The origin of the pterosaurs

The first tetrapods to evolve powered flight were the pterosaurs. These were a group of archosaurs related to the dinosaurs, but not dinosaurs themselves. The earliest known pterosaur was Eudimorphodon, who lived in what is now Italy around 230-220 million years ago, in the late Triassic. However, while the earliest known pterosaur, Eudimorphodon had specialised multi-cusped teeth not found in any of the later pterosaurs, so it would not have been ancestral to them but rather part of a distinct pterosaur lineage that died out in the Triassic. Furthermore, both Eudimorphodon and other late Triassic pterosaurs are "completely" developed, having all the typical pterosaur skeletal characteristics. This suggests the origins of pterosaurs may lie even further back in the past, in the earlier Triassic or perhaps even in the Permian (Wellnhofer, 1991).

Figure 1: Triassic pterosaurs Eudimorphodon (right) and Peteinosaurus (left). In the background a Ticinosuchus wanders about. Artist: John Sibbick.

No fossils of the pterosaurs' immediate ancestors are known. The most likely theory on their origins is that they evolved from arboreal creatures that would leap from branch to branch, flapping their forelimbs to stay airborne longer. Pterosaur hips had great freedom of movement, their knees and ankles were hinge-like and their feet were plantigrade. The knees and ankles did not permit the necessary rotation for them to move bipedally, so pterosaurs were obligate quadrupeds (though they may have had bipedal ancestors). A possible explanation for these features is that the early pterosaurs or proto-pterosaurs were arboreal creatures that evolved powerful leaping from branch to branch as an active mode of transport not dissimilar to that of arboreal leaping primates (Christopher, 1997). These arboreal leapers would not have been gliders, who merely fall slowly downwards and forwards with the help of special flaps, but rather creatures utilising a quite different form of locomotion, one that led them to eventually having their forelimbs evolve into more and more sophisticated flapping airfoils.

Figure 2: A hypothetical series of pterosaur ancestors. Artist: Maija Karala.

The next blog post shall delve into more detail on the anatomical and physiological features that made the pterosaurs such great fliers.

References
Christopher, B. S. (1997). The arboreal leaping theory of the origin of pterosaur flight. Historical Biology. 12 (3).
Wellnhofer, P. (1991). The Illustrated Encyclopedia of Pterosaurs. Crescent Books: New York.

Image sources
Figure 1: http://www.moensklint.dk/media/37420/eudimorphodon_large.jpg
Figure 2: https://gwawinapterus.files.wordpress.com/2014/02/tumblr_mrseawh0qk1sx3tcvo1_1280.jpg