The more evidence we gather about the evolution of birds, the more likely it seems that they evolved from dinosaurs. But when, exactly, did birds split from their awe-inspiring cousins? A new discovery will help you imagine what it was like to live during the age of birds and dinosaurs.

As Brian Switek explains over at his National Geographic blog Laelaps, there was probably a long period when the earliest birds looked almost exactly like dinosaurs. The truth is that speciation, an event where one species diverges into two or more, is often a murky process. There was no single moment when dinosaurs became birds, but instead a gradual process of evolution.

Switek writes:

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 Biologypaper 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 anArchaeopteryx-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 andArchaeopteryx than between other closely-related groups of dinosaurs, such as the parrot-like oviraptorosaurs and bizarre therizinosaurs.

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Here's a chart that can help you think about the relationships between T. rex and today's birds. At the bottom, you can see the forerunners of birds are basically romping around in the same jungles as dinosaurs.

Still, we aren't sure which dinosaurs were really the forebears of birds. There are a lot of contenders, partly because so many dinosaurs looked similar to birds and shared skeletal features with them like wishbones.

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Read more about the ongoing mystery of bird evolution at Laelaps

Chart via Stephen Brusatte