70-year-old T. rex mystery solved by ‘Dueling dinosaurs’ fossil

For many years, paleontologists have debated about whether there was a smaller relative of the Tyrannosaurus rex, or if those smaller fossils were really just awkward teenage versions of the king of dinosaurs. Now, one skeleton has stopped the debate and changed how scientists think about one of the most recognizable animals to roam the Earth.

The fossil, found in Montana’s Hell Creek Formation and notoriously known as part of the Andrew's Dueling Dinosaurs, preserves two prehistoric fighters engaged in their last struggle: a Triceratops and a small-bodied tyrannosaur. After years of study, the researchers established that the predator was not a juvenile T. rex at all but a fully grown species called Nanotyrannus lancensis.

“This fossil does not just settle the debate. It flips decades of T. rex research on its head," said Dr. Lindsay Zanno, associate research professor at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences. She co-authored the new study in Nature with James Napoli, an anatomist at Stony Brook University.

The story starts nearly 80 years ago with a small tyrannosaur skull found in Montana that was named Gorgosaurus lancensis. In the 1980s, it was re-named Nanotyrannus lancensis as a tiny cousin of T. rex. Some researchers acceded with the idea of “pygmy tyrant,” while others thought the fossils were juvenile versions of Tyrannosaurus rex.

The debate took a new turn in 2001 with the discovery of “Jane,” a nearly complete skeleton that looked very similar to the earlier skull. A histological study, which analyzes bones using microscopes, confirmed that Jane was still growing.

This was the final piece of evidence for most biologists studying this issue. Nanotyrannus specimens were simply juvenile T. rexes. This explanation was accepted without equivocation for years, and the development of T. rex in the various media--textbooks, museum displays, or documentaries--was shaped by it.

However, not everyone agreed with this explanation. A vocal minority insisted that the anatomical differences between Jane and T. rex were too significant to be explained by growth alone. These differences were categorized as "fringe" until their research team's investigation.

The team led by Zanno and Napoli re-analyzed nearly the entire tyrannosaur from the Dueling Dinosaurs site and conducted one of the most extensive bone histology investigations of a dinosaur ever. They examined growth rings, spinal fusion, and the microscopic structure of the bone, somewhat like how tree rings are observed to identify age. There was no doubt this animal was physically mature at the time of death: about 20 years old.

"For Nanotyrannus to be a juvenile T. rex, it would need to violate everything we know about vertebrate growth," says Napoli. "It is not just improbable, it is impossible."

The specimen, called NCSM 40000, had a more elongated forelimb, more teeth, and a smaller overall body size than any recorded T. rex, even with age considerations of the specimen. The estimated weight of an adult would be approximately one-tenth that of an adult T. rex.

Even more surprising is the size of the hands and forearms, which are proportionately larger than other known Tyrannosaurs, and these features do not shrink as animals grow. These anatomical features, along with variations in the skull's nerves and tail characteristics, indicated that this specimen was not a juvenile version of the king but rather a separate predator species.

As part of their research, Zanno and Napoli studied more than 200 tyrannosaur fossils, through which they discovered yet another individual distinct from the Dueling Dinosaurs’ Nanotyrannus lancensis. They named it Nanotyrannus lethaeus, referencing the River Lethe of Greek mythology, which represents how this species had been ‘forgotten’ for decades.

Their research suggests that during the last million years of the Cretaceous that multiple species of tyrannosaurs lived in North America’s ancient floodplains. Additionally, the research has upended the long-held belief that Tyrannosaurus rex was the only predator at the top of its food chain. Ecosystems near the end of the dinosaur age were much more complex and competitive than previously understood.

"This discovery paints a richer, more competitive picture of the last days of the dinosaurs," Zanno says. "With a massive size and bone-crushing power, T. rex was indeed an incredibly terrifying predator, but it was not at the top without competition. At the same time, darting alongside was the smaller, faster, and more agile Nanotyrannus."

The study also raises questions of how we come together for consensus in science and how things can go wrong. For decades, researchers simply accepted that all small tyrannosaurs were juvenile T. rexes, despite unresolved challenges to that view. Part of it was just common sense; for years, scientists had wondered why very few juvenile T. rexes were found in the fossil record. Attribution to smaller specimens as young rexes fits the bill snugly.

But science evolves from challenge, not the simple explanation. New data meaningfully uncovers how groupthink, restricted access to fossils, and even academic hierarchy constraints themselves might have prevented rethinking. Some of the key Nanotyrannus fossils happen to be privately owned collections, which complicates the study or verification of those fossils, leading to discussions about research ethics and the transparency of data, divisions that extend to paleontology today.

Upon widely furnishing the Dueling Dinosaurs specimens publicly at the North Carolina Museum of Natural Sciences, the opportunity to address this question of species status in an open scientific approach now exists. This time, every layer of bone and every ring of growth could be scrutinized and passed along to the public.

Accepting the existence of Nanotyrannus rethinks how scientists might conceptualize late Cretaceous ecosystems. That is, the landscape of predator diversity in North America sported far more ecological competition complexity than would have been considered previously. Moreover, it means that prior research enlisting Nanotyrannus fossils as proxies for growth camouflage or behavior in T. rex must be re-evaluated.

“This changes everything about how we understand tyrannosaur evolution,” Zanno explains to me. “It’s not just a new species, it’s an entire branch of the family tree we didn’t know existed.”

The discovery also highlights the virtue of patience in science. Zanno and Napoli cautiously and quietly studied the case and specimen while there was public excitement about the fossils being auctioned off. Their eventually successful commitment to a thorough and transparent analysis restored respect in paleontology’s usually intense disagreements.

The implications of the findings go beyond expanding and rethinking dinosaur narratives. Both scientists and the public are reminded that new discovery is usually uncomfortable and can mean re-thinking is necessary.

If Nanotyrannus is not what we think it is, the models of predator-prey relationships, competition in ecological habitats, and evolutionary adaptations towards the end of the Cretaceous should also come into focus. Alongside considerations of how many “missing species” there may be due to adhering to other say, narratives.

For museums, educators, and anyone interested in dinosaurs, this reinstates the plausibility of one of the most well-known dinosaur stories in science, demonstrating that even after 66 million years, the past and what we think we know still holds secrets untold.

Research findings are available online in the journal Nature.

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