"I think what we have to do now is re-model dinosaurs," said Jack Horner of the Museum of the Rockies in Bozeman, Montana.

Lawrence Witmer of Ohio University's College of Osteopathic Medicine and colleagues used computed tomography, a type of X-ray also known as CT or CAT scans, to look at the skulls of more than 100 dinosaur fossils.

"It turns out that inner ear provides some very important clues about behaviors (and) also about their relative movements - how agile they were or how stately they were," Witmer told a news conference at a meeting of the American Association for the Advancement of Science in St Louis.

"The hearing part is long and delicate in T. Rex, suggesting that it could potentially discriminate sounds effectively and that hearing was important behaviorally."

In comparison, Diplodocus, a large, four-legged herbivore, had a smaller ear canal. "Dinosaurs are famous for their small brains and Diplodocus is a great example of that," Witmer said. "It has an ear that reflects that. It is a very stubby or dumpy-looking ear."

The inner ear also gives clues as to posture and shows that T. Rex held its head in an alert, forward-looking position, while Diplodocus looked down, presumably to graze.

And T. Rex seems to have an enlarged brain region that is associated with a sense of smell in modern animals, Witmer said.

"This is interesting because T. Rex was a gigantic animal. T. Rex actually had heightened senses," Witmer said. "It also strongly employed relatively rapid turning movements of its eyes and head."

NOT SO FAST

But Horner found evidence that T. Rex was not as lithe as such measurements would suggest. His team has been cutting into the fossilized leg bones of the dinosaurs, which lived during the last part of the Cretaceous period, 85 million to 65 million years ago, in what is now western North America.

They found soft bone tissue and within it, the nuchal ligament, which connects the vertebrae. "We took a microsection through the bone and there it was - soft tissue," Horner said in an interview. "We were surprised."

The structure appeared to have been very stiff, Horner said.

"I am not arguing with Larry (Witmer) on his information about the ear. But it is really odd that we have an animal that looks like it should be agile but isn't. It is one of those puzzles that we have with dinosaurs."

Horner believes that T. Rex was a scavenger, in part because its fossil skeletons are so common. "Top predators are rare," Horner said.

And it had bone-crushing teeth.

"If you are an animal that does the killing, you don't need to crush bone. You just eat the meat and leave," Horner added.

Perhaps T. Rex was a predator as a juvenile and turned into a scavenger as it aged, Horner suggested.

"I don't have any problem with it moving its head around," he added.

But Horner, one of the world's leading experts on dinosaurs, said there is no evidence they were ever as fast or agile as, say, a modern ostrich.