The large, big-beaked "terror birds" (Phorusrhacidae) didn't need flight to snag a Miocene meal. Some of these extinct, flightless fowl likely used their massive rigid skulls and hooked beaks to chomp into prey with strong, successive pulls, concluded a research team after performing a biomechanical analysis of fossilized skulls.
More than a dozen species of terror birds roamed South America starting about 60 million years ago; the biggest, Kelenken, reached 2.1 meters tall. But with no close living relatives, researchers have not been sure how these giant avian carnivores lived—and killed.
As a model, the researchers examined Andalgalornis steulleti, a 1.4-meter tall, 40-kilogram Phorusrhacid that lived and died some 6 million years ago in what is now Argentina. This heavyweight fowl was no average ancient bird—its stiff 0.37-meter skull was mostly beak.
"This guy had a strong skull," Lawrence Witmer, of Ohio University College of Osteopathic Medicine and coauthor of the new study, said in a prepared statement. The new work was published online August 18 in PLoS ONE.
Witmer and his colleagues used CT scans to analyze fossilized skulls of Andalgalornis, other terror birds and some modern fowl. The team also examined the skulls and biting power of hook-beaked eagles and Argentinian birds in the seriemas group, the closest living—if diminutive—descendant of true terror birds.
"Combining all this information, we discovered that the bite force of Andalgalornis was a little lower than we expected and weaker than the bite of many carnivorous mammals of about the same size," Frederico Degrange, of the Museo de La Plata and lead study author, said in a prepared statement.
Based on the skull structure alone (not taking into account other jaw or head muscles), the researchers estimated Andalgalornis to have a bite force of about 133 Newtons, which is only slightly more than that of the relatively slight South American gray fox (Pseudalopex griseus). Many modern birds, however, also have relatively weak bite forces, such as the eagle's 50-Newton clamp, the researchers noted.
"Andalgalornis may have compensated for this weaker bite by using its powerful neck muscles to drive its strong skull into prey like an axe," Degrange said.
Terror birds had strikingly different cranial bone structures than modern fowl. "Birds generally have skulls with lots of mobility between the bones, which allows them to have light but strong skulls," Witmer said. "We found that Andalgalornis had turned these mobile joints into rigid beams."
The strength of Andalgalornis's skull lie especially in forward and backward movements, rather than side-to-side shaking, which would put severe strain on the fused skull sections, the research team surmised. This discovery led the group to propose that the bird might not have been stalking monster prey that could put up a thrashing fight. Rather, Andalgalornis seems likely to have gone after slightly smaller fare, and used repeated, pulling chomps to quickly subdue prey and prevent any dangerous lateral movements.
"Relative to the other birds considered in the study, the terror bird was well-adapted to drive the beak in and pull back with that wickedly recurved tip of the beak," Stephen Wroe, of the Computational Biomechanics Research Group at the University of New South Wales and study coauthor, said in a prepared statement.
Once researchers can pinpoint what prey terror birds were likely to hunt, they can drill down to the birds' more specific place in the ancient ecosystem. "We need to figure out the ecological role that these amazing birds played if we really want to understand how the unusual ecosystems of South America evolved over the past 60 million years," Degrange said.
Video of terror bird bite
Image of terror bird attacking a cat-sized Hemihegetotherium courtesy of Marcos Cenizo/Museo de la Plata; image of Andalgalornis skull, golden eagle skull and human skull model courtesy of Ohio University
Video of CT scan courtesy of Ridgely & Witmer/Witmer Lab at Ohio University