Quirks & Quarks

Tiny tyrannosaur fossil helps scientists understand how T-rex grew so large

This finding helps to fill in a big gap in the evolutionary history of the fearsome T-rex
A life reconstruction of North America’s newest tyrannosaur - Moros intrepidus (Lindsay Zanno)

Scientists have discovered a fossil from oldest and tiniest Cretaceous tyrannosaur ever found in North America — one that's baby-sized compared to its descendant, the ferocious and massive Tyrannosaurus rex.

But even a baby-sized tyrannosaur would have been intimidating. "It would have been 5 feet high at the head, so this is an animal that would have been able to stare you right in the eyes," said Dr. Lindsay Zanno, the head of paleontology at the North Carolina Museum of Natural Sciences and a research professor at N.C. State University in Raleigh.

Her team had been working in Utah looking for fossils for ten years before they discovered what's turned out to be a hind leg of a fully grown new mini tyrannosaur.

"When I first found these bones sticking out of the side of a hill and recognized them as being from a small theropod, it was really, really exciting for the team," said Zanno.

Figuring out it was a fully grown dinosaur

Like tree rings, Zanno said they were able to cut inside the bone to see the preserved rings that represent growth cycles for each year that the dinosaur was alive.

"We're able to look at those rings and count up that this animal was older than seven years old and also that its growth was slowing down," said Zanno about how they were able to determine this tyrannosaur was approaching its adult body size and confirm it was a tiny tyrannosaur and not a juvenile of a bigger species."

How it evolved into the T-rex — the apex giant

Prior to this discovery, there was a 70 million year gap in the fossil record in North America leading up to the evolution of the T-rex. By studying this mini tyrannosaur, which dates to around 96 million years ago, Zanno says they can now start to fill in that gap in the tyrannosaur evolution.

Before the mighty T-rex's showed up, a different kind of dinosaur "ruled the roost in North America" — the giant allosaur, which were almost as big as the T-rex.

This animal would have been an incredibly efficient hunter.- Dr. Lindsay Zanno, North Carolina Museum of Natural Sciences

"The tyrannosaurs that were living in the same ecosystems as these giant allosaurs were really quite small. They were secondary hunters," said Zanno. "They weren't able to really do much until actually an extinction wiped out these allosaurs around this time. And that freed the tyrannosaurs of the day up for ascending to take over the food chain in the late Cretaceous."

Mini, yet ferocious

This fossil gives paleontologists a look back in time to see how the tyrannosaurs became the successful top predator of its time.

With this fossil and other teeth Zanno previously had found that also belong to this mini tyrannosaur, along with skulls that were found in Asia before the tyrannosaurs likely walked over on a land bridge to North America, they can see this mini version would have had sophisticated sensory systems, speed, and agility.

"This slender leg bone was highly adapted for running very quickly," said Zanno. "This animal would have been an incredibly efficient hunter."


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