A legendary fossil housed at the North Carolina Museum of Natural Sciences in Raleigh features skeletons apparently locked in prehistoric combat — an epic meeting of two of the world’s favorite dinosaurs: Triceratops and Tyrannosaurus rex. Or so paleontologists thought.
Researchers who spent the past five years studying the stunning collection of bones called the “dueling dinosaurs” say they have uncovered a case of mistaken identity, determining that the small-bodied dinosaur was not a juvenile T. Rex but rather a fully grown example of a hotly debated species known as Nanotyrannus lancensis.
“We have the growth record preserved in the microstructure of the bone, which shows that it’s an adult,” said James Napoli, a vertebrate paleontologist at Stony Brook University and coauthor of the new study published Thursday in the journal Nature. The discovery has triggered a rethink of many other fossils previously identified as teenage T. rex remains, Napoli added.
While similar in appearance, the two dinosaur species would have been very different: Nanotyrannus was 18 feet long, agile and built for speed, with long legs and strong arms to grasp prey, while 42-foot-long T. rex had stocky legs and used its devastating bite to devour huge, slow-moving dinos.
Despite its relatively diminutive size, Napoli said that Nanotyrannus had larger upper limbs than a fully grown T. rex, which had famously tiny arms. “Bones don’t shrink when animals grow so this could not have possibly become an (adult) T. rex,” he added.
The finding “flips decades of T. rex research on its head,” according to study coauthor Lindsay Zanno, associate research professor at North Carolina State University and head of paleontology at the North Carolina Museum of Natural Sciences.
“A wealth of studies on the biology of T. rex over the past three decades have unknowingly mixed data from Nanotyrannus with that of T. rex.
Those studies need to be reevaluated in light of this discovery,” Zanno said in an email.
The dueling dinosaurs fossil was first discovered 2006, exposed as sedimentary rock eroded from the Hell Creek Formation, which dates back 65.5 million years and stretches across parts of Montana, North Dakota, South Dakota and Wyoming.
Nanotyrannus lancensis, which like T. rex is part of the larger tyrannosaurid family in taxonomic classification, was first identified from a fossil unearthed in the 1940s from the same rock formation as the dueling dinosaurs.
However, scientists later shifted the narrative and interpreted that initial find and many other small Tyrannosaur fossils uncovered in the following decades to be juvenile T. rex specimens. The idea that another species might have produced the fossils fell out of favor, though the possibility was still debated at scientific conferences and other forums.
The new research, which examined and compared over 200 Tyrannosaur fossils, suggests that juvenile T. rex fossils were not as common in the fossil record as paleontologists previously thought. For years, the study authors reported, paleontologists have misidentified Nanotyrannus fossils as juvenile T. rex specimens and used the morphological features of the remains to model T. rex growth and behavior.
“What’s exciting is that this discovery opens the door to a whole series of new questions about how these different predators — one built for brute strength and one built for speed — interacted in the twilight of the dinosaurs,” Zanno said.
The study authors said they also had identified a fossil called “Jane” as a second species of Nanotyrannus, which they dubbed Nanotyrannus lethaeus. The name is a reference to the River Lethe, the river of forgetfulness in Greek mythology, a nod to how this dinosaur remained hidden in plain sight and “forgotten” for decades.
The finding raises “uncomfortable” questions about why a scientific consensus came together so quickly around the idea that all Nanotyrannus specimens are juvenile T. rexes, said Larry Witmer, Chang Ying-Chien Professor of Paleontology at Ohio University. Witmer didn’t take part in the study.
Many T. rex fossils, which can sell for tens of millions of dollars at auction, were commercially collected and are in private hands, making them difficult to study, he noted in a commentary published alongside the research.
“This exceptionally researched study by Zanno and Napoli puts Nanotyrannus on a solid foundation,” Witmer said.
He added that the implications of the findings go beyond settling a scientific disagreement.
“There is much more at stake here than declaring a victor in the debate, because there are literally decades of research and probably hundreds of publications based on a premise that this article overturns,” he said. “These analyses will all need revision.”
Steve Brusatte, a professor of paleontology at the University of Edinburgh said the finding should prompt “a fundamental reassessment of tyrannosaur classification and evolution.”
“For many years in my research on tyrannosaurs, I’ve considered a set of smaller skeletons found in the same rocks as the famous skeletons of huge T. rexes to be juveniles of T. rex rather than a distinctive smaller species,” Brusatte, who wasn’t involved with the new study, explained.
“Reading this new paper, I think new evidence from this exquisite new specimen in the North Carolina Museum of Natural Sciences shows that I was wrong — at least in part,” he added.
However, not every smaller tyrannosaur skeleton should be classified as Nanotyrannus, according to Brusatte. “T. rex was the size of a bus as an adult, and bones of adult T. rexes are very common across western North America, from Saskatchewan down to New Mexico,” he said.
“Some of these must be juvenile T. rexes, and I think it is ultimately going to be very hard to tell apart adult or near-adult Nanotyrannus from teenage T. rex, as they were probably both very similar in size, and in their skeletal features,” he noted. “That will be the challenge for future paleontologists.”
The evidence presented in the study that the smaller animal in the dueling dinosaurs fossil is in fact Nanotyrannus is “pretty conclusive,” according to Thomas Carr, a senior scientific advisor at the Dinosaur Discovery Museum and an associate professor of biology at Carthage College. However, fresh fossils are needed to understand the differences between Nanotyrannus lancensis and the newly named species of Nanotyrannus lethaeus, he said.
Carr also said he isn’t sure about whether the fossil “Jane” should be reclassified as Nanotyrannus lethaeus, as the new research suggests. “They’ve done strong work, but we must be cautious,” said Carr, who wasn’t involved in the study.
Napoli said dueling dinosaurs, which is still partially encased in rock, has many more secrets to spill. Scientists still don’t know how the two animals died — or why they were found entangled. But the Nanotyrannus specimen is 100% complete, which is extraordinarily rare in the fossil record, Napoli noted.
“There are some bones that are partially broken with a piece missing, but every individual bone is represented to some degree,” he said.
The researchers hope skin or feathers might also be preserved in the sediment surrounding the remains and that there are injuries on the specimen, such as a broken finger, they haven’t yet explored.
“There is a tremendous amount of science to do on the dueling dinosaurs,” Napoli said. “It’s an amazing fossil to work on.”