Pterosaurs: dragons of the air

What are pterosaurs?

They are extinct volant Mesozoic ornithodirans. This is a meaningless statement to all but a few experts, not all of whom agree with it anyway. What all do agree is that pterosaurs are extinct, were restricted to the Mesozoic era and could fly. However, not everyone agrees that pterosaurs are ornithodirans, partly because not everyone is sure what an ornithodiran is. So, do we really know what a pterosaur is?

In 1801 French anatomist and palaeontologist Baron von Cuvier (1769–1832) established that pterosaurs were reptiles, describing features of the skull, most notably two holes in the side of the skull behind the eye. This placed pterosaurs in a reptilian group called Diapsida, along with lizards, snakes, crocodiles, dinosaurs, birds and many other fossil reptiles.

Not everyone agreed with Cuvier. Some suggested that pterosaurs were birds and others even a type of marsupial bat, but for no good reason other than that they had fur and therefore were probably warm-blooded. However, Cuvier had sound anatomical reasons for his claim and it is now well established that pterosaurs were reptiles.

Skull of a Jurassic long-tailed pterosaur

The two green holes behind the eye indicate that pterosaurs are diapsid reptiles, the group that today includes snakes, lizards, crocodiles and birds.

Skull of the Cretaceous pterosaur Santanadactylus

This skull shows the large nasoantorbital fenestra coloured in purple. This hole contains the nostril and possibly a salt gland or air sac, or all three.

What sort of reptiles?

Of the many groups of reptiles, one is characterised by yet another hole in the skull, this time between the eye socket and the nostrils. This hole, called the antorbital fenestra or a nasoantorbital fenestra in those pterosaurs where it had merged with the nostril, is characteristic of the Archosauria, a group which contains crocodiles, dinosaurs and birds. However, pterosaurs clearly differ greatly from these animals. So, to which group of archosaurs – dinosaurs, birds or crocodiles – are pterosaurs most closely related?

It is now established that birds are highly evolved theropod (meat eating) dinosaurs, so the question is really, are pterosaurs more closely related to dinosaurs or crocodiles? An analysis of the bones of the ankle shows that they are indeed closer to dinosaurs, but this does not take into account the many fossil archosaur groups. Presently, it appears that pterosaurs are closely related to dinosaurs and scientists call this group the Ornithodira (Pterosauria + Dinosauria). There are other hypotheses and the answer will only be solved with the discovery of new fossils, probably from the Middle Triassic.

Why are scientists struggling to answer these questions?

Back in the 1700s the very first pterosaur fossil was thought to be a type of shrimp (it only seems daft in hindsight), so it seems these animals have baffled scientists for more than 200 years. The reason scientists struggle to answer this question is simple: the pterosaur skeleton was so highly modified for flight, including many modifications that aided weight reduction or redistribution, strengthened the landing gear, allowed flapping, as well changes related to feeding and sexual display, that there are few clues left to their evolutionary origins. It is even difficult to identify individual bones in the skull because, like birds, most of the bones fuse together.

Reptile evolutionary tree