This ancient reptile is not a lizard. Don’t call him a lizard
150 million years ago, a prehistoric reptile unlike modern lizards snuck around what is now Wyoming. An ancient rhynchocephalus, the discovery of the insectivorous animal could shed light on the persistence of its living relative, the tuatara.
The reptile is named Opisthiamimus gregori. It looks like a lizard, but like the New Zealand tuatara, it is not. Lizards are squamates, an order of reptiles that includes snakes and worm lizards. Rhynchocephali are a distinct group that diverged from lizards in the Triassic.
The fossils of Opisthiamimus come from Wyoming, where they sat above what was once an allosaurus nest. Paleontologists found four specimens at the site, including an almost complete articulated skeleton of the reptile. The newly discovered species is described in a study published today in the Journal of Systematic Paleontology.
“What [the fossil] fact is to hammer home the fact that the rhynchocephali were a very diverse group for much of their evolutionary history,” study co-author Matthew Carrano, curator of Dinosauria at the Smithsonian’s National Museum of Natural History, said in a statement. email to Gizmodo. “There’s probably more ‘hidden diversity’ there, because a lot of the fossils are small and fragmentary, and hard to identify.”
Last year, scientists described a rhynchocephalus called Taytalura alcoberihelping to clarify the evolutionary discrepancy between their reptilian order and the squamates. Taytalura is known only from a well-preserved skull, but the youngest Opisthiamimus has an almost complete skeleton. His discovery adds to that of Taytalura by showing that their reptilian order was diverse relatively early in deep time.
“I agree with the authors that this is an important discovery from the Morrison Formation,” said Tiago Simões, an evolutionary biologist at Harvard University who was unaffiliated. to the recent article, in an email to Gizmodo. Simões was one of the researchers who worked on Taytalura.
Opisthiamimus is very old; its existence precedes tyrannosaurus rex by 60 million years. He lived at the end of the Jurassic, alongside Archeopteryx and stegosaurus (although much lower to the ground than the first two, and much smaller, measuring only 6 inches from nose to tail.)
The only extant rhynchocephalus is the tuatara, which is part of the subgroup called the sphenodonts, of which there are two species. The tuatara can live for over 100 years and has the fastest cum of any reptile. In particular, it has a parietal eye in the center of its forehead and three rows of teeth: two in its upper jaw and one in the lower jaw. Unlike other reptiles, rhynchocephalic teeth are part of their jaws, rather than separate, replaceable parts.
Due to its unique anatomy, the tuatara is often referred to as a “living fossil.” He persisted when all the other members of his order could not. But don’t call it primitive: it simply found a winning formula for survival and stuck to it.
“I would be careful with the phylogenetic interpretation that the authors provided for this species,” added Simões, noting that the characteristics of Opisthiamimus are more typical of the sphenodontians that later appear in the fossil record.
Finding more fossils of ancient reptiles could help explain why squamates persist on Earth in abundance while rhynchocephali do not.
“One theory is that one or more of the unique characteristics of squamates allowed them to outperform rhynchocephalians,” Carrano said. “There is a broad pattern of gradual decline in rhynchocephali alongside a gradual increase in squamate diversity. But competition happens within environments, and right now we don’t have enough fossils to really investigate that idea, even though in a place like the Morrison Formation we’re getting close.
Now the team is sifting through the remains of the Allosaurus nest found just below Opisthiamimus. More rhynchocephalic fossils await discovery, in the Morrison Formation and beyond. When they are discovered, they could help us discover their reptilian family tree.
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