PTERODACTYLS, ter'O-clalc'tilz, an order of extinct flying reptiles technically known as Pterosauria. They lived exclusively during the Mesozoic period, from the beginning of the Jurassic to near the close of the Cretaceous. Their fossil remains have been found in Europe and North America, but are best known from the chalk deposits of western Kansas, where many hundred specimens have been collected. Of known reptiles they are nearest allied to the dinosaurs (q.v.) ; they are only distantly related to birds. They are especially char acterized by their very hollow bones and the extraordinary elongation of the fourth (accord ing to some authors, the fifth) finger, which supported a thin bat-like membrane that ex tended to the sides of the body and the legs. Not only were the bones very hollow, more so than in any living birds, but they were com posed of very dense osseous tissue and were pneumatic, that is, had openings, usually near the ends, for the ingress and egress of air, as in birds. The nature and extent of the wing membrane, or patagium, are known from Impressions in the rocks, but the covering of the body elsewhere is still in doubt. That the body is not clothed with feathers is certain, but it may have had, like that of most reptiles, horny scales.
The neck was more or less elongated and very flexible; the legs were relatively feeble and of but little use in locomotion upon land; the breastbone, which is poorly developed in other reptiles and wholly absent in early reptiles, was of great size, covering the whole under side of the thorax. Unlike birds, however, it had no real keel but only a stout projection in front for the attachment of muscles. The pel vic or hip bones were well developed for the support of the hind legs, which were of ma terial use in flight. The fifth finger was absent; the first three fingers were short and useful only for clinging to rocks and branches.
In all the early kinds, those from the Juras sic, the skull, of moderate length, was provided with long and sharp teeth. The tail was long
and flexible and had at its extremity a diamond shaped expansion for use in controlling flight. The bones of the skeleton were less hollow and the wing was proportionally less elongated. The first bone of the wing-finger, that ts, its metacarpal, was relatively short, never longer than the forearm, and the first three fingers were articulated directly with the wrist. There were five separate toes and the feet were of some use upon land. None was of large size, not exceeding five or six feet in expanse. The best known form is Rhamphorhynchus (Fig. 1), and the group or suborder is known as the Pterodermata.
The later and much more specialized group, beginning near the close of the Jurassic, known as the Pterodactyloidea, had the skull more elongated, the wing-finger longer, with its meta carpal longer than the forearm, the first three fingers of the hand loosely attached in the flesh, the tail vestigial, the pelvis larger and firmer and the feet with only four toes. The earliest of these, of which Pterodactylus is the best known example, were small, some with a body not larger than that of a sparrow, and they had teeth in the jaws. The later ones, however, those from the Upper Cretaceous, of which so many are found in America, reached a size of more than 20 feet from tip to tip of wings. The skull was very long and slender, in some with an enormously developed crest upon the back of the head, and the jaws were without teeth. The neck was longer and more flexible, in many with additional articulations, giving greater strength; the bones of the shoulder were united into a firmly ossified ring, which, unlike all other known animals, articu lated directly with the spinal column; the wrist was firmer; the wing-finger still more elon gated; the small bone of the legs had disap peared, as in birds; and the first bone of the ankle, the astragalus, had become indistinguish ably united with the leg bone, as in birds. The most noted member of thisgroup is Pteranodon.