June 17, 2009—The fossil hand of a long-necked, ostrich-like dinosaur recently found in China may help solve the mystery of how bird wings evolved from dinosaur limbs, according to a new study.
The ancient digits belonged to a 159-million-year-old theropod dinosaur dubbed Limusaurus inextricabilis. Theropods are two-legged dinos thought to have given rise to modern birds.
Although it was a distant relative of Tyrannosaurus rex, the newfound dinosaur was a small herbivore, said study co-author James Clark, a biologist at George Washington University in Washington, D.C. The animal was about 5.6 feet (1.7 meters) long and had relatively short, clawless forearms.
"Its head is [also] unusual because it doesn't have any teeth, so it would have had a beak of some sort, although not a sharp one," Clark said. (Related: "New Dinosaur Was Nut-Cracking 'Parrot.'")
Primitive feathers may have covered the dinosaur's body, but there is no direct evidence for that, noted Clark, whose work was funded in part by the National Geographic Society. (The National Geographic Society owns National Geographic News.)
L. inextricabilis appears to be an evolutionary snapshot of the transition between dinosaur fingers and the digits in modern bird wings, according to the study authors.
Theropod hands and bird wings each have three bones that appear to have evolved from the digits on a common five-fingered ancestor. But the dinosaurs were thought to have retained the first, second, and third fingers, while birds kept the second, third, and fourth. (Read more and see an illustration of dino and bird finger positions at the Stones, Bones 'n Things blog.)
"This didn't really add up, because all the other evidence is suggesting that birds are related to dinosaurs," Clark said.
L. inextricabilis, however, has four digits. The dinosaur's first finger is greatly reduced, while the second finger is enlarged.
"So here you have a form that's reduced the first finger, and it is right in this period of transition in the evolution of theropods," Clark said.
The find helps fill in what had been seen as a "chink" in the otherwise widely supported theory that birds descended from dinosaurs, said Hans Dieter Sues, a paleontologist at Smithsonian's National Museum of Natural History who was not involved in the research.
"Having a fossil that shows that there are three digits plus this sort of little residual digit
is almost a perfect structural intermediate" linking theropods and birds, he said.
Findings appear this week in the journal Nature.