As paleontologist Lawrence Witmer runs his eyes over the remains of T. rex, triceratops or any other dinosaur for that matter, he's most interested in what he can't see. Muscle, blood vessels and other tissues aren't preserved in the fossil record. The only evidence they leave behind are notches in bone.

With the aid of technology and studies of dinosaurs' modern-day relatives, Witmer has filled in those empty spaces, reconstructing the facial features of all types of dinosaurs. His work has allowed him to offer likely theories about how animals breathed, ate, thwarted predators and attracted mates.

During his career, Witmer has stripped the lips from T. rex and the cheeks from triceratops and lowered the nostril positions of dozens of dinosaurs, creating a more accurate physiological picture of these prehistoric animals. His reputation has led to collaborations with scientists around the world and media coverage that has included live interviews on CNN and front page stories in The New York Times.

Witmer's latest study offered the most comprehensive depiction ever of the brains of pterosaurs, more commonly known as pterodactyls.

"Pterosaurs are in many ways like birds in that they are very delicate creatures that don't tend to fossilize very well," says Witmer, an associate professor of biomedical sciences. So when the scientist received a call from researchers in Texas working with intact skulls from two types of pterodactyls, he knew the sort of valuable data such specimens might offer.

His collaborators took CT scans of the skulls (making plaster casts could have damaged the fragile fossils) and built a virtual brain scan -- a computerized mold of the inside of the animals' skull cavities. Witmer reconstructed the sections of the animals' brains using this virtual scan, which allowed him to figure out how much the brain weighed and how much space it occupied in the skulls. This alone made the study exciting, Witmer says, but there was something else. Preserved in the fossils was evidence of three large semi-circular canals in the inner ear, a sensory organ the animal used to maintain equilibrium during flight.

"It looks as if they had an even better developed sense of equilibrium than birds," Witmer says. What's more, the structure of the canals inside the animals' skulls gave scientists new information about the way these animals preferred to hold their heads -- important data to anyone trying to figure out how pterodactyls ate and steered their enormous bodies as they flew. All of this caused quite a stir at a fall paleontology meeting at which Witmer presented his findings.

"People were pretty excited because not only did we have the best data on pterodactyl brains to date, but we found links to behavior that we didn't think we could get," Witmer says. "Behavior is one of the hardest things to recover from the fossil record, but we were able to get some behaviorally relevant data from these brain casts."

Witmer plans to use the same techniques that gave him this evidence of pterosaurs' behavior to collect similar information on other dinosaurs.