This artistic interpretation shows a pack of Nanotyrannus attacking a juvenile Tyrannosaurus rex (front left).Credit: Anthony Hutchings
A fossil once assumed to be of a young Tyrannosaurus rex is in fact that of a different species altogether, and the dinosaur it belongs to was a fully grown adult at the time of its death, palaeontologists have found.
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The small tyrannosaur — named Nanotyrannus — is about half the length and one-tenth of the body mass of a fully grown T. rex — which led scientists to initially suspect the fossil belonged to a teenage T. rex. But the specimen has several distinct physical features, too.
“Nanotyrannus is a small-bodied predator designed for speed. It’s very agile and has long powerful arms [that are] larger than those of the T. rex,” says Lindsay Zanno, a palaeontologist at North Carolina State University in Raleigh. Nanotyrannus also had a shorter tail than did T. rex and sharper, less curved teeth.
The findings, published in Nature on 30 October1, come from the ‘Duelling Dinosaurs’ fossils — a Triceratops and a small tyrannosaur found tangled in combat in the Hell Creek Formation in Montana in 2006. The specimens date back to the latest Cretaceous period, around 67 million years ago.
Small but ferocious
To determine the Nanotyrannus’s age and growth rate, Zanno and her colleague James Napoli, a palaeontologist at Stony Brook University in New York, studied the growth rings present in a thin slice of one of the fossilized bones. These marks show how bone growth has slowed down and sped up with the seasons, creating layers like a tree trunk’s annual rings. The researchers compared the dinosaur’s growth rate with that of a crocodile, one of its closest living relatives.
