The skull of the Andalgalornis, or "terror bird," seen here… (Ohio University )
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 an pickax and repeatedly chopping at prey until it succumbed.
The 5-foot-tall, 90-pound Andalgalornis steulleti, 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.
The new study "allows researchers to get down to the real nitty-gritty of the animal and be more specific about some of its behavior techniques," said vertebrate paleontologist Bob Chandler of the Georgia College & State University in Milledgeville, Ga., who did not take part in the study. "This allows us to add another layer to the biomechanics of paleontology."
Andalgalornis lived in northwestern Argentina about 6 million years ago. It was one of a family of at least 18 species collectively known as phorusrhacids, but popularly called "terror birds" because of their size and fearsome skull. The largest was the 10-foot-tall Kelenken.
All the species went extinct 5 million years ago and there are no close relatives alive today, so their habits have been mysterious. Vertebrate paleontologist Lawrence Witmer of the Ohio University College of Osteopathic Medicine and colleagues from the Universidad Nacional de La Plata in Argentina used CT scans to study the interior of an Andalgalornis skull, and also measured the bite strength of an eagle and a red-winged seriema, a descendant of the phorusrhacids, for comparison.
They published their results in the online journal PLoS One.
Modern birds have light skulls with considerable internal flexibility, Witmer noted. But the terror birds had heavier, much more rigid structures, "really changing the architecture of the skull," he said. "It was more rigid than anything we had anticipated."
The skull was extremely strong from front to back and up and down, he noted, but weak from side to side. Muscle attachments also suggest that the bite, while "respectable, probably wasn't delivering killing bites to large animals."
Computerized simulations of the skull further indicated that it wouldn't withstand the stress of grasping prey and shaking it from side to side, like a dog does a rabbit, he said.
The authors concluded that the bird probably bobbed and weaved like Muhammad Ali and used a repeated attack and retreat strategy, jabbing straight down with the skull to use the beak like a pickax, repeatedly driving the bill tip into the prey. "It was using its powerful neck like the handle of a pickax," Witmer said. The bird could then use its beak to pull meat off the carcass.
Considering all the strengths and weaknesses of the skull, he concluded, "this is the only strategy this animal could adopt."