Nanotyrannus was not a teenaged T. rex

For decades, researchers have debated whether a fossil of a small, fierce theropod belonged to a teenaged Tyrannosaurus rex or was a separate species. Now they know: Nanotyrannus can stand on its own two feet. The tiny tyrannosaur wasn’t just a younger T. rex, scientists report October 30 in Nature.

An exquisitely preserved skeleton of a small tyrannosaur from Montana’s 67-million-year-old Hell Creek formation can end the debate, researchers say. The skeleton is part of a famous fossil known as Dueling Dinosaurs, featuring a small tyrannosaur entangled with its possible prey, a horned ceratopsian dinosaur, entombed together in rock for millions of years.

That tyrannosaur, the new analysis reveals, is a kind of holy grail, a long-sought-after missing link for researchers who’ve hoped to demonstrate that Nanotyrannus exists: It’s the first identified adult specimen of Nanotyrannus. And that is allowing researchers to at long last disentangle it from T. rex.

The identity mystery started decades ago. In 1942, researchers unearthed the skull of a small, sharp-toothed dinosaur initially thought to be Gorgosaurus. But in 1988, scientists reinterpreted that fossil as a new type of tyrannosaur, which they dubbed Nanotyrannus lancensis.

Others contested that description, noting features in the 60-centimeter-long skull that strongly resembled T. rex; this skull, they suggested, belonged to a juvenile version of that dinosaur. Since then, several other small tyrannosaur fossils discovered in the Hell Creek Formation have been assumed to be young T. rexes.

The burden of proof was on Nanotyrannus to forge its own identity.

Now, paleontologists Lindsay Zanno of North Carolina State University in Raleigh and James Napoli of Stony Brook University in New York report that the newly analyzed fossil is the first demonstrably adult specimen of N. lancensis. The tyrannosaur is just about six meters long, but analyses of growth rings within its leg and arm bones reveal that it was skeletally mature, and therefore fully grown. By comparison, adult T. rex can get up to 14 meters long from snout to tail.

It also contains the first preserved tail and arm bones of a Nanotyrannus, both of which were distinct from T. rex. A T. rex tail had about 40 vertebrae while N. lancensis had just 35.

And perhaps the most obvious difference, Zanno says, is in the arms. “The arm of our Nanotyrannus is already [a bit] bigger than a T. rex arm.”

Other telling differences in anatomy between Nanotyrannus and T. rex include traces of cranial nerves on the skull and the structures of sinuses and the respiratory system. These are features that don’t change as the creatures grow, Zanno says.

With the aid of the newly identified adult N. lancensis specimen, the team also reexamined another long-debated dinosaur, known as Jane. Jane’s bones had revealed that the animal was a juvenile. But by comparing Jane’s anatomical features with those of the Dueling Dinos fossil — as well as over 100 other previously analyzed tyrannosaur specimens — the team determined that Jane was a young Nanotyrannus, not a young T. rex.

Jane was also, they suggest, a new, slightly larger species than N. lancensis. They dubbed the species N. lethaeus after the River Lethe of Greek mythology, which was said to bring forgetfulness to those who drink from it. That alludes to the idea that Jane isn’t a new discovery, but has been hiding in plain sight for decades, Zanno says.

The bone analysis of the Dueling Dinosaur tyrannosaur “appears to show that the specimen is approaching adult size, and I am fine with that conclusion,” says Holly Ballard, a paleontologist at Oklahoma State University in Tulsa.

But Ballard, who published a previous analysis of the Jane specimen, is not convinced that Jane represents a new species of Nanotyrannus, or that it was a Nanotyrannus at all. Even as a juvenile, Jane was already bigger than N. lancensis — and the animal was still growing, she notes.

Nanotyrannus and Tyrannosaurus could have lived alongside one another at the twilight of the dinosaur era — not just coexisting in time but occupying different ecological niches in the same Hell Creek region, Zanno says. “Nanotyrannus was a completely different kind of predator: small, slender, extremely fast, with large predatory arms,” while T. rex was bulky, heavily built, with a huge head and powerful bite force. That adds to growing evidence that dinosaurs were still diverse and flourishing right up to the end of the Cretaceous Period 66 million years ago, when an asteroid slammed into Earth.

“Every other tyrannosaur-bearing community had a couple of different tyrannosaurs in it,” says Thomas Holtz, a paleontologist at the University of Maryland in College Park who was not involved in the new study. “It actually makes Hell Creek less weird.”

Holtz says he has been a Nanotyrannus doubter, but the new study has “done a far better job advocating for [it] than anyone in the past. That’s all we wanted, those of us who were skeptics.”

The discovery has implications far beyond identifying a new species or even highlighting the enduring diversity of the tyrannosaur line. It throws a massive wrench into much of what we’ve come to understand about the life and times of T. rex, Zanno says.

“Several decades of basic research [on T. rex] — locomotion, diet, life history and growth — all contain data that comes from two different types of dinosaurs,” she says. “These need to be pulled apart and reexamined in light of this new conclusion.”

Then there’s the other side of the coin, Holtz adds: “If Nanotyrannus is real, and it looks like it is, we now once again do not know what a teen T. rex looks like.”

But we might soon. He and Zanno both alluded to a fossil currently in preparation at the Denver Museum of Nature & Science that is thought to be a true teen T. rex. If so, it’ll offer up yet another line of evidence on anatomical differences for researchers to wrangle over.