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

Abstract

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.

Comparative cranial biomechanics in two lizard species: impact of variation in cranial design

“…Therefore, bone shape and bone mass distribution in the cranium do not necessarily reflect an adaptation to feeding loads (Ross, 2001). In lepidosaurs, biomechanical simulations have demonstrated the importance of certain components of the skull, such as the lower temporal bar and the quadrate-pterygoid joint, in the structural resistance of the whole system (Moazen et al, 2009a,b;Wilken et al, 2019). By contrast, other features of the lepidosaur cranium appear to have no effect on its structural resistance to external loads.…”

“…Computer-based biomechanical simulation techniques offer the opportunity to test in silico hypotheses of the function of biological structures. These approaches can further be used to create artificial morphologies (Gröning et al, 2013a;Lautenschlager et al, 2013;Moazen et al, 2009b;Nakashige et al, 2011;Sharp and Rich, 2016) and change the material properties of the tissues (Jones et al, 2017;Moazen et al, 2009a;Reed et al, 2011;Wilken et al, 2019) to assess the effect of a given structure in different scenarios. In the present study, we investigated cranial mechanics during feeding in two lizard species, the Argentine black and white tegu (Salvator merianae Duméril and Bibron 1839) and the African Nile monitor (Varanus niloticus Fitzinger 1826), by combining in vivo measurements with two in silico modelling techniques: multibody dynamic analysis (MDA) and finite element analysis (FEA).…”

Cranial Structure of Varanus komodoensis as Revealed by Computed-Tomographic Imaging

“…The cranium of the genus Varanus is a complex structure that has received some attention in morphofunctional studies [12,17], perhaps due to the enormous disparity in the form that evolved among varanid lizards [18]. The head of this iconic varanid represents a complex structure, composed of various tissues with varying degrees of attenuation in radiographic images, making it a challenging object to assess.…”

“…These bones, called cephalic osteoderms, vary in shape and complexity and serve primarily as a defensive anatomical system to protect individuals during aggressive confrontations with other specimens [11]. To avoid this low resolution, some investigations reported the use of micro-CT scanners [17], although this equipment is not usually available in veterinary clinics [15].…”

COMPARATIVE EVALUATION OF THE KOMODO DRAGON (Varanus komodoensis) AND THE GREEN IGUANA (Iguana iguana) SKULL BY THREE DIMENSIONAL COMPUTED TOMOGRAPHIC RECONSTRUCTION

“…This technique requires multiple thin section images, and the advantages of this process are that anatomical detail is improved, and can image bony structures with different degrees of rotation (6). CT reconstruction has already been used in morphofunctional studies performed in the Savannah monitor (7) and American Alligator (8). However, there are no reports concerning comparative anatomy of the skull of different species of reptiles by three-dimensional reconstructed CT to the author´s knowledge.…”