Giant flying reptiles known as pterosaurs dominated the skies above Earth 60 million years ago, and a species characterized as toothless "dragons" were among the most successful, researchers say.

Scientists have been studying a family of pterosaurs known as Azhdarchidae, from Azdarha, a Persian word for dragon.

It included a number of the largest flying animals ever to soar above the Earth, with wingspans reaching almost 40 feet, they say. 

These giants had long legs and even longer necks, and may have resembled modern storks, although when not flying they walked on four legs, the back ones and their winged forearms.

Unlike pterosaurs of earlier periods, the Azhdarchidan pterosaurs of the Late Cretaceous were noted for being without teeth. Large, toothless flying creatures? Fans of DreamWorks' "How to Train Your Dragon" movies might be interested in these toothless "dragons".

Fossil studies suggest they began to dominate over their toothed predecessors starting around 90 million years ago when elevated levels of carbon dioxide caused important microscopic marine creatures to go extinct, interrupting food chains and triggering mass extinctions.

"This shift in dominance from toothed to toothless pterodactyloids apparently reflects some fundamental changes in Cretaceous ecosystems, which we still poorly understand," says Alexander Averianov from Russia's Academy of Sciences.

Writing in the journal ZooKeys, the researchers said the toothless "dragons" were distributed all around the globe and were the last to survive on the planet, until around 60 million years ago.

Although they almost certainly inhabited a wide variety of environments, fossil evidence suggests they were most abundant near large rivers and lakes and shoreline marine environments, says Averianov.

Fossil remains of pterosaurs are hard to come by, researchers note, as their light and thin-walled bones necessary for flight are fragile and do not preserve well.

Most Azhdarchidae fossils are found in a deposits of a certain type of soft sediment rarely found for the Late Cretaceous period when the majority of the evolutionary saga of the species occurred, making a determination of their place on the pterosaur family tree difficult, Averianov says.

"Azhdarchidae currently represent a real nightmare for paleontologists," he says. Most are identified from only a few bone fragments, and the few discovered complete skeletons are not well preserved, he explains.

Still, he says, the multitude of locations around the world where azhdarchidan pterosaur fossils have been found is remarkable.

It is evidence the giant flying reptiles played a significant role in Cretaceous ecosystems in all parts of the world, he says.