The modern tuatara (Sphenodon punctatus) found in New Zealand may look like a lizard, but it is actually the last remnant of a mysterious and ancient order of reptiles known as the Rhynchocephalians. The Rhynchocephalians peaked in the Jurassic period and then mostly vanished from the fossil record. These odd creatures with jaws that slide back and forth and a third eye on the top of their heads can live for more than a century, and they prefer a chilly climate.
The decline following the Jurassic period created a patchy fossil record making it difficult for researchers fully study tuatara and the evolution of its unusual traits. In a new study in Communications Biology postdoctoral fellow Tiago Simões and Professor Stephanie Pierce examine a new sphenodontian species from the Early Jurassic of North America and show that the modern tuatara had changed very little from their 190 million year old ancestor.
Simões and Pierce discovered the specimen in the Harvard Museum of Comparative Zoology (MCZ) where Pierce is Curator of Vertebrate Paleontology. Pierce was sorting through fossils at the MCZ and found the specimen, which had been sitting in a museum drawer for decades. The fossil was discovered in 1982 in northern Arizona in the Kayenta Formation. Pierce and Simões named the animal Navajosphenodon sani, which means "old age" in the Navajo language of the Navajo Tribe who live in the area where the fossil was found.
They used micro-CT scans to examine the fossil in three dimensions. They then digitally pieced together the flattened skull to reveal a full structure. Although the body was lizardlike, the skull structure resembled a tuatara. N. sani had similar rows of sharp, interlocking teeth extending directly from the jawbone. It also had two holes behind the eye which is a key trait that differentiates tuatara from lizards, who have only one hole. “All of these features are quite conspicuous to modern tuatara and are unlike what’s seen in any other modern reptiles,” Simões said.
N. sani provides the first nearly complete skull of any fossil sphenodontine anywhere in the world. It is also among the oldest known sphenodontines along with Cynosphenodon from the Early-Middle Jurassic of Mexico, making its cranial data key to recognizing skull features of possible spenodontines fossils. Simões and Pierce place N. sani near the base of the tuatara lineage as the earliest diverging and oldest eilenodontine, thus extending the origin of the clade into Late Triassic.
Simões and Pierce's study shows that the bodies of modern tuatara appeared in the Jurassic era and have changed little in 190 million years, supporting these reptiles as "living fissils." The researchers suggest the lack of change may represent natural selection on overdrive. “Slow rates of evolution don’t necessarily mean absence of evolution,” Simões said.
“Why the modern tuatara and their lineages have evolved so slowly for such a long period of time is a bigger question and a bit harder to get at,” Pierce said. “We need more fossils.”