Cranial sutures work collectively to distribute strain throughout the reptile skull

The skull is composed of many bones that come together at sutures. These sutures are important sites of growth, and as growth ceases some become fused while others remain patent. Their mechanical behaviour and how they interact with changing form and loadings to ensure balanced craniofacial development is still poorly understood. Early suture fusion often leads to disfiguring syndromes, thus is it imperative that we understand the function of sutures more clearly. By applying advanced engineering modelling techniques, we reveal for the first time that patent sutures generate a more widely distributed, high level of strain throughout the reptile skull. Without patent sutures, large regions of the skull are only subjected to infrequent low-level strains that could weaken the bone and result in abnormal development. Sutures are therefore not only sites of bone growth, but could also be essential for the modulation of strains necessary for normal growth and development in reptiles.

Tooth and cranial disparity in the fossil relatives of Sphenodon (Rhynchocephalia) dispute the persistent 'living fossil' label

The tuatara (Sphenodon punctatus) is the only living representative of Rhynchocephalia, a group of small vertebrates that originated about 250 million years ago. The tuatara has been referred to as a living fossil; however, the group to which it belongs included a much greater diversity of forms in the Mesozoic. We explore the morphological diversity of Rhynchocephalia and stem lepidosaur relatives (Sphenodon plus 13 fossil relatives) by employing a combination of geometric morphometrics and comparative methods. Geometric morphometrics is used to explore cranium size and shape at interspecific scale, while comparative methods are employed to test association between skull shape and size and tooth number after taking phylogeny into account. Two phylogenetic topologies have been considered to generate a phylomorphospace and quantify the phylogenetic signal in skull shape data, the ancestral state reconstruction as well as morphological disparity using disparity through time plots (DTT). Rhynchocephalia exhibit a significant phylogenetic signal in skull shape that compares well with that computed for other extinct vertebrate groups. A consistent form of allometry has little impact on skull shape evolution while the number of teeth significantly correlates with skull shape also after taking phylogeny into account. The ancestral state reconstruction demonstrates a dramatic shape difference between the skull of Sphenodon and its much larger Cretaceous relative Priosphenodon. Additionally, DTT demonstrates that skull shape disparity is higher between rather than within clades while the opposite applies to skull size and number of teeth. These results were not altered by the use of competing phylogenic hypotheses. Rhynchocephalia evolved as a morphologically diverse group with a dramatic radiation in the Late Triassic and Early Jurassic about 200 million years ago. Differences in size are not marked between species whereas changes in number of teeth are associated with co-ordinated shape changes in the skull to accommodate larger masticatory muscles. These results show that the tuatara is not the product of evolutionary stasis but that it represents the only survivor of a diverse Mesozoic radiation whose subsequent decline remains to be explained.

Structure and compositional characteristics of caniniform dental enamel in the tuatara Sphenodon punctatus (Lepidosauria: Rhynchocephalia)

OBJECTIVE

The evolution of dental tissues in relation to tooth function is poorly understood in non-mammalian vertebrates. We studied the dentition of Sphenodon punctatus, the sole remaining member of the order Rhynchocephalia in this light.

METHODS

We examined 6 anterior maxillary caniniform teeth from adult Sphenodon by scanning electron microscopy, nano-indentation and Raman spectroscopy.

RESULTS

The elastic modulus (E) for tuatara enamel was 73.17 (sd, 3.25) GPa and 19.52 +/- 0.76 Gpa for dentine. Hardness (H) values for enamel and dentine were 4.00 (sd, 0.22) and 0.63 +/- 0.02 Gpa respectively. The enamel was thin (100 gm or less), prismless and consisted of grouped parallel crystallites. Incremental lines occurred at intervals of about 0.5 to 1 rm. There were tubular structures along the enamel dentine junction running from the dentine into the inner enamel, at different angles. These were widened at their base with a smooth, possibly inorganic lining. Enamel elastic modulus and hardness were lower than those for mammals.

CONCLUSIONS

The presence of enamel tubules in the basal part of the enamel along the EDJ remains speculative, with possible functions being added enamel/dentinal adhesion or a role in mechanosensation.

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

BACKGROUND

Today Rhynchocephalia, the sister tax-on to Squamata (snakes, lizards and amphisbaenians), is only represented by the tuatara (Sphenodon) of New Zealand. However, for much of the Mesozoic, the group was speciose and globally distributed. Historically, the Rhynchocephalia were considered to be homogenous and unspecialized but new fossils and new research are overturning this view. As well as differences in body size, body proportions, habit (aquatic vs. terrestrial), and skull structure, their teeth show variation in shape, size, number, arrangement and enamel thickness. This suggests differences in diet and mode of feeding. The teeth of basal taxa tend to be relatively simple and conical, whereas those of derived taxa possess complex flanges and wear facets.

METHODS

Dimensions of the dentary tooth bases were measured in apical view for a large sample of rhynchocephalian taxa.

RESULTS

These measurements reveal three general tooth types: small ovoid teeth, large wide teeth, and large elongate teeth.

CONCLUSION

These three categories correspond to food processing as inferred from tooth wear (puncturing+crushing, grinding+shredding and tearing+cutting, respectively). A phylogenetic signal is also present as the teeth of basal taxa generally conform to the first category. The larger tooth bases of derived taxa provide stronger attachment and contribute to a stouter tooth shape more resistant to loading and torsional forces. This in turn corresponds to skull architecture because the skulls of derived taxa could accommodate larger jaw muscles with a greater leverage relative to basal taxa.