Life in the air
By Mike Habib
Flight is one of the most distinguishing characteristics of pterosaurs, and arguably the feature for which they are most famous. Pterosaurs were the first group of vertebrates to evolve flight. The first pterosaurs appear in the Late Triassic, while the first birds did not appear until the Late Jurassic, nearly 80 million years later. Bats did not arrive on the scene until much later still.
Even after the appearance of birds, pterosaurs maintained a position as the dominant large-bodied flying vertebrates, as well as the primary flyers in coastal and open ocean habitats. The success of pterosaurs for nearly 135 million years likely had much to do with their very well developed, and unique, flight-related anatomy. We now know that the flight machinery of pterosaurs was efficient and powerful. The specific manner in which they took off and flew also tells us a great deal about pterosaur evolution. Now that we understand more about pterosaur flight, we have a much more complete idea of how some species became so large, and why pterosaurs were especially diverse in particular habitats.
The primary wing is referred to as a brachiopatagium (arm wing), and though it was the primary source of lift and thrust, pterosaurs also had other important lifting surfaces. In between the hind limbs was a stretch of flight surface called the uropatagium. This surface falls into two major types within pterosaurs. The more basal, paraphyletic “ramphorhynchoids” had a broad uropatagium that linked across the two hind limbs. By contrast, the pterodactyloids had a uropatagium that was split, such that a roughly triangular membrane ran along each hind limb. The uropatagium would have had several uses during flight. As a lifting surface, it could help support the legs during level flight, as well as help the animal roll or pitch for tighter maneuvers. The split uropatagium of pterodactyloids would have been especially useful for producing strong turns: kicking a leg out would generate lift at an angle to the body, and thus make the animal roll or yaw to the side. In flying animals, a roll induces a turn, so pterodactyloids may have been quite maneuverable for their size.
The propatagium has been the source of a great deal of controversy among pterosaur biologists recently. It was the section of the wing that stretched along in front of the elbow and made up the leading edge of the wing between the body and the wrist. The extent of this patagium has been debated to a fair degree, but the real source of consternation has been in deciphering how this membrane was controlled. A bone, called the pteroid, projected from the side of the wrist and helped support, and move, the propatagium. There has been a great deal of debate regarding how the pteroid was attached, and thus what motions it was capable of. Most recently, Wilkinson (2007) and Bennett (2007) have published competing views of this articulation. At present, the model of Bennett (2007) seems to be more consistent with what is seen in fossil specimens, but uncertainty continues to exist.
Regardless, the propatagium would have been important in altering the camber (or curvature) of the wing, and the flow of air over the leading edge. The propatagium could also be utilized to tense the wing, since it could take up slack produced when the wing was partly retracted during rapid glides. The position of the propatagium would change depending speed and conditions; it would likely take on the configuration for greatest lift during launch and landing, when a high lift coefficient is most important.
There are a number of common errors made when reconstructing pterosaur wings. One error is to reconstruct the tip as being very sharp. This was an unlikely configuration, because such sharp tips can be prone to a particular form of stall (which will lead to the animal suddenly doing much more falling than flying). Living birds, for example, have somewhat rounded wing tips, and pterosaurs probably did, as well. Another common mistake is to depict pterosaurs as “ultralight” animals (see anatomy section) devoid of much muscle mass and almost floating through the air. In reality, it is the ratio of power to weight that is truly important for flight, and a super-light animal simply will not have enough power to push itself through the air. In fact, contrary to common perceptions, being somewhat heavy, relative to wing area, can be helpful for certain types of flyers. In particular, it is an advantage for rapid gliding in open areas, such as the ocean.
Pteranodon soars over a stormy ocean. A relatively high wing loading may have helped such ocean soarers penetrate into high winds and glide rapidly to search for food.
This is not to say that all pterosaurs lived over the ocean. Many of them seem to have lived more inland, such as the famous giant, Quetzalcoatlus, or the tiny, insect-munching anurognathids. There were a wide range of flight patterns demonstrated by pterosaurs, showing the versatility of their wing structure, and yet, they never seem to have generated the degree of wing and flight diversity seen in birds, which was probably the result of certain physical constraints on the pterosaur wing. Pterosaurs could not, for example, split the end of their wing into separate tips as many birds do; which is a very useful trick for slow flying (especially slow soaring).
Both the similarities and differences between pterosaurs and living flying species are telling. As we learn more about how pterosaurs flew, we see that they were not weak, super-light animals barely capable leaving the ground. Pterosaurs were powerful, accomplished flyers, that achieved body plans that no other animals have ever matched.