This 74-Million-Year-Old Dinosaur Bone May Have Belonged to a Surprisingly Large Ancestor of Tyrannosaurus Rex

The massive reptile may have weighed more than 4.5 tons and been 35 feet long—much bigger than its related peers at the time

Margherita Bassi | Daily Correspondent

March 16, 2026

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A huge 74-million-year-old shin bone found in New Mexico may have belonged to a close relative of the famous Tyrannosaurus rex, according to a study published March 12 in the journal Scientific Reports. The finding hints at the origins of the Tyrannosaurus genus, whose members became the largest known land predators—until an asteroid strike killed them 66 million years ago.

The ancient bone came from the Kirtland Formation, a fossil site in the San Juan Basin, and researchers suspect it belonged to a tyrannosaur. Tyrannosaurs were a broad group of meat-eating dinosaurs that emerged around 170 million years ago, but didn’t give rise to massive animals like those in the Tyrannosaurus genus until tens of millions of years later. (T. rex, for instance, roamed the Earth starting about 68 million years ago.) Many tyrannosaurs before then may have been roughly human-size.

That’s why the newly uncovered shin bone, or tibia, came as such a surprise. It belonged to a massive creature, as it measures nearly 38 inches long and about 5 inches in diameter. That’s more than three-quarters the size of the equivalent bones on one of the largest known T. rexes, Sue, who’s housed at the Field Museum in Chicago. The fossil’s location in the rock layers led researchers to date its owner to 74 million years ago, which corresponds with the Late Campanian age.

“It is a very large tyrannosaur for its time, and much older geologically than anybody would have predicted,” study co-author Spencer Lucas, a paleontologist at New Mexico Museum of Natural History and Science, tells Popular Science’s Laura Baisas.

Given the tibia’s dimensions, giant size, straight shaft and triangular-shaped lower end, Lucas and his colleagues suggest that it may have been part of an animal closely related to Tyrannosaurus. And based on weight estimates for Sue, they calculated that the ancient relative may have weighed more than 4.5 tons and stretched about 35 feet long, earning it the title of biggest dinosaur among its tyrannosaur peers.  

It was probably “small by Tyrannosaurus standards, but maybe 50 percent more than anything we know of from that time period,” study co-author Nick Longrich, a paleontologist at the University of Bath in England, tells Science News’ Carolyn Gramling. “Just really chunky.”

While analyses hint that the dinosaur was a direct ancestor of T. rex, finding a skull would really help clinch whether that’s the case or if it was instead a close cousin of the famed predator, Longrich tells Reuters’ Will Dunham.

Still, the finding could support the idea that the Tyrannosaurus genus emerged in what is now North America, rather than in modern-day Asia. Its members were the top dogs in both locations during the Late Cretaceous, but researchers aren’t sure where they first popped up.

But not everyone is convinced. Thomas Carr, a vertebrate paleontologist at Carthage College who didn’t participate in the study, tells Science News that the bone may have belonged to a different tyrannosaur genus called Bistahieversor. They’re the only type known from the geologic unit where the fossil came from, he notes.

The study authors suggest that the tibia is too big to belong to this genus, but Carr says that tyrannosaur leg bones can be tricky to place. For instance, the leg bones of young tyrannosaurs were often narrower and more bowed than those of adults.

Regardless, the research team agrees that more specimens of this strange dinosaur are needed. “We are developing our plans to expand to search for more material,” study co-author Anthony Fiorillo, a paleontologist at the New Mexico Museum of Natural History and Science, tells Reuters.

Margherita Bassi | Read More

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.