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The "terror bird" Andalgalornis.
A fossil skull of a "terror bird" species dwarfs those of a modern golden eagle and a human.

Photograph courtesy Ohio University

Brian Handwerk

for National Geographic News

Published August 19, 2010

Phorusrhacids have been extinct for millions of years, yet the so-called terror birds just got a bit more frightening.

The flightless birds stood up to ten feet (three meters) tall and had hook-beaked heads the size of horse heads. Now a new study has apparently deciphered how the birds used those fearsome skulls—employing a fighting style like that of legendary boxer Muhammad Ali.

Researchers used CT scans of terror-bird skulls and biomechanical computer models to conclude that the birds likely used a speedy, graceful, strike-and-retreat style, killing their prey with a succession of punishing, hatchet-like blows.

The technique probably landed the birds a top ranking in the food chain of prehistoric South America, according to the study, published August 18 in the journal PLoS ONE.

"South America was a strange and different place back then," said paleontologist Lawrence Witmer of the Ohio University College of Osteopathic Medicine.

"It was an island continent and had been so since around the time of the dinosaurs, so there was a lot of evolution that took place in its own microcosm," said Witmer, who co-authored the new study.

After rising about 60 million years ago, terror birds evolved into at least 18 species, all of which disappeared by about two to three million years ago, around the time North and South America collided.

Because no living animals like the phorusrhacids exist today, terror bird behavior is something of a mystery.

(See also "Terror Birds: Predators With a Kung Fu Kick?")

Terror Bird Floated Like a Butterfly, Stung Like a Bee

To unlock clues to the terror-bird lifestyle, the international team looked to specimens of Andalgalornis, a 4.5-foot-tall (1.4-meter-tall) terror bird that lived in northwestern Argentina some six million years ago. (Related: "Largest 'Terror Bird' Fossil Found in Argentina.")

The scientists used CT scans to examine the architecture of the skull from the inside out, finding a structure far more rigid than that seen in most birds.

The data were used to create engineering-based, 3-D computer models, which revealed the stresses created when the digital skull was put through a series of biting, thrashing, and shaking movements.

Anatomy and biomechanics pointed to the same behaviors—those of an "outside fighter," Witmer said.

"This animal had a hatchet-like skull that was very strong and rigid when being driven straight down into prey but was weak from side to side," he explained.

"Consequently these animals weren't sluggers. They couldn't grapple with prey, couldn't handle those twisting movements.

"So these things were more like a Muhammad Ali, dancing around, using their speed and surgically precise jabs with that hatchet-like skull coming straight down over and over again," he said. "That's how they killed their prey."

Terror Bird: Latter-Day T. Rex

This system likely proved extremely effective—at least for some tens of millions of years—because the birds appear to have chopped their way to a rarefied role.

"The conventional dinosaurs went extinct," Witmer added. "But these terror birds were like avian dinosaurs which in many ways filled the ecological niche that predatory dinosaurs filled.

"T. rex was a large, head-dominated, bipedal predator, and that's kind of what we're talking about with these terror birds—though both the birds and their prey were smaller than animals of the dinosaur era."

(Also see: "Seven of the Biggest Beasts of All Time.")

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