Image copyright F. Degrange Image caption So-called "terror birds" had very large, hooked beaks and the largest species grew to 3m

A 90%-complete "terror bird" skeleton found on an Argentinean beach suggests these big-beaked predators had good low-frequency hearing and deep voices.

It is the most complete skeleton ever discovered for one of these menacing beasts, and represents a new species.

Scientists have even been able to reconstruct the shape of its inner ear.

This offers clues about the animal's hearing, which was probably lower than that of modern birds and suggests they used low-pitched calls to communicate.

The find is reported in the Journal of Vertebrate Paleontology.

Argentinian palaeontologists made the discovery in the cliffs of La Estafeta beach, not far from the popular tourist city of Mar del Plata.

Federico Degrange, one of the study's authors, said dealing with the tide had presented a challenge.

"The sea can actually take the fossil and destroy it in the sea. It's a nice place to work, but you have to be fast," he told BBC News.

Terror birds, or "phorusrhacids", were the top predators on the South American land mass in the era following the dinosaurs' extinction some 65 million years ago.

Image copyright H. Santiago Druetta Image caption Deadly beaks and low-pitched grunts: An artist's impression of the new species in full voice

The flightless beasts stood up to 3m tall, boasting long legs and devastating hooked beaks. A previous study of this weaponry suggested that the birds could have despatched their prey with a single blow, before setting to work on its flesh.

"They evolved very unique forms, with huge skulls, huge beaks with hooks, and long hindlimbs," said Dr Degrange, a terror bird specialist who works at the National University of Cordoba.

"They lost their ability to fly and they developed very unusual predatory capabilities that were not present in any comparable animals."

The newly discovered species is dubbed Llallawavis scagliali ("Scaglia's Magnificent Bird") after the study's senior author Fernando Scaglia.

It stood about 1.2m tall and probably weighed 18kg, making it a medium-sized addition to the terror bird family. And it lived towards the end of that family's long period of dominance, some 3.5 million years ago.

Image copyright M. TAGLIORETTI/F. SCAGLIA Image caption It is the most complete terror bird specimen ever found

This means it probably ate mammals or other birds; pretty much anything smaller than itself, Dr Degrange suggested.

Perhaps most intriguing among the well-preserved details of the fossil is its skull, which allowed the researchers to make some educated guesses about the animal's sensory capabilities - and even its voice.

"A very interesting thing is that we could reconstruct the shape of the inner ear," Dr Degrange said.

Based on comparisons with living species, these measurements suggested that the ears of terror birds like Llallawavis were most sensitive to low-pitched sounds.

"We are able to say that terror birds had low frequency sensitivity - so it seems reasonable to suggest that they also produced low-frequency sounds."

Again, by comparing their anatomy with birds that are alive today, you might imagine that they sounded something like an ostrich or an emu, Dr Degrange said.

"But it's not possible to say for sure."

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