“You guys are brave,” I told Baron on the phone.

“Or stupid,” he says, laughing, with a tinge of nervousness.

If Baron is right, how could such a well-established fact about dinosaurs turn out to be wrong?

Partly, people may have been blinded by their preconceptions. The split between saurischians and ornithischians was first proposed in 1887 by British paleontologist Harry Seeley, who divided the groups based on the shape of their hips—a seductively obvious difference. “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,” says Baron. “People then go into their studies with that mindset, and Seeley’s idea has never been rigorously tested.”

That’s not to say that previous researchers did sloppy science. They worked with what they had. It certainly didn’t help them that the earliest dinosaurs, which tell us most about how the group first evolved, are also the rarest.

If you collapsed the entire reign of the dinosaurs into a single calendar year, icons like Tyrannosaurus and Triceratops only appeared after Christmas, Velociraptor was only alive in mid-December, and Stegosaurus and Diplodocus arose in early July. There are far fewer fossils from the first half of the dinosaurs’ history in the Triassic and early Jurassic periods. And among the species from those pioneering millennia, the saurischians commanded most of the attention. Early ornithischians are seldom unearthed.

Those were the species Baron originally set out to study. As he pored over the few existing specimens, he got a sense that they were oddly theropod-ish. “Other people had been sniffing around this but because the [saurischian/ornithischian split] is so dogmatic, they didn’t follow it up,” says Baron. “When I talked to David Norman, my supervisor, he had this wry smile, and confessed that he always had the same hunch. And he said: Wouldn’t it be interesting to test that?”

Baron spent three years gathering a huge data set on 74 species of dinosaurs and dinosaur-adjacent species. He largely ignored the late-arriving celebrities and focused from the Triassic and early Jurassic. For each species, Baron painstakingly noted and measured 457 physical characteristics. He then used these traits to create his own family tree. “The more species that are included in these analyses, the more robust the result,” says Susannah Maidment, from the University of Brighton. And Baron “examined many, many more species than have previously been used,” with computing power that just wasn’t available in previous decades.

Baron’s tree rewrites the first chapters of the dinosaur story in several ways. It suggests that they first arose around 247 million years ago, slightly earlier than the 231 to 243 million year range that’s typically cited. It hints that they might have originated in the northern half of the world rather than the southern half. And most importantly, it says that the ancestral dinosaurs split into two major groups—just not the ones we traditionally recognize.