T. Rex

  • Posted 04.01.05
  • NOVA scienceNOW

(This video is no longer available for streaming.) Perhaps the greatest mystery about dinosaurs is how did they get so big? "Sue," for example, a Tyrannosaurus rex fossil at Chicago's Field Museum, is fully 20 feet tall and 40 feet long. How she got that big, it turns out, is tied up in yet another mystery: How long did she live? By studying the bones of both extinct dinosaurs and their living reptilian cousins—alligators and lizards—Greg Erickson of Florida State University has been able to answer both questions: Like a reptilian James Dean, T. rex lived fast and died young.

Running Time: 08:00



PBS Air date: April 2005

So many of us, when we're little, think about and dream about and play with these celebrated reptiles, you'd think we'd know a whole lot about them. And yet, there are things about tyrannosaurs, basic, bottom line things that nobody knows, like, "How long did they live?" Apparently, there is no way to tell. That's what the experts said.

But as our correspondent Chad Cohen reports, the experts were wrong.

CHAD COHEN (Correspondent): For over 150 million years, dinosaurs roamed the earth. Sixty five million years ago they became extinct. For scientists looking to piece together the mysteries of these animals, fossilized bones are among the only clues left.

Perhaps the greatest mystery about dinosaurs, like Sue here at Chicago's Field Museum, is how did they get so big? Over 20 feet tall and 40 feet long, Sue is the world's largest Tyrannosaurus rex. How she got that big is tied up in yet another mystery: "How long did she live?"

DR. GREG ERICKSON (Florida State University): It really is just like forensic biology.

CHAD COHEN: ...a mystery Greg Erickson of Florida State University wanted to solve. Greg is a biologist. His expertise is living reptiles like lizards and alligators.

GREG ERICKSON: If you really want to understand the lives of dinosaurs, you need people that understand the lives of living animals in the first place.

CHAD COHEN: Today's huge animals, elephants and whales, generally live long lives. To find out if this was also true for T-rex, well, of course, Greg was going to need to know its age, and that's not so easy. Scientists typically age dinosaurs by counting growth rings in the weight-bearing leg bones—yup, just like trees—but not in tyrannosaurs.

GREG ERICKSON: I was always told you can't age tyrannosaurs, the reason being that tyrannosaurs have hollow bones, just like birds.

CHAD COHEN: That was the problem Greg was pondering when, a little over four years ago, he made an unexpected discovery at the Field Museum. Greg wasn't interested in what was on display here, but what the museum might have tucked away in the maze of corridors behind the scenes.

PETE MAKOVICKY (Dinosaur Curator): These are all pieces of Sue, and as you can see, it's really a jigsaw puzzle that no one can put together.

CHAD COHEN: It's a nightmare.

Dinosaur curator Pete Makovicky's nightmare turned out to be Greg Erickson's dream come true. It was here where he found his first clue.

GREG ERICKSON: And I pulled open the drawer and started looking at some of these rib chunks. I realized how solid they were.

CHAD COHEN: And when he looked closely, he thought he saw what shouldn't be on a rib bone: separate, distinct lines that looked a lot like those coveted growth rings. Maybe everything he needed was recorded in these tiny chunks.

GREG ERICKSON: I was elated! And for the first time I realized that we might be able to crack the code and actually age some of the tyrannosaurs for the first time.

CHAD COHEN: But before he could celebrate, Greg had to make sure, back in his lab in Florida, that his discovery could accurately measure a tyrannosaur's age.

CHAD COHEN: He needed something living, or at least something that hasn't been extinct for the last 65 million years. And this being Florida, what better than the American alligator, a not-too-distant cousin of dinosaurs. It turns out these guys have growth rings too, in those same non-weight-bearing bones as T-rex did.

GREG ERICKSON: Since we know their age, we can see if their ages based on growth rings, match up with the actual ages of the animals. And our study showed that you can use the fibula, which is a shin bone; you can use the pubis, which is one of the hip bones, here; the ribs work quite well.

CHAD COHEN: Is it exact? I mean, is it, "Count the growth rings, and that's how old this alligator was?"


CHAD COHEN: That's amazing.

With that, Greg went to work. His first order of business, assemble a large enough scientific sample of backroom bones.

GREG ERICKSON: This is what we were looking for. We would look for broken ends like this, and we'd see very nice growth line record.

CHAD COHEN: He ended up with the remains of 20 different tyrannosaurs of different ages and sizes. Seven were T- rexes, including Sue, and the rest, smaller cousins that lived millions of years earlier.

GREG ERICKSON: So Gorgosaurus, Daspletosaurus, Albertosaurus...

CHAD COHEN: With these fragments, Greg believed he would not only be able to calculate the ages of different tyrannosaurs, but determine how T-rex grew to be so much bigger.

GREG ERICKSON: Basically, we'll take a promising specimen like this, we'll pour an epoxy resin over it and essentially re-entomb the specimen here.

CHAD COHEN: The hardened specimen is then sliced, sanded, polished to a thickness of just seven microns and put on a microscope slide. And with the flick of a switch, the life of an animal that lived more than 65 million years ago is revealed.

For the first time science can accurately determine the age of tyrannosaurs. Since one growth ring equals one year, all you have to do is count.

Let me see if I can age this guy on my own. One, two, three, four, five...five years old?

GREG ERICKSON: It was in its sixth year, yeah, 'cause there's actually a sixth, right on the rim, right there.

CHAD COHEN: Six years old.

GREG ERICKSON: So if you went to school for 11 years, then you'd find that last one.

CHAD COHEN: It's great. It's so clear. It's so easy to see.

After repeating the procedure on some 60 different tyrannosaur bones, Greg knew the exact ages when each had died. Most were in their teens. And in the case of Sue...

GREG ERICKSON: The actual age of Sue turned out to be 28. It doesn't seem right. You know, it seems like such a short life for such a magnificent animal. And this has led me to say, T-rex lived fast and died young. It's the James Dean of dinosaurs, there's no doubt about it.

CHAD COHEN: Sue not only lived fast, she grew fast.

GREG ERICKSON: You're seeing here, on Sue, you're seeing some of these broader, broader growth bands here. And as you head out towards the outer part here, all of a sudden they start getting really tight.

CHAD COHEN: Narrow rings mean slow growth, wider rings, fast, same as trees. And by comparing the ages when all the different T-rexes died to their projected weights at the time, Greg could finally show how Sue grew from a 10-pound hatchling to a six-ton giant in just 20 years, with most of the growth occurring as a teenager between the ages of 14 and 18.

GREG ERICKSON: It's just mind-boggling. This is an animal that's putting on five pounds a day. So obviously, it was eating just an enormous amount of flesh and bone to pull this off.

CHAD COHEN: But unlike today's huge animals, whose life spans can be as long as ours, T-rexes only got to enjoy their bigness for a relatively short time. And that's because we're pretty sure that Sue here died of old age. Arthritis crippled her tail; her rib bones, which were broken, had had plenty of time to heal; an infection in her mouth left holes in the back of her jaw. It was a tough life, no doubt, and now, for the first time, we know just how long a life it was.

Tyrannosaurus rex, the supreme killer of the Cretaceous, terrorized some of the largest creatures ever to walk the earth and earned a permanent place in our imagination, all before the age of 30.

ROBERT KRULWICH: Correspondent Chad Cohen who already, by the way, is older than most T-rex's ever got to be.


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This material is based upon work supported by the National Science Foundation under Grant No. 0229297.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

© 2005 WGBH Educational Foundation

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Image credit: (T. rex skeleton) © Corbis Images

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