160 - Reconstructing Jaw Muscles and Predatory Function of Saber-toothed Mammal Precursor Inostrancevia Through Extant Musculature Homologues

Poster Board Number: 160
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Abstract Body : This study’s objective is to reconstruct the jaw muscles of Inostrancevia, a saber-toothed gorgonopsian therapsid. These distant mammal relatives stalked the earth 265 to 252 million years ago as apex predators of the late Permian period. This investigation will help to occupy the current research vacuum for this taxon, infer jaw muscles of gorgonopsians, and inform current work detailing evolution of visceral arch muscles from early synapsids to mammals.

To ground our muscle inferences, we examined contrast-stained, microCT scans of disparate extant species whose phylogenies converge to bracket Inostrancevia: Varanus prasinus, Chelonia mydas, and Lontra canadensis. We created 3D virtual reconstructions by segmenting the jaw muscles and skull for each species in Avizo.

From each segmented muscle’s surface mesh, we collected volume, global pennation angle, fiber lengths, and contribution to total force for individual muscle heads in Avizo and MeshLab. From these variables, we calculated the approximate force for each muscle head using the standard isometric force per area for vertebrates, 32 N/cm^2.

We measured an Inostrancevia’s subtemporal fenestra area in Fiji as a natural anatomical limit on jaw adductor forces, and subdivided that force potential by relative contributions from individual inferred muscles. Total potential force for Inostrancevia is 20912.864 N on the right and 20214.227 N on the left.

We applied these calculations to a finite element analysis (FEA) of an Inostrancevia skull using Strand7 to test the effects of musculature respectively more resembling each of the three extant species.

A trial Chelonia mydas-based analysis created von Mises stresses between 20 MPa and 200 MPa at the constraints, a range typically experienced in vertebrate bone. We found high tension on superior surface of the dentary and compression on the inferior surface, as expected from three-point bending induced by the posterior adductor muscle insertions and tooth and joint constraints. The body of the mandible experienced much greater stress than the cranium, except at constraints.

A key and unexpected finding was stress distributed in a wave pattern on the cranium from anterior to posterior; on the dorsal side of the frontals/parietals and the ventral side of the orbit roof. These thin areas of the cranium are under the most stress as the cranium is pulled inferiorly while the mandible is pulled superoposteriorly by the adductor muscles while biting.

These results indicate that the body of the mandible, the thinnest bony area of the jaw, is under the highest stresses from this muscular topology. The forces are not extreme enough to break the bone, but the relatively slender mandible is the limiting factor in delivery of adductor-driven bite force. Further analyses employing muscles of Varanus prasinus and Lontra canadensis are ongoing.

Determining bite force and skull stresses of Inostrancevia anchors a broader foray into understanding feeding evolution in radiations of Permian and Triassic synapsids along, and divergently specialized, from the mammalian line. This project’s focus on gorgonopsians will advance the field as a template for further investigations into reconstructive evolutionary morphology. In the future, we will further improve our feeding force and stress models by exploring recruitment modalities of ventral neck muscles in conjunction with traditional craniofacial jaw adductors.