A paleontologist cleans the fossilized bird cranium at Peru's National History Museum in Lima on Friday. (Martin Mejia/Associated Press)

The unusually intact fossilized skull of a giant, toothed sea bird that died millions of years ago has been found on Peru's arid southern coast, researchers said Friday.

The fossil is the best-preserved cranium ever found of a pelagornithid, a family of large seabirds believed to have gone extinct some three million years ago, said Rodolfo Salas, head of vertebrate paleontology at Peru's National History Museum.

The museum said in a statement the birds had wingspans of up to six metres and may have used tooth-like projections on their beaks to prey on slippery fish and squid. But studying members of the Pelagornithidae family has been difficult because their extremely thin bones — while helpful for keeping the avian giants aloft — tended not to survive as fossils.

"Its fossils are very strange, very rare and very hard to find," Salas said.

The cranium discovered in Peru is 40 centimetres long and is believed to be eight million to 10 million years old, based on the age of the rock bed in which it was found.

"Rarely are any bones of these gigantic, marine birds found fossilized uncrushed, and to find an uncrushed skull of this size is very significant," said Ken Campbell, curator of vertebrate zoology at the Natural History Museum of Los Angeles.

Campbell, who examined photos of the find but was not involved in the dig, said he knows of "no specimen of comparable quality."

Bird an enigma

Dan Kepska, a paleontology researcher at North Carolina State University who also was not part of the project, agreed the skull is the most complete ever reported.

He called the birds "one of the great enigmas of avian paleontology."

With fossils discovered in North America, North Africa and even the Antarctic, Kepska said, the birds were ubiquitous only a few million years before humans evolved, and scientists puzzle over why they died out. Some believe they are related to gannets and pelicans while other say they are related to ducks.

Campbell said the Peru find "will undoubtedly be of great importance to our understanding of these gigantic birds and it will help clarify the relationships of the other fossil pelagornithids found in the Pisco Formation."

The formation, a coastal rock bed south of the capital Lima, is known for yielding fossils of whales, dolphins, turtles and other marine life dating as far back as 14 million years.