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

Abstract

Cranial morphology in lepidosaurs is highly disparate and characterized by the frequent loss or reduction of bony elements. In varanids and geckos, the loss of the postorbital bar is associated with changes in skull shape, but the mechanical principles underlying this variation remain poorly understood. Here, we seek to determine how the overall cranial architecture and the presence of the postorbital bar relate to the loading and deformation of the cranial bones during biting in lepidosaurs. Using computer-based simulation techniques, we compare cranial biomechanics in the varanid Varanus niloticus and the teiid Salvator merianae, two large, active foragers. The overall strain magnitudes and distribution across the cranium is similar in both species, despite lower strain gradients in Varanus niloticus. In Salvator merianae, the postorbital bar is important for the resistance of the cranium to feeding loads. The postorbital ligament, which partially replaces the postorbital bar in varanids, does not affect bone strain. Our results suggest that the reduction of the postorbital bar impaired neither biting performance nor the structural resistance of the cranium to feeding loads in Varanus niloticus. Differences in bone strain between the two species might reflect demands imposed by feeding and non-feeding functions on cranial shape. Beyond variation in cranial bone strain related to species-specific morphological differences, our results reveal that similar mechanical behaviour is shared by lizards with distinct cranial shapes. Contrary to mammals, the morphology of the circumorbital region, calvaria and palate appears to be important for withstanding high feeding loads in these lizards.

Modeling tooth enamel in FEA comparisons of skulls: Comparing common simplifications with biologically realistic models

“…Several previous FEA studies have investigated bite force and feeding stress in lizards. Dutel et al. (2021) measured an in vivo bite force of 211 and 314 N and ex vivo bite force of 213 and 308 N in anterior bilateral and posterior unilateral bites of Salvator merianae (skull length = 81mm).…”

True colours or red herrings?: colour maps for finite-element analysis in palaeontological studies to enhance interpretation and accessibility

“…A recent study has used a variety of colour maps to display the results from FEAs [64]. Similarly, for palaeontological studies using other engineering tools, different colour maps have started to appear in publications.…”

A new stem-varanid lizard (Reptilia, Squamata) from the early Eocene of China

“…The South American teiid lizard Salvator merrianae is of similar size and lifestyle to V. niloticus, and the two species have a comparable bite force [69]. However, the skull of Salvator differs from that of Varanus in snout structure, and in having a complete postorbital bar and weakly developed cristae cranii on the frontal.…”

“…However, the skull of Salvator differs from that of Varanus in snout structure, and in having a complete postorbital bar and weakly developed cristae cranii on the frontal. Dutel et al [69] found that the postorbital bar of Salvator experienced very high strain magnitudes during biting, especially in bites by the posterior teeth, and that removal of the bar significantly increased strain magnitudes in other parts of the skull. The postorbital bar is therefore important in maintaining skull integrity during biting in Salvator.…”

“…Artificially modelling a postorbital bar on to Varanus decreased strain magnitudes across the skull to some degree, whereas removing the ventrally united cristae cranii (subolfactory processes) in the Varanus model significantly increased strain levels, particularly in the frontal. Extension and fusion of the cristae cranii in Varanus therefore strengthen the interorbital region of the skull, permitting loss of the postorbital bar without compromising skull rigidity [69]. The interorbital region experiences high levels of strain in Varanus during feeding movements like prey shaking and pull-back cutting that draws teeth through prey items [70].…”