In the course of evolution, three separate groups of vertebrates became active fliers: pterosaurs, birds and bats.

The word 'pterosaur' means 'flying lizard'. The oldest pterosaur fossils date back some 220 million years, and these creatures dominated the skies between the Jurassic and Cretaceous periods, thus spanning the entire time period of the dinosaurs.

Pterosaurs belong to a group of reptiles known as archosauria, which includes crocodiles, dinosaurs and birds, because they have a specialised ankle joint that improves locomotive ability.

The earliest and most primitive pterosaurs were the Rhamphorhynchoidea (Greek: 'beak snout'). These were small animals, with jaws well-equipped with teeth, and long tails. A typical example of the Rhamphorhychoidea was Rhamphorhynchus, which had a wingspan of about 76cm (30 inches). The tail was stiffened by ligaments and expanded at the end into a lozenge-shaped rudder.

The more advanced group of pterosaurs was the Pterodactyloidea (Greek: 'winged fingers'). These differed from the Rhamphorhynchoidea in having few teeth and a much shorter tail. Pteranodon (Greek: 'wing without teeth') was a typical member of this group and lived in North America during the Late Cretaceous period, about 80 million years ago. Pteranodon was a large pterosaur with a wingspan of about 7.6m (25 feet); however, it was built so lightly that it probably only weighed about 18kg (40lb), thus giving it a wingspan-to-weight ratio of 0.42¹. Pteranodon had no teeth at all and virtually no tail. The back of the skull was extended upwards and backwards into an enormous crest of unknown function².

The largest of the pterosaurs was Quetzalcoatlus northropi, which lived in what is now Texas, USA about 65 million years ago. Partial remains found in Big Bend National Park, West Texas in 1971 were of a creature with a wingspan of 11 - 12m (36 - 39 feet) and weighing an estimated 86kg (190lb). Its wingspan-to-weight ratio was therefore 0.13. Pterosaur trackways discovered recently in Mexico suggest some had wingspans of 18m (60ft) — the size of a fighter aircraft such as the General Dynamics F-111 — while fossils from Romania and Brazil are from creatures that reached 13 or 14m (42-45ft) across.

Compare this with the bird credited with having the largest wingspan today, the Wandering Albatross, Diomedea exulans. Adult males have an average wing span of 3.15m (10 feet, 4 inches) and weigh 8 - 12kg (3.6-5.5lb). The wingspan-to-weight ratio of this bird is 0.26 - 0.39.

Skeleton

The bones of pterosaurs, like those of modern birds, were pneumatic (contained air spaces) and were therefore very light. Modern birds, however, have a well-developed keel on their breastbones for the attachment of the powerful flight muscles. Pterosaurs, on the other hand, had comparatively small keels, suggesting that their flight could not have been very active. Thus, it has been thought that their flight would have consisted simply of gliding and soaring on air currents. However, according to Dr David Martill of the University of Portsmouth, their ability to utilise air currents, thermals and ground effects would astonish modern aircraft designers³.

A major topic of wonder has been the question of how pterosaurs got airborne. Their legs were small and weak and, if their wing muscles were also weak, neither would be of much use. One theory was that they may have lived on cliffs and exploited rising air currents in order to launch themselves. A more recent suggestion is that, because of their exceptional lightness, only very low speeds were required for take-off. In the case of Pteranodon, this is thought to have been in the region of 24kph (15mph; a 'Moderate Breeze' on the Beaufort Scale). If a wind of this strength was blowing, the animal need only to turn to face into it and spread its wings to achieve 'lift off'.

Bird Brain?

Scientists once believed that flying reptiles were primitive and died out because they couldn't compete with birds. This theory has been revised in the light of information from CAT scanning which has enabled researchers to create three-dimensional reconstructions of their brains. From this, the scientists were able to deduce that pterosaur brains were very bird-like, with reduced olfactory lobes and large optic lobes — suggesting that, like modern birds, they were more interested in what they could see than what they could smell. However, a surprising finding was that the pterosaur brains had two very pronounced balance-related regions called 'floccular lobes'. It is thought that these may have gathered information from the wing membranes, which functioned as sense organs, to enable the reptile to build up a detailed map of the forces experienced by its wings. A creature with such senses would have excellent flight control and could become a highly adapted, visually-oriented flying predator. This would confirm the notion of scientists working in the pterosaur field who have long suspected that pterosaurs were good fliers and may have hunted fish and insects whilst in flight.

The Wings of Flying Creatures

In the cases of pterosaurs, birds and bats, the wings are formed by modification of the forelimbs, but the manner of their modification is different in each case. In the case of the pterosaur, the wing was a web of skin supported by an enormously elongated fourth digit of the hand, the rest of the arm, the side of the body and the thigh. The wings of birds consist essentially of rows of feathers arising from the back of the forearm and from the reduced second and third digits — mainly the second. The wings of bats, like that of pterosaurs, are a web of skin; but instead of being supported by one digit alone, is supported by all the digits, except the first (thumb). Furthermore, it runs back not only to the sides of the body and the legs, but often to the tail as well.

Were Pterosaurs Ancestors of Modern Birds?

Although the membranous wings attached to their legs are reminiscent of bats and their long beaks look like some modern bird species, pterosaurs are unrelated to any living creatures. Scientists hope that future discoveries will reveal just where pterosaurs should be placed in the evolutionary tree of life forms on Earth.