A study of the running ability of Tyrannosaurus Rex, reported last week in the journal Nature, suggests that the king of the prehistoric jungle wasn't quite the speedster we once thought. Researchers found that T. rex (as imagined in an early 20th century painting, left, by Charles Knight) was too massive to sprint like the giant-size cheetah it appears to be in movies. This is just the latest in a series of humbling revisions made over the past decade or so, as new fossils were unearthed and old ones re-examined. But one thing hasn't changed: this 42-ft.-long, 14,000-lb. toothy predator would still have been plenty scary. --By Andrea Dorfman and Roy B. White
--NEW POSTURE The classic images of T. rex showed him standing upright, with tail near the ground for stability. The correct posture is shown in this skeleton from Chicago's Field Museum: the beast balanced its weight over its hips like a seesaw.
--THE SMELL OF THE KILL Studies of skulls and braincases indicate that T. rex had a keen sense of smell, a key weapon in tracking prey. The dinosaur's nostrils have been repositioned as well: closer to the end of its snout rather than its eyes.
--NO GAZELLE A new biomechanical study, validated in living chickens and alligators, shows that T. rex's muscle mass would allow it to run only 25 m.p.h., not the 45 m.p.h. previously estimated. And the mighty hunter, it turns out, may have also been a scavenger.
--FEATHERED FOE Most scientists now believe T. rex and its close relatives, like this coelurosaur, were more closely related to birds than to other dinosaurs. Juveniles may have had feather-like coats (like chicks) that they shed as adults.