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Sphenodon skull and teeth compared to Mesozoic relatives



From: Ben Creisler
bcreisler@gmail.com

A new online paper:

C. Meloro and M. E. H. Jones (2012)
Tooth and cranial disparity in the fossil relatives of Sphenodon
(Rhynchocephalia) dispute the persistent ‘living fossil’ label.
Journal of Evolutionary Biology (advance online publication)
DOI: 10.1111/j.1420-9101.2012.02595.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2012.02595.x/abstract


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.