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shape of bananas, poised to tear into a hapless adversary.

Lowering its head, T. rex chomps down on the back of the horned dinosaur. T. rex's stubby, powerful claws lock into the tough hide of the victim, securing it while jaws and teeth shake the flailing prey, tearing away huge, bloody hunks of flesh.

This T. rex is fast, and nimble. It might hunt in packs as well as alone, butting heads with its rivals. To kill, it leaps out to kick or pinion its prey with a massive hind leg or crunch it with a lethal bite. It is one fleet-footed, mean, killing son-of-a-gun.

Another day, another imagined view of T. rex. This is an equally graceful monster. It, too, is fast enough to catch prey, but rather than get into a nasty struggle to kill its dinner, it uses its monstrous jaws to tear huge hunks of meat from the many carcasses littering the landscape. It's the vulture of its day: a huge, efficient leftover-eater.

Which of these T. rexes comes closer to the truth? Either one, depending on how you interpret the evidence. Each vision of T. rex is based on reasonable inferences, unlike the antiquated vision of T. rex as a fat and sluggish cold-blooded reptile. Each of these modern scenarios is based on the best information we have about T. rex. That information is growing fast. In the past few years, we've found out more about T. rex then we ever knew before. My co-workers and I at the Museum of the Rockies dug up one of the two most complete T. rexes of all in 1990 and found some surprises, among them that T. rex's front arms, long imagined to be puny and useless, weren't weak after all.

We'll tell you here what we paleontologists now know about T. rex, how we dug out bones and figured out how T. rex looked and acted. But for all we know about T. rex, much more remains a mystery. Some questions may never be answered. Others require us to speculate, to make reasonable guesses based on the good information we do have.

Some people don't like to hear scientists use the word speculate. They think science is all hard data and certain answers. It's not, especially when you've got a science with as many gaps and as little data as we have for the evolution and behavior of dinosaurs.

It's as if we're detectives investigating a murder, only we weren't there, and we don't have the culprit or much of the evidence. Speculation is healthy, very healthy, as long as it is grounded in evidence. Where you get into trouble is when you start believing the speculation simply because it appeals to you.

Dinosaurs are awesome. All of us would really like to know what they looked like, how they acted and moved. That leads to speculation. That doesn't mean any speculation we make is good science. If we do speculate, we need to identify it as speculation, not fact. And in our speculation we need to try for the neatest, most parsimonious solution to fit the facts we do have.

Since there is speculation in all science, it is important to understand that just because one particular scientist says this is what happened, it isn't necessarily so. That's only his or her speculation.

Science is constantly changing. We're always learning new things. The first person who comes up with an idea always has the least information that will ever be known about that idea. But we can never solve all the mystery.

We'll never know the absolute, complete truth about dinosaurs. Certainly not about T. rex. In the first ninety years that we knew of T. rex's existence, scientists had uncovered only eight skeletons of the animal, none of them more than 60 percent complete.

Now that's all changed. Here's how and why.

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