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Ancient 'Terror Bird' Used Powerful Beak to Jab Like Boxer

Posted on 27. September, 2010.

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The ancient "terror bird" Andalgalornis could not fly, but it used its unusually large, rigid skull - coupled with a hawk-like hooked beak – to fight like a boxer.

According to a study published 18 August  2010, in the journal PLoS ONE, the agile creature repeatedly attacked and retreated, landing well-targeted, hatchet-like jabs to take down its prey. The study is the first detailed look at the predatory style of a member of an extinct group of large, flightless birds known scientifically as Phorusrhacids but popularly labelled "terror birds" because of their fearsome skull and often imposing size.

Terror birds evolved about 60 million years ago in isolation in South America, an island continent until the last few million years, radiating into about 18 known species ranging in size up to more than 2 metres). Little was known about their habits since they have no close analogs among modern-day birds.

The multinational team of scientists used CT scanning and advanced engineering methods to study the e form, function and predatory behaviour of a terror bird, Andalgalornis. "No one has ever attempted such a comprehensive biomechanical analysis of a terror bird," says study lead author Federico Degrange of the Museo de La Plata/CONICET in Argentina. "We need to figure out the ecological role these amazing birds played if we really want to understand how the unusual ecosystems of South America evolved over the past 60 million years."

Andalgalornis lived in northwestern Argentina about six million years ago. It was a mid-sized terror bird, standing about 1.4 metres tall and weighing in at a fleet-footed 40 kg. Like all terror birds, its skull was relatively large (37 centimetres) with a deep narrow bill armed with a powerful, hawk-like hook.

Paper co-author Lawrence Witmer of the Ohio University College of Osteopathic Medicine, USA, ran a complete skull of Andalgalornis through a CT scanner. The scans revealed that Andalgalornis was unlike other birds because it had evolved a highly rigid skull. "Birds generally have skulls with lots of mobility between the bones, which allows them to have light but strong skulls," said Witmer. "But we found that Andalgalornis had turned these mobile joints into rigid beams. This guy had a strong skull, particularly in the fore-aft direction, despite having a curiously hollow beak." The evolution of this large and rigid bony weapon was presumably linked to the loss of flight in terror birds, as well as to their sometimes gigantic sizes.

From the CT scans, Stephen Wroe, director of the Computational Biomechanics Research Group at the University of New South Wales, Australia, assembled sophisticated 3-D models of the terror bird and two living species for comparison (an eagle, and the terror bird's closest living relative, the seriema).
The models were used to simulate and compare the biomechanics of biting straight down (as in a killing bite), pulling back with its neck (as in dismembering prey) and shaking the skull from side to side (as in thrashing smaller animals, or when dealing with larger struggling prey).

The simulations supported the CT-based anatomical results. "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," says Wroe.

The study also looked at how hard a bite Andalgalornis could deliver. To examine bite force in birds in general, they worked with zookeepers at the La Plata Zoo to get a seriema and an eagle to chomp down on their bite meter. "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," Degrange says. "Andalgalornis may have compensated for this weaker bite by using its powerful neck muscles to drive its strong skull into prey like an axe."

The team's results give new insight into the lifestyle of a unique avian predator. Its skull, though strong vertically, was weak from side to side; its hollow beak was in danger of catastrophic fracture if Andalgalornis grappled too vigorously with large struggling prey. Instead, the study shows that the terror bird engaged in a repeated attack-and-retreat strategy with well-targeted, hatchet-like jabs.

Once killed, the prey would have been ripped into bite-sized morsels by the powerful neck pulling the head straight back or, if possible, swallowed whole.

The complete article can be found in PLoS ONE, 18 August 2010 10.1371/journal.pone.0011856.

In picture: The terror bird Andalgalomis, an extinct flightless bird found as a fossil in Argentina. Photo courtesy of Witmer Lab.

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