Abstract
BackgroundCrocodilians exhibit a spectrum of rostral shape from long snouted (longirostrine), through to short snouted (brevirostrine) morphologies. The proportional length of the mandibular symphysis correlates consistently with rostral shape, forming as much as 50% of the mandible’s length in longirostrine forms, but 10% in brevirostrine crocodilians. Here we analyse the structural consequences of an elongate mandibular symphysis in relation to feeding behaviours.Methods/Principal FindingsSimple beam and high resolution Finite Element (FE) models of seven species of crocodile were analysed under loads simulating biting, shaking and twisting. Using beam theory, we statistically compared multiple hypotheses of which morphological variables should control the biomechanical response. Brevi- and mesorostrine morphologies were found to consistently outperform longirostrine types when subject to equivalent biting, shaking and twisting loads. The best predictors of performance for biting and twisting loads in FE models were overall length and symphyseal length respectively; for shaking loads symphyseal length and a multivariate measurement of shape (PC1– which is strongly but not exclusively correlated with symphyseal length) were equally good predictors. Linear measurements were better predictors than multivariate measurements of shape in biting and twisting loads. For both biting and shaking loads but not for twisting, simple beam models agree with best performance predictors in FE models.Conclusions/SignificanceCombining beam and FE modelling allows a priori hypotheses about the importance of morphological traits on biomechanics to be statistically tested. Short mandibular symphyses perform well under loads used for feeding upon large prey, but elongate symphyses incur high strains under equivalent loads, underlining the structural constraints to prey size in the longirostrine morphotype. The biomechanics of the crocodilian mandible are largely consistent with beam theory and can be predicted from simple morphological measurements, suggesting that crocodilians are a useful model for investigating the palaeobiomechanics of other aquatic tetrapods.
Highlights
Large aquatic predators operate in a physical environment that has driven remarkable morphological convergence, notably the independent evolution of a tunniform body form in ichthyosaurs, lamnids, thunnids and odontocetes [1,2,3,4,5]
Image segmentation was largely straightforward, with the exception of the Crocodylus intermedius scan; this specimen had wire embedded in several positions within the mandible, resulting in refraction artefacts in the CT data; the affected slices were manually processed in a bitmap editor (Paintshop Pro v8, JASC) to improve image quality and reduce the influence of the artefacts (Figure 6)
Even though the present study focuses on mandibular biomechanics, crania were included within the model to provide accurate boundary conditions
Summary
Large aquatic predators operate in a physical environment that has driven remarkable morphological convergence, notably the independent evolution of a tunniform body form in ichthyosaurs (reptiles), lamnids (sharks), thunnids (bony fish) and odontocetes (mammals) [1,2,3,4,5]. The Gharial (Gavialis gangeticus) is the longest snouted form and is described as a specialist fish eater [7,9], whilst the saltwater (Crocodylus porosus) and Nile (C. niloticus) crocodiles have shorter, more robust snouts and are capable of taking terrestrial prey much larger than themselves [10]. This relationship between head shape and diet has been considered reliable enough to serve as a basis to infer diet in fossil species of marine reptiles and mammals [2,5,11]. We analyse the structural consequences of an elongate mandibular symphysis in relation to feeding behaviours
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