If you’re a dinosaur geek, #Ornithoscelida will rock your world. Dinosaur Twitter (yes, it is a thing) went wild this week with the news that a new study has uprooted the dino family tree.

For 130 years, our teachers, textbooks and museums have all taught us the same thing. Dinosaurs could be divided into one of two main groups, based on a seemingly obvious physical difference: the shape of their hips.

But what if that theory—first proposed by British paleontologist Harry Seeley in 1887—was all wrong? A new study, led by Ph.D. candidate Matthew Baron of Cambridge University (U.K.) and published in the journal Nature this week, is arguing exactly that, effectively shattering everything we thought we knew about dinosaurs by radically reorganizing their family tree.

It also suggests that dinosaurs first emerged around 247 million years ago, as many as 15 million years earlier than previously believed. And because some of the earliest dinosaurs on the reworked family tree were found in the northern hemisphere, Baron’s work contradicts existing thinking on the geographic origins of dinosaurs as well. As he told BBC News, “The northern continents certainly played a much bigger role in dinosaur evolution than we previously thought and dinosaurs may have originated in the UK.”

Model of a Triceratops skeleton.
Model of a Triceratops skeleton. (Credit: Hulton Archive/Getty Images)

Saurischians, whose hip bones were more similar to reptiles, included both meat-eating theropods like Tyrannosaurus rex and long-necked, plant-eating sauropodomorphs like Brontosaurus. Ornithischians, from horned Triceratops to armored Stegosaurus, were strictly vegetarian, and had more bird-like hips. Fossils of earlier dinosaurs, from the Triassic and early Jurassic periods, are relatively rare—but they provide the most info about dino origins and evolution. When Baron originally began his research a few years ago, he studied the few existing specimens of early ornithischians, and quickly got the sense that they resembled theropods.

He wasn’t the first to have this idea. Back in 1870, the biologist Thomas Henry Huxley had argued (correctly, it turns out) that modern birds descended from theropods, and included the bird-hipped dinosaurs in a group he called Ornithoscelida, meaning bird-limbed. At the time, however, Huxley’s ideas were dismissed in favor of Seeley’s.

Why did Seeley’s theory become so dominant over so many years? According to Baron, paleontologists might have been seduced by the seemingly obvious distinction between hip shapes. “When you look at specimens, it’s very quick and easy to say: that’s a saurischian, that’s an ornithischian, and never the two shall meet,” Baron told the Atlantic. “People then go into their studies with that mindset, and Seeley’s idea has never been rigorously tested.”

The world's most complete Stegosaurus fossil on display at London's Natural History Museum. (
The world’s most complete Stegosaurus fossil on display at London’s Natural History Museum. (Credit: Carl Court/Getty Images)

Baron set out to do just that. Acting on his hunch, he built a huge data set with info on 74 species of dinosaurs and dinosaur-related species, mostly from the Triassic and early Jurassic. He then measured 457 physical characteristics from each and used traits to draw a new family tree.

The ornithischian-theropod grouping, which Baron also calls Ornithoscelida, is definitely the biggest bombshell in the new research. No single feature, such as birdlike hips, unites the newly defined group. Instead, members of Ornithoscelida share at least 21 features, which are much more difficult to pin down. When taken together, this mishmash of features means that T. rex is more closely related to Triceratops than to Brontosaurus, contrary to what paleontologists have always believed. On the other side of the family tree is what Baron calls Saurischia “v 2.0,” which groups Brontosaurus and other sauropodomorphs with herrerasaurids, a more obscure group of early meat-eaters.

Much of the reaction to this potential revolution in dino research has been positive, but it remains to be seen whether Baron’s theories will hold up under further testing. At least one paleontologist, Professor Hans Sues of the Smithsonian Museum in Washington, D.C., is not on board quite yet. “I am skeptical as none of the other recent analyses obtained similar results,” Sues told BBC News. “But I keep an open mind.”