NPR has a story about Nanotyrannus lamcensis and why it’s a separate species from Tyrannosaurus rex. When the “Dueling Dinosaurs” fossil, which consists of two entangled dinorsaur skeletons, was discovered in 2006, it launched a quite a bit of debate as to whether the tyrannosaur was a juvenile T. rex or a new species of tyrannosaur. New research in the journal Nature by paleontologists Lindsay Zanno and James Napoli shows why they have concluded the tyrannosaur specimen is a Nanotyrannus lancensis instead of a juvenile T. rex.

When that work began, Zanno also figured the carnivorous dinosaur was the world’s most complete juvenile T. rex ever found. “But as soon as we started actually studying this specimen, we realized there were a lot of red flags,” she says.

For context, paleontologists had previously debated whether small-bodied tyrannosaurs were teenage T. rexes or a different species altogether. The existence of a two-legged tyrannosaur species that lived alongside T. rex was proposed back in the 1940s, but had long since fallen out of favor.

The consensus converged on juvenile T. rex, “meaning it would have been the only major predator in end-Cretaceous ecosystems right before the asteroid impact,” says Zanno. Most researchers believed that T. rex — which could be 12 feet high, 40 feet long and weigh around 15,000 pounds — had outcompeted all other meat-eating dinosaurs.

The completeness of the new skeleton provided Zanno with the ideal specimen to test these long-standing conclusions.

“This is such a controversial topic,” she says. “And we don’t want to write another paper that just stirs the pot. We want to write the paper that ends the debate.”