Nanotyrannus is Distinct Species of Small Tyrannosaur, Paleontologists Say

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Tyrannosaurs are among the most intensively studied and best-known dinosaurs. Despite this, their relationships and systematics are highly controversial. An ongoing debate concerns the validity of Nanotyrannus lancensis, interpreted either as a distinct genus of small-bodied tyrannosaur or a juvenile of Tyrannosaurus rex. In new research, paleontologists from the University of Bath and the University of Chicago examined multiple lines of evidence and showed that the evidence strongly supports recognition of Nanotyrannus lancensis as a distinct species.

Nanotyrannus attacking a juvenile T. rex. Image credit: Raul Martin.

The first skull of Nanotyrannus was found in Montana in 1942, but for decades, paleontologists have gone back and forth on whether it was a separate species, or simply a juvenile of the much larger T. rex.

In the new study, University of Bath paleontologist Nick Longrich and University of Chicago paleontologist Evan Saitta re-analyzed the fossils, looking at growth rings, the anatomy of Nanotyrannus, and a previously unrecognized fossil of a young T. rex.

Measuring the growth rings in Nanotyrannus bones, they showed that they became more closely packed towards the outside of the bone — its growth was slowing. It suggests these animals were nearly full size; not fast-growing juveniles.

Modeling the growth of the fossils showed the animals would have reached a maximum of around 900-1,500 kg and 5 m — about 15% of the size of the giant T. rex, which grew to 8,000 kg and 9 m or more.

“When I saw these results I was pretty blown away. I didn’t expect it to be quite so conclusive,” Dr. Longrich said.

“If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that.”

“We tried modeling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex.”

Supporting the existence of distinct species, the researchers found no evidence of fossils combining features of both Nanotyrannus and T. rex — which would exist if the one turned into the other.

Every fossil they examined could be confidently identified as one species or the other.

Neither did the patterns of growth in other tyrannosaurs fit with the hypothesis that these were young T. rex.

“If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults. A very young Tarbosaurus — a close relative of T. rex — shows distinctive features of the adults,” Dr. Longrich said.

“In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. And Nanotyrannus just doesn’t look anything like a T. rex.”

“It could be growing in a way that’s completely unlike any other tyrannosaur, or any other dinosaur — but it’s more likely it’s just not a T. rex.”

But that raises a mystery — if Nanotyrannus isn’t a juvenile Tyrannosaurus, then why hasn’t anyone ever found a young T. rex?

“That’s always been one of the big questions. Well, it turns out we actually had found one,” Dr. Longrich said.

“But the fossil was collected years ago, stuck in a box of unidentified bones in a museum drawer, and then forgotten.”

Nanotyrannus was more lightly-built and long-limbed than its thick-set relative. It also had larger arms, unlike the famously short-armed T. rex.

“The arms are actually longer than those of T. rex. Even the biggest T. rex, has shorter arms and smaller claws than in these little Nanotyrannus. This was an animal where the arms were actually pretty formidable weapons. It’s really just a completely different animal — small, fast, agile,” Dr. Longrich said.

T. rex relied on size and strength, but this animal relied on speed.”

“The long arms and other features suggest it was only distantly related to T. rex — and may have sat outside the family Tyrannosauridae, which T. rex is part of, in its own family of predatory dinosaurs.”

A paper on the findings was published in the journal Fossil Studies.

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Nicholas R. Longrich & Evan T. Saitta. 2024. Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) – A Distinct Taxon of Small-Bodied Tyrannosaur. Foss. Stud 2 (1): 1-65; doi: 10.3390/fossils2010001

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