pterosaurs Pterosaurs are one of the three groups of terrestrial vertebrates that have evolved flight independently (the other two are the birds and the bats). They were the first to evolve, in the Late Triassic, but also the first to become extinct, dying out at the end of the Cretaceous. Pterosaurs were archosaurs, closely related to dinosaurs, and both groups seem to have evolved from small agile bipedal reptiles in the Middle Triassic.
Pterosaurs are generally divided into two groups. The earliest of these is paraphyletic (it consists of a common ancestor and some but not all of its descendants) and is referred to as the Rhamphorhynchoidea. The later, monophyletic group (having a common ancestor and all its descendants) is called the Pterodactyloidea. In both these groups the forelimb was transformed into a wing support by the enormous elongation of the fourth digit (the ‘little’ finger). The remaining three digits projected forward from the leading edge of the wing and bore functional claws; medial to them was a small projecting bone, the pteroid, which probably supported a small anterior flight membrane. The wing itself was a tough membrane that was supported internally by elastic threads attached to muscles. The sternum (breastbone) was greatly enlarged to provide an attachment area for the large flight muscles that powered the propulsive stroke, and the shoulder girdle was modified to allow a powerful downstroke. The body was fairly short, but the legs were long and very similar to those of a bird. It therefore seems likely that the pterosaurs were active bipedal animals when on the ground. The bones were hollow and thin-walled, but also pneumatic: that is, they had openings in their walls that allowed air sacs from the respiratory system to enter the bones. This would have had the effect of lightening the skeleton but would also have provided a greater area for gas exchange. The rhamphorynchoids retained a long tail which had a small rudder as its tip. The pterodactyloids lost the tail and also attained great size.
Pterosaurs were originally considered to have been adapted for gliding and were not thought to have been capable of flapping flight; however, more recent analyses of their skeletons indicate that this view is incorrect. The attachment areas for the flight muscles are much larger than would be necessary for a gliding organism, and, in addition, the shoulder girdle is adapted for the forward and downward motion necessary in active flight. The wing appears to have been narrow and gull-like, attached at its base to the side of the body and the upper thigh, but not to the lower leg or ankle. Estimated body weights for pterosaurs suggest that their wing loading was much lower than in modern birds, which would have made take-off much easier and allowed them to have been efficient gliding hunters once airborne.
Teeth and jaw shapes were very diverse; many of the smaller forms had small pointed teeth but the large species were often toothless. The genus Pterodaustro from the Lower Cretaceous of Argentina had numerous bristle-like teeth that might have acted as a straining device, which suggests a mode of life similar to that of the modern flamingo. Despite their efficiency in flight and apparent adaptation to a variety of ways of life, the pterosaurs dwindled through the Cretaceous. The last was the largest of all,
Quetzalcoatlus from the Late Cretaceous of Texas, which had a wingspan of possibly 11 m, making it the largest flying organism that has ever existed.
David K. Elliott
