Famed for their large hooked beaks and a presumed taste for meat, flightless phorusrhacids, also known as “terror birds,” were among South America’s top predators before going extinct about 2.5 million years ago. Now paleontologists have unearthed one of the most complete fossils of a phorusrhacid to date. The skeleton of the new species, dubbed Llallawavis scagliai, is approximately 95% complete, giving scientists the ability to study a terror bird’s anatomy in unprecedented detail. Analyses of the well-preserved remains are already providing insights into the bird’s hearing ability, scientists say.

“It’s rare to find such a complete fossil of anything, let alone a bird,” says Lawrence Witmer, a vertebrate paleontologist at Ohio University, Athens, who wasn’t involved in the new study. “This is a very exciting find.”

The fossil, which is missing only a few wing and toe bones and the tip of its stubby tail, was excavated in northeastern Argentina in 2010 from material laid down as sediment about 3.5 million years ago. L. scagliai likely lived in an open environment, possibly a grassland or a sparse forest through which small rivers flowed, says Federico Degrange, a paleo-ornithologist at the National University of Córdoba in Argentina and lead researcher on the study.

The new species of terror bird weighed an estimated 18 kilograms (about 40 pounds) and stood about 1.2 meters (4 feet) tall, Degrange’s team reports in the current issue of the Journal of Vertebrate Paleontology. That’s a moderate size for a terror bird, Degrange notes; at least one other species in the group grew more than 2 meters tall and weighed 70 kg or more.

Unlike most birds, many of the joints between bones in a terror bird’s skull are typically fused, Witmer says. In this species, the joints in the birds’ upper palate, as well as some of those near the beak, are much less flexible than they are in other types of birds, which may have helped them pummel their prey and more effectively rip apart carcasses.

But the most interesting information from the new fossil came from CT scans of its inner ear. The shape and orientation of the semicircular canals in that structure suggest the bird could swivel its head quickly, as it might when tracking or striking at prey, Degrange says. Moreover, he notes, analyses of the scans even provide information about the bird’s sense of hearing, which was most likely limited to frequencies between 380 and 4230 hertz—approximately the same range as keys on the right half of a standard piano keyboard. That is substantially lower than the range measured for L scagliai’s closest flightless relatives, the team reports.

“That’s to be expected for a large animal,” says Luis Chiappe, a vertebrate paleontologist at the Natural History Museum of Los Angeles County in California. “The general rule of thumb is, the bigger you are, the lower the sounds you produce and hear.”

The emphasis on low-frequency hearing is very interesting, Witmer says, because it may suggest something about how the birds tracked prey. “Low frequencies tend to propagate across long distances with little attenuation.”

But in recent years, some scientists have proposed that not all terror birds were good hunters. This suggests that the members of some species may have spent a good fraction of their time scavenging carcasses rather than chasing prey. By measuring the proportions of various carbon and nitrogen isotopes in terror bird fossils, researchers could better assess the birds’ dietary habits, Chiappe says. In particular, they could get a better idea about where in the food chain the terror birds’ main sources of nutrition were coming from. “I’m surprised that no one has done a careful study of such isotopes yet,” he notes.