Found back in 1982 in the United States, a 190-million-year-old tuatara (or tautara) fossil revealed the secret of one of the most mysterious reptiles on Earth. The lack of change in species over such a huge amount of time may indicate an excessive acceleration of natural selection, the researchers say.

New Zealand tuatars look like iguanas, but in fact, these spiny reptiles are not lizards, but the last representatives of the mysterious and ancient order of reptiles known as rhynchocephals, which disappeared at the end of the Jurassic period.

Tuataria have many unusual characteristics: they can live for over 100 years, live in fairly cold climates, and are able to move their jaws back and forth to gnaw through insects and seabirds (and sometimes each other). They even have a vestigial third eye under the scales on the top of their head that helps them track the sun. These bizarre features make the tuatara an evolutionary mystery, and the scattered fossil record of its prehistoric relatives has baffled paleontologists. Probably, having lost in the evolutionary race to lizards and snakes, almost all rhynchocephals died out at the end of the Mesozoic era. Many left behind only fragments of teeth and jaws.

It turns out that the key piece to this puzzle has been in museum storage for decades. While browsing the fossil collection at the Museum of Comparative Zoology at Harvard University, Stephanie Pearce, curator at the Museum of Vertebrate Paleontology, and her team discovered a near-complete skeleton of a lizard-like animal on a palm-sized stone slab. This fossil was found in 1982 in Northern Arizona.

Scientists named the discovered animal Navajosphenodon sani. Both the genus name and the species name (meaning “old age” in the Navajo language) refer to the Navajo tribe that lived in the area where the fossil was found.

Scientists used micro-CT scans to examine the fossil in three dimensions and digitally assembled the flattened skull like a jigsaw puzzle. Although its body was similar to that of a lizard, the skull structure of Navajosphenodon sani resembled that of a tuatara. It had the same rows of sharp, intertwining teeth that radiated straight from the jawbone. The skull also had two openings behind the animal’s parietal eye – this configuration is one of the key features that distinguishes tuatara from lizards, which have only one such opening. According to scientists, the extra hole helps the tuatara “stabilize the skull” when it bites and chews on prey.

“All of these features are highly visible in modern tuataras and are unlike anything seen in any other modern reptiles,” Simoes said.

The fossil showed that modern tuatars appeared in the Jurassic and have changed little in 190 million years. But Simões, at the same time, drew attention to the differences: for example, the jaws of modern tuataras end in a set of beak-shaped fused teeth, which Navajosphenodon sani does not have.

According to Kelsey Jenkins, a Yale University doctor who specializes in the early evolution of reptiles, many features of rhynchocephals have undergone only minor changes throughout their history. However, 200 million years is the limit.

“The only animals that have been so well preserved in this amount of time are horseshoe crabs and cockroaches, not decent-sized reptiles,” Jenkins said.

The researchers argue that the lack of change may indicate an excessive acceleration of natural selection.

“Slow pace of evolution does not necessarily mean no evolution. In fact, this is the evolutionary equivalent of the proverb “If it ain’t broke, don’t fix it” (“Don’t fix it if it’s not broken”),” Simões joked.

While the discovery of Navajosphenodon sani helps flesh out an important chapter in the evolution of the tuatara, much of this reptile’s prehistory remains obscure.

“Why modern tuatars and their lineages have evolved so slowly over such a long period of time is a very serious question and a bit harder to understand. We need more fossils,” said Dr. Pierce.