Friday, October 12, 2012

First known as early as the 1780s from the Late Jurassic SOLNHOFEN limestones of Bavaria, pterosaurs, the ``winged reptiles'' of the MESOZOIC ERA, have been regarded as biological oxymorons ever since. That they flew, using wings of skin stretched from the body and supported by the forelimb and a tremendously elongated outer finger, is generally accepted, but nearly every other idea about their paleobiology has been contested at one time or another. Currently, their known stratigraphic range is from the latest Triassic (Norian) through the latest Cretaceous (Maastrichtian), and they have now been found on every continent. They are known to range from sparrow-sized to giant forms with wingspans exceeding 10 or 11 m (Quetzalcoatlus), and as far as the available record indicates, they seem generally to have occupied many adaptive zones that the birds took over during the Tertiary, after coexisting with them for approximately 85 million years.

Pterosaurs, like birds and bats, used a down-and-forward stroke in flight that creates a ring-shaped vortex wake that provides the forward thrust component of flight (Padian, 1983, Padian and Rayner, 1993). Pterosaurs had expanded, keeled breastbones like those of birds, and the coracoids braced the shoulder girdle to the sternum. The humerus also had an expanded, proximally concentrated deltopectoral crest for insertion of the flight muscles. Even the earliest known pterosaurs had a fully developed flight apparatus; but the largest pterosaurs, like the largest birds, undoubtedly spent most of their time soaring (Padian, 1987).

Sereno (1991) defined Pterosauria as a list of commonly accepted taxa and all the descendants of their most recent common ancestor; hence it may be regarded as a node-based. According to cladistic analyses by Padian, Gauthier, and Sereno (see Sereno, 1991), pterosaurs are the closest major sister group to dinosaurs within the ornithodiran archosaurs, and the small, poorly 614 Pterosauria preserved Late Triassic (Carnian) form Scleromochlus is their closest known sister group. Scleromochlus has a large skull and long limbs in which the humerus is longer than the scapula and the forearm is longer still. The bowed femur is exceeded in length by the lower leg, in which the fibula is greatly reduced and fused to the tibia. The ankle is mesotarsal and there are four elongated, closely appressed metatarsals, plus an aberrant, somewhat reduced fifth metatarsal. These features are only found otherwise in pterosaurs, but most other bones of the pterosaurian skeleton are so modified for flight that it is difficult to establish many skeletal comparisons to other ornithodirans.

Pterosaurs have been traditionally divided into the Rhamphorhynchoidea (long tailed, with moderately long metacarpals and a long fifth toe of two tapering, curved phalanges) and the Pterodactyloidea (short tailed, with elongated metacarpals and a fifth toe reduced to only a nubbin of the metatarsal); the former group ranged from the Late Triassic to the Late Jurassic and the latter from the Late Jurassic to the Late Cretaceous (Wellnhofer, 1991). The ``Rhamphorhynchoidea'' has now been generally abandoned as a taxon because it is not monophyletic: The Pterodactyloidea evolved from this general group of basal pterosaurs, and Rhamphorhynchus itself is one of its closest known.

If the indications of their biology are correct, pterosaurs could not have flown as soon as they hatched and must have been cared for while they grew rapidly to fledging size. Some evidence indicates that they lived in large terrestrial colonies (Bell and Padian, 1995). By the end of the Maastrichtian their diversity had apparently dwindled to little over a few species in a subclade, the Azhdarchidae, that included both the giant Quetzalcoatlus and smaller forms. The record is too sparse at the species level to provide any indication of catastrophic extinction or rapid decline, although through the Late Cretaceous the other pterodactyloid subclades disappeared from the record one by one (Wellnhofer, 1991). Whatever their ultimate fate, pterosaurs were the first vertebrate fliers, and their geological time span of success is only now being matched by the birds.


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