2D and 3D visualizations of archosaur jaw muscle mechanics, ontogeny and phylogeny using ternary diagrams and 3D modeling.

Comparing patterns of performance and kinematics across behavior, development and phylogeny is crucial to understand the evolution of complex musculoskeletal systems such as the feeding apparatus. However, conveying 3D spatial data of muscle orientation throughout a feeding cycle, ontogenetic pathway or phylogenetic lineage is essential to understanding the function and evolution of the skull in vertebrates. Here, we detail the use of ternary plots for displaying and comparing the 3D orientation of muscle data. First, we illustrate changes in 3D jaw muscle resultants during jaw closing taxa the American alligator (Alligator mississippiensis). Second, we show changes in 3D muscle resultants of jaw muscles across an ontogenetic series of alligators. Third, we compare 3D resultants of jaw muscles of avian-line dinosaurs, including extant (Struthio camelus, Gallus gallus, Psittacus erithacus) and extinct (Tyrannosaurus rex) species to outline the reorganization of jaw muscles that occurred along the line to modern birds. Finally, we compare 3D resultants of jaw muscles of the hard-biting species in our sample (A. mississippiensis, T. rex, P. erithacus) to illustrate how disparate jaw muscle resultants are employed in convergent behaviors in archosaurs. Our findings show that these visualizations of 3D components of jaw muscles are immensely helpful towards identifying patterns of cranial performance, growth and diversity. These tools will prove useful for testing other hypotheses in functional morphology, comparative biomechanics, ecomorphology and organismal evolution.

The roles of joint tissues and jaw muscles in palatal biomechanics of the savannah monitor (Varanus exanthematicus) and their significance for cranial kinesis.

Numerous vertebrates exhibit cranial kinesis, or movement between bones of the skull and mandible other than at the jaw joint. Many kinetic species possess a particular suite of features to accomplish this movement, including flexible cranial joints and protractor musculature. Whereas the musculoskeletal anatomy of these kinetic systems is well understood, how these joints are biomechanically loaded, how different soft tissues affect joint loading and kinetic capacity, and how the protractor musculature loads the skull remain poorly understood. Here, we present a finite element model of the savannah monitor, Varanus exanthematicus, a modestly kinetic lizard, to better elucidate the roles of soft tissue in mobile joints and protractor musculature in cranial loading. We describe the 3D resultants of jaw muscles and the histology of palatobasal, otic and jaw joints. We tested the effects of joint tissue type, bite point and muscle load to evaluate the biomechanical role of muscles on the palate and braincase. We found that the jaw muscles have significant mediolateral components that can impart stability across palatocranial joints. Articular tissues affect the magnitude of strains experienced around the palatobasal and otic joints. Without protractor muscle loading, the palate, quadrate and braincase experience higher strains, suggesting this muscle helps insulate the braincase and palatoquadrate from high loads. We found that the cross-sectional properties of the bones of V. exanthematicus are well suited for performing under torsional loads. These findings suggest that torsional loading regimes may have played a more important role in the evolution of cranial kinesis in lepidosaurs than previously appreciated.

Palatal Biomechanics and Its Significance for Cranial Kinesis in Tyrannosaurus rex.

The extinct nonavian dinosaur Tyrannosaurus rex, considered one of the hardest biting animals ever, is often hypothesized to have exhibited cranial kinesis, or, mobility of cranial joints relative to the braincase. Cranial kinesis in T. rex is a biomechanical paradox in that forcefully biting tetrapods usually possess rigid skulls instead of skulls with movable joints. We tested the biomechanical performance of a tyrannosaur skull using a series of static positions mimicking possible excursions of the palate to evaluate Postural Kinetic Competency in Tyrannosaurus. A functional extant phylogenetic bracket was employed using taxa, which exhibit measurable palatal excursions: Psittacus erithacus (fore-aft movement) and Gekko gecko (mediolateral movement). Static finite element models of Psittacus, Gekko, and Tyrannosaurus were constructed and tested with different palatal postures using anatomically informed material properties, loaded with muscle forces derived from dissection, phylogenetic bracketing, and a sensitivity analysis of muscle architecture and tested in orthal biting simulations using element strain as a proxy for model performance. Extant species models showed lower strains in naturally occurring postures compared to alternatives. We found that fore-aft and neutral models of Tyrannosaurus experienced lower overall strains than mediolaterally shifted models. Protractor muscles dampened palatal strains, while occipital constraints increased strains about palatocranial joints compared to jaw joint constraints. These loading behaviors suggest that even small excursions can strain elements beyond structural failure. Thus, these postural tests of kinesis, along with the robusticity of other cranial features, suggest that the skull of Tyrannosaurus was functionally akinetic. Anat Rec, 303:999-1017, 2020. © 2019 Wiley Periodicals, Inc.