A deer-sized T. rex ancestor shows how fast tyrannosaurs became giants

A new dinosaur shows that even Tyrannosaurus rex had humble beginnings.

Dubbed Moros intrepidus, or “the harbinger of doom,” the new species is one of the smallest tyrannosaurs yet discovered from the Cretaceous Period. Analyses of the animal’s fossilized leg show that the creature would have stood only 1.2 meters at the hip, and weighed an estimated 78 kilograms — about the size of a mule deer, researchers report February 21 in Communications Biology.

Dating to around 96 million years ago, the fossil is the oldest Cretaceous tyrannosaur found in North America. Its discovery helps fill in a 70-million-year gap in the evolution of tyrannosaurs leading up to the ferocious giants like T. rex.

Teeth from early, petite tyrannosaurs have been found in rocks in North America dating to the Late Jurassic Period around 150 million years ago, when larger predator dinosaurs called allosaurs topped the food chain. But the next time tyrannosaurs are seen in the North American fossil record is 70 million years later, when they’ve become the colossal top predators. When, and how, the dinosaurs sized up within that period is a mystery.

Paleontologist Lindsay Zanno of North Carolina State University in Raleigh and her colleagues dug for 10 years around Emery County in Utah, searching for clues to solve that mystery. That’s where the team discovered M. intrepidus’ long, thin leg, a characteristic indicative of a swift runner, quite unlike later titanic tyrannosaurs.

“What Moros shows is that the ancestral stock of the big tyrannosaurs was small and fast,” says Thomas Carr, a vertebrate paleontologist at Carthage College in Kenosha, Wis., who wasn’t involved in the study. And it “suggests that the tyrannosaurs became giant some time in that 16-million-year stretch between Moros and the earliest of the big guys.”

Comparing M. intrepidus’ fossil traits with other tyrannosaurs to see where it fit in the tyrannosaur family tree, the researchers determined that M. intrepidus emerged from Asia. It was part of a great migration that included mammals, lizards and dinosaurs moving between modern Siberia and Alaska during occasional drops in sea levels, the authors suggest.

Eventually, the Cretaceous Period’s warming climate probably killed off the allosaurs, Zanno says, but not tyrannosaurs. “They rapidly increase in size and go on really quickly to become the dominant predators of Late Cretaceous ecosystems,” she says.

Even with the discovery of M. intrepidus, the picture of tyrannosaur evolution remains incomplete. “It’s great that [the new fossil] helps fill in part of the history,” says vertebrate paleontologist and tyrannosaur expert Thomas Holtz Jr. at the University of Maryland in College Park. But scientists need to find the rest of the skeleton of M. intrepidus as well as other tyrannosaurs in the narrow 16-million-year span between M. intrepidus and its giant descendants to help pinpoint when the creatures grew in size. “The story of tyrannosaurs is definitely not over,” he says.

Editor's note: This story was updated February 23, 2019, to clarify that the newly discovered tyrannosaur is the oldest Cretaceous Period tyrannosaur fossil found in North America, not the oldest overall.