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LOS ANGELES — From the size and shape of the beak, researchers have always known that the massive South American “terror bird” was a predator. Now, they know precisely how the bird killed — wielding its huge skull and hooked beak like a pickax and repeatedly chopping at prey until it succumbed.

The 5-foot-tall, 90-pound Andalgalornissteulleti, whose skull was nearly twice the size of a human’s, went extinct millions of years ago, but Argentine and U.S. researchers have been using CT scans and biomechanical reconstructions to deduce how the flightless predators killed. Their findings were announced Wednesday in the online journal PLoS One.

Andalgalornis lived in northwestern Argentina about 6 million years ago. There are no close relatives alive today, so their habits have been mysterious.

Vertebrate paleontologist Lawrence Witmer of Ohio University College of Osteopathic Medicine and colleagues from the Universidad Nacional de La Plata in Argentina studied the Andalgalornis skull.

Modern birds have light skulls with considerable internal flexibility, Witmer noted. But the terror birds’ skulls were heavier, much more rigid structures, “really changing the architecture of the skull,” he said. “It was more rigid than anything we had anticipated.”