciated, aspect of their feeding apparatus.