Primitive dinosaur species found in New Mexico

By analyzing nearly complete skeletons of a dinosaur found in northern New Mexico, researchers have obtained new insights into how the creatures lived and evolved more than 213 million years ago. The newly identified primitive dinosaur species, Tawa hallae, was not gigantic, with specimens attaining the size of medium-sized to large dogs. Their skeletons demonstrate the presence of air sacs in the braincase and neck areas, an evolutionary wrinkle later found in birds.

A report on Tawa will be published in the December 10, 2009 issue of the journal Science.

Tawa probably was a meat-eater. Its fossils were retrieved in the same deposits as two other carnivorous dinosaur species that are not closely related to it. This finding suggests to researchers that the animals didn't originate in North America, but likely migrated in from elsewhere. The fossils were excavated by scientists from several universities and museums, including Chicago's Field Museum and University of Chicago, in 2006. Scenes of the excavation of Tawa were filmed for the 3D feature movie Dinosaurs Alive! currently being shown at The Field Museum.

Nathan D. Smith, a researcher in The Field Museum's geology department who participated in the excavation, said that the completeness of Tawa's skeleton and lack of damage to the fossilized bones enabled scientists to see where air sacs were located, providing insights into the evolutionary path taken as dinosaurs became more like birds.

The specimens were found in the Hayden Quarry on the Ghost Ranch in northern New Mexico, near the town of Abiquiu. Their condition suggests that the animals likely died near where they were found and were buried quickly after death. "We were lucky to find skeletons nearly intact," Smith said.

Comparisons of the fossils from New Mexico with dinosaur fossils found in South America suggests that dinosaurs in the Late Triassic Period had varying degrees of ecological success in different locations. The absence of large plant-eating dinosaurs in New Mexico at a time when they were prevalent in South America raises questions of what caused the differences, Smith said.

Dinosaurs arose at the same time as several other animals, including crocodile relatives, mammals, pterosaurs, turtles, frogs and lizards. It appears that in some locales dinosaurs dominated, but elsewhere, they didn't.

"We can see there were no physical barriers to migration," he said. Climate may have played a role, as the Hayden Quarry was much nearer to the equator at the time the dinosaurs died than it is now. At that time the Earth had a single huge land mass, Pangea, that later broke into separate continents that moved to locations where they are now.

Tawa's discovery raises the possibility that dinosaurs orignated in South America, Smith said. Large, long-necked plant-eating dinosaurs apparently didn't live in North America during the Triassic, but may have had to pass through it when dispersing between the land masses that became South America, Africa and Europe, Smith said.

Source: Field Museum