T. Rex, Other Dinosaurs Had Heads Full of Air

Ker Than

Dinosaurs were airheads—and that's not just because they had tiny brains, a new study says.

New 3-D scans of the skulls of Tyrannosaurus rex and other dinosaurs reveal the creatures had more empty space inside their heads than previously thought.

These air spaces made the skulls light but strong and could have helped dinosaurs breathe, communicate, and hunt.

The extra room may even have paved the way for flight in some species.

"Air is a neglected system that is actually an important contributor to what animals do," said study co-author Lawrence Witmer, a paleontologist at Ohio University in Athens.

The research is detailed in a recent issue of the journal The Anatomical Record.

Empty-Headed

Witmer and colleague Ryan Ridgely made detailed CT scans of air cavities in the skulls of two predators, T. rex and Majungasaurus; and two ankylosaurs, Panoplosaurus and Euoplocephalus, both plant-eaters with armored bodies and short snouts. (See an illustration of another ankylosaur species.)

The results mark the first time scientists were able to accurately estimate the weight of a dinosaur's head.

A T. rex head, for example, would have weighed more than 1,100 pounds (about 500 kilograms), close to the average weight of an adult cow, Witmer and colleagues found.

Until now, paleontologists had to make do with estimates for the weight of dinosaur heads, said Tom Holtz, a paleontologist at the University of Maryland who was not involved in the research.

"Larry's team is able to calculate a volume for the skull, so they can constrain the weight far more securely," Holtz said. "This is the next best thing to having a fleshy T. rex head to dissect."

Witmer estimates that T. rex's head would have been 18 percent heavier if not for the air spaces in its skull.

This savings may have allowed T. rex to pack more muscle onto its head, which possibly strengthened its bite and allowed it to tackle bigger prey.

"Crazy Straws"

The nasal airways in the ankylosaurs, however, were surprisingly convoluted. It was as if "crazy straws" had been rammed up the creatures' snouts, Witmer said.

These winding airways were often located next to large blood vessels.

"Whenever we see that, it raises the possibility that we're looking at heat transfer," Witmer said.

This setup would have allowed hot blood circulating through the creatures' heads to dump excess heat into the airways, helping to cool their brains and the rest of their bodies.

The transferred heat also could have warmed up air the dinosaurs breathed, making gas exchange in the lungs easier.

In addition, the twisty nasal passages may have acted as resonating chambers for sounds.

The two ankylosaur species examined had slightly different airways, so their voices would have been subtly different, Witmer said.

Hollow Bones

The research could provide new clues about how dinosaurs achieved flight.

Some of the new study's research subjects were theropods, the group of dinosaurs from which modern birds are descended.

(Related: "T. Rex Protein "Confirms" Bird-Dinosaur Link" [April 24, 2008].)

"Very often people have thought that birds have hollow bones because they fly, but it could be the other way around," Witmer said.

"They could have evolved hollow bones for other reasons, and that gave them the lower body mass necessary to take to the air."

Hans-Dieter Sues is a dinosaur expert at the Smithsonian National Museum of Natural History in Washington, D.C., who also did not participate in the research.

Witmer "certainly makes a strong case for paranasal sinuses [air-filled spaces within the skull] reducing the weight of the skull in certain dinosaurs," Sues said.

Sues cautioned, however, that "such functional hypotheses are difficult to test even in living species, including our own."