The age of dinosaurs on land was also the era of pterosaurs in the skies - but part way through a strange change happened, with toothless species replacing toothed ones as the dominant flying creatures.

Despite their numbers and diversity, we are only starting to learn about pterosaurs, as their thin bones did not fossilize well. It was only this year that enough of their eggs have been found to help us start to understand their breeding.

However, we do know that until 90 million years ago the dominant pterosaurs (Ornithocheiridae) had teeth. From the late Cretaceous all the fossils we have are toothless, predominantly members of the Azhdarchidae family (from the Persian for dragon).

In ZooKeys Dr Alexander Averianov of the Russian Academy of Sciences has published a Review of taxonomy, geographic distribution, and paleoenvironments of Azhdarchidae, investigating the 51 known specimens (often just a few bones) and three preserved tracks.

Based on the diversity of geological sites where the bones have been found Averianov concludes, “Azhdarchids likely inhabited a variety of environments, but were abundant near large lakes and rivers and most common in nearshore marine paleoenvironments.”

The reason the Ornithocheiridae died out is “still poorly understood” according to Averianov, but “apparently reflects some fundamental changes in Cretaceous ecosystems.” This change appears to have been associated with a surge in carbon dioxide levels. Simultaneously, crustal production reached its highest rate for 100 million years and the Earth's oceans became deprived of oxygen. In combination these wiped out a quarter of the microscopic sea creatures at the base of the food chain.

However, it is not clear why these dramatic events would wipe out toothed pterosaurs, leaving room for toothless species to flourish. The changeover did nothing to diminish the size of these fearsome creatures with some of the late Azhdarchidae having 12 meter wingspans.

^{ZooKeys. Although their bones fossilized poorly, Azhdarchidae remains have been found around the world, indicating how widespread they were in the late Cretaceous.}

H/T LiveScience