A new study turns perceived wisdom on its head, concluding that some dinosaurs may actually have descended from birds.
A new analysis used three-dimensional models to study the possible flight potential of an unusual fossil specimen discovered in 2003 called microraptor, and concluded that it must have been a glider that came down from trees.
The weight of the evidence is now suggesting that not only did birds not descend from dinosaurs, said John Ruben, a professor of zoology at Oregon State University, but that some species now believed to be dinosaurs may have descended from birds.
"There are just too many inconsistencies with the idea that birds had dinosaur ancestors, and this newest study adds to that," he said.
New research, Ruben said, is actually much more consistent with a different premise – that birds may have had an ancient common ancestor with dinosaurs, but that they evolved separately. After millions of years of separate evolution, he says, it was the birds that gave rise to the raptors.
Small animals such as velociraptor that have generally been thought to be dinosaurs are more likely flightless birds, he said.
"Raptors look quite a bit like dinosaurs but they have much more in common with birds than they do with other theropod dinosaurs such as Tyrannosaurus," Ruben said. "We think the evidence is finally showing that these animals which are usually considered dinosaurs were actually descended from birds, not the other way around."
Another study last year from Florida State University raised similar doubts, Ruben said.
In the newest study, scientists examined a remarkable fossil specimen that had feathers on all four limbs. Glide tests based on its structure concluded it would not have been practical for it to have flown from the ground up, but that it could have glided from the trees down, somewhat like a modern-day flying squirrel. Many researchers have long believed that gliders such as this were the ancestors of modern birds.
"This model was not consistent with successful flight from the ground up, and that makes it pretty difficult to make a case for a ground-dwelling theropod dinosaur to have developed wings and flown away," Ruben said. "On the other hand, it would have been quite possible for birds to have evolved and then, at some point, have various species lose their flight capabilities and become ground-dwelling, flightless animals – the raptors."
OSU scientists found that the position of the thigh bone and muscles in birds is critical to their ability to have adequate lung capacity for sustained long-distance flight, a fundamental aspect of bird biology. Theropod dinosaurs did not share this feature.
Perhaps most significant, birds appear in the fossil record before the elaboration of the dinosaurs they supposedly descended from. That would be consistent with raptors descending from birds, Ruben said, but not the reverse.
The study appears in PNAS.