Middle Jurassic fossils document an early stage in salamander evolution.

Salamanders are an important group of living amphibians and model organisms for understanding locomotion, development, regeneration, feeding, and toxicity in tetrapods. However, their origin and early radiation remain poorly understood, with early fossil stem-salamanders so far represented by larval or incompletely known taxa. This poor record also limits understanding of the origin of Lissamphibia (i.e., frogs, salamanders, and caecilians). We report fossils from the Middle Jurassic of Scotland representing almost the entire skeleton of the enigmatic stem-salamander Marmorerpeton. We use computed tomography to visualize high-resolution three-dimensional anatomy, describing morphologies that were poorly characterized in early salamanders, including the braincase, scapulocoracoid, and lower jaw. We use these data in the context of a phylogenetic analysis intended to resolve the relationships of early and stem-salamanders, including representation of important outgroups alongside data from high-resolution imaging of extant species. Marmorerpeton is united with Karaurus, Kokartus, and others from the Middle Jurassic-Lower Cretaceous of Asia, providing evidence for an early radiation of robustly built neotenous stem-salamanders. These taxa display morphological specializations similar to the extant cryptobranchid "giant" salamanders. Our analysis also demonstrates stem-group affinities for a larger sample of Jurassic species than previously recognized, highlighting an unappreciated diversity of stem-salamanders and cautioning against the use of single species (e.g., Karaurus) as exemplars for stem-salamander anatomy. These phylogenetic findings, combined with knowledge of the near-complete skeletal anatomy of Mamorerpeton, advance our understanding of evolutionary changes on the salamander stem-lineage and provide important data on early salamanders and the origins of Batrachia and Lissamphibia.

Clade-wide variation in bite-force performance is determined primarily by size, not ecology.

Performance traits are tightly linked to the fitness of organisms. However, because studies of variation in performance traits generally focus on just one or several closely related species, we are unable to draw broader conclusions about how and why these traits vary across clades. One important performance trait related to many aspects of an animal's life history is bite-force. Here, we use a clade-wide phylogenetic comparative approach to investigate relationships between size, head dimensions and bite-force among lizards and tuatara (lepidosaurs), using the largest bite-force dataset collated to date for any taxonomic group. We test four predictions: that bite-force will be greater in larger species, and for a given body size, bite-force will be greatest in species with acrodont tooth attachment, herbivorous diets, and non-burrowing habits. We show that bite-force is strongly related to body and head size across lepidosaurs and, as predicted, larger species have the greatest bite-forces. Contrary to our other predictions, tooth attachment, diet and habit have little predictive power when accounting for size. Herbivores bite more forcefully simply because they are larger. Our results also highlight priorities for future sampling to further enhance our understanding of broader evolutionary patterns.

Reliable quantification of bite-force performance requires use of appropriate biting substrate and standardization of bite out-lever.

Bite-force performance is an ecologically important measure of whole-organism performance that shapes dietary breadth and feeding strategies and, in some taxa, determines reproductive success. It also is a metric that is crucial to testing and evaluating biomechanical models. We reviewed nearly 100 published studies of a range of taxa that incorporate direct in vivo measurements of bite force. Problematically, methods of data collection and processing vary considerably among studies. In particular, there is little consensus on the appropriate substrate to use on the biting surface of force transducers. In addition, the bite out-lever, defined as the distance from the fulcrum (i.e. jaw joint) to the position along the jawline at which the jaws engage the transducer, is rarely taken into account. We examined the effect of bite substrate and bite out-lever on bite-force estimates in a diverse sample of lizards. Results indicate that both variables have a significant impact on the accuracy of measurements. Maximum bite force is significantly greater using leather as the biting substrate compared with a metal substrate. Less-forceful bites on metal are likely due to inhibitory feedback from mechanoreceptors that prevent damage to the feeding apparatus. Standardization of bite out-lever affected which trial produced maximum performance for a given individual. Indeed, maximum bite force is usually underestimated without standardization because it is expected to be greatest at the minimum out-lever (i.e. back of the jaws), which in studies is rarely targeted with success. We assert that future studies should use a pliable substrate, such as leather, and use appropriate standardization for bite out-lever.

A juvenile pleurosaurid (Lepidosauria: Rhynchocephalia) from the Tithonian of the Mörnsheim Formation, Germany.

Late Jurassic rhynchocephalians from the Solnhofen Archipelago have been known for almost two centuries. The number of specimens and taxa is constantly increasing, but little is known about the ontogeny of these animals. The well-documented marine taxon Pleurosaurus is one of such cases. With over 15 described (and many more undescribed) specimens, there were no unambiguous juveniles so far. Some authors have argued that Acrosaurus, another common component of the Solnhofen Archipelago herpetofauna, might represent an early ontogenetic stage of Pleurosaurus, but the lack of proper descriptions for this taxon makes this assignment tentative, at best. Here, we describe the first unambiguous post-hatchling juvenile of Pleurosaurus and tentatively attribute it to Pleurosaurus cf. P. ginsburgi. The new specimen comes from the Lower Tithonian of the Mörnsheim Formation, Germany. This specimen is small, disarticulated, and incomplete, but preserves several of its craniomandibular bones and presacral vertebrae. It shares with Pleurosaurus a set of diagnostic features, such as an elongated and triangular skull, a low anterior flange in its dentition, and an elongated axial skeleton. It can be identified as a juvenile due to the presence of an unworn dentition, well-spaced posteriormost dentary teeth, a large gap between the last teeth and the coronoid process of the dentary, and poorly ossified vertebrae with unfused neural arches. Acrosaurus shares many anatomical features with both this specimen and Pleurosaurus, which could indicate that the two genera are indeed synonyms. The early ontogenetic stage inferred for the new Pleurosaurus specimen argues for an even earlier ontogenetic placement for specimens referred to Acrosaurus, the latter possibly pertaining to hatchlings.

Multiple pathways to herbivory underpinned deep divergences in ornithischian evolution.

The extent to which evolution is deterministic is a key question in biology,1,2,3,4,5,6,7,8,9 with intensive debate on how adaptation6,10,11,12,13 and constraints14,15,16 might canalize solutions to ecological challenges.4,5,6 Alternatively, unique adaptations1,9,17 and phylogenetic contingency1,3,18 may render evolution fundamentally unpredictable.3 Information from the fossil record is critical to this debate,1,2,11 but performance data for extinct taxa are limited.7 This knowledge gap is significant, as general morphology may be a poor predictor of biomechanical performance.17,19,20 High-fiber herbivory originated multiple times within ornithischian dinosaurs,21 making them an ideal clade for investigating evolutionary responses to similar ecological pressures.22 However, previous biomechanical modeling studies on ornithischian crania17,23,24,25 have not compared early-diverging taxa spanning independent acquisitions of herbivory. Here, we perform finite-element analysis on the skull of five early-diverging members of the major ornithischian clades to characterize morphofunctional pathways to herbivory. Results reveal limited functional convergence among ornithischian clades, with each instead achieving comparable performance, in terms of reconstructed patterns and magnitudes of functionally induced stress, through different adaptations of the feeding apparatus. Thyreophorans compensated for plesiomorphic low performance through increased absolute size, heterodontosaurids expanded jaw adductor muscle volume, ornithopods increased jaw system efficiency, and ceratopsians combined these approaches. These distinct solutions to the challenges of herbivory within Ornithischia underpinned the success of this diverse clade. Furthermore, the resolution of multiple solutions to equivalent problems within a single clade through macroevolutionary time demonstrates that phenotypic evolution is not necessarily predictable, instead arising from the interplay of adaptation, innovation, contingency, and constraints.1,2,3,7,8,9,18.

Reproductive phenotype predicts adult bite-force performance in sex-reversed dragons (Pogona vitticeps).

Sex-related differences in morphology and behavior are well documented, but the relative contributions of genes and environment to these traits are less well understood. Species that undergo sex reversal, such as the central bearded dragon (Pogona vitticeps), offer an opportunity to better understand sexually dimorphic traits because sexual phenotypes can exist on different chromosomal backgrounds. Reproductively female dragons with a discordant sex chromosome complement (sex reversed), at least as juveniles, exhibit traits in common with males (e.g., longer tails and greater boldness). However, the impact of sex reversal on sexually dimorphic traits in adult dragons is unknown. Here, we investigate the effect of sex reversal on bite-force performance, which may be important in resource acquisition (e.g., mates and/or food). We measured body size, head size, and bite force of the three sexual phenotypes in a colony of captive animals. Among adults, we found that males (ZZm) bite more forcefully than either chromosomally concordant females (ZWf) or sex-reversed females (ZZf), and this difference is associated with having relatively larger head dimensions. Therefore, adult sex-reversed females, despite apparently exhibiting male traits as juveniles, do not develop the larger head and enhanced bite force of adult male bearded dragons. This pattern is further illustrated in the full sample by a lack of positive allometry of bite force in sex-reversed females that is observed in males. The results reveal a close association between reproductive phenotype and bite force performance, regardless of sex chromosome complement.

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.

Neutron scanning reveals unexpected complexity in the enamel thickness of an herbivorous Jurassic reptile.

Eilenodontines are one of the oldest radiation of herbivorous lepidosaurs (snakes, lizards and tuatara) characterized by batteries of wide teeth with thick enamel that bear mammal-like wear facets. Unlike most reptiles, eilenodontines have limited tooth replacement, making dental longevity particularly important to them. We use both X-ray and neutron computed tomography to examine a fossil tooth from the eilenodontine Eilenodon (Late Jurassic, USA). Of the two approaches, neutron tomography was more successful and facilitated measurements of enamel thickness and distribution. We find the enamel thickness to be regionally variable, thin near the cusp tip (0.10 mm) but thicker around the base (0.15-0.30 mm) and notably greater than that of other rhynchocephalians such as the extant Sphenodon (0.08-0.14 mm). The thick enamel in Eilenodon would permit greater loading, extend tooth lifespan and facilitate the establishment of wear facets that have sharp edges for orally processing plant material such as horsetails (Equisetum). The shape of the enamel dentine junction indicates that tooth development in Eilenodon and Sphenodon involved similar folding of the epithelium but different ameloblast activity.

The biomechanical role of the chondrocranium and sutures in a lizard cranium.

The role of soft tissues in skull biomechanics remains poorly understood. Not least, the chondrocranium, the portion of the braincase which persists as cartilage with varying degrees of mineralization. It also remains commonplace to overlook the biomechanical role of sutures despite evidence that they alter strain distribution. Here, we examine the role of both the sutures and the chondrocranium in the South American tegu lizard Salvator merianae We use multi-body dynamics analysis (MDA) to provide realistic loading conditions for anterior and posterior unilateral biting and a detailed finite element model to examine strain magnitude and distribution. We find that strains within the chondrocranium are greatest during anterior biting and are primarily tensile; also that strain within the cranium is not greatly reduced by the presence of the chondrocranium unless it is given the same material properties as bone. This result contradicts previous suggestions that the anterior portion (the nasal septum) acts as a supporting structure. Inclusion of sutures to the cranium model not only increases overall strain magnitudes but also leads to a more complex distribution of tension and compression rather than that of a beam under sagittal bending.

Bite force in the horned frog (Ceratophrys cranwelli) with implications for extinct giant frogs.

Of the nearly 6,800 extant frog species, most have weak jaws that play only a minor role in prey capture. South American horned frogs (Ceratophrys) are a notable exception. Aggressive and able to consume vertebrates their own size, these "hopping heads" use a vice-like grip of their jaws to restrain and immobilize prey. Using a longitudinal experimental design, we quantified the ontogenetic profile of bite-force performance in post-metamorphic Ceratophrys cranwelli. Regression slopes indicate positive allometric scaling of bite force with reference to head and body size, results that concur with scaling patterns across a diversity of taxa, including fish and amniotes (lizards, tuatara, turtles, crocodylians, rodents). Our recovered scaling relationship suggests that exceptionally large individuals of a congener (C. aurita) and extinct giant frogs (Beelzebufo ampinga, Late Cretaceous of Madagascar) probably could bite with forces of 500 to 2200 N, comparable to medium to large-sized mammalian carnivores.

The importance of accurate muscle modelling for biomechanical analyses: a case study with a lizard skull.

Computer-based simulation techniques such as multi-body dynamics analysis are becoming increasingly popular in the field of skull mechanics. Multi-body models can be used for studying the relationships between skull architecture, muscle morphology and feeding performance. However, to be confident in the modelling results, models need to be validated against experimental data, and the effects of uncertainties or inaccuracies in the chosen model attributes need to be assessed with sensitivity analyses. Here, we compare the bite forces predicted by a multi-body model of a lizard (Tupinambis merianae) with in vivo measurements, using anatomical data collected from the same specimen. This subject-specific model predicts bite forces that are very close to the in vivo measurements and also shows a consistent increase in bite force as the bite position is moved posteriorly on the jaw. However, the model is very sensitive to changes in muscle attributes such as fibre length, intrinsic muscle strength and force orientation, with bite force predictions varying considerably when these three variables are altered. We conclude that accurate muscle measurements are crucial to building realistic multi-body models and that subject-specific data should be used whenever possible.

Shearing mechanics and the influence of a flexible symphysis during oral food processing in Sphenodon (Lepidosauria: Rhynchocephalia).

The New Zealand tuatara, Sphenodon, has a specialized feeding system in which the teeth of the lower jaw close between two upper tooth rows before sliding forward to slice food apart like a draw cut saw. This shearing action is unique amongst living amniotes but has been compared with the chewing power stroke of mammals. We investigated details of the jaw movement using multibody dynamics analysis of an anatomically accurate three-dimensional computer model constructed from computed tomography scans. The model predicts that a flexible symphysis is necessary for changes in the intermandibular angle that permits prooral movement. Models with the greatest symphysial flexibility allow the articulation surface of the articular to follow the quadrate cotyle with the least restriction, and suggest that shearing is accompanied by a long axis rotation of the lower jaws. This promotes precise point loading between the cutting edges of particular teeth, enhancing the effectiveness of the shearing action. Given that Sphenodon is a relatively inactive reptile, we suggest that the link between oral food processing and endothermy has been overstated. Food processing improves feeding efficiency, a consideration of particular importance when food availability is unpredictable. Although this feeding mechanism is today limited to Sphenodon, a survey of fossil rhynchocephalians suggests that it was once more widespread.

The head and neck anatomy of sea turtles (Cryptodira: Chelonioidea) and skull shape in Testudines.

BACKGROUND: Sea turtles (Chelonoidea) are a charismatic group of marine reptiles that occupy a range of important ecological roles. However, the diversity and evolution of their feeding anatomy remain incompletely known. METHODOLOGY/PRINCIPAL FINDINGS: Using computed tomography and classical comparative anatomy we describe the cranial anatomy in two sea turtles, the loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii), for a better understanding of sea turtle functional anatomy and morphological variation. In both taxa the temporal region of the skull is enclosed by bone and the jaw joint structure and muscle arrangement indicate that palinal jaw movement is possible. The tongue is relatively small, and the hyoid apparatus is not as conspicuous as in some freshwater aquatic turtles. We find several similarities between the muscles of C. caretta and L. kempii, but comparison with other turtles suggests only one of these characters may be derived: connection of the m. adductor mandibulae internus into the Pars intramandibularis via the Zwischensehne. The large fleshy origin of the m. adductor mandibulae externus Pars superficialis from the jugal seems to be a characteristic feature of sea turtles. CONCLUSIONS/SIGNIFICANCE: In C. caretta and L. kempii the ability to suction feed does not seem to be as well developed as that found in some freshwater aquatic turtles. Instead both have skulls suited to forceful biting. This is consistent with the observation that both taxa tend to feed on relatively slow moving but sometimes armoured prey. The broad fleshy origin of the m. adductor mandibulae externus Pars superficialis may be linked to thecheek region being almost fully enclosed in bone but the relationship is complex.

Predicting muscle activation patterns from motion and anatomy: modelling the skull of Sphenodon (Diapsida: Rhynchocephalia).

The relationship between skull shape and the forces generated during feeding is currently under widespread scrutiny and increasingly involves the use of computer simulations such as finite element analysis. The computer models used to represent skulls are often based on computed tomography data and thus are structurally accurate; however, correctly representing muscular loading during food reduction remains a major problem. Here, we present a novel approach for predicting the forces and activation patterns of muscles and muscle groups based on their known anatomical orientation (line of action). The work was carried out for the lizard-like reptile Sphenodon (Rhynchocephalia) using a sophisticated computer-based model and multi-body dynamics analysis. The model suggests that specific muscle groups control specific motions, and that during certain times in the bite cycle some muscles are highly active whereas others are inactive. The predictions of muscle activity closely correspond to data previously recorded from live Sphenodon using electromyography. Apparent exceptions can be explained by variations in food resistance, food size, food position and lower jaw motions. This approach shows considerable promise in advancing detailed functional models of food acquisition and reduction, and for use in other musculoskeletal systems where no experimental determination of muscle activity is possible, such as in rare, endangered or extinct species.

Skull shape and feeding strategy in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria).

The Rhynchocephalia are a group of small diapsid reptiles that were globally distributed during the early Mesozoic. By contrast, the only extant representatives, Sphenodon punctatus and S. guntheri (Tuatara), are restricted to New Zealand off-shore islands. The Rhynchocephalia are widely considered to be morphologically uniform but research over the past 30 years has revealed unexpected phenotypic and taxonomic diversity. Phylogenetically "basal taxa" generally possess relatively simple conical or columnar teeth whereas more derived taxa possessed stouter flanged teeth and sophisticated shearing mechanisms: orthal in some (e.g., Clevosaurus hudsoni) and propalinal in others (e.g., S. punctatus). This variation in feeding apparatus suggests a wide range of feeding niches were exploited by rhynchocephalians. The relationship of skull shape to skull length, phylogenetic grouping, habit, and characters relating to the feeding apparatus are explored here with geometric morphometric analysis on two-dimensional landmarks. Principle components analysis demonstrates that there are significant differences between phylogenetic groups. In particular, Sphenodon differs significantly from all well known fossil taxa including the most phylogenetically basal forms. Therefore, it is not justifiable to use Sphenodon as a solitary outgroup when studying skull shape and feeding strategy in squamates; rhynchocephalian fossil taxa also need to be considered. There are also significant differences between the skull shapes of aquatic taxa and those of terrestrial taxa. Of the observed variation in skull shape, most variation is subsumed by variation in dentary tooth base shape, the type of jaw movement employed (e.g., orthal vs. propalinal) and the number of palatal tooth rows. By comparison, the presence or absence of flanges, dentary tooth number and palatal tooth row orientation subsume much less. Skull length was also found to be a poor descriptor of overall skull shape. Compared to basal rhynchocephalians members of more derived terrestrial radiations possess an enlarged postorbital area, a high parietal, and a jaw joint positioned ventral to the tooth row. Modification of these features is closely associated with increased biting performance and thus access to novel food items. Some of these same trends are apparent during Sphenodon ontogeny where skull growth is allometric and there is evidence for ontogenetic variation in diet.