The Sciences

This New Extinct Species of Lizard Was Alive During the Dinosaurs

The discovery of Opisthiamimus gregori may one day illuminate the decline of the rhynchocephalian reptiles.

By Sam WaltersSep 15, 2022 10:00 AM
Opisthiamimus
(Credit: Julius Csotonyi/Smithsonian Institution)

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Millions of years ago, in the middle of the Mesozoic, reptile development and diversification began to surge. It broke out in a stunning variety of reptilian shapes, sizes, structures and traits. But, while many reptiles maintained their morphological diversity throughout the years, many others dwindled or disappeared, including the rhynchocephalians and the better-known dinosaurs.

A paper published in the Journal of Systematic Palaeontology states that researchers recently discovered and described the fossils of Opisthiamimus gregori, a small new species of extinct rhynchocephalian. It ambled through the North American terrain alongside the dinosaurs around 150 million years ago.

According to the team, the discovery may solve a persisting problem within the world of reptile research. Future studies of the fossil species may one day reveal why once-diverse reptile groups declined so dramatically with time.

The Rhynchocephalians

According to the research team, the rhynchocephalians are a group of reptiles closely related to the lizards. At their peak, approximately 200 to 145 million years ago, these lizard-like reptiles roamed all around the world. They ranged in shape and size, and fulfilled a wide variety of functions, from that of the aquatic predator to the terrestrial prey. But for some strange reason that remains a mystery to reptile researchers, the rhynchocephalians soon started to decline, tumbling closer and closer to completely disappearing.

"These animals may have disappeared partly because of competition from lizards but perhaps also due to global shifts in climate and changing habitats," says Matthew Carrano, a member of the research team and a dinosaur curator at the Smithsonian National Museum of Natural History, in a press release.

In fact, only one species of rhynchocephalian, Sphenodon punctatus, still survives today. Similar in appearance to a stout, stocky iguana, this species — commonly called a tuatara — forms one of the only avenues for scientists to study the winnowing down of the rhynchocephalians.

The discovery and description of O. gregori opens up another valuable source of information, which, according to the researchers, could one day show why rhynchocephalians declined to only a single surviving species.

"It’s fascinating when you have the dominance of one group giving way to another group over evolutionary time," Carrano says in press release. "We still need more evidence to explain exactly what happened, but fossils like this one are how we will put it together."

Reptile Revelations

Found at a site in Wyoming, the small fossil specimen features almost all of the O. gregori skeleton, save for sections of its tail and legs. This, the research team says, makes the specimen particularly precious, since most similarly sized fossils are found shattered or in fragments thanks to the tough process of fossilization or the wearing away of rocks over time.

Opting to take full advantage of the completeness of the fossil, the team scanned the specimen from an assortment of angles to create an almost fully complete digital rendering. This rendering, in turn, revealed that O. gregori reached approximately 6 inches in length and likely lived on a steady flow of insects and other small invertebrates. The rendering also revealed that the rhynchocephalian species looked surprisingly similar to the surviving tuatara, despite being around five times smaller.

"Such a complete specimen has huge potential for making comparisons with fossils collected in the future and for identifying or reclassifying specimens already sitting in a museum drawer somewhere," concludes David DeMar Jr., another member of the team and a researcher at the Smithsonian National Museum of Natural History, in a press release.

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