How Flight Evolved in Pterosaurs: When Flying Got Big

Article by: Adam Manning
Edited by: Harry T. Jones, J. D. Dixon, Lewis Haller, and Jack Wood

In the previous article of this miniseries, we explained how flight first evolved in insects, but now we move to the next chapter in the aerial saga.

A deceased Eudimorphodon ranzii. This species, much like other pterosaurs, has been found fossilised in a shale (aquatic deposits), so probably ended up falling into water after its death. Some plant matter has even ended up caught in its mouth. Illustration by Jack Wood.

The first pterosaur was identified in 1784 by Cosimo Collini, who initially thought that the winglike structures were paddles for swimming, as the pterosaur was found with fossilised marine organisms. We now recognise pterosaurs as, in fact, flying reptiles, and they have become one of the most iconic groups of prehistoric animals. This is well deserved, as they are the first known flying vertebrates to ever evolve, and grew to enormous sizes. But some information regarding pterosaurs is unclear. How, and why, did they evolve the ability to fly?

The oldest known pterosaurs, such as Eudimorphodon, date back to the Late Triassic, and lasted until the K-Pg mass extinction, the same event that wiped out the dinosaurs ~ 65 million years ago. They were also widespread, having been found on every continent except Antarctica. There are roughly two main groups of pterosaurs; Rhamphorynchoids, which were typically smaller with longer tails and differentiated teeth, and the bigger Pterodactyloids, which had shorter tails, longer wrist bones and bigger head crests, with simpler or no teeth.

A skeletal reconstruction of the Late Jurassic pterosaur, Rhamphorhynchus. Illustration by Scott Hartman. Available at: https://www.skeletaldrawing.com/non-dinosaurs/rhamphorynchus.

The adaptations of pterosaurs for flight are clear from their anatomy. They had light hollow bones like a modern-day bird, and an elongated fourth finger, which supported the skin that formed the wing membrane. This skin also attached to the femur. They also had a pteroid, a small pointed bone attached to the wrist that supported a small anterior flight membrane. The two latter adaptations gave pterosaurs a broader wingspan. Moreover, pterosaurs had large eyes, and reconstructions of their brains have even suggested that they may have the good vision and sense of balance needed for flight.

Whilst the adaptations of pterosaurs are clear, it is much harder to understand how these adaptations evolved. This is because the earliest known pterosaurs already had these adaptations. There are three main theories on how the group evolved the ability of flight. One, the cursorial hypothesis suggests that the ancestors of pterosaurs were terrestrial bipeds that leapt into the air to catch insects, and used their forelimbs to help control themselves. Over many generations, they evolved a patagium, or skin membrane between their limbs, that made them better at doing this, which over time would have evolved into wings. The other theories suggest that the ancestors of pterosaurs were arboreal, or lived in trees. The arboreal leaping hypothesis states that these ancestors first evolved a patagium to leap from tree to tree more easily. This would eventually form gliding membranes and then wings for powered flight. The arboreal parachute hypothesis, however, suggests that pterosaur ancestors moved around the trees by falling to lower branches rather than leaping, and thus evolved a patagium to make this easier and safer, which in turn developed into wings.

A skeletal reconstruction of the Late Cretaceous pterosaur, Pteranodon. Illustration by Scott Hartman. Available at: https://www.skeletaldrawing.com/non-dinosaurs/pteranodon.

So how do we know which hypothesis is correct? Well, we know from analyses of closely related groups that they evolved their wings last, and already had all of their other adaptations. The cursorial hypothesis is flawed. Fossil footprints show that pterosaurs were quadrupedal when walking on the ground, so it is unlikely they had a bipedal ancestor. Furthermore, for this hypothesis to be correct, the patagium wouldn’t connect to the hindlimbs, but it has been reconstructed that the wing membranes of pterosaurs did. The arboreal parachuting hypothesis is supported by the patagium being connected to the hindlimbs, and by various morphological characteristics of pterosaurs that suggest they could have done well in an arboreal environment, but it doesn’t explain specialised adaptations in the hindlimbs of pterosaurs. This leaves the arboreal leaping hypothesis, which does explain these adaptations, as they would have been used to generate the power needed for leaping. More recent work suggests that a combination of the terrestrial and arboreal leaping hypotheses could have led to the evolution of pterosaurs.

In sum, the theory that pterosaur ancestors were tree-dwellers that leapt from tree to tree seems like the most viable hypothesis, but there is little direct evidence to support any of these hypotheses. Like pterosaurs themselves, these ancestors were likely lightly built, and so didn’t fossilise well. What we do know is that pterosaurs represent a landmark in evolutionary history, as they were the largest and first group of flying vertebrates in Earth’s history.

Image References
[1] A deceased Eudimorphodon ranzii. This species, much like other pterosaurs, has been found fossilised in a shale (aquatic deposits), so probably ended up falling into water after its death. Some plant matter has even ended up caught in its mouth. Illustration by Jack Wood.
[2] A skeletal reconstruction of the Late Jurassic pterosaur, Rhamphorhynchus. Illustration by Scott Hartman. Available at: https://www.skeletaldrawing.com/non-dinosaurs/rhamphorynchus.
[3] A skeletal reconstruction of the Late Cretaceous pterosaur, Pteranodon. Illustration by Scott Hartman. Available at: https://www.skeletaldrawing.com/non-dinosaurs/pteranodon.

Information References and Further Sources
[1] Bennet, S. C. (1997). ‘The arboreal leaping theory of the origin of pterosaur flight’, Historical Biology: An International Journal of Paleobiology, 12 (3-4), pp. 265-290. DOI: https://doi.org/10.1080/08912969709386566. Accessed 28th October 2019. Click Here.
[2] Benton, M. J. (2015). ‘Vertebrate Paleontology’, 4th ed. Oxford: Wiley Blackwell pp. 236-241.
[3] Hone, D. W. E., and Buffetaut, E. (2008). ‘Flugsaurier: pterosaur papers in honour of Peter Wellnhofer’, Zitteliana: An International Journal of Palaeontology and Geobiology Series B, 28. Accessed 28th October 2019. Click Here.
[4] Langston Jr., W. (1981). ‘Pterosaurs’, Scientific American, 244 (2), pp. 122-137. Accessed 28th October 2019. Click Here.
[5] Monastersky, R. (Unknown). ‘Pterosaurs-Lords of the Ancient Skies’, National Geographic. Accessed 24th January 2020. Click Here.
[6] Unwin, D. M. (1999). ‘Pterosaurs: back to the traditional model?’, Trends in Ecology and Evolution, 14 (7), pp. 263-268. Accessed 28th October 2019. Click Here.