DAVID GREENE, HOST:

OK, picture the biggest, fiercest meat-eating animal to ever live.

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GREENE: OK, glad he's not in the studio. That's a Tyrannosaurus rex. And since the first T. rex skeleton was discovered just over a century ago, books and movies have thrilled us with portrayals of this terrifying predator. But some scientists have argued that the beast was actually more like a vulture, a lowly scavenger, far less threatening.

NPR's Christopher Joyce reports on a discovery that might resolve the argument.

CHRISTOPHER JOYCE, BYLINE: Hollywood certainly has no doubt about T. rex's dietary habits. In "Jurassic Park," a T. rex ambushed a herd of horse-sized Gallimimus and gobbled one up.

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JOYCE: It chased a jeep filled with human snacks.

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UNIDENTIFIED MAN #1: Must go faster.

JOYCE: And it even ate a lawyer.

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UNIDENTIFIED MAN #2: No.

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JOYCE: And in this case, most scientists agreed with Hollywood, T. rex pursued and killed what it ate. It was a predator, rather than scavenging leftovers like some kind of terrestrial carp.

Greg Erickson, at Florida State University, has looked for evidence and says it's hard to come by.

GREG ERICKSON: Bones fossilize really well but unfortunately behavior doesn't.

JOYCE: To understand dinosaur behavior, scientists now look way beyond the shape and size of bones.

ERICKSON: We started finding bite marks of these animals. And we started finding coprolites; there is their fecal matter, and we could see what they were eating. And it's from such evidence we've gained most of our understanding of what this animal is doing.

JOYCE: And that leads us to the latest piece of evidence that appears to confirm T. rex's predatory nature. Paleontologist David Burnham and one of his students at the University of Kansas found it buried in between two tail bones from a fossilized hadrosaur, a duck-billed, plant-eating dinosaur. It looked like a tooth so they scanned it with a medical imaging machine.

DAVID BURNHAM: Once we realized it was a tooth, and we looked at each other; wouldn't that be just great if it was a T. rex? You know... (Chuckling) We'd finally, you know, be able to put the nail in the coffin for the scavenger theory.

JOYCE: It was a T. rex tooth. They're unique: sharp, serrated and long. Scientists sometimes call them lethal bananas. Now, a T. rex tooth in the tail doesn't prove a T. rex actually killed that duckbill. Maybe the duckbill was already dead and the T. rex just found it, chowed down on it and broke off a tooth in the carcass.

But in this case, the duckbill's bone had fused around the tooth, a sign that the wound had healed. So apparently, the duckbill survived an attack and got away with a tooth stuck in its tail.

BURNHAM: The bullet from the smoking gun, you know, here you have attempted murder and here we are able to identify the perpetrator.

JOYCE: Another damning piece of evidence: Burnham notes that the wound is on the tail, which is typical of where predators bring down running prey.

BURNHAM: So this gives us the food pyramid and puts T. rex back on top as the apex predator.

JOYCE: However, Florida State's Greg Erickson says it's not as simple as it is in the movies. Like most predators today, it probably was both predator and scavenger.

ERICKSON: So what T. rex did more of is really the key and to me this debate is not solved. From this, all we can say is that, yes, T. rex acted like a predator.

JOYCE: But also like a scavenger.

ERICKSON: We have cases where there are hundreds, sometimes thousands of duckbilled dinosaurs that died. And these animals were clearly fed upon by tyrannosaurs. We find tooth marks and shed teeth among the skeletons.

JOYCE: This all might seem to be an academic debate, but scientists want to know who T. rex ate and when. It explains how the food chain worked millions of years ago, and how one great big dinosaur influenced it. The research appears in the Proceedings of the National Academy of Sciences.

Christopher Joyce, NPR News.

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