The dinosaur world is buzzing this week because of a new paper that suggests the way we classify dinosaurs may be very wrong.

Back in 1888, a paleontologist named Harry Seeley said that all dinosaurs fall into two categories: bird-hipped (Ornithischia) and lizard-hipped (Saurischia). This has been accepted ever since. But a study published this week in Nature suggests that this division is wrong, essentially challenging the most basic division of dinosaur genealogy.

Under Seely’s original organization, the bird-hipped dinosaurs include ones with horns and armor, like the Triceratops and Stegosaurus. The lizard-hipped ones include theropods like the T-rex and sauropodomorphs like Brontosaurus. Matthew Baron, a doctoral candidate at the University of Cambridge, studied dinosaur fossils from around the world and catalogued them depending on 457 different features. Then, he used a computer program to analyze them. Based on these features, the computer program suggested the arrangement that makes the most sense — and it wasn’t the one that already existed.

The new tree puts the theropods — which were originally considered lizard-hipped — together with the bird-hipped dinosaurs into a combined group. As Ed Yong of The Atlantic put it, “This is like someone telling you that neither cats nor dogs are what you thought they were, and some of the animals you call ‘cats’ are actually dogs.”

This proposal isn’t written in stone, and it might not be fully accepted. “It’s a radical proposal with a reasonable basis but no one expects it will be the last word,” Kevin Padian, a biologist at the University of California, Berkeley, told the New York Times.

He continued that dinosaur classification can be tricky, because there have been so many new discoveries int he past few decades and a lot of these dinosaurs have evolved in ways that make it hard for them to place on the family tree. So don’t be too quick to think that just because this key division is wrong, dinosaur experts don’t know what they’re talking about. But it may time to look a little more closely.