Bird in the hand

Fossilized fingers strengthen evolutionary link between dinosaurs and avian relatives

The hands of a newly discovered dinosaur species provide fresh support for the notion that birds are closely related to dinosaurs, researchers say.

BIRDY DIGITS The arrangement of bones (inset) in the hands of Limusaurus inextricabilis, a theropod dinosaur that lived in China about 159 million years ago, is similar to that seen in the wings of modern-day birds. P. Sloan (Inset: Clark)

Many paleontologists contend that theropods — a group of bipedal dinosaurs that, with rare exception, dined on meat — didn’t die out 65 million years ago with the rest of their kin. Analyses, those scientists say, show that this group lived on and gave rise to modern-day birds. But hardcore skeptics of that theory have long noted that the bone arrangement in birds’ wings doesn’t match the arrangement of bones in the hands of dinosaurs, says James M. Clark, a vertebrate paleontologist at George Washington University in Washington, D.C. Now, fossils of a new theropod species, described by Clark and his colleagues in the June 18 Nature, reveal that some theropods indeed had birdlike hand-bone arrangements.

The new species, an ostrich-sized dinosaur that lived in China about 159 million years ago, has been dubbed Limusaurus inextricabilis, the “mire lizard who could not escape.” Fossils of several of these creatures indicate that they became mired in a mudflat when its hard, thin crust gave way under their weight. Unlike most theropods, L. inextricabilis probably was a vegetarian: The species lacked teeth and had a beak. The clincher, Clark notes, is that some of the new fossils include gastroliths, or stomach stones — rocks swallowed to help grind vegetation and aid digestion.

But the most unusual feature of the newly described species is the arrangement of the fingers on its hands. Its innermost digit on each hand is much smaller than the other three digits, a trait not seen in any other theropods, Clark says.

While early theropods and their ancestors had four or five digits on their hands, the more-recent members of the group had three or less. Analyses of previously discovered fossils hinted that the “fingers” missing in later theropods had been lost from the outside of the hand, leaving only the three innermost digits, Clark says. But studies of developing bird embryos suggest that the bones within the wings of adult birds are equivalent to the second, third and fourth digits in dinosaur ancestors, he notes. Detractors of the evolutionary link between dinosaurs and birds say that this disparity proves that the two groups aren’t closely related.

Clark and his colleagues, however, contend that the new species weakens the skeptics’ argument by showing that at least some theropods lost their innermost finger.

The new species reveals diversity among theropods, especially among early members of the group, that wasn’t previously recognized, says Thomas R. Holtz Jr., a vertebrate paleontologist at the University of Maryland in College Park. “It turns out there were many ways to be a theropod,” he notes.

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