If you were to take a stroll through the Late Jurassic forest, roundabout 150 million years ago, you might spot little feathery dinosaurs hopping through the undergrowth. They’d look like miniature Velociraptor, complete with tiny sickle claws held off the ground. One might even briefly flutter in the air to nab an insect, or take a short glide down off a toppled tree trunk. And that would offer a critical clue to their real identity. They wouldn’t look very different from the famous “raptors”, but these meek dinosaurs mark the beginning of where birds start.

That’s what University of Edinburgh paleontologist Stephen Brusatte and his coauthors found when they looked at the big picture of bird evolution, throwing in a little scifi speculation along the way. In a new Current Biology paper on the grand evolutionary transition between birds and dinosaurs, the researchers write “we surmise that a Mesozoic naturalist would make no immediate distinction between a Velociraptor-type animal and an Archaeopteryx-type animal.”

The origin of the first birds is one of the most celebrated evolutionary transitions. Scores of fossils – with more found all the time – have confirmed that birds are dinosaurs. That’s why it might seem a little counter-intuitive that Archaeopteryx and other early birds were not very much different from their dinosaur ancestors.

After cataloging 853 skeletal characteristics in 150 dinosaurs and analyzing the rate at which these characters change, Brusatte and coauthors found that “there was no grand jump between nonbirds and birds in morphospace.” To put it another way, there was less difference between Velociraptor and Archaeopteryx than between other closely-related groups of dinosaurs, such as the parrot-like oviraptorosaurs and bizarre therizinosaurs.

View Images The relationships of coelurosaurian dinosaurs, including birds. Courtesy Stephen Brusatte, silhouettes by Scott Hartman, Nobumichi Tamura, Brad McFeeters, J. J. Harrison, “FunkMonk”, and Mike Keesey.

The only sticking point is that there are now so many bird-like nonbird dinosaurs that determining which group the first birds evolved from is a tricky task. Dromaeosaurids and troodontids – roughly, “raptors” that were quite similar to each other – are the strongest contenders. More fossils and new analyses will be needed to parse the split, but this is a happy problem to have. The exact jumping-off point for birds is so difficult to pin down simply because there are so many feathery dinosaurs perched right near the split. In fact, the problem mostly exists because the earliest members of that bird stem are the spitting image of their forebears.

Modern birds are quite different from any other vertebrates alive today. They seem that way because all of their close relatives are extinct, masking the fact that many of their unusual traits actually have a very deep history. A wishbone was a widespread trait among theropod dinosaurs. Allosaurus, a dinosaur not particularly close to birds, had one. The same goes for the air sacs that extend from the avian respiratory system. They were present in theropods as well as the long-necked sauropods, pointing to a common origin more than 70 million years before the first birds. Feathers are quite ancient, too, with an accumulating number of finds hinting that some kind of feather-like body covering might have been present in the very earliest dinosaurs, or at least evolved several times early in their history.

These traits – as well as a reduction in size and some incipient aerodynamic abilities – culminated in dinosaurs like Archaeopteryx, reaffirmed by the new study as the most archaic known member of the bird lineage. (In technical terms, this makes Archaeopteryx an avialan, or on the “stem” leading up to Aves.) From there, though, birds evolved faster than any of their close dinosaurian relatives. They proliferated into new forms and molded new niches, including toothy little flappers and loon-like diving birds by the Late Cretaceous, and continuing after the great K/Pg extinction until today.

The spark for this evolutionary explosion isn’t yet known. Flight could be a major factor, allowing birds to be adapted in starkly different ways from their earthbound ancestors. (Although, of course, nonavian dinosaurs like Microraptor found their own way into the air.) Paleontology thrives on such mysteries. For now, though, the new study underscores the fact that the change from dinosaur to bird is one of the most surprising and best-documented evolutionary transformations of all time. There is no sharp dividing line between dinosaur and bird. “Birds”, Brusatte and coauthors conclude, “are a contiuum of millions of years of theropod evolution.”

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