Scientists thought this fossil was a teen T. rex. Turns out it's a new tyrannosaur

"It's known as the "Dueling Dinosaurs" fossil: A triceratops and a tyrannosaur, skeletons entangled, locked in apparent combat right up until the moment of their mutual demise. Even in the Hell Creek Formation in eastern Montana, a spot known for great finds, this specimen was, in a word, "fantabulous," says Clayton Phipps, a self-described rancher, cowboy and dinosaur hunter. That discovery in 2006 now appears to have overturned decades of dinosaur dogma about Tyrannosaurus rex, the fearsome giant long thought to be the sole top"

"In a paper in the journal Nature, paleontologists Lindsay Zanno and James Napoli conclude that some of the bones from that specimen belong not to a teenage T. rex, but to a fully grown individual of a different tyrannosaur species Nanotyrannus lancensis. The majority of paleontologists, after some debate, had dismissed the idea that any other tyrannosaurs existed besides T. rex. And yet, the sands of Montana seem to have disgorged a second one."

""We knew the specimen was exceptional," says Zanno. "We did not realize it would turn decades of research about T. rex on its head." "Just insanely good" The Montana dig began on a breezy summer day when Phipps' friend, Mark Eatman, found an exposed pelvis and femur at the bottom of a canyon. Phipps thought maybe there was half a dinosaur hidden below. But after two weeks of digging, Phipps and his older cousin revealed that those bones belonged to a stunning triceratops."

The "Dueling Dinosaurs" specimen from Hell Creek contains an entangled triceratops and tyrannosaur whose bones reveal unexpected taxonomy. Some bones previously attributed to a teenage Tyrannosaurus rex match a fully grown Nanotyrannus lancensis, and further analysis identifies a third tyrannosaur species once thought to be juvenile T. rex. The discovery challenges the long-standing view that T. rex was the only large tyrannosaur predator in the late Cretaceous. The Montana excavation revealed an exceptionally complete triceratops skeleton, and careful digging exposed additional tyrannosaur material that clarified species-level differences.

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