The Relationship between Cranial Structure, Biomechanical Performance and Ecological Diversity in Varanoid Lizards

Abstract

Skull structure is intimately associated with feeding ability in vertebrates, both in terms of specific performance measures and general ecological characteristics. This study quantitatively assessed variation in the shape of the cranium and mandible in varanoid lizards, and its relationship to structural performance (von Mises strain) and interspecific differences in feeding ecology. Geometric morphometric and linear morphometric analyses were used to evaluate morphological differences, and finite element analysis was used to quantify variation in structural performance (strain during simulated biting, shaking and pulling). This data was then integrated with ecological classes compiled from relevant scientific literature on each species in order to establish structure-function relationships. Finite element modelling results showed that variation in cranial morphology resulted in large differences in the magnitudes and locations of strain in biting, shaking and pulling load cases. Gracile species such as Varanus salvadorii displayed high strain levels during shaking, especially in the areas between the orbits. All models exhibit less strain during pull back loading compared to shake loading, even though a larger force was applied (pull =30N, shake = 20N). Relationships were identified between the morphology, performance, and ecology. Species that did not feed on hard prey clustered in the gracile region of cranial morphospace and exhibited significantly higher levels of strain during biting (P = 0.0106). Species that fed on large prey clustered in the elongate area of mandible morphospace. This relationship differs from those that have been identified in other taxonomic groups such as crocodiles and mammals. This difference may be due to a combination of the open ‘space-frame’ structure of the varanoid lizard skull, and the ‘pull back’ behaviour that some species use for processing large prey.

The biomechanical consequences of longirostry in crocodilians and odontocetes

“…As well as qualitative visual comparisons, 95% values were generated using the R code following the methods of McCurry et al (2015b) and Walmsley et al (2013b). These values represent the peak strain within the model if the top 5% of strain values are removed.…”

“…Finite element analysis is a modeling technique that can predict the response of complex structures to applied load using numerical methods. Previous studies using this technique have shown that the cranial morphology of a species often relates to its preferences in feeding (Dumont et al, 2005;McCurry et al, 2015b;McHenry et al, 2007;Moreno et al, 2008;Soons et al, 2010). Here we aim to use finite element analysis to: 1) Determine whether longirostry has similar effects on structural performance during biting, shaking and twisting in crocodilians and odontocetes.…”

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

“…The Varanus cranium is a broad, mediolateral and highly fenestrated braced frame (McCurry et al, 2015) and supports its braincase with various skeletal elements such as the epipterygoids (Metzger, 2002), muscular elements such as the protractor musculature (Holliday and Witmer, 2008;Moazen et al, 2009), and a complex network of cranial joints (Holliday and Witmer, 2008).…”

“…FEA has been used to explore hypotheses of cranial form and function (e.g. Soons et al, 2010;Santana and Dumont, 2011;McCurry et al, 2015), and the effects of soft tissue on non-mobile cranial joints (e.g. Curtis et al, 2013;Jones et al, 2017), and to create artificial morphologies (e.g.…”

Geometric Morphometrics Provides an Alternative Approach for Interpreting the Affinity of Fossil Lizard Jaws

“…This facilitates standardization between specimens of different size via Procrustes superimposition and permits visualizations that aid interpretation. Over the past decade, geometric morphometrics has become an increasingly accessible approach for morphological analyses and has been used to characterize and compare shape variation amongst two-dimensional images of reptile skulls (e.g., Stayton, 2005;Jones, 2008;Meloro and Jones, 2012;Sanger et al, 2013;Fabre et al, 2014;Openshaw et al, 2016); however, it also has been applied to three-dimensional reptile anatomy using X-ray computed tomography (CT) (Parr et al, 2012;McCurry et al, 2015). In at least one case, it has been used as an approach to associate fossils with modern taxa (Dollion et al, 2015).…”