Ancient Anatomical Evidence Shows That T. Rex Kept a Cool Head, Literally

Tyrannosaurus rex has a reputation as a mighty predator — but it turns out the lizard wasn’t exactly hot-headed. The king of lizards had an “air conditioner” in its skull, and the dino A/C helped to keep the apex predator from becoming overheated.

Two large holes at the top of T. rex’s head allowed it to regulate its body temperature, as described in a study published in July the journal The Anatomical Record. The findings increase researchers’ understanding of how some dinosaurs’ bodies functioned, and disprove a century-old understanding of the holes’ purpose.

Having “internal thermostat” was key for T. rex, the researchers say, since the mighty reptiles tended to generate a lot of body heat.

“If you’re an active predator, that we know T. rex probably was, at some point you want to be able to shed heat, as much as you want to be able to gather heat,” says Casey Holliday, lead author of the study and professor of anatomy at the University of Missouri School of Medicine.

For more than a century, researchers thought the holes were filled with muscles that helped to move the jaw.

These latest findings overturn that theory; instead, the holes were home to blood vessels that helped T. rex regulate temperature. Big warm-blooded animals like hippos, elephants, cows, and antelope cool off in water, or with the help of big ears or special vessels in their throats.

“Our findings show that T.rex had a few vascular devices that helped it handle body temperature,” Holliday tells Inverse. “This adds to a growing body of evidence that suggests T.rex and other species of dinosaurs were warmer-blooded than we used to think.”

Researchers used thermal imaging to study living relatives of dinosaurs, including birds and alligators, which have a similar “internal thermostat” in their heads. They observed changes in the holes — called dorsotemporal fenestra — based on the temperature outside.

In cooler temperatures, as the cold-blooded reptiles need to warm up, the holes showed up as big hot spots, indicating temperature increase. But when it was warmer outside, the holes appeared dark, as if they were “turned off to keep cool.”

Most dinosaurs, crocodilians and birds have similar vascular structures for temperature regulation, Holliday says. “These same blood vessels supply the frills of Ceratopsian dinosaurs, crests of pterosaurs, and skull roofs of weird crocodiles like Aegisuchus, and fleshy display structures of birds like turkeys and vultures,” he wrote in an email.

“Our discovery shows that dinosaurs and crocodiles share a lot of anatomy, despite some superficial differences.”