John David Fortner

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.

High-resolution imaging, 3D modeling, and quantitative analyses are equipping evolutionary biologists with new approaches to understanding the variation and evolution of the musculoskeletal system. However, challenges with interpreting DiceCT data and higher order use of modeled muscles have not yet been fully explored, and the error in and accuracy of some digital methods remain unclear. West Indian Anolis lizards are a model clade for exploring patterns in functional adaptation, ecomorphology, and sexual size dimorphism in vertebrates. These lizards possess numerous jaw muscles with potentially different anatomies that sculpt the adductor chamber of the skull. Here we test approaches to quantifying the musculoskeletal shape of the heads of two species of Anolis: A. pulchellus and A. sagrei. We employ comparative approaches such as DiceCT segmentation of jaw muscles, 3D surface attachment mapping, and 3D landmarking with the aim of exploring muscle volumes, 3D muscle fiber architecture, and sexual dimorphism of the skull. We then compare sources of measurement error in these 3D analyses while also presenting new 3D musculoskeletal data from the Anolis feeding apparatus. These findings demonstrate the accessibility and repeatability of these emerging techniques as well as provide details regarding the musculoskeletal anatomy of the heads of A. pulchellus and A. sagrei which show potential for further research of comparative biomechanics and evolution in the clade.