Arizona find links tuatara to 190-million-year-old fossils

The skeletal structure of the tuatara (Sphenodon punctatus), a reptile species found only in New Zealand, may have evolved 190 million years ago and has hardly changed since, according to a new study.

Researchers have studied a nearly complete articulated skeleton, and dozens of upper and lower jaws, of a species of ancient sphenodontian reptile – whose single surviving species today are tuatara – found in Arizona, USA.

The specimens were collected during the 1982–1983 field seasons and remained undescribed for decades in the vertebrate paleontology collections of the Museum of Comparative Zoology, Harvard University.

“The discovery of the nearly complete sphenodontid fossil from North America is a WOW moment in palaeontology, the stuff cover images are made of,” says Dr Nic Rawlence, Director of the Otago Palaeogenetics Laboratory and senior lecturer in ancient DNA in the Department of Zoology, University of Otago, New Zealand.

Previously, little has been known about the tuatara’s origins because fossils have been highly fragmented. This fossil collection forms the most complete series in the sphenodontid fossil record.

The fossils, which date back to the Early Jurassic period, have a remarkably similar skeletal structure and jaw to modern tuatara. The species has been named Navajosphenodon sani.

The genus name comes from a combination of “Navajo” in honour of the native people from North America that inhabit the Colorado Plateau where the specimens were found, and “sphenodon,” in reference to the modern tuatara. Fittingly, the species epithet “sani,” means “old age” in the Navajo language.

The name tuatara comes from the Maori language and means “peaks on the back”, referring to the spiny crest along the reptiles’ backs, which are especially pronounced in males. Looks can be deceiving because although tuatara might resemble lizards, they aren’t.

The only surviving member of sphenodontian reptiles – an extremely old evolutionary lineage – tuataras are, however, the closest living relatives to squamates (lizards and snakes), which they diverged from some 250 million years ago.

Sphenodontids were globally distributed and more diverse than squamates during the first half of their evolutionary history, and fossils have been recovered from various locations in the UK, USA, Germany, Brazil, Argentina and Zimbabwe.

However, most of their fossils are highly fragmentary, especially within sphenodontines – the group that includes the tuatara – which has severely hindered the development of our understanding of its origin.

Until now.

Fossil of the fully articulated skeleton of navajosphenodon sani.
Navajosphenodon sani, preserved in the sedimentary matrix in ventral view. Credit: the study “An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny” DOI:10.1038/s42003-022-03144-y

Using high-resolution micro-CT scanning – a 3D imaging technique that uses X-rays to see inside an object, slice by slice – they were able to unambiguously place this species as one of the earliest evolving and oldest-known sphenodontids.

“By comparing the new fossil to tuatara they have shown that the body form of tuatara has been conserved for at least 190 million years… a great example of if it ain’t broke, don’t fix it,” says Rawlence. “This is remarkable and fits with what scientists know of slow rates of genome evolution in tuatara.

“There is still a lot to discover about the evolution of tuatara but this is a first step,” adds Rawlence. “New fossil discoveries will shed further light on this remarkable animal, whether they are collected in the field or have been hiding in museum collections for 30 years like Navajosphenodon sani.”

The research was published in Communications Biology.

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