A new raptor-like species of dinosaur found in New Mexico is providing answers to lingering questions about the early evolution of meat-eating dinosaurs.
The find included fossils of several members of the new species, named Tawa hallae, but the most complete skeleton was that of a young dinosaur about two metres long from nose to tail. Its body was about the size of a large dog, standing 70 centimetres tall at the hips.
Tawa lived about 214 million years ago, in the Triassic period, when the first dinosaurs evolved. Tawa is part of the theropods, a group of meat-eating, bipedal dinosaurs that includes the famous T. rex and velociraptor.
Volunteers taking a paleontology seminar at the Ruth Hall Museum of Paleontology in Abiquiu, New Mexico, found the first fossils of the new species in 2004.
Sterling Nesbitt, of the University of Texas, and his colleagues at Columbia University, the Field Museum and the American Museum of Natural History began the full excavation of the fossils in 2006.
They describe Tawa in this week's issue of Science. The researchers named the dinosaur after the Hopi word for the Puebloan sun god.
The dinosaur's fossil bones as remarkably well preserved, with very little of the flattening associated with the fossilization process.
"When we saw them, our jaws dropped," said 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 is important to the study of the evolution of dinosaurs because of its similarities to another dinosaur, Herrerasaurus, discovered in the 1960s in Argentina.
While Herrerasaurus has some traits of the theropods, such as the large claws and teeth, it lacks other characteristics, such as pockets of air in the vertebrae.
Some paleontologists place Herrerasaurus among the earliest theropods while others claim it wasn't part of the theropod lineage.
Tawa shows a mix of features seen in theropods, including the air sacs in the spine, and those seem in Herrerasaurus, particularly in the pelvis. The similarities suggest that Herrerasaurus is, indeed, a theropod.
Herrerasaurus is an early dinosaur — it lived about 230 million years ago — and it was found in a layer that included specimens of the two other main dinosaur groups: the sauropods and the ornithischians.
The researchers say this means that dinosaurs diverged rapidly into the three groups shortly after they appeared.
"Tawa pulls Herrerasaurus into the theropod lineage, so that means all three lineages are present in South America pretty much as soon as dinosaurs evolved," said Nesbitt. "Without Tawa, you can guess at that, but Tawa helps shore up that argument."