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

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.

Evolution of Cranial Shape in Caecilians (Amphibia: Gymnophiona)

“…In several examples, this is taken to the extreme, where different lineages have traversed major areas of morphospace to occupy space otherwise occupied by relatively distantly related species, illustrating strikingly convergent evolution of shape in often ecologically similar taxa (Figueirido et al 2010;Price et al 2011;Sanger et al 2012). A number of studies of other taxa (Nicola et al 2003;Sidlauskas 2008;Figueirido et al 2010;Klingenberg and Gidaszewski 2010;Fortuny et al 2011;Klingenberg et al 2012;Meloro and Jones 2012) using similar methods to ours have insufficient taxonomic sampling to offer good comparison.…”

A distinctive Late Triassic microvertebrate fissure fauna and a new species of Clevosaurus (Lepidosauria: Rhynchocephalia) from Woodleaze Quarry, Gloucestershire, UK

“…This group of lepidosaurian reptiles is known today only from Sphenodon, the New Zealand tuatara, often termed a 'living fossil' because it is apparently the solitary surviving genus from a lineage that extends back to the Late or Middle Triassic (Jones et al, 2013), and has seemingly changed little morphologically. The epithet 'living fossil' is misleading, and has been disputed (Whiteside, 1986;Meloro and Jones, 2012). The Bristol fissure rhynchocephalians were first notified by Swinton (1939), who reported the type species of Clevosaurus, C. hudsoni, from Cromhall quarry, and the extensive material was described by Fraser (1988).…”

Why coelacanths are not ‘living fossils’

“…As a consequence, the idea that the coelacanth is a biological 'living fossil' is a long held but false belief which should not bias the interpretation of molecular data in extant Latimeria populations. The same reasoning could be generalised to other extant species (such as hagfish, lamprey, shark, lungfish and tatuara, to cite few examples of vertebrates) that for various reasons are often presented as 'ancient', 'primitive', or 'ancestral' even if a lot of recent data has shown that they have many derived traits [58][59][60][61][62][63][64]. We hope that this review will contribute to dispelling the myth of the coelacanth as a 'living fossil' and help biologists keep in mind that actual fossils are dead.…”