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The fossilised remains of two dinosaurs locked in combat have unleashed a fresh drama, suggesting diminutive specimens thought to be teenagers of Tyrannosaurus rex could instead belong to 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.
However 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 simply a juvenile T rex, as many have 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, co-author of the study from North Carolina State University and head of Paleontology 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.”
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 dubbed “Jane” and 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,” said Zanno.
Zanno added the results have 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 for many years in his research on tyrannosaurs that he had considered a set of smaller skeletons found in the same rocks as T rex fossils to be juveniles of T. rex.
“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 the analysis of the duelling tyrannosaur offers “solid evidence” Nanotyrannus is real.
But Brusatte said he is not convinced there were multiple species of Nanotyrannus, while he also pointed out the multitude of fossilised T rex adults that have been unearthed suggests 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.”
