From the tottering penguin to the scurrying kiwi, flightless birds can seem a bit helpless on the ground. But one species of bird seems to have made aggressive use of its front appendages. The Xenicibis xympithecus had clublike wings that might have been used to deliver a powerful slug other animals.

"It's the most specialized weaponry of any bird I've ever seen," Nicholas Longrich, of Yale University's Department of Geology and Geophysics, said in a prepared statement. Other birds have been seen using their wings as weapons, including the pugnacious steamer duck (Tachyeres) and the aptly named spur-winged goose (Plectropterus), but the level of specialized skeletal adaptation in Xenicibis packed a big surprise to researchers. "No one could believe it was actually that bizarre," said Longrich, who was part of the team that described this odd adaptation.

"When I first saw it, I assumed it was some sort of deformity," Longrich noted—or as the team noted in the paper, "some inexplicable pathology." But after seeing more of the same skeletal patterns in other individuals, the researchers began to piece together the implications: "Xenicibis may have used the club-shaped hand to deliver blows," they concluded in the paper, which was published online January 4 in Proceedings of the Royal Society B: Biological Sciences.

The specimens, which had been recovered from a site in Jamaica, are related to living ibises and would have been about the size of a chicken (likely weighing about two kilograms). The especially long forearm might have functioned like a handle and the large, thick hand bones as a blunt club head. And an unusual joint in the wrist would be troublesome for flight, but excellent for a good fight, Longrich and his colleagues suggested.

The wing bones of Xenicibis are hollow, but they have thicker walls than the birds' leg bones. The researchers also suggested that, "the hollow metacarpal also allows the hand to achieve greater strength for a given amount of material, much like an aluminum baseball bat."

And like any seasoned fighters, these birds seem to have received their fair share of blows. Among the examined specimens, two bones "show evidence of healed fractures," including a humerus and a hand bone, the researchers noted.

The assessment presents an unusual example of functional adaptation. "Although the appendages of vertebrates have repeatedly become specialized for walking, running, swimming, burrowing and flying," the researchers concluded, "Xenicibis is unique in having modified its pectoral appendage into a jointed club that can be swung to increase the speed and energy of the blow."

Of course these wing-weapons were not enough to ensure survival in the long run. Xenicibis seems to have gone extinct some 10,000 years ago.

Images courtesy of Nicholas Longrich/Yale University