By Victoria Gill
Science reporter, BBC News
Tawa hallae was a small, meat-eating dinosaur
Researchers have unveiled a new species of dinosaur from the late Triassic period - a small, early relative of T.rex and Velociraptor.
The 2m-long dinosaur, named Tawa hallae, was found in a "bone bed" on the Ghost Ranch in New Mexico, US.
The discovery of this early theropod, reported in the journal Science, sheds light on early dinosaur evolution.
The team says the find also highlights how dinosaurs dispersed across what was then the "supercontinent" Pangaea.
Sterling Nesbitt, a researcher at the University of Texas at Austin in the US, led a team from a number of US research institutes that studied the fossilised dinosaur bones.
The researchers named the 215 million-year-old dinosaur Tawa after the Native American Hopi word for the sun god.
Dr Nesbitt told BBC News that the bone bed was first excavated in 2004, but his team made a larger excavation in 2006, discovering articulated dinosaur skeletons that were between 90% and 95% complete.
These remarkable specimens enabled the researchers to confirm, without doubt, that Tawa was a new type of dinosaur.
"When we saw the [specimens], our jaws dropped," said Dr Nesbitt. "A lot of these theropods have really hollow bones, so when they get preserved, they get really crunched. But these were in almost perfect condition."
"Tawa has an interesting combination of different characteristics," he said. "There's no single huge difference, but in combination, the characteristics show that Tawa is brand new."
The bipedal dinosaur had relatively short forelimbs with sharp claws, and downward curving teeth.
"The teeth have little serrations - like a steak knife - so we're fairly confident that it was a carnivore," said Dr Nesbitt.
The bone bed yielded remarkably complete skeletons
"Tawa is a little bit of a relic of the early evolution of dinosaurs," Dr Nesbitt told BBC News. "It's about 215 million years old and our oldest dinosaurs are about 230 million years old."
He explained that it filled a gap in the fossil record, demonstrating that dinosaurs split into their three major groups - theropods, sauropodomorphs and ornithischians - very early in their evolution.
"These three groups then persisted until at least 65 million years ago," said Dr Nesbitt.
The finding provides strong evidence for an existing hypothesis that dinosaurs originated in what is now South America, and very soon diverged into three major lines.
The theropods were bipedal dinosaurs, and were mainly carnivores. The line included the iconic Tyrannosaurus rex and velociraptor.
Sauropodomorphs included ground-shaking giants like Apatasaurus, and ornithischians included a range of dinosaurs, such as Stegosaurus and Triceratops.
Dr Nesbitt and his team found other theropods in the same bone bed as Tawa. These simultaneous discoveries allowed them to reconstruct a picture of how the early dinosaurs dispersed throughout the world.
The location of the discovery shows how early dinosaurs dispersed
"The closest relatives of the other theropods [we found] were dinosaurs that are found in South America.
"So Tawa shows us that dinosaurs moved between South and North America," he said.
This was at the time of the supercontinent Pangaea, when "you could walk from the North to the South Pole," Dr Nesbitt told the Science podcast.
David Martill, a palaeontologist from the University of Portsmouth in the UK, who was not involved in this study, said this was a "very exciting discovery".
"This... rewrites the evolutionary tree for meat-eating dinosaurs," Dr Martill told BBC News.
"This beast shows how important it is to keep going in to the field looking for fossils.
"Just one lucky discovery can make such a difference to the way we perceive the evolution of dinosaurs, and any other creature for that matter."