Mysterious dinosaur skeleton thought to be a teenage T. rex is actually a rival species, researchers say

Scientists have long puzzled over the origins of a mysterious dinosaur excavated in the 1940s: Was it a young T. rex or another type of dinosaur?

At first, researchers had only a tyrannosaur skull to go by, making it hard to tell if it belonged to a child or adult. Another skull and skeleton nicknamed Jane added to the debate, but didn’t settle the controversy.

Now, a research team says new evidence resolves the case. The latest clue comes from a complete skeleton — first uncovered in Montana in 2006 — that scientists say identifies the mystery reptile as its own species and not a juvenile Tyrannosaurus rex.

The discovery “rewrites decades of research on Earth’s most famous predator,” said study co-author Lindsay Zanno with the North Carolina Museum of Natural Sciences and North Carolina State University.

Growth rings within the bones found in Montana’s Hell Creek Formation told scientists the new dinosaur was an adult about half the size of a fully fledged T. rex. From growth comparisons to other reptiles like crocodiles, they also found that the major differences between the creature’s skull and an adult T. rex’s — changes in bone structure, nerve patterns and sinuses — were unlikely to form from simply going through puberty.

Signs pointed to a dinosaur that’s a distant T. rex cousin known as Nanotyrannus lancensis, the researchers reported in a study published Thursday in the journal Nature, calling it a rival species. 

“Nanotyrannus is a small-bodied predator designed for speed. It’s very agile and has long powerful arms [that are] larger than those of the T. rex,” Zanno told Nature.

There’s now “more support and evidence than there ever has been” that this T. rex relative could exist, said Holly Woodward, a fossil bone expert from Oklahoma State University who had no role in the new study. But she’s not yet convinced that the other mystery skeletons like Jane are something new.

Case of mistaken identity  

Other independent scientists also said the debate isn’t over. The new skeleton is indeed an adult, but it could be a sister species to T. rex and not a distant relative, said vertebrate paleontologist Thomas Carr of Carthage College.

There are similarities between the shape of T. rex’s skull and the mystery specimens that keep him from switching camps.

“I don’t think this study settles everything,” he said.

Resolving this case of mistaken identity is important to understanding how T. rex grew up, said study co-author James Napoli with Stony Brook University. Another big question is whether T. rex was the main predator prowling toward the end of the age of dinosaurs 67 million years ago — or whether a tinier, but still mighty predator also roamed.

“I suspect that these two species would have occasionally come into conflict, as predators tend to do, but the long legs of Nanotyrannus, and its small size, suggest that it mostly hunted smaller, faster prey than Tyrannosaurus,” Napoli told the Reuters news agency.

The new skeleton is dubbed “Dueling Dinosaurs” because it was found intertwined with the bones of a Triceratops, and is currently on display at the North Carolina Museum of Natural Sciences.

The study comes just days after new research suggested dinosaur populations were still thriving in North America before the asteroid strike. The evidence came from analyzing a portion of the Kirtland Formation in northern New Mexico that’s been known for around 100 years to contain several interesting dinosaur fossils.

In June, scientists identified a smaller-bodied ancestor of the T. rex that roamed the plains of Mongolia as a new tyrannosauroid named Khankhuuluu mongoliensis. Apex predators, including the T. rex, eventually arose from these smaller-bodied tyrannosauroids, the study’s researchers said.