The fossilised remains of two dinosaurs locked in combat have unleashed a fresh drama, suggesting diminutive specimens thought to be Tyrannosaurus rex teenagers could instead be separate, smaller species.
The “duelling dinosaurs” fossil, which reveals a triceratops in battle with a medium-sized tyrannosaur, was unearthed in Montana by commercial fossil hunters in 2006 and dates to shortly before the asteroid strike that ended the reign of the dinosaurs 66m years ago.
It only became available for scientific research after it was acquired by the North Carolina Museum of Natural Sciences (NCMNS) in recent years.
Now researchers say a detailed analysis of the fighting tyrannosaur reveals it is not a juvenile T rex as many had thought but an adult of a different species, Nanotyrannus lancensis.
“Our specimen is a fully grown Nanotyrannus weighing only 1,500 pounds after two decades of growth,” said Dr Lindsay Zanno, a co-author of the study from North Carolina State University and the head of palaeontology at NCMNS.
“The anatomy of Nanotyrannus, from its higher tooth count, enlarged hands, shorter tail, unique pattern of cranial nerves and sinuses and smaller adult body size, is incompatible with the hypothesis that this skeleton is a teenage T rex,” Zanno said.
The name Nanotyrannus lancensis was previously given to a small skull that was reported in the Hell Creek Formation of Montana in 1946. However, experts later argued that specimen, known as the Cleveland skull, was actually a juvenile T rex.
Now the study by Zanno and colleagues, published in the journal Nature, reveals Nanotyrannus lancensis was indeed a species in its own right that lived at the same time and inhabited the same ecosystems as T rex.
What’s more, the team say the skeleton of a juvenile dinosaur named Jane found in the Hell Creek Formation in 2001 is not a young T rex either but a new species of Nanotyrannus.
“Our study suggests some specimens previously argued to represent juveniles of T rex are instead Nanotyrannus,” Zanno said.
She said the results had important implications. “For decades, palaeontologists have unknowingly used Nanotyrannus specimens as a model for teenage T rex to understand the biology of Earth’s most famous dinosaur – studies of its locomotion, growth, diet and life history. Those studies need a second look,” she said.
Prof Steve Brusatte, of the University of Edinburgh, who was not involved in the work, said that for many years in his research on tyrannosaurs he had considered a set of smaller skeletons found in the same rocks as T rex fossils to be T rex juveniles.
“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 said, adding that the analysis of the duelling tyrannosaur offered “solid evidence” Nanotyrannus was real.
But Brusatte said he was not convinced there were multiple species of Nanotyrannus, while he also pointed out that the multitude of fossilised T rex adults that had been unearthed suggested there should be fossilised juveniles too.
“So I’m not yet ready to proclaim every smaller tyrannosaur skeleton to be Nanotyrannus,” 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.”
