Imagine an albatross with a hacksaw for a mouth. Set that strange creature about 50 million years in the past and you&rsquove got the image of a pelagornithid, a group of ancient avians that included some of the largest flying birds of all time. And now paleontologists have uncovered in that group what may be the largest known flying birds ever, with wingspans of roughly 21 feet.

Though a much smaller pelagornithid fossil dates from 62 million years ago, one newly described fossil a 50 million-year-old portion of a bird&rsquos foot shows that the larger pelagornithids arose just after life rebounded from the mass extinction 65 million years ago, when the relatives of birds, the dinosaurs, went extinct.

The last known pelagornithid is from 2.5 million years ago, a time of changing climate as Earth cooled, and the ice ages began, they said. Pelagornithids are known as &lsquobony-toothed&rsquo birds because of the bony projections, or struts, on their jaws that resemble sharp-pointed teeth, though they are not true teeth, like those of humans and other mammals.

The bony protrusions were covered by a horny material, keratin, which is like our fingernails, the researchers said. Called pseudoteeth, the struts helped the birds snag squid and fish from the sea as they soared for perhaps weeks at a time over much of Earth&rsquos oceans, they said.

Large flying animals have periodically appeared on Earth, starting with the pterosaurs that flapped their leathery wings during the dinosaur era and reached wingspans of 33 feet, according to the researchers.