Skull evolution and functional morphology in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria).

The tuatara, Sphenodon, is the sole extant representative of the Rhynchoccphalia, a group of diapsid reptiles that were extremely widespread during the Mesozoic. Traditionally, Sphenodon was considered to be "primitive", and its fossil relatives are frequently disregarded as conservative. However, a detailed review shows that the group was diverse in terms of both morphology and lifestyle. In particular, it demonstrates a range of different tooth morphologies and arrangements. Geometric morphomctric analysis shows that differences between the skull shape of different taxa is related to feeding (e.g. muscle volume, jaw joint position). Derived taxa possess stouter teeth, an increase in space for adductor musculature, a larger skull size and in turn a greater potential bite force. A survey of suture morphology reveals that by comparison to basal taxa (Diphydontosaurus, Gephyrosaurus), derived taxa (e.g. Clevosaurus, Sphenodon) have more complicated sutures including extensive overlaps. These observations correspond with research indicating that sutures are important for controlling and reducing stresses within the skull. Variation in sutures is also found between different derived taxa. For example in Clevosaurus the most complex sutures are found in the palate by contrast in Sphenodon. the most complex sutures surround the postfrontal bones. These differences are probably related to the extent and distribution of forces experienced by the skull. A contributing factor is the different mode of shearing mechanism employed by each taxon: a precise orthal scissor-like cut in Clevosaurus and a prooral rip in Sphenodon, each of which required a specific muscle arrangement. The Rhynchocephalia as a whole demonstrate a progressive evolutionary trend in their diet toward larger and harder food items this allowed at least one clade to become herbivorous. This to some extent echoes Sphenodon ontogeny. The rhynchocephalian skull is highly integrated suture complexity increased in parallel with increasing complexity of feeding apparatus. Key Words: skull design, functional morphology, bite force, jaws, teeth, palaeoecology

Identification of a new snake fossil from the Canary Islands using micro-CT techniques

Abstract and keywords in English and SpanishThere are no native snakes on the Canary Islands today. The recovery of a boid vertebra from Miocene deposits on Fuertaventura suggested snakes could have been present in the past, but this single small vertebra could have reached the island from the nearby African continent in the gut of a bird. Now, however, the articulated remains of a snake have been found in a volcanic cave on Fuerteventura. The specimen is covered by a calcitic matrix and is of uncertain age. Given the fragility of the remains and the difficulty of removing the matrix, we used micro-Ct scans to make three-dimensional digital models for study. These reveal that the bones belong to a 'colubrid' snake. = En el Mioceno de las Islas Canarias se ha citado la presencia de una vértebra de boido, que por su pequeño temaño pudo haber llegado a las islas desde el cercano continente africano en el tracto digestivo de un ave. Sin embargo, en un tubo volcánico de Fuerteventura se han encontrado restos de vértebras y costillas articuladas, cubiertas por una capa de calcita y de edad incierta, que pertenecen a una seriente de la familia 'Colubridae'. Para su estudio, dadas la fragilidad de los restos y la dificultad para eliminar la calcita, se utilizó un escáner micro CT para obtener modelos digitales tridimensionales.Evans, S.E., Martín-González, E., Jones, M.E.H., Sánchez-Pinto, L. & García-Talavera, F

Dentary tooth shape in sphenodon and its fossil relatives (Diapsida: Lepidosauria: Rhynchocephalia)

Volume 13 title: Comparative Dental Morphology: 14th International Symposium on Dental Morphology, Greifswald, August 2008: Selected papers. ISBN 9783805592291Abstract not availableM.E.H. Jone

A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)

We describe a new, small-bodied rhynchocephalian reptile, Opisthiamimus gregori gen. et sp. nov., from the Upper Jurassic Morrison Formation of Wyoming, USA. Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the holotype of O. gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America. We used micro-computed tomography to examine its skeletal anatomy in detail and to develop a three-dimensional reconstruction of the skull. The skull of O. gregori is similar to that of several non-neosphenodontian rhynchocephalians such as Planocephalosaurus (e.g. large orbits) and Clevosaurus (e.g. parietal parasagittal crests) yet exhibits a suite of other features related to the proal shearing mechanism that becomes increasingly elaborated among more phylogenetically nested taxa such as Sphenodon (e.g. lateral palatine tooth row parallels maxillary tooth row along its entire length, pyramidal dentary teeth with mesial shearing crests). The postcranial skeleton of O. gregori exhibits characteristics typical of a terrestrial rhynchocephalian. Our phylogenetic analyses use a substantially updated data set of 118 characters and 46 taxa, and both maximum parsimony and Bayesian frameworks. Results place O. gregori inside Eusphenodontia but outside Neosphenodontia, and therefore in a key position for contributing to character polarity for more deeply nested clades such as Clevosauridae, Sphenodontidae and Pleurosauridae. We also erect Leptorhynchia taxon nov., composed primarily of aquatically adapted taxa (e.g. Pleurosaurus, Sapheosaurus), which is supported by both cranial and postcranial characters. Because O. gregori is not particularly closely related to the other named Morrison rhynchocephalians (e.g. Opisthias rarus), it increases both the alpha and beta taxonomic diversities within the formation. Similarly, major differences in body size and inferred diet of the Morrison taxa imply considerable concomitant palaeoecological diversity just prior to a major global decline in rhynchocephalian diversity around the close of the Jurassic.David G. DeMar Jr., Marc E. H. Jones, Matthew T. Carran

Redescription of the skull of the Australian flatback sea turtle, Natator depressus, provides new morphological evidence for phylogenetic relationships among sea turtles (Chelonioidea)

Abstract: Chelonioidea (sea turtles) are a group where available morphological evidence for crown-group relationships are incongruent with those established using molecular data. However, morphological surveys of crown-group taxa tend to focus on a recurring subset of the extant species. The Australian flatback sea turtle, Natator depressus, is often excluded from comparisons and it is the most poorly known of the seven extant species of Chelonioidea. Previous descriptions of its skull morphology are limited and conflict. Here we describe three skulls of adult N. depressus and re-examine the phylogenetic relationships according to morphological character data. Using X-ray micro Computed Tomography we describe internal structures of the braincase and identify new phylogenetically informative characters not previously reported. Phylogenetic analysis using a Bayesian approach strongly supports a sister-group relationship between Chelonia mydas and N. depressus, a topology that was not supported by previous analyses of morphological data but one that matches the topology supported by analysis of molecular data. Our results highlight the general need to sample the morphological anatomy of crown-group taxa more thoroughly before concluding that morphological and molecular evidence are incongruous.Ray M Chatterji, Mark N Hutchinson, Marc E H Jone

Changes in ontogenetic patterns facilitate diversification in skull shape of Australian agamid lizards

Background: Morphological diversity among closely related animals can be the result of differing growth patterns. The Australian radiation of agamid lizards (Amphibolurinae) exhibits great ecological and morphological diversity, which they have achieved on a continent-wide scale, in a relatively short period of time (30 million years). Amphibolurines therefore make an ideal study group for examining ontogenetic allometry. We used two-dimensional landmark-based geometric morphometric methods to characterise the postnatal growth patterns in cranial shape of 18 species of amphibolurine lizards and investigate the associations between cranial morphology, and life habit and phylogeny. Results: For most amphibolurine species, juveniles share a similar cranial phenotype, but by adulthood crania are more disparate in shape and occupy different sub-spaces of the total shape space. To achieve this disparity, crania do not follow a common post-natal growth pattern; there are differences among species in both the direction and magnitude of change in morphospace. We found that these growth patterns among the amphibolurines are significantly associated with ecological life habits. The clade Ctenophorus includes species that undergo small magnitudes of shape change during growth. They have dorsoventrally deep, blunt-snouted skulls (associated with terrestrial lifestyles), and also dorsoventrally shallow skulls (associated with saxicolous lifestyles). The sister clade to Ctenophorus, which includes the bearded dragon (Pogona), frill-neck lizard (Chlamydosaurus), and long-nosed dragon (Gowidon), exhibit broad and robust post-orbital regions and differing snout lengths (mainly associated with scansorial lifestyles). Conclusions: Australian agamids show great variability in the timing of development and divergence of growth trajectories which results in a diversity of adult cranial shapes. Phylogenetic signal in cranial morphology appears to be largely overwritten by signals that reflect life habit. This knowledge about growth patterns and skull shape diversity in agamid lizards will be valuable for placing phylogenetic, functional and ecological studies in a morphological context.Jaimi A. Gray, Emma Sherratt, Mark N. Hutchinson and Marc E. H. Jone

Comparative skull biomechanics in Varanus and Salvator 'Tupinambis'

The lizard species Salvator ‘Tupinambis’ merianae and Varanus ornatus evolved independently in South America and Africa but share similar ecology and feeding behaviour, despite having notable differences in their skull structure. Tupinambis has a compact, relatively short and wide snout, whereas that of Varanus is more slender and narrow. In addition, a postorbital bar (POB) is present in Tupinambis but absent in Varanus, and the former lacks the mid-frontal suture that is present in the latter. Here, we explore the biomechanical significance of these differences using 3D computer-based mechanical simulations based on micro-computed tomography, detailed muscle dissections, and in vivo data. First, we simulated muscle activity and joint-reaction forces during biting using Multibody Dynamics Analysis. Then, the forces calculated from these models were used as an input for Finite Element Analysis, to investigate and compare the strains of the skull in these two species. The effects of the presence/absence of structures, such as the POB, were investigated by constructing artificial models which geometry was altered. Our results indicate that strains in the skull bones are lower in Tupinambis than in Varanus, in particular at the back of the skull. The presence of a POB clearly reduces the strains in the bones during posterior biting in Tupinambis, but not in Varanus. Our results hence highlight how the morphological differences between these two taxa affect the mechanical behaviour of their respective skulls during feeding.Hugo Dutel, Susan E. Evans, Michael J. Fagan, Flora Gröning, Marc E. H. Jones, Alana Shar

A sphenodontine (Rhynchocephalia) from the Miocene of New Zealand and palaeobiogeography of the tuatara (Sphenodon)

Jaws and dentition closely resembling those of the extant tuatara (Sphenodon) are described from the Manuherikia Group (Early Miocene; 19–16 million years ago, Mya) of Central Otago, New Zealand. This material is significant in bridging a gap of nearly 70 million years in the rhynchocephalian fossil record between the Late Pleistocene of New Zealand and the Late Cretaceous of Argentina. It provides the first pre-Pleistocene record of Rhynchocephalia in New Zealand, a finding consistent with the view that the ancestors of Sphenodon have been on the landmass since it separated from the rest of Gondwana 82–60 Mya. However, if New Zealand was completely submerged near the Oligo-Miocene boundary (25–22 Mya), as recently suggested, an ancestral sphenodontine would need to have colonized the re-emergent landmass via ocean rafting from a currently unrecorded and now extinct Miocene population. Although an Early Miocene record does not preclude that possibility, it substantially reduces the temporal window of opportunity. Irrespective of pre-Miocene biogeographic history, this material also provides the first direct evidence that the ancestors of the tuatara, an animal often perceived as unsophisticated, survived in New Zealand despite substantial local climatic and environmental changes

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

In biomechanical investigations, geometrically accurate computer models of anatomical structures can be created readily using computed-tomography scan images. However, representation of soft tissue structures is more challenging, relying on approximations to predict the muscle loading conditions that are essential in detailed functional analyses. Here, using a sophisticated multi-body computer model of a reptile skull (the rhynchocephalian Sphenodon), we assess the accuracy of muscle force predictions by comparing predicted bite forces against in vivo data. The model predicts a bite force almost three times lower than that measured experimentally. Peak muscle force estimates are highly sensitive to fibre length, muscle stress, and pennation where the angle is large, and variation in these parameters can generate substantial differences in predicted bite forces. A review of theoretical bite predictions amongst lizards reveals that bite forces are consistently underestimated, possibly because of high levels of muscle pennation in these animals. To generate realistic bites during theoretical analyses in Sphenodon, lizards, and related groups we suggest that standard muscle force calculations should be multiplied by a factor of up to three. We show that bite forces increase and joint forces decrease as the bite point shifts posteriorly within the jaw, with the most posterior bite location generating a bite force almost double that of the most anterior bite. Unilateral and bilateral bites produced similar total bite forces; however, the pressure exerted by the teeth is double during unilateral biting as the tooth contact area is reduced by half. © 2010