A fossil reveals the secrets of one of nature’s most mysterious reptiles


New Zealand tuataras look like dark iguanas. But these spiny reptiles aren’t really lizards. Instead, they are the last remnant of a mysterious and ancient order of reptiles known as the Rhynchocephali that mostly died out after their Jurassic peak.

And they really are the eccentrics of the reptile family. Tuataras can live for over a century, live in cold climates, and are able to slide their jaws back and forth to shear insects, seabirds, and One and the other. They even have a rudimentary third eye under the scales on top of their heads that can help them track the sun.

These bizarre traits make the tuatara an evolutionary enigma, and a patchy fossil record of its long-lost relatives has baffled paleontologists. Probably overtaken by lizards and snakes, virtually all Rhynchocephali disappeared by the end of the Mesozoic era. Many left behind only dusty fragments of teeth and jaws.

It turns out that a crucial piece of this puzzle has been sitting in a museum drawer for decades. Sifting through a backlog of fossils at Harvard University’s Museum of Comparative Zoology, Stephanie Pierce, the museum’s curator of vertebrate paleontology, and her team recently discovered the nearly complete skeleton of a lizard-like animal. on a stone slab small enough to fit in the palm. from their hands.

The remarkable fossil was discovered in 1982 during an expedition to the Kayenta Formation, a fossil-rich outcrop in northern Arizona. This strip of red rock was laid down during the early Jurassic period when the reign of the dinosaurs was in its infancy. Around this primeval floodplain, early dinosaurs like the crested Dilophosaurus mingled with sturdy crocodile-like creatures wrapped in armor. Primitive shrew-like mammals and this strange new reptile raced under their feet.

While the site’s early mammal fossils generated much of the initial interest, Dr. Pierce and Tiago Simões, a Harvard postdoctoral paleontologist specializing in the early evolution of lizards, eventually studied this specimen in depth.

In a published article Thursday in Communications Biology, the scientists named the new animal Navajosphenodon sani. The genus and species name (meaning “old age” in the Navajo language) refers to the Navajo tribe, which lives in the area where the fossil was found.

Scientists used microscanners to study the crushed fossil in three dimensions and digitally assembled the flattened skull like a puzzle.

Although its body resembled a lizard, its skull structure resembled a tuatara. It sported similar rows of sharp, interlocking teeth extending directly from the jawbone. The skull also had two holes behind the animal’s eye. This configuration is one of the key features differentiating tuataras from lizards, which only have one hole. The extra hole helps stabilize the skull when the tuatara bites and saws its prey.

“All of these features are quite obvious for modern tuataras and unlike what you see in any other modern reptile,” Dr Simões said. After a series of statistical tests, the team spotted Navajosphenodon near the base of the tuatara lineage.

The fossil illustrates that the bodies of modern tuataras emerged in the Jurassic era and have changed little in 190 million years. This confirms the popular distinction that these reptile remains are “living fossils”. But Dr. Simões pointed out the differences: for example, the jaws of modern tuatara end in a set of fused, beak-like teeth that are absent in Navajosphenodon.

According to Kelsey Jenkins, a Yale doctoral student specializing in early reptile evolution, many lineages of Rhynchocephali have changed little over their history. However, 200 million years is extreme. “The only things that are highly conserved are things like horseshoe crabs and cockroaches – not a decent sized reptile,” said Ms Jenkins, who was not involved in the new study.

The researchers argue that this lack of change may represent natural selection in overdrive. “Slow rates of evolution do not necessarily mean no evolution,” Dr. Simões said. Basically, it’s the evolutionary equivalent of the adage, “If it ain’t broke, don’t fix it.”

While the discovery of Navajosphenodon helps flesh out a crucial chapter in the evolution of the tuatara, much of this reptile’s history remains unclear. Without more fossil discoveries, it will be difficult for scientists to pinpoint exactly why these solitary survivors appear to possess evolutionary cheat codes.

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


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