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The earliest stages of dinosaur evolution remain buried under eons of rock, but the discovery of a new primitive carnivore fossil in the U.S. Southwest promises to dispel some of the debate about how these beasts spread across the globe—and about the origins of the group that eventually led to modern birds.
"It gives us new information about early evolution of dinosaurs," Sterling Nesbitt, a postdoctoral researcher at the University of Texas at Austin's Jackson School of Geosciences, said about the discovery during a Web cast with reporters on Wednesday. The diminutive, two-meter-long Tawa hallae fills a big gap in the fossil record between the earliest carnivores, such as Herrerasaurus, and later theropod lines that include Tyrannosaurus rex and the first birds. "Tawa helps us reconstruct those relationships," Nesbitt said. The findings were published online Thursday in Science.
"Discovering these really basal forms are always one of the most important things," says Mark Norell, chairman of the Division of Paleontology at the American Museum of Natural History in New York City and senior author of a paper on the discovery also co-authored by Nesbitt.
Dinosaurs first appear in the fossil record in the late Triassic period (which ranges from about 250 million to 200 million years ago), so the unearthing of the 214 million-year-old Tawa in New Mexico brings a new understanding about where many of the earliest dinosaurs evolved. Found in the fossil-rich Hayden Quarry at Ghost Ranch, Tawa and some of its contemporaries actually appear to be more closely related to different dinosaur relatives in South America—rather than each other. This finding suggests that early dinosaur lineages likely diverged in what is now South America (as the supercontinent Pangaea was breaking up) before moving to North America, a conclusion that Norell calls "rather unexpected." Previous theories about this poorly understood period had assumed a North American origin for some of these earlier lines.
Even though the new meat-eater lived some 139 million years before the vicious Velociraptor, Tawa already looked quite a bit like it, with a big tail, long, sharp claws and serrated, meat-ripping teeth, revealing the speed with which the earliest dinosaurs evolved. Despite its familiar body, however, Tawa retains some primitive features—including its hips and parts of its forearms—surprising the researchers when it turned up at such a late date in New Mexico. It seemed to be living "about 15 million years later than it should have been," Nesbitt said.
Although Tawa is not part of the precise lineage that led directly to birds, it appears to be the first dinosaur known to have specialized hollow air sacs in its neck vertebrae, a version of which can still be found in modern birds. The purpose of these structures is still unknown, but, says Norell, "it shows us how primitive air sacs are." Tawa also already had open hip sockets that remain characteristic in birds we see today—right down to a holiday turkey, Nesbitt noted in the Webcast.
Nesbitt asserts that this new theropod will also at last settle a debate about the even older Herrerasaurus, which some paleontologists had thought belonged to a separate non-theropod early line, or was not even a dinosaur. Nesbitt and his colleagues examined Tawa's pelvis, claws and teeth and found they were similar enough those of Herrerasaurus that the latter was indeed a very early theropod.
Tawa hallae is represented by fossils from some five to seven well-articulated individuals of different sizes, all unearthed at Ghost Ranch over the course of four years. "The preservation of Tawa is exquisite," Nesbitt said, noting that the level of detail found in the bones and other surrounding fossils is "unprecedented [from] the late Triassic." That the bones and skeletons were well preserved allows the researchers to examine many of the specimens in three dimensions. As Norell notes, any Triassic find is exciting but the condition of these discoveries, he says, is especially exhilarating.
Tawa also helps to trace the evolution of some morphological features back to the very first dinosaurs. It provides a much clearer picture of the basal theropods, Norell says. And, as Nesbitt noted, it "looks more like what we think the common ancestor of dinosaurs looked like."
