Five Discoveries That Changed Our Understanding of T. Rex Forever, From Warm Blood to Lips

No dinosaur fires our collective imagination quite like Tyrannosaurus rex. Ever since it was named in 1905, the Cretaceous carnivore has been the subject of public awe, silver screen celebrity and intense scientific study. Decades of research and debate have investigated everything from what the infamous predator ate to its walking speed, body temperature, social life and body coverings.

But the initial image of a monstrous, ever-hungry T. rex has since transformed into a predator very different from most other giant, carnivorous dinosaurs. T. rex was originally envisioned as a slow-moving, tail-dragging terror of ancient swamps, facing off against Triceratops in epic, blood-spattered battles. Now, the T. rex we know and love is understood as a hot-blooded, possibly feathery carnivore that played a critical role in its ancient ecosystem, feeding on naive and injured herbivores while also cleaning up the carcasses of adult Triceratops and Edmontosaurus that were too large to hunt.

The shift didn’t happen all at once, but it was gradually pieced together, discovery by discovery. Here are five findings that contributed to the ever-changing image of the tyrant lizard. The tiny arms of T. rex were a mystery as old as the dinosaur’s name.

The 1905 paper that coined the name Tyrannosaurus rex also pointed out how surprisingly puny the carnivore’s forelimbs were. The appendages wouldn’t have been any help in capturing prey, so perhaps they were only useful in helping one T. rex cling onto another while mating, one scientist suggested. But no one really knew for sure.

The puny arms of T. rex and related tyrannosaurs simultaneously became a puzzle and a joke, as if small, two-fingered arms somehow contradicted an animal that was otherwise big and stout from nose to tail. Paleontologists originally thought T. rex had three fingers on each stubby forearm, aligning with earlier predators like Allosaurus. They didn’t have fossil evidence of the carnivore’s hand or claws.

But after more closely related dinosaurs, such as Gorgosaurus, were found with two claws in Canada, experts realized T. rex must have had two fingers, too. The T. rex forearm fossil that confirmed this was found in 1988. But small arms make sense for a predator with a massive head, heavily threaded with muscle.