Some 'Dinosaurs' Evolved from Birds?

Sunday, June 26, 2011

Keep in mind that animals can evolve similar traits independently. The accepted transition from dinosaur to bird, or in this proposed case—bird to dinosaur—didn't necessary follow a simple path from large beast to tiny, feathered flier. For example, some dinosaurs are thought to have had feathers and beaks, traits we now tend to associate with birds. It's also believed that some dinosaurs increased in size, shrunk, and then became large again. The evolutionary paths, in other words, don't always follow certain, predictable courses, since animals are constantly adapting to ever-changing habitats and climates.

The new PNAS paper doesn't entirely surprise me, because there have been recent discoveries of very bird-like dinosaurs that weren't even very closely related to birds. Check out our story on Haplocheirus sollers, for example. I tend to agree with Jonah Choiniere, lead author of that Science paper, who believes the first birds emerged out of the Maniraptora, aka "hand snatcher," clade, but birds and dinosaurs from that point on then went down different evolutionary paths.

John Ruben, a professor of zoology at Oregon State University, authored a commentary on the PNAS paper. Ruben doesn't dispute that birds and dinosaurs likely shared a common ancestor. Per the study, however, he suggests that once birds started down their own evolutionary path they may have given rise to raptors. This is where the debate heats up because he and others contend that very bird-like 'dinosaurs,' such as Velociraptor, may have actually been more bird than dinosaur.

"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."

He believes birds, on the other hand, may not have descended from ground-dwelling theropod dinosaurs, but instead from a gliding animal that moved somewhat like a modern day flying squirrel.

(An image drawn in 1915 by naturalist William Beebe suggests a hypothetical view of what early birds may have looked like, gliding down from trees - and it bears a striking similarity to a fossil discovered in 2003 that is raising new doubts about whether birds descended from ground-dwelling theropod dinosaurs. Photo courtesy of Oregon State University.)

"We're finally breaking out of the conventional wisdom of the last 20 years, which insisted that birds evolved from dinosaurs and that the debate is all over and done with," Ruben said. "This issue isn't resolved at all. There are just too many inconsistencies with the idea that birds had dinosaur ancestors, and newest study adds to that."

He added, ""Pesky new fossils...sharply at odds with conventional wisdom never seem to cease popping up. Given the vagaries of the fossil record, current notions of near resolution of many of the most basic questions about long-extinct forms should probably be regarded with caution."

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