Extinct prehistoric reptile that lived among dinosaurs discovered by Smithsonian researchers

The discovery sheds light on the tuatara, the last living member of a once-diverse reptilian group almost entirely supplanted by lizards.

Smithsonian researchers have discovered a new extinct species of lizard-like reptile belonging to the same ancient lineage as New Zealand’s living tuatara. The new way Opisthiamimus gregorithat once inhabited

” data-gt-translate-attributes=”[{” attribute=””>Jurassic North America about 150 million years ago alongside dinosaurs like Stegosaurus and Allosaurus, is described in a paper published on September 15, 2022, in the Journal of Systematic Palaeontology. In life, this prehistoric reptile would have been about 16 centimeters (about 6 inches) from nose to tail—and would fit curled up in the palm of an adult human hand. It likely survived on a diet of insects and other invertebrates.

A team of scientists, including the National Museum of Natural History’s curator of Dinosauria Matthew Carrano and research associate David DeMar Jr. as well as University College London and Natural History Museum, London scientific associate Marc Jones, contributed to the research.

“What’s important about the tuatara is that it represents this enormous evolutionary story that we are lucky enough to catch in what is likely its closing act,” Carrano said. “Even though it looks like a relatively simple lizard, it embodies an entire evolutionary epic going back more than 200 million years.”

Fossil skeleton of the new lizard-like reptile Opisthiamimus gregori. The fossil was discovered in the Morrison Formation of the Bighorn Basin, north-central Wyoming, and dates to the Late Jurassic Period, approximately 150 million years ago. Researchers named the new species after Smithsonian’s National Museum of Natural History volunteer Joseph Gregor who spent hundreds of hours meticulously scraping and chiseling the bones from a block of stone that first caught museum fossil preparator Pete Kroehler’s eye back in 2010. The fossil has been added to the museum’s collections where it will remain available for future study. A team of scientists describes the new species, which once lived alongside dinosaurs like Stegosaurus and Allosaurus, in a paper published today in the Journal of Systematic Palaeontology. In life, this prehistoric reptile would have been about 16 centimeters (about 6 inches) from nose to tail—and would fit curled up in the palm of an adult human hand—and likely survived on a diet of insects and other invertebrates. Credit: David DeMar for the Smithsonian Institution

The discovery comes from a handful of specimens, one of which was an incredibly complete and well-preserved fossil skeleton excavated from a site centered around an Allosaurus nest in northern Wyoming’s Morrison Formation. Further investigation of the find could help reveal why this animal’s ancient order of reptiles was winnowed down from being diverse and numerous in the Jurassic to only New Zealand’s tuatara surviving today.

Although the tuatara looks a bit like a particularly stout iguana, the tuatara and its newly discovered relative are in fact not lizards at all. They are actually rhynchocephalians, an order that diverged from lizards at least 230 million years ago, Carrano said.

The research team set out to scan the fossil using high-resolution computed tomography (CT), a method that uses multiple X-ray images from different angles to create a 3D representation of the specimen. The team used three separate CT scanning facilities, including one at the Smithsonian National Museum of Natural History, to capture everything they could about the fossil. After the bones of the fossil were digitally rendered, the team set about using software to reassemble the digitized bones of the skull, some of which were crushed, misplaced or missing on one side, to eventually create a near-complete lifelike 3D reconstruction create. A team of scientists describes the new species Opisthiamimus gregori, which once lived alongside dinosaurs like Stegosaurus and Allosaurus, in a paper published today in the Journal of Systematic Palaeontology. In life, this prehistoric reptile would have been about 16 centimeters (about 6 inches) long from nose to tail — and would fit in the palm of an adult human’s palm when curled up — and likely survived on a diet of insects and other invertebrates. Photo credit: D. DeMar

In their heyday during the Jurassic period, rhynchocephalians were found almost worldwide and came in large and small sizes. They filled ecological roles ranging from aquatic fish hunters to bulky herbivores. But for reasons that are still not fully understood, rhynchocephalians all but disappeared as lizards and snakes became the more common and diverse reptiles around the world.

This evolutionary gap between lizards and rhynchocephalians helps explain the tuatara’s odd features. These include a lifespan of over 100 years, teeth fused to the jawbone, a unique chewing motion that slides the lower jaw back and forth like a saw blade, and a tolerance to colder climates.

3D reconstruction of the skull of Opisthiamimus gregori, a new extinct species of lizard-like reptile from the Late Jurassic of Wyoming, USA. Individual bones are color coded. Photo credit: D. DeMar

Following O. gregoriIn his formal description, Carrano said the fossil has been added to the museum’s collections, where it will be available for future study. Perhaps one day it will help scientists figure out why the tuatara is all that remains of the rhynchocephalians, while lizards are now found all over the world.

“These animals may have disappeared in part due to competition from lizards, but perhaps also due to global climate change and changing habitats,” Carrano said. “It’s fascinating when, over the course of evolution, one group’s dominance gives way to another, and we still need more evidence to explain exactly what happened, but fossils like this are how we’re going to piece it together.”

The new species is named after Joseph Gregor, a museum volunteer who spent hundreds of hours meticulously scraping and chiseling the bones from a block of stone that caught the attention of Pete Kroehler, a museum fossil taxidermist, in 2010.

“Pete is one of those people who has kind of x-ray vision for things like that,” Carrano said. “He noticed two tiny patches of bone on the side of that block and marked it to bring back without really knowing what was inside. As it turns out, he hit the jackpot.”

Photo (above) and interpretation drawing (below) of the skull and skeleton of Opisthiamimus gregoria new extinct species of lizard-like reptile from the Late Jurassic of Wyoming, US source: D. DeMar (top photo), James Morrison (bottom image).

The fossil is almost complete except for the tail and part of the hind legs. Such a complete skeleton is rare for small prehistoric creatures like this one, Carrano said, because their fragile bones were often destroyed, either before they fossilized or when emerging from an eroding rock formation today. As a result, rhynchocephalians are known to paleontologists primarily from small fragments of their jaws and teeth.

After Kroehler, Gregor and others extracted as much of the tiny fossil from the rock as possible, the team led by DeMar set about scanning the fossil with high-resolution computed tomography (CT). This is a method that uses multiple X-ray images from different angles to create a 3D representation of the sample. The research team used three separate CT scanning facilities, including one at the National Museum of Natural History, to capture everything they could about the fossil.

After the bones of the fossil were digitally rendered with a

” data-gt-translate-attributes=”[{” attribute=””>accuracy smaller than a millimeter, DeMar set about reassembling the digitized bones of the skull. Some of them were crushed, out of place, or missing on one side, so software was used to eventually create a nearly complete 3D reconstruction. This reconstructed 3D skull now provides scientists with an unprecedented look at this Jurassic-age reptile’s head.

Given Opisthiamimus’s diminutive size, tooth shape, and rigid skull, it likely ate insects, said DeMar, adding that prey with harder shells such as beetles or water bugs might have also been on the menu. Broadly speaking, the new species looks quite a bit like a miniaturized version of its only surviving relative (tuataras are about five times longer).

“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,” DeMar said. “With the 3D models we have, at some point, we could also do studies that use software to look at this critter’s jaw mechanics.”

Reference: “A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)” 15 September 2022, Journal of Systematic Palaeontology.
DOI: 10.1080/14772019.2022.2093139

Funding and support for this research were provided by the Smithsonian and the Australian Research Council.