PTEROSAUR

member of a group of flying reptiles that flourished during the Jurassic (208 to 144 million years ago) and Cretaceous (144 to 66.4 million years ago) periods of the Mesozoic Era. The order Pterosauria is one component of the major reptile group of the Archosauria, or ruling reptiles, to which dinosaurs and crocodilians also belong and from which birds are descended. Archosaurs of the Triassic Period (245 to 208 million years ago) tended toward a bipedal gait; thus the forelimbs were free for use in some other fashion; both birds and pterosaurs converted them into wings. Like bats rather than birds, pterosaurs formed a wing surface by means of a membrane of skin. In bats all of the fingers except the thumb support the membrane. In pterosaurs, however, the membrane was attached solely to one elongated fingerthe fourthand extended thence back along the flank to the knee; an accessory membrane lay between the neck and the arm. The first three fingers were slender, clawed, clutching structures. The pterosaur membrane appears to have been well adapted to soaring and gliding but less maneuverable and more fragile than bat or bird wings. The body was compact; the hind legs were long but slender, and their structure suggests that pterosaurs were little adapted to upright locomotion or perching but hung by the hind limbs when at rest. The neck appears to have been held upright in flight, with the head attached to it at right angles and pointing forward. The skull was lightly but strongly built, with fusion of most of the component bones; there was a long, slender beak. The eyes were large, and the eyeball, as in many birds, was reinforced by a series of bony plates (sclerotic ring) lying in its walls. The brain was large and apparently comparable to that of birds in pattern; as in that group, sight rather than smell appears to have been the dominant sense. Most remains of pterosaurs are found in marine sediments, hence it is probable that they obtained food by diving for fish; but it is difficult to understand how they could have risen from land or water after alighting. Two major groups of pterosaurs are known. Rhamphorhynchus (q.v.) of the Late Jurassic Solenhofen slates is typical of the more primitive division, although earlier forms are present in the Late Triassic (Norian, or Carnian). Characteristics of this group include strong, sharply pointed teeth, relative shortness of the bones supporting the fingers (the metacarpals), and a long tail, which in Rhamphorhynchus had a diamond-shaped rudder at its tip. Rhamphorhynchus had a wingspread of about 1 m (3 feet). A second group of pterosaurs appeared in the Late Jurassic and continued on into the Cretaceous. The typical Jurassic form is Pterodactylus, of which numerous examples are known from Solenhofen. The Jurassic pterodactyl (q.v.) was usually a small reptile, some specimens being no larger than a sparrow. Features include a few small teeth, long metacarpal bones, and a short tail. Descendants of the pterodactyl type continued on into the Cretaceous and were generally much larger. They are best represented by Pteranodon (q.v.) of the Kansas chalk. Pteranodon was larger than an albatross and had a wingspan of 7 m or more . Parts of the skeletons of three very large pterosaurs in Late Cretaceous nonmarine sediments in Big Bend National Park, Texas, were discovered in 1975. From the size of these bones, the wingspan of the largest specimen was about 15.5 m, making it by far the largest flying animal of which there is knowledge. The anatomy of these reptiles and the landform of the region in which their remains were found indicate that they fed on carrion rather than on fish. No pterosaur remains are more recent than the Cretaceous; their place in nature was taken over by true birds.