Biomechanical assessment of evolutionary changes in the lepidosaurian skull

Abstract

The lepidosaurian skull has long been of interest to functional morphologists and evolutionary biologists. Patterns of bone loss and gain, particularly in relation to bars and fenestrae, have led to a variety of hypotheses concerning skull use and kinesis. Of these, one of the most enduring relates to the absence of the lower temporal bar in squamates and the acquisition of streptostyly. We performed a series of computer modeling studies on the skull of Uromastyx hardwickii, an akinetic herbivorous lizard. Multibody dynamic analysis (MDA) was conducted to predict the forces acting on the skull, and the results were transferred to a finite element analysis (FEA) to estimate the pattern of stress distribution. In the FEA, we applied the MDA result to a series of models based on the Uromastyx skull to represent different skull configurations within past and present members of the Lepidosauria. In this comparative study, we found that streptostyly can reduce the joint forces acting on the skull, but loss of the bony attachment between the quadrate and pterygoid decreases skull robusticity. Development of a lower temporal bar apparently provided additional support for an immobile quadrate that could become highly stressed during forceful biting.biomechanics ͉ Lepidosauria ͉ lower temporal bar ͉ streptostyly L epidosauria is composed of 2 subgroups: Rhynchocephalia (Sphenodon and its extinct relatives) and Squamata (lizards, snakes, and amphisbaenians). Several stem taxa (as part of the more inclusive Lepidosauromorpha) are known from fossil deposits of Permian to Jurassic age (Ϸ250 million to 164 million years ago; ref. 1). In relation to lepidosaurian evolution, paleontologists, comparative anatomists, and functional morphologists have focused particularly on changes in cranial morphology, most notably with respect to the evolution of streptostyly (anteroposterior quadrate movement) and cranial kinesis. A longstanding hypothesis held that the ancestral lepidosaur possessed a fully diapsid skull (upper and lower temporal openings) with a complete lower temporal bar (2-6). Loss of that bar supposedly ''freed'' the quadrate, resulting in streptostyly. Elements of this hypothesis persist in the literature, even though it has been demonstrated unequivocally (1, 7-9) that the ancestral lepidosauromorph, the ancestral lepidosaur, and the ancestral rhynchocephalian (7, 8) all had a fixed quadrate (strong quadratepterygoid and squamosal-quadrate joints) but no lower temporal bar (Fig. 1). Indeed, the same seems to have been true for the last common ancestor of lepidosaurs and archosaurs ( Fig. 1) (9). A complete lower temporal bar was developed de novo one or more times within Rhynchocephalia (7,8). The ancestral squamate, on the other hand, inherited a skull without a lower temporal bar but underwent a reduction of the palatoquadrate and its derivatives, as well as a reduction of the connections between those derivatives (quadrate and epipterygoid) and the rest of the skull (quadrate-pterygoid, quadrate-squamosal, and epip...

Functional Relationship between Skull Form and Feeding Mechanics in Sphenodon, and Implications for Diapsid Skull Development

“…The lower temporal bar, secondarily acquired in Sphenodon [66], [76]–[80] as well as in the common ancestor of archosaurs like crocodiles [66], [80], [81], is under compressive strain during all bites. This is consistent with previous suggestions that it provides a brace [66], [79], [82] that contributes to skull robusticity, and in large theropods such as Tyrannosaurus rex Osborn, 1905 and Allosaurus fragilis Marsh, 1877 this would be important as they would likely generate extremely large biting forces and experience heavy cranial loading [4], [83]. The corollary is that reptiles that lack a lower bar do not need a brace in this location.…”

Comparison between in vivo and theoretical bite performance: Using multi-body modelling to predict muscle and bite forces in a reptile skull

“…Curtis et al, 2010;Koolstra and Tanaka, 2009;Moazen et al, 2009;Rayfield, 2007;Strait et al, 2007). Such computer models are often based on high resolution computed-tomography (CT) scan data and are thus geometrically accurate.…”

Is solid always best? Cranial performance in solid and fenestrated caecilian skulls

“…These values have been used in previous studies that involved FEA to assess patterns in nonmammalian tetrapod skull evolution (e.g. Moazen et al, 2008;Moazen et al, 2009). Further, Dumont et al (Dumont et al, 2009) demonstrated that FEA can be a useful tool for comparisons of shapes, even in the absence of knowledge on the exact material attributes in vivo.…”