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Meet Moros intrepidus: A Tiny Ancestor of Tyrannosaurus Rex

Scientists predict that Moros intrepidus is the first of many discoveries that will help to paint a picture of North America during the early Cretaceous period.

By Gabe Allen
Aug 22, 2023 3:00 PM
Moros intrepidus
Artistic interpretation of Moros intrepidus. (Credit: Jorge Gonzalez)


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During the late Jurassic period around 150 million years ago, a small tyrannosauroid called Stokesosaurus lived in North America. This tiny carnivore had to keep an eye out for the much larger Allosaurus while hunting and scavenging.

But by the late Cretaceous period, around 66 million years ago, Allosaurus was long gone and tyrannosauroids had evolved into hulking, ferocious top predators. Case in point: the infamous Tyrannosaurus rex.  

For many decades, scientists could not find any North American tyrannosauroid fossils between these two periods. It was as if these dinosaurs had disappeared in the late Jurassic and reappeared larger and more ferocious 84 million years later.

Then, in 2019, a group of American paleontologists announced they had unearthed a remarkable specimen in the Utah desert.

Who Was Moros intrepidus?

The team, led by the North Carolina Museum of Natural History’s Lindsey Zanno, found leg bones and teeth from a tyrannosauroid that they named Moros intrepidus. The fossils dated back to the middle of the Cretaceous period, 30 million years before T. Rex showed up.

Moros intrepidus fossils (Credit: Lindsay Zanno/Communications Biology/Open Access)

M. intrepidus appears to have had powerful hind legs, suggesting that it was fast and agile. Like its Jurassic ancestors, however, it was not large.

“This guy was a small carnivore that probably would have had to worry about other carnivores,” says College of Eastern Idaho paleontologist L.J. Krumenacker.

This realization, that tyrannosauroids were still small and unassuming during the middle of the Cretaceous, makes late-Cretaceous tyrannosaurids like T. Rex even more remarkable; the dinosaurs somehow evolved from marginal scavengers to top predators in just 30 million years.

When M. intrepidus walked the Earth, present-day Utah was at the humid edge of a shallow sea that divided the west part of North America from the east. It lived in a lush ecosystem that supported many other animals.

Read More: Is the T. Rex Three Different Species?

Untapped Treasure Trove?

Unfortunately, the identities of the dinosaur’s neighbors and relatives remained largely unknown, as few other specimens had been recovered in the region — that is, until two years ago, when Krumenacker found one more piece of the puzzle.

Ever since Zanno’s team had discovered the fleet-footed M. intrepidus, Krumenacker says, he had been on the lookout for similar fossils at his research sites in Eastern Idaho.

“I’ve always worked these rocks in Idaho that are about the same age as the ones Moros was found in,” he says. “I remember thinking, ‘Wouldn’t that be cool if we could find something similar there?’”

And sure enough, in 2021 he unearthed a thigh bone from another tyrannosauroid about the same size as M. intrepidus. His finding, which was published in Cambridge University Press’ Journal of Paleontology, suggests that Moros may be the first of many similar discoveries — if scientists look in the right places.

“Rocks of this age in Utah and Idaho and Southwest Montana are just starting to get looked at,” Krumenacker says. “There’s a lot of potential left and a lot of extra-weird animals to find.”

Read More: These 7 New Dinosaur Species Were Unearthed In 2022

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