It's a New Dinosaur, Not a Young T. Rex

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 there's new evidence that resolves the case, per the AP. The latest clue comes from a complete skeleton—first uncovered in 2006 in Montana—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," says 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. 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.

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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. 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.