Why did birds get dinosaur status?

Show a young child a feather and they will proudly say, “bird”. Show an older child the feather and they might gleefully say, “dinosaur”. It’s a source of smugness my generation missed out on because, back then, dinosaurs were supposed to be covered in scales, not to mention extinct. What changed, and if birds really are dinosaurs, why did it take so long to make the connection?

Archaeopteryx / by Peter Montgomery / CC BY-NC 2.0

The first “missing link” between reptiles and birds was found way back in the 1860’s. Archaeopteryx had a long tail with many vertebrae, as well as strong curved claws - like a dinosaur - but its feathered wings were enough to define it as a primitive bird by simply stretching the definition of birds. When teeth were found on some specimens (the first discovery was headless), the solution was to stretch the definition of birds a little further.

Enter Thomas Henry Huxley, the outspoken champion of Darwin’s freshly minted theory of evolution. In a New York lecture in 1876, Huxley described Archaeopteryx as an example of “animals which overstepped the bounds of existing groups”. It was evidence that evolution could produce a wide range of creatures, but he thought the links in the chain from reptiles to birds lay not in Archaeopteryx but elsewhere.

Compsognathus / by Nobu Tamura / CC BY 2.5

Huxley was intrigued by the striking resemblance between the skeletons of turkey-sized dinosaurs named Compsognathus and modern birds. While on the one hand not ruling out that birds may have existed long before the 150-million-year-old dinosaur, making the two only distantly related, he also recognized that one feature would suggest a closer family link.

“There is no evidence that Compsognathus possessed feathers,” said Huxley. “But, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.”

It came down to feathers. Archaeopteryx had them, and was considered a primitive form of bird; Compsognathus lacked them, and thus was a dinosaur.

Compsognathus probably did have feathers, but for Huxley and generations of paleontologists after, defining an extinct creature by the presence or absence of feathers was a problem. That's because feathers rot away too quickly to fossilize; the best that can be found is the impression of feathers, and that is rare. Even fossilization of skeletons is a rare occurrence. A dying creature must first fall into mud or silt to cover the body and delay rotting. Water flowing over the dead body then gradually dissolves minerals in the bone, creating a space in which minerals from the water can crystallize. Eventually a new rock hardens that contains these crystals and preserves the shape of the skeleton. Only on the most detailed fossils will the impression of feathers be preserved, which means the lack of feathers on a fossil does not mean a lack of feathers in life.

Deinonychus / by Eloy Manzanero Criado / CC BY-SA 3.0

With no feathered fossils to force the issue, Huxley's ideas were all but forgotten for much of the 1900’s. Birds were thought to have evolved from a reptilian ancestor who lived before the first dinosaur. The two groups were therefore thought to have shared a common ancestor.

The idea that birds might be more closely related to dinosaurs, might even have evolved from them, only started to gain supporters in the 1960’s when the paleontologist John Ostrom pointed out the similarities between the agile, high-energy predator Deinonychus and birds. Even then, almost a century after Huxley, those that were reluctant to accept the idea needed only point out the crucial missing feature: feathers.

So the debate continued until 1996, when a 125-million-year-old fossil of Sinosauropteryx was discovered in northeast China. This “Chinese lizard wing” looked like a dinosaur, but its primitive feathers led to its initial classification as a bird.

Sinosauropteryx /  CC BY 4.0 /
by Fiann M. Smithwick, Robert Nicholls, Innes C. Cuthill, Jakob Vinther

Soon this Yixian formation in China was revealing the remains of many more feathered creatures, thanks to a fine volcanic ash that had fossilized in intricate detail. That feathers did not belong to birds alone quickly became indisputable. Of course, feathers themselves do not make flight, but the extinct dinosaurs used their feathers in a variety of other ways, from insulation to attraction to egg warming to camouflage. Some feathers were primitive, mere filaments jutting out from a reptile’s skin, others were the barbed feathers that enable birds to fly today.

The old classification system that held dinosaurs and birds apart could stretch no further. Sinosauropteryx was even re-labelled a dinosaur. But if many dinosaurs had feathers, where was left to draw a line between them and the birds?

In 1998, the highly respected journal Nature made its pronouncement: “Birds and dinosaurs - the debate is over.” The article declared that recent finds were enough to “close the debate on whether or not birds and dinosaurs share a close evolutionary heritage. The answer is a resounding ‘yes’.”

Today it is accepted that birds are dinosaurs. They evolved from a group of bipedal dinosaurs with bird-like hollow bones and three toes called the theropods, a group that included T. rex - as any cheeky kid will tell you.

Further reading:

How Dinosaurs Shrank and Became Birds, Quanta magazine

Dinosaurs' Living Descendants, Smithsonian.com

The Origin and Diversification of Birds, Current Biology