The Origin of Pterosaurs

Where did pterosaurs come from? This has been a question that scientists have considered for a long time.
It is clear from the skull of pterosaurs that they belong to a group of reptiles called the Diapsids. This just happens to be the biggest group of reptiles that there is. This group is classified on the basis of individuals having a pair of openings in the back of the skull. It can be seen in the sketch of Eudimorphodon, a very early pterosaur, that there are two openings behind the orbit. There is also a sclerotic ring within the orbit.

Another feature of early pterosaurs is their tricuspate teeth. Later pterosaur teeth are single cusped. These features link the pterosaurs to basal dinosaur groups, making them Dinosauromorphs. This is still a very big group to pin them down within.

The problem with trying to examine pterosaur origins is a complex one. Firstly, the earliest pterosaurs that are recorded in the fossil record are very well adapted for flight. Their bone shape and structure have significantly changed with adaptations for flight, being thinner, lighter and structurally strengthened. These changes cannot easily be reconciled with an ancestral form. Secondly, pterosaurs are limited in their adaptation by the physical requirements for flight, so any evolutionary changes tend to have a convergent element. Similar wing shapes, structures and balance are needed to be able to compete and survive.

Pterosaurs had air sacs and flow through lungs, similar to those found in birds and in some dinosaurs. It seems likely that these structures developed only once in these animals and they had common ancestors. The most likely candidate for this ancestry would be within the Ornithodirs. This is a view supported by several researchers.

It is likely that the pterosaur ancestors evolved as a small isolated community in a localised area of the world. It may be that the habitat did not produce fossils easily (erosional environment) or that any sediments holding such fossils have not yet been found. It may also be the case that the sediments containing such fossils have been eroded away in the recent past. Whatever the reason, the elusive missing link has not been found.

Of course, the other factor in the equation is the choice of characteristics to compare when looking for an ancestral species. Depending upon which characters are given dominance, different conclusions can be made. For instance, if you classify based on neck structure, the pterosaurs are more likely to seem like Archosauromorphs. If the dominant characteristic is the foot, then they come out ar Dinosauromorphs. At present, there is not enough evidence to identify an ancestral species and make the link back to any specific group. Ornithodire ancestry is just a best guess based on sound, but incomplete, evidence.

Padian K., 1984, The Origin of Pterosaurs, Third Symposium on Mesozoic Terrestrial Ecology, Pp.163-168 Tubingen, Attempto Verlag

Bennett S. C., 1996, The origin of pterosaurs and their systematic position within the Diapsida. Journal of Vertebrate Paleontology, 16; Sup.3

Hone D. W. E. and Benton M.J., 2007, An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles, Journal of Systematic Palaeontology, 5: 465-469.

Claessens L. P. A. M., O'Connor P. M. and Unwin D. M., 2009, Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PLoS ONE 4(2): e4497. doi:10.1371/journal.pone.0004497