Extinct prehistoric reptile that lived among dinosaurs discovered by Smithsonian researchers

An artistic rendition of a recently discovered extinct species of lizard-like reptile belonging to the same ancient lineage as the living New Zealand tuatara. The newly discovered Opisthiamimus gregori feeds on a now extinct water bug (morrisonnepa jurassica), while in the background the predatory dinosaur Allosaurus jimmadseni keep its nest. The scene is the floodplain of a Late Jurassic river in Wyoming, about 150 million years ago. A team of scientists describes the new species, which once inhabited Jurassic North America around 150 million years ago alongside dinosaurs like Stegosaurus and Allosaurus, in a paper published today in the Journal of Systematic Palaeontology. In life, this prehistoric reptile would have measured about 16 centimeters (about 6 inches) from nose to tail and would have sat curled up in the palm of an adult human hand. The find comes from a handful of specimens, including an extraordinarily complete and well-preserved fossil skeleton excavated from a site centered around an Allosaurus nest in the Morrison Formation of northern Wyoming. Credit: Julius Csotonyi for the Smithsonian Institution

The discovery sheds light on the tuatara, the last living member of a once diverse group of reptiles that has been almost entirely supplanted by lizards.

A new, extinct species of lizard-like reptile belonging to the same ancient lineage as the living New Zealand tuatara has been discovered by Smithsonian researchers. The new species Opisthiamimus gregoriwho once lived[{” 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

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 with 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 scan facilities, including one housed at the Smithsonian’s National Museum of Natural History, to capture everything they could about the fossil. Once the fossil bones were digitally rendered, the team set about reassembling the scanned bones of the skull, some of which were crushed, displaced or missing from one side, using software to ultimately create a realistic 3D reconstruction. almost complete. 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 measured about 16 centimeters (about 6 inches) from nose to tail – and could have been curled up in the palm of an adult human hand – and likely survived on a diet of insects and other invertebrates. Credit: D. DeMar

At their peak during the Jurassic period, rhynchocephali were found almost worldwide and came in large and small sizes. They fulfilled ecological roles ranging from aquatic fish hunters to heavy plant eaters. But for reasons that are not yet fully understood, rhynchocephalians have all but disappeared as lizards and snakes have become the most common and diverse reptiles throughout the world.

This evolutionary chasm between lizards and rhynchocephalans helps explain the tuatara’s strange characteristics. These include a lifespan of over 100 years, teeth fused to the jaw bone, a unique chewing motion that slides the lower jaw back and forth like a saw blade, and tolerance to colder climates.

Opisthiamimus gregori skull

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

Next O. gregoriIn his official description, Carrano said the fossil has been added to the museum’s collections where it will remain available for future study. Maybe one day this will help scientists understand why the tuatara is all that remains of the rhynchocephali, when lizards are now found all over the world.

“These animals may have gone extinct partly because of competition from lizards, but possibly also because of global climate change and shifting habitats,” Carrano said. “It’s fascinating when the dominance of one group gives way to another group during evolution, and we still need more evidence to explain exactly what happened, but fossils like this these are the way we’re going to put it together.”

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

“Pete is one of those people who has kind of an x-ray vision for this stuff,” Carrano said. “He noticed two tiny pieces of bone on the side of this block and marked it to be brought back without really knowing what was in it. Turns out he hit the jackpot.

Opisthiamimus gregori Skeleton

Photo (top) and interpretive drawing (bottom) of the skull and skeleton of Opisthiamimus gregoria new extinct species of lizard-like reptile from the Late Jurassic of Wyoming, USA Credit: D. DeMar (photo, top), James Morrison (illustration, bottom).

The fossil is almost entirely complete except for the tail and parts of the hind legs. Such a complete skeleton is rare for small prehistoric creatures like this, Carrano said, because their frail bones were often destroyed either before they fossilized or when they emerged from an eroded rock formation in our lands. days. Accordingly, rhynchocephali are best known to paleontologists from small fragments of their jaws and teeth.

After Kroehler, Gregor and others freed as much of the tiny fossil from the rock as possible given its fragility, the team, led by DeMar, set about scanning the fossil with computerized tomography (CT) at high resolution. 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 scan facilities, including one housed at the National Museum of Natural History, to capture everything they could about the fossil.

Once the bones of the fossil have been digitally rendered with a precision smaller than a millimeter, DeMar set about reassembling the digitized bones of the skull. Some of them were crushed, moved or missing on one side. Software was therefore used to create an almost complete 3D reconstruction. This reconstructed 3D skull now offers scientists an unprecedented look into the head of this Jurassic-age reptile.

Given OpisthiamimusDue to its small size, the shape of its teeth and its rigid skull, it likely ate insects, DeMar said, adding that harder-shelled prey such as beetles or water bugs could also have been eaten. appear on the menu. Generally speaking, the new species looks a bit like a miniaturized version of its only surviving relative (tuataras are about five times longer).

“Such a complete specimen has enormous potential for making comparisons with fossils collected in the future and for identifying or reclassifying specimens already sitting somewhere in a museum drawer,” DeMar said. “With the 3D models we have, at some point we could also do studies that use software to examine the mechanics of this creature’s jaw.”

Reference: “Nearly complete skeleton of a new eusphenodont from the Late Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephaly (Reptilia: Lepidosauria)” September 15, 2022 , Journal of Systematic Paleontology.
DOI: 10.1080/14772019.2022.2093139

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