Decades-Long Debate on ‘Teenage’ Tyrannosaur Fossils Takes Another Turn

A new paper adds to evidence suggesting a group of disputed fossils, identified by many scientists as young T. rex, are actually another species

two dinosaurs, one larger than the other, fighting in an illustration
An illustration of Nanotyrannus attacking a juvenile T. rex Raul Martin

From the familiar silhouette adorning the “Jurassic Park” logo to Sue, the 40-foot-long specimen that greets visitors to Chicago’s Field Museum, the adult Tyrannosaurus rex skeleton is one of the most recognizable and beloved fossils in popular culture and the scientific community.

But when it comes to juveniles, a debate has raged among paleontologists, spurred by a 1942 discovery of a small, T. rex-like skull in Montana. Some believed the fossil belonged to a T. rex that had failed to reach maturity—a “teenager,” as some would later call it—though others argued the fossil represented a new species altogether, called Nanotyrannus lancensis.

Over the decades since, a growing number of researchers have weighed in on the discussion and examined additional fossils, with studies piling supporting evidence to both sides of the debate.

“It’s ultimately a quite in-the-weeds question of the taxonomy and the classification of one very particular type of dinosaur,” Steve Brusatte, a paleontologist at the University of Edinburgh, tells the New York Times’ Julia Jacobs and Zachary Small. “However, it involves T. rex, and the debate always gets a little bit more ferocious when the king of dinosaurs is involved.”

Now, in a paper published Wednesday in the journal Fossil Studies, paleontologists Nicholas Longrich and Evan Saitta have submitted the latest entry into the debate’s domain—and it’s another score for team Nanotyrannus.

The researchers compared the skull from Montana and other disputed fossils to T. rex skulls already in the fossil record. They identified more than 150 distinct ways in which the groups differ, including a slimmer snout and smoother teeth on Nanotyrannus. In all, the duo arrived at six lines of evidence that “strongly supports recognition of Nanotyrannus as a distinct species,” per the study.

a larger t-rex skull compared to the Nanotyrannus specimen
A comparison of T. rex and Nanotyrannus lancensis skulls. Nick Longrich

Bone maturity analyses factored heavily into the researchers’ findings. Growth rings, which form in the bones each year, appeared more tightly packed toward the surface in the disputed fossils, which suggests the animals’ growth rates had been slowing down.

“When I saw these results I was pretty blown away,” Longrich, of the University of Bath in England, says in a statement. “I didn’t expect it to be quite so conclusive. If they were young T. rex, they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that.”

The researchers suggest the disputed Nanotyrannus specimens, estimated to reach a maximum of 900 to 1,500 kilograms, couldn’t have matured into an 8,000-kilogram adult T. rex, based on modeled growth rates.

Additionally, the team analyzed how predators coexisted in the Late Cretaceous and made comparisons between these fossils and the typical development patterns seen in other tyrannosaurs. They also found a frontal bone stored in a San Francisco museum, which they interpret to be from a juvenile T. rex skull. This fossil resembles T. rex but notably differs from Nanotyrannus.

The new results contradict findings published in Science in 2020 by paleontologist Holly Woodward of the University of Oklahoma, who used growth ring analyses to support the juvenile T. rex argument.

“I have no problem with Nanotyrannus being a real thing, if science shows that,” Woodward tells New Scientist’s James Dinneen. “I’m not convinced that their interpretation is more accurate than ours.”

Thomas Carr, a paleontologist at Carthage College who first suggested Nanotyrannus was a juvenile T. rex, also objected to the new study, per the New York Times’ Asher Elbein. “With T. rex and tyrannosaurs in general, differences between juveniles and adults are quite extreme and people are easily thrown,” he tells the publication.

t-rex skeleton in museum with a t-rex illustration behind it
Sue, the largest and most complete T. rex skeleton, on display at the Field Museum in Chicago in 2014. Brett T. Roseman for the Washington Post via Getty Images

This isn’t the first time a T. rex-related dispute has shaken the paleontology world. In early 2022, a study controversially suggested the T. rex was actually three different species: T. rex, T. regina and T. imperator. Just months later, other scientists refuted that idea with a rebuttal paper.

The results from this latest study don’t put an end to the Nanotyrannus debate. However, recent news suggests the disagreement’s central question has, potentially, a multi-million-dollar answer.

A dinosaur specimen called Chomper—with a skeleton that’s 55 percent complete and a skull more than 90 percent intact—is currently on sale for $20 million in a London art gallery. Chomper is listed as a juvenile T. rex, though the dino is another subject of controversy in the fossil identity debate—and according to the New York Times, some see a young T. rex skeleton as more lucrative than a Nanotyrannus.

Would a reclassification—or the ongoing dispute of its species—affect Chomper’s value? Much like the discussion paleontologists are carrying on, a definitive answer has yet to be unearthed.

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