WEDNESDAY, March 4 (HealthDay News) -- Meat-eating dinosaurs that walked on their hind legs (theropods) and held their arms with their palms faced inward -- just like birds -- were roaming the Earth at least 198 million years ago.
It's long been known that theropods, such as Tyrannosaurus rex and Velociraptor, held their arms this way. But a lack of clear fossil evidence has made it difficult to determine when this bird-like posture evolved.
A report published online this week in the journal PLoS One details fossilized handprints and footprints made by a large, meat-eating dinosaur about 198 million years ago. The tracks in rocks in St. George, Utah, were made by a theropod that sat down and extended its arms far enough that they left marks in the ground.
"The crouching tracks are preserved on the shore of an ancient lake," the paper's lead author, Andrew Milner, a paleontologist at the St. George Dinosaur Discovery Site at Johnson Farm museum, said in a journal news release. "The theropod that made the tracks may have been in the lake but walked out and up a shallow slope and sat down for some reason."
"These tracks are important because the handprints have inward-pointing fingers, showing that even very early theropods had bird-like arms and hands with inward-facing palms," Milner said.
Co-author Jerry Harris, of Dixie State College in St. George, said in the news release that "while theropod arms couldn't pivot to make the palms face up or down the way a human's hands can, they certainly could move."
"In particular, the wrist could pivot up and down, allowing the outside of the hand to move toward the side of the arm, a motion impossible for humans," Harris said. "But this is the same motion birds have that allows them to fold their wings."
"These tracks show that this ability evolved long before feathery wings did, and much earlier than this posture is known from theropod skeletons," he said. "Birds only inherited this ability from their ancestors."
The theropod tracks were found in a large area that also included the fossilized tracks of more than 1,000 large and small dinosaurs and small, early relatives of alligators and crocodiles.
The University of California Museum of Paleontology has more about theropods.
SOURCE: PLoS One, news release, March 3, 2009