The aim of this analysis was to establish the basic mechanical principles of simple archosaur cranial form. In particular we estimated the influence of two key archosaur innovations, the secondary palate and the antorbital fenestra, on the optimal resistance of biting-induced loads. Although such simplified models cannot substitute for more complex cranial geometries, they can act as a clearly derived benchmark that can serve as a reference point for future studies incorporating more complex geometry. We created finite element (FE) models comprising either a tall, domed (oreinirostral) snout or a broad, flat (platyrostral) archosaur snout. Peak von Mises stress was recorded in models with and without a secondary palate and/or antorbital fenestra after the application of bite loads to the tooth row. We examined bilateral bending and unilateral torsion-inducing bites for a series of bite positions along the jaw, and conducted a sensitivity analysis of material properties. Pairwise comparison between different FE morphotypes revealed that oreinirostral models are stronger than their platyrostral counterparts. Oreinirostral models are also stronger in bending than in torsion, whereas platyrostral models are equally susceptible to either load type. As expected, we found that models with a fenestra always have greatest peak stresses and by inference are “weaker,” significantly so in oreinirostral forms and anterior biting platyrostral forms. Surprisingly, although adding a palate always lowers peak stress, this is rarely by large magnitudes and is not significant in bilateral bending bites. The palate is more important in unilateral torsion-inducing biting. Two basic principles of archosaur cranial construction can be derived from these simple models: (1) forms with a fenestra are suboptimally constructed with respect to biting, and (2) the presence or absence of a palate is significant to cranial integrity in unilaterally biting animals. Extrapolating these results to archosaur cranial evolution, it appears that if mechanical optimization were the only criterion on which skull form is based, then most archosaurs could in theory strengthen their skulls to increase resistance to biting forces. These strengthened morphotypes are generally not observed in the fossil record, however, and therefore archosaurs appear subject to various non-mechanical morphological constraints. Carnivorous theropod dinosaurs, for example, may retain large suboptimal fenestra despite generating large bite forces, owing to an interplay between craniofacial ossification and pneumatization. Furthermore, living crocodylians appear to strengthen their skull with a palate and filled fenestral opening in the most efficient way possible, despite being constrained perhaps by hydrodynamic factors to the weaker platyrostral morphotype. The future challenge is to ascertain whether these simple predictions are maintained when the biomechanics of complex cranial geometries are explored in more detail.
The first ichthyosaur to be recorded from the Pliensbachian Stage of the English Lower Liassic is described as Leptonectes moorei sp. nov., extending the geological range of Leptonectes to the Pliensbachian. According to criteria for assessing the maturity of ichthyosaurs, it is concluded that L. moorei is an immature individual of a relatively small, slender and short snouted species close to the earlier long‐snouted L. tenuirostris (Conybeare) which ranges from the Rhaetian to the Sinemurian. The presence of a short‐snouted leptonectid in the Pliensbachian suggests two contrasting patterns of rostral morphology within the clade Leptonectidae in the latest Early Jurassic, rostral reduction within the genus Leptonectes and rostral elongation in Excalibosaurus Eurhinosaurus .
ABSTRACT Spinosaurid theropod dinosaurs appear to represent convergent morphological evolution toward a croc-odylian-like cranial morphology, previously linked to the possibility that spinosaurs adopted a similar, partially piscivorous, trophic niche. Further conclusions are hindered by a lack of quantitative evidence, and an incomplete understanding of the functional significance of key crocodylian cranial characters. A comparative biomechanical analysis of function in the snout of the spinosaurid theropod Baryonyx walkeri has been performed, comparing B. walkeri with a generalised large theropod dinosaur and two extant crocodylians (Alligator, Gavialis) that represent different endpoints of extant crocodylian morphological diversity. The aims of the analysis were (a) to determine which group is the closest functional analogue to B. walkeri, and (b) investigate the mechanical influence on cranial function of the antorbital fenestra and the secondary palate; morphological characters that appear to be of importance in both crocodyliform and spinosaur functional morphology. Results demonstrate that the closest structural and biomechanical analogue to B. walkeri is the extant gharial, rather than the alligator or conventional theropods. The secondary palate confers strength to the alligator skull in torsion, but provides resistance to bending in gharials and B. walkeri. Loss of the antorbital fenestra strengthens narrow or tubular theropod and gharial snouts, but has limited influence on the broader-snouted alligator morphotypes. Consequently, with their large antorbital fenestrae and lack of secondary palate, most theropod skulls were surprisingly suboptimally constructed to resist feeding-related bite loads. The mechanical impetus for archosaur palatal development and fenestral closure appears more complex than previously thought.
Abstract A continuous sequence of terrestrial sediments bracketing the Cretaceous-Tertiary (K-T) boundary is known only in the western interior of North America. Documentation of terrestrial faunal diversity, and patterns of survival and extinction, demonstrate that dinosaurs (i.e. non-avian dinosaurs) declined in the Late Maastrichtian and were the only major terrestrial vertebrate group, along with the pterosaurs, that became extinct at or near the K-T boundary. There was a varying degree of turnover but no mass extinction in other vertebrate groups. Both gradualist and catastrophe scenarios have been advocated as the main cause. There is much less evidence in other parts of the world that can be brought to bear on the timing and nature of dinosaur extinction. None the less, there are dinosaur remains from Maastrichtian horizons on every continent and none are recorded from overlying Paleocene strata. This supports the axiom that dinosaurs died out world-wide at the end of the Cretaceous although there is no means of determining whether the extinction pattern in western North America was a local or global phenomenon. The disruption of reproductive patterns in herbivorous dinosaurs by trace element contamination derived from an impact, volcanic activity or both events is one factor which might correlate with their global extinction.
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