A legendary fossil at the North Carolina Museum of Natural Sciences in Raleigh contains skeletons that were apparently locked in prehistoric combat, an epic encounter between two of the world’s favorite dinosaurs, Triceratops and Tyrannosaurus rex. Or so paleontologists thought.
Researchers over the past five years studying an astonishing collection of bones known as “dueling dinosaurs” say they have discovered a case of mistaken identity, determining that the diminutive dinosaur was not a baby T. Rex, but a full-grown specimen of the hotly debated species known as Nanotyrannus lancensis.
“The microstructure of the bone preserves records of growth, indicating that this is an adult,” said James Napoli, a vertebrate paleontologist at Stony Brook University and co-author of the new study, published Thursday in the journal Nature. The discovery has prompted a rethinking of many other fossils previously identified as juvenile T. rex remains, Napoli added.
In this illustration, a pack of Nanotyrannus brazenly attacks a baby T. rex. – Anthony Hutchings
Although similar in appearance, the two dinosaur species would have been very different: the Nanotyrannus was 18 feet long, agile and built for speed with long legs and strong arms to grab prey, while the 42-foot-long T. rex had thick legs and used its devastating bite to swallow huge, slow-moving dinos.
Despite its relatively small size, Napoli said Nanotyrannus had larger upper limbs than a fully grown T. rex, which had very small arms. “Bones don’t shrink when animals grow, so it couldn’t have become (adult) T. rex,” he added.
According to study co-author Lindsay Zann, associate professor at North Carolina State University and director of paleontology at the North Carolina Museum of Natural Science, the discovery “turns decades of T. rex research on its head.”
“Many studies of T. rex biology over the past three decades have unwittingly mixed data from Nanotyrannus and T. rex.
These studies need to be re-evaluated in light of this finding,” Zanno said in an email.
The dueling dinosaur fossil was first discovered in 2006, exposed in sedimentary rocks quarried from the Hell Creek Formation, which dates back 65.5 million years and spans parts of Montana, North Dakota, South Dakota and Wyoming.
Mistaken identity
Nanotyrannus lancensis, which like T. rex is taxonomically part of the larger tyrannosaur family, was first identified from a fossil discovered in the 1940s from the same rock as the dueling dinosaurs.
However, scientists later changed the narrative, explaining that the original find and many other small tyrannosaur fossils discovered in the following decades were young specimens of T. rex. The idea that another species could have produced the fossils has fallen away, although the possibility is still debated at scientific conferences and other forums.
The new study, which examined and compared more than 200 tyrannosaur fossils, suggests that baby T. rex fossils were not as common in the fossil record as paleontologists previously thought. For years, paleontologists misidentified Nanotyrannus fossils as juvenile T. rex specimens and used the remains’ morphological features to model T. rex growth and behavior, the study authors reported.
“What’s exciting is that this discovery opens the door to a lot of new questions about how these different predators—one built for brute force and one for speed—interacted during the twilight years of the dinosaurs,” Zanno said.
Lindsay Zanno, head of paleontology at the North Carolina Museum of Natural Science, poses with a famous dueling dinosaur fossil. – NC State University
The study authors said they also identified the fossil, called “Jane,” as a second species of Nanotyrannus, which they named Nanotyrannus lethaeus. The name is a reference to the River Lethe, the River of Oblivion in Greek mythology, a nod to how this dinosaur was hidden in plain sight and “forgotten” for decades.
Awkward questions
The discovery raises “uncomfortable” questions about why scientific consensus has so quickly emerged on the idea that all Nanotyrannus specimens are young T. rex, said Larry Witmer, the Chang Ying-Chien Professor of Paleontology at Ohio University. Witmer was not involved in the investigation.
Many T. rex fossils, which can sell for tens of millions of dollars at auction, have been collected commercially and are in private hands, making them difficult to study, he noted in a commentary published with the study.
“This exceptionally well-researched study by Zanno and Napoli puts Nanotyrannus on solid footing,” said Witmer.
The implications of the findings go beyond resolving a scientific disagreement, he added.
“There’s a lot more to this than declaring a winner of the debate, because there are literally decades of research and probably hundreds of publications based on the premise that this article overturns,” he said. “All these analyzes will need to be reviewed.
Although baby T. rex and Nanotyrannus look similar in the fossil record, they were two separate species, a new study has found. – North Carolina Museum of Natural Sciences
Steve Brusatte, professor of palaeontology at the University of Edinburgh, said the find should lead to a “fundamental reassessment of the classification and evolution of tyrannosaurs”.
“For many years, when I studied a set of smaller tyrannosaur skeletons found in the same rocks as the famous skeletons of large T. rexes, I considered T. rex to be juveniles and not a distinct smaller species,” explained Brusatte, who was not involved in the new study.
“After reading this new paper, I think the new evidence from this exceptional new specimen at the North Carolina Museum of Natural Science shows me wrong — at least partially,” he added.
But not every smaller tyrannosaur skeleton should be classified as a nanotyrann, according to Brusatte. “The adult T. rex was the size of a bus, and adult T. rex bones are very common in western North America, from Saskatchewan to New Mexico,” he said.
“Some of these must be juvenile T. rex, and I think it will ultimately be very difficult to distinguish adult or near-adult Nanotyrannus from juvenile T. rex, as they were probably very similar in size and skeletal features,” he noted. “This will be a challenge for future paleontologists.
The finger bones and claws of Nanotyrannus are larger than even the largest-bodied T. rex, which had short arms and hands due to its size. – NC Museum of Natural Sciences
According to Thomas Carr, senior research advisor at the Dinosaur Discovery Museum and associate professor of biology at Carthage College, the evidence presented in the study that the smaller animal in the dueling dinosaur fossil is actually a Nanotyrannus is “pretty convincing.” But understanding the differences between Nanotyrannus lancensis and the newly named species Nanotyrannus lethaeus requires fresh fossils, he said.
Carr also said he wasn’t sure if the Jane fossil should be reclassified as Nanotyrannus lethaeus, as the new study suggests. “They did a strong job, but we have to be careful,” said Carr, who was not involved in the investigation.
Naples said the dinosaur duel, which is still partially encased in rock, has many more secrets to shed. Scientists still don’t know how the two animals died — or why they were found entangled. However, the Nanotyrannus specimen is 100% complete, which is extremely rare in the fossil record, Napoli noted.
“There are some bones that are partially broken and a piece is missing, but every single bone is represented to some extent,” he said.
The researchers hope that the sediment surrounding the remains may also preserve skin or feathers, and that there are injuries on the specimen, such as a broken finger, which they have not yet examined.
“There’s a lot of work to be done on the Dukova dinosaurs,” Napoli said. “It’s an amazing fossil to work on.”
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