Scientists Say 'Dueling Dinosaurs' Fossil Confirms a Smaller Tyrannosaur Species, Not a Teenaged T. Rex

An anonymous reader shared this report from NPR:

It's known as the 'Dueling Dinosaurs' fossil: A triceratops and a tyrannosaur, skeletons entangled, locked in apparent combat right up until the moment of their mutual demise... That discovery in 2006 now appears to have overturned decades of dinosaur dogma about Tyrannosaurus rex, the fearsome giant long thought to be the sole top predator stalking the late Cretaceous. In a paper in the journal Nature, paleontologists Lindsay Zanno and James Napoli conclude that some of the bones from that specimen belong not to a teenage T. rex, but to a fully grown individual of a different tyrannosaur species — Nanotyrannus lancensis....

One of the first of those red flags in the new specimen was the arm bones. They looked completely different than T. rex's puny appendages... 'These are powerful arms with large claws, large hands. They were using them for prey capture.' Contrast that with T. rex, 'an animal that's a mouth on legs.' There were additional clues. The animal had fewer tail vertebrae and more teeth than T. rex. Zanno and Napoli considered other lines of evidence. They created 3D models of numerous purported T. rexes against which they compared their specimen. They looked at the growth stages of the cranial nerves and sinuses of close living relatives of dinosaurs, features that were visible in the fossilized skeleton.

'But maybe the most important and damning thing that we did was we were able to figure out that our animal is not a juvenile at all,' she says. This conclusion was based on slicing through the fossil's limb bones to examine the growth rings. That work demonstrated that this animal was mature and done growing when it died around the age of 20. 'That means it's half the size and a tenth of the mass of a full grown Tyrannosaurus rex,' says Zanno... In addition, while making models of all those other alleged T. rex skeletons, Zanno says they identified another new species of tyrannosaur, one they're calling Nanotyrannus lethaeus...

'It tells us that these end-Cretaceous ecosystems right before the asteroid hit were flourishing,' says Zanno. 'They had an abundance of different predators. And refutes this idea that dinosaurs were in decline before the asteroid struck.'

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