The biomechanical role of the chondrocranium and sutures in a lizard cranium.

Marc E H Jones,Michael J Fagan,Susan E Evans,Flora Gröning,Hugo Dutel,Alana Sharp

doi:10.1098/rsif.2017.0637

Abstract

The role of soft tissues in skull biomechanics remains poorly understood. Not least, the chondrocranium, the portion of the braincase which persists as cartilage with varying degrees of mineralization. It also remains commonplace to overlook the biomechanical role of sutures despite evidence that they alter strain distribution. Here, we examine the role of both the sutures and the chondrocranium in the South American tegu lizard Salvator merianae. We use multi-body dynamics analysis (MDA) to provide realistic loading conditions for anterior and posterior unilateral biting and a detailed finite element model to examine strain magnitude and distribution. We find that strains within the chondrocranium are greatest during anterior biting and are primarily tensile; also that strain within the cranium is not greatly reduced by the presence of the chondrocranium unless it is given the same material properties as bone. This result contradicts previous suggestions that the anterior portion (the nasal septum) acts as a supporting structure. Inclusion of sutures to the cranium model not only increases overall strain magnitudes but also leads to a more complex distribution of tension and compression rather than that of a beam under sagittal bending.

Highlights

Lizards and tuatara are ideal taxa for investigating the evolution of skull mechanics as they exhibit a wide range of skull shapes, muscle arrangements, feeding behaviours and life styles (e.g. [1,2,3])
The chondrocranium is derived from six components: the parachordals; the occipital and preoccipital arches; the otic capsules; the orbital cartilages
We investigate the role of both the chondrocranium and the sutures in the South American tegu lizard, Salvator merianae using load cases generated by a biomechanical model with wrapped muscles and minimal constraints in addition to a finite element model comprising both bone and soft tissue

Summary

Introduction

Lizards and tuatara are ideal taxa for investigating the evolution of skull mechanics as they exhibit a wide range of skull shapes, muscle arrangements, feeding behaviours and life styles (e.g. [1,2,3]). The role of soft-tissue structures such as the cartilaginous chondrocranium and fibrocellular cranial sutures, which are known to vary dramatically in structure among lepidosaurs The chondrocranium is the portion of the braincase represented by cartilage [2,9,10,11]. In the majority of vertebrate taxa, the chondrocranium is largely replaced by bone during skull development but in tetrapods this replacement occurs relatively late [12]. The chondrocranium is derived from six components: the parachordals (which provide the posterior base of the braincase); the occipital and preoccipital arches (which support the posterior part of the brain); the otic capsules (which house the inner ear); the orbital cartilages

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