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And I'm Melissa Block.

Scientists have identified the largest skull of a bird ever found. It's about the size of a horse's skull, and it was found in a small museum in the mountains of Argentina. It belonged to a creature that ruled the grasslands of South America for millions of years, a creature called the terror bird.

NPR's Christopher Joyce has more.

CHRISTOPHER JOYCE: A bird 10 feet tall with a beak like a battering ram that could take down a small horse. Who knew? Actually, scientists have known about this creature but not much. They never had a skull so complete.

This bird didn't fly. It put its biological resources into growing bigger and faster than anything else on the continent. Its weapon was a massive hooked beak on a head that makes the California Condor look like a pigeon. It was the largest bird ever and it was the top predator, sort of like the saber-toothed cat of South America.

Dr. LUIS CHIAPPE (L.A. Natural History Museum): A vicious meat eater, no question about that.

JOYCE: Luis Chiappe is the paleontologist who identified the skull. It was originally found by a high school student. He says this creature was puckish to say the least.

Dr. CHIAPPE: This is not like a crocodile. This was a, you know, a warm blooded, enormous, very active bird. I mean with the size of the skull, imagine the damage that that skull could have done just by hitting something.

JOYCE: The bird comes from a large family. Scientists call them the Phorusrhacids. Chiappe, who runs the Natural History Museum of Los Angeles, says there were many types running around South America after the dinosaurs bowed out.

But bones of the really big ones are very rare. Scientists assumed the big species were just bulky versions of their smaller cousins. But Chiappe says this skull and a surprisingly slender leg-bone found with it suggest something not just bigger but different.

Dr. CHIAPPE: We are challenging this traditional view that these birds, as they grew bigger, they became less agile.

JOYCE: Instead, Chiappe writes in journal Nature, they were probably greyhound quick. That quickness gave them an advantage. Paleontologist John Flynn of the American Museum of Natural History in New York notes South America was an island at the time. It was a place where evolution had taken a different turn from other parts of the world after the dinosaurs died out.

Dr. JOHN FLYNN (American Museum of Natural History): There were very limited numbers of predators. Most of the mammalian predators were a kind of dog or lion-like marsupial.

JOYCE: Basically carnivorous relatives of possum. There were also big crocodiles, but none of these hunters had the speed and agility that these new bones suggest the terror birds had. And there were plenty of grazing animals to prey on, so the terror birds climbed to the top of the food chain. Flynn says they remind him of their very successful predecessors.

Dr. FLYNN: You look at it, it's really interesting because they do, they resemble, at least in a crude way, large predatory dinosaurs like t-rex that had gigantic heads, very small forelimbs, very long legs. And so they've got that same kind of large meat eater adaptation and they obviously did what they did very well and they persisted for tens of millions of years.

JOYCE: The big birds died out about two million years ago, about the time that North and South American continents hooked themselves together at the Isthmus of Panama. Flynn notes that the climate was changing at the time and could have contributed to the terror birds' extinction. Another possibility, even more terrible predators. Lions and tigers and bears may have moved in from the north and knocked them off their perch.

Christopher Joyce, NPR News.

SIEGEL: You can see a picture of the 10-foot terror bird at NPR.org.

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