A FOSSIL uncovered on a Melbourne beach belonged to a giant prehistoric bird with a five-metre wing span and serrated beak, scientists report.

The 5 million-year-old lower leg bone is the first evidence the ancient sea bird once soared across Australia's skies.

A Museum Victoria palaeontologist, Erich Fitzgerald, who led analysis of the specimen, said the discovery meant the species, Pelagornis, which lived for more than 50 million years before becoming extinct about 2.5 million years ago, existed on all continents.

The ancient feathered creatures, known as bony-toothed birds for the sharp, teeth-like serrations along their beak, was the largest flying animal to exist on Earth after the extinction of Pterosaurs, a flying reptile that lived during the time of the dinosaurs, 65 million years ago.