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.
Click to increase image sizeClick to decrease image size ACKNOWLEDGMENTS We thank the Iziko South African Museum for allowing histological analysis and CT scanning of Lystrosaurus specimens. Clint Davies Taylor (Simulia, U.K.) and Adrian Gill are also thanked for assisting with Abaqus and engineering principles. Thanks go to B. D. Reddy (Cerecam, UCT) and the two anonymous reviewers for constructive comments on an earlier version of the manuscript. The following grants and scholarships supported this research: Bob Savage Memorial grant, Mid-American Paleontological Society grant, National Science and Engineering Research Council (PGS-D) scholarship, Sigma Xi Grant-in-Aid of Research, Sir James Laughed Award of Distinction, Stephen J. Gould Grant-in-Aid of Research, and University of Bristol Transferable Skills Training fund. E.J.R. and A.C. wish to acknowledge the University of Bristol, Institute of Advanced Studies, Benjamin Meaker Fast Track Award.
The origin and function of a biomineralised skeleton in many of the non-motile groups of plankton remains an open question. Morphological diversity within these groups has often been explained by its relevance to hydrodynamic behaviour, principally buoyancy and settling. Consequently, ecological and evolutionary patterns of morphology have been associated with changes in surface water properties, but these hypotheses have rarely been critically assessed. Computational Fluid Dynamics simulations present a way to quantify the relative effect of size (maximum diameter), shape of the test and density (ratio between calcite and cavity volumes) of the specimen on settling velocity, as all variables can be manipulated independently. Here we interrogate the morphological diversity in planktic foraminifera as model organisms to explore the range of evolutionary options open to plankton to modulate settling velocity under varying environmental conditions. The evolutionary changes in morphology required to accommodate physical changes in the upper water column due to environmental changes, such as increased temperature, are small compared to the ecophenotypic variability of the population. In the modern ocean, the pattern of species distribution with depth is not likely to be determined by hydrodynamics as it is inconsistent with predictions based on settling velocity. These results suggest that intrinsic constraints on size, shape and calcification, such as heritage, exposure of the symbionts to light or oxygen diffusion into the cell, are likely to be more important than hydrodynamic function in determining the depth distribution and test morphology of planktic foraminifera.
The extinct sthenurine (giant, short-faced) kangaroos have been proposed to have a different type of locomotor behavior to extant (macropodine) kangaroos, based both on physical limitations (the size of many exceeds the proposed limit for hopping) and anatomical features (features of the hind limb anatomy suggestive of weight-bearing on one leg at a time). Here, we use micro computerised tomography (micro-CT) scans of the pedal bones of six kangaroos, three sthenurine, and three macropodine, ranging from ~50 to 150 kg, to investigate possible differences in bone resistances to bending and cortical bone distribution that might relate to differences in locomotion. Using second moment of area analysis, we show differences in resistance to bending between the two subfamilies. Distribution of cortical bone shows that sthenurines had less resistant calcaneal tubers, implying a different foot posture during locomotion, and the long foot bones were more resistant to the medial bending stresses. These differences were the most pronounced between Pleistocene monodactyl sthenurines (Sthenurus stirlingi and Procoptodon browneorum) and the two species of Macropus (the extant M. giganteus and the extinct M. cf. M. titan) and support the hypothesis that these derived sthenurines employed bipedal striding. The Miocene sthenurine Hadronomas retains some more macropodine-like features of bone resistance to bending, perhaps reflecting its retention of the fifth pedal digit. The Pleistocene macropodine Protemnodon has a number of unique features, possibly indicative of a type of locomotion unlike the other kangaroos.
Dinosaurs evolved a remarkable diversity of dietary adaptations throughout the Mesozoic, but the origins of different feeding modes are uncertain, especially the multiple origins of herbivory. Feeding habits of early dinosaurs have mostly been inferred from qualitative comparisons of dental morphology with extant analogs. Here, we use biomechanical and morphometric methods to investigate the dental morphofunctional diversity of early dinosaurs in comparison with extant squamates and crocodylians and predict their diets using machine learning classification models. Early saurischians/theropods are consistently classified as carnivores. Sauropodomorphs underwent a dietary shift from faunivory to herbivory, experimenting with diverse diets during the Triassic and Early Jurassic, and early ornithischians were likely omnivores. Obligate herbivory was a late evolutionary innovation in both clades. Carnivory is the most plausible ancestral diet of dinosaurs, but omnivory is equally likely under certain phylogenetic scenarios. This early dietary diversity was fundamental in the rise of dinosaurs to ecological dominance.
Abstract Previous studies of the morphology of the humerus in kangaroos showed that the shape of the proximal humerus could distinguish between arboreal and terrestrial taxa among living mammals, and that the extinct “giant” kangaroos (members of the extinct subfamily Sthenurinae and the extinct macropodine genus Protemnodon ) had divergent humeral anatomies from extant kangaroos. Here, we use 2D geometric morphometrics to capture the shape of the distal humerus in a range of extant and extinct marsupials and obtain similar results: sthenurines have humeral morphologies more similar to arboreal mammals, while large Protemnodon species ( P. brehus and P. anak ) have humeral morphologies more similar to terrestrial quadrupedal mammals. Our results provide further evidence for prior hypotheses: that sthenurines did not employ a locomotor mode that involved loading the forelimbs (likely employing bipedal striding as an alternative to quadrupedal or pentapedal locomotion at slow gaits), and that large Protemnodon species were more reliant on quadrupedal locomotion than their extant relatives. This greater diversity of locomotor modes among large Pleistocene kangaroos echoes studies that show a greater diversity in other aspects of ecology, such as diet and habitat occupancy.
The early tetrapod Crassigyrinus scoticus was a large aquatic predator known from the lower- to mid-Carboniferous (upper Tournasian to upper Visean/lower Serpukovian, approximately 350–330 Ma) of Scotland and Canada. Crassigyrinus is enigmatic in terms of its phylogenetic position due to its unusual morphology, which features a mixture of primitive and derived characters. Previous reconstructions, based on five incomplete and deformed specimens, have suggested a dorsoventrally tall skull with a short and broad snout, large orbits and external nares, and an extended postorbital region. In this study, we scanned four specimens using computed tomography and segmented imaging data to separate bone from matrix and individual bones from each other. Based on these data, we present a revised description of the upper and lower jaws, including sutural morphology and abundant new anatomical information. Damage was repaired and the skull retrodeformed to create a hypothetical three-dimensional reconstruction of the skull of Crassigyrinus that is dorsoventrally flatter than earlier reconstructions, yet still morphologically unique amongst early tetrapods. Overall skull shape, the size and distribution of the teeth, sutural morphology, and the specialized anatomy of the jaw joint and mandibular symphysis all suggest that Crassigyrinus was a powerful aquatic predator capable of hunting and subduing large prey.
Extant crocodilian jaws are subject to functional demands induced by feeding and hydrodynamics. However, the morphological and ecological diversity of extinct crocodile-line archosaurs is far greater than that of living crocodilians, featuring repeated convergence towards disparate ecologies including armoured herbivores, terrestrial macropredators and fully marine forms. Crocodile-line archosaurs, therefore, present a fascinating case study for morphological and functional divergence and convergence within a clade across a wide range of ecological scenarios. Here, we build performance landscapes of two-dimensional theoretical jaw shapes to investigate the influence of strength, speed and hydrodynamics in the morphological evolution of crocodile-line archosaur jaws, and test whether ecologically convergent lineages evolved similarly optimal jaw function. Most of the 243 sampled jaw morphologies occupy optimized regions of theoretical morphospace for either rotational efficiency, resistance to Von Mises stress, hydrodynamic efficiency or a trade-off between multiple functions, though some seemingly viable shapes remain unrealized. Jaw speed is optimized only in a narrow region of morphospace whereas many shapes possess optimal jaw strength, which may act as a minimum boundary rather than a strong driver for most taxa. This study highlights the usefulness of theoretical morphology in assessing functional optimality, and for investigating form–function relationships in diverse clades.
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