The discovery of extinct prehistoric reptiles that lived among dinosaurs is a field of study known as paleontology. Paleontologists study fossils of extinct animals, including dinosaurs, to learn about their anatomy, behavior, and how they lived in their environments. Many new dinosaur species have been discovered in recent years through paleontological expeditions and research. Some examples include the T-Rex, Velociraptor, and Triceratops. With each discovery, scientists are able to better understand the diversity of life that existed during the Mesozoic era and how these creatures interacted with one another.
Smithsonian scientists have discovered a new extinct species of lizard-like reptile that is related to New Zealand’s living tuatara. In a paper published today in the Journal of Symbiosis, a team of scientists led by National Museum of Natural History 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, describe the new species Opisthiamimus gregori, which once inhabited Jurassic North America around 150 million years ago alongside dinosaurs like Stegosaurus and Allosaurus. 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.
“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.”
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. Even though it looks like a relatively simple lizard, it embodies an entire evolutionary epic going back more than 200 million years.
Dinosauria Matthew Carrano
A handful of specimens were excavated from a site centered around an Allosaurus nest in northern Wyoming’s Morrison Formation, including an extraordinarily complete and well-preserved fossil skeleton. Further research into the discovery could help explain why this animal’s ancient order of reptiles was reduced from being diverse and numerous in the Jurassic to only New Zealand’s tuatara surviving today.
The tuatara resembles a particularly large iguana, but it and its newly discovered relative are not lizards at all. According to Carrano, they are rhynchocephalians, an order that split from lizards at least 230 million years ago.
Rhynchocephalians were found nearly everywhere in the Jurassic period, came in all sizes, and filled ecological roles ranging from aquatic fish hunters to bulky plant munchers. However, for reasons that are still unknown, rhynchocephalians all but vanished as lizards and snakes became the more common and diverse reptiles on the planet.
This evolutionary gap between lizards and rhynchocephalians explains why the tuatara has teeth fused to the jaw bone, a unique chewing motion that slides the lower jaw back and forth like a saw blade, a 100-year lifespan, and tolerance for colder climates.
Following O. gregori’s formal description, Carrano stated that the fossil has been added to the museum’s collections and will be available for future study, possibly one day helping researchers 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 died as a result of competition from lizards, but they may also have died as a result of global climate change and changing habitats,” Carrano said. “It’s fascinating to see one group’s dominance give way to another group over evolutionary time, and we still need more evidence to explain exactly what happened, but fossils like this one will help us piece it together.”
The new species was named after museum volunteer Joseph Gregor, who spent hundreds of hours meticulously scraping and chiseling the bones from a block of stone that first caught the eye of museum fossil preparator Pete Kroehler back in 2010.
“Pete is one of those people who has a kind of X-ray vision for this sort of thing,” Carrano said. “He noticed two tiny specks of bone on the side of this block and marked it to be brought back with no idea what was inside. He actually hit the jackpot.”
The fossil is almost entirely complete, with the exception of the tail and parts of the hind legs. Carrano said that such a complete skeleton is rare for small prehistoric creatures like this because their frail bones were often destroyed either before they fossilized or as they emerge from an eroding rock formation in the present day. As a result, rhynchocephalians are mostly known to paleontologists from small fragments of their jaws and teeth.
After Kroehler, Gregor, and others had extracted as much of the tiny fossil as possible given its fragility, the team, led by DeMar, began scanning the specimen with high-resolution computerized tomography (CT), a method that uses multiple X-ray images from different angles to create a 3D representation of the specimen. To capture as much information about the fossil as possible, the team used three separate CT scanning facilities, one of which was housed at the National Museum of Natural History.
Once the fossil’s bones had been digitally rendered with accuracy smaller than a millimeter, DeMar set about reassembling the digitized bones of the skull, some of which were crushed, out of place or missing on one side, using software to eventually create a nearly complete 3D reconstruction. The reconstructed 3D skull now provides researchers an unprecedented look at this Jurassic-age reptile’s head.
Given Opisthiamimus’ small size, tooth shape, and rigid skull, it most likely ate insects, according to DeMar, who added that prey with harder shells, such as beetles or water bugs, could also have been on the menu. In general, the new species appears to be a miniature version of its only living relative (tuataras are about five times longer).
“Such a complete specimen has enormous potential for making comparisons with future fossils collected as well as identifying or reclassifying specimens already sitting in a museum drawer somewhere,” DeMar said. “With the 3D models we have, we could do studies that use software to look at this critter’s jaw mechanics at some point.”
