“What’s important about the tuatara is that it represents this huge evolutionary story that we’re lucky enough to be caught up in that is likely to end,” Carrano said. “Although it looks like a relatively simple lizard, it embodies an entire evolutionary saga that goes back more than 200 million years.” The discovery comes from a handful of specimens, including a remarkably complete and well-preserved fossil skeleton excavated from a site centered around an Allosaurus nest in the Morrison Formation of northern Wyoming. Further study of the find could help reveal why this animal’s ancient reptile order went from being diverse and numerous in the Jurassic to just New Zealand’s tuatara surviving today. The tuatara looks a bit like a particularly burly iguana, but the tuatara and its recently discovered relative aren’t actually lizards at all. They’re actually rhinoceros, an order that diverged from lizards at least 230 million years ago, Carrano said. In their Jurassic heyday, cephalopods were found nearly all over the world, came in sizes large and small, and had ecological roles ranging from hunters of aquatic fish to massive chewers of plants. But for reasons that are still not fully understood, the cephalopods died out, as lizards and snakes became the most common and diverse reptiles around the world. This evolutionary gap between lizards and cephalopods helps explain the tuatara’s strange features, such as teeth fused to the jawbone, a unique chewing motion that slides the lower jaw back and forth like a saw blade, a lifespan of 100 years, and a tolerance for colder climates. After the formal description of O. gregori, Carrano said the fossil has been added to the museum’s collections where it will remain available for future study, perhaps one day helping researchers understand why the tuatara is the only tuatara left. whereas now lizards are found all over the planet. “These animals may have become extinct in part because of competition from lizards, but also perhaps because of global climate change and habitat change,” Carrano said. “It’s fascinating when you have the dominance of one group giving way to another group over evolutionary time, 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 researchers named the new species after museum volunteer Joseph Gregor, who spent hundreds of hours painstakingly scraping and carving the bones from a boulder that first caught the eye of museum fossil preparator Pete Kroehler in 2010. “Pete is one of those people who has a kind of X-ray vision for these kinds of things,” Carrano said. “He noticed two tiny flecks of bone on the side of this block and marked that it should be returned with no real idea what was inside. As it turned out, he hit the jackpot.” The fossil is almost entirely complete, with the exception of the tail and parts of the hind legs. Carrano said such a complete skeleton is rare for small prehistoric creatures like this because their fragile bones were often destroyed either before they fossilized or as they emerged from a rock formation that is eroding today. As a result, rhinoceros are mostly 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 was practical given its fragility, the team, led by DeMar, began scanning 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 sample. The team used three separate CT scanning 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 had been digitally rendered to an accuracy of less than a millimeter, DeMar began reassembling the digitized skull bones, some of which were crushed, in place or missing on one side, using software to finally create an almost complete 3D reconstruction. The reconstructed three-dimensional skull now offers researchers an unprecedented look at the head of this Jurassic-era reptile. Given Opisthiamimus’ small size, tooth shape and rigid skull, it likely ate insects, DeMar said, adding that harder-shelled prey such as beetles or water bugs may also have been on the menu. In general, the new species looks quite like a miniature of its only surviving relative (the tuatara is about five times longer). “Such a complete specimen has tremendous potential for comparisons with fossils collected in the future and for identifying or reclassifying specimens that are 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 the mechanics of the jaws of this creature.” Funding and support for this research was provided by the Smithsonian and the Australian Research Council.
