Pterosaur

Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger.

Traditionally, pterosaurs were divided into two major types. Basal pterosaurs (also called non-pterodactyloid pterosaurs or 'rhamphorhynchoids') were smaller animals, up to two meter wingspan, with fully toothed jaws and, typically, long tails. Their wide wing membranes probably included and connected the hindlimbs. On the ground, they would have had an awkward sprawling posture due to short metacarpals, but the anatomy of their joints and strong claws would have made them effective climbers, and some may have lived in trees. Basal pterosaurs were insectivores, piscivores or predators of small land vertebrates. Later pterosaurs (pterodactyloids) evolved many sizes, shapes, and lifestyles. Pterodactyloids had narrower wings with free hindlimbs, highly reduced tails, and long necks with large heads. On the ground, they walked well on all four limbs due to long metacarpals with an upright posture, standing plantigrade on the hind feet and folding the wing finger upward to walk on the metacarpals with the three smaller fingers of the hand pointing to the rear. They could take off from the ground, and fossil trackways show that at least some species were able to run, wade, and/or swim. Their jaws had horny beaks, and some groups lacked teeth. Some groups developed elaborate head crests with sexual dimorphism. Since 2010 it is understood that many species, the basal Monofenestrata, were intermediate in build, combining an advanced long skull with long tails.

Pterosaurs sported coats of hair-like filaments known as pycnofibers, which covered their bodies and parts of their wings. Pycnofibers grew in several forms, from simple filaments to branching down feathers. These may be homologous to the down feathers found on both avian and some non-avian dinosaurs, suggesting that early feathers evolved in the common ancestor of pterosaurs and dinosaurs, possibly as insulation. They were warm-blooded (endothermic), active animals. The respiratory system had efficient unidirectional "flow-through" breathing using air sacs, which hollowed out their bones to an extreme extent. Pterosaurs spanned a wide range of adult sizes, from the very small anurognathids to the largest known flying creatures, including Quetzalcoatlus and Hatzegopteryx, which reached wingspans of at least nine metres. The combination of endothermy, a good oxygen supply and strong muscles made pterosaurs powerful and capable flyers.

Pterosaurs are often referred to by popular media or the general public as "flying dinosaurs", but dinosaurs are defined as the descendants of the last common ancestor of the Saurischia and Ornithischia, which excludes the pterosaurs. Pterosaurs are nonetheless more closely related to birds and other dinosaurs than to crocodiles or any other living reptile, though they are not bird ancestors. Pterosaurs are also colloquially referred to as pterodactyls, particularly in fiction and journalism. However, technically, pterodactyl may refer to members of the genus Pterodactylus, and more broadly to members of the suborder Pterodactyloidea of the pterosaurs.

Pterosaurs had a variety of lifestyles. Traditionally seen as fish-eaters, the group is now understood to have also included hunters of land animals, insectivores, fruit eaters and even predators of other pterosaurs. They reproduced by eggs, some fossils of which have been discovered.

The anatomy of pterosaurs was highly modified from their reptilian ancestors by the adaptation to flight. Pterosaur bones were hollow and air-filled, like those of birds. This provided a higher muscle attachment surface for a given skeletal weight. The bone walls were often paper-thin. They had a large and keeled breastbone for flight muscles and an enlarged brain able to coordinate complex flying behaviour. Pterosaur skeletons often show considerable fusion. In the skull, the sutures between elements disappeared. In some later pterosaurs, the backbone over the shoulders fused into a structure known as a notarium, which served to stiffen the torso during flight, and provide a stable support for the shoulder blade. Likewise, the sacral vertebrae could form a single synsacrum while the pelvic bones fused also.

Pterosaurs were highly diverse in size, and some were the largest flying organisms in earth's history. Early pterosaurs of the Triassic and Jurassic periods were typically small animals with wingspans only up to 2 metres (6.6 ft), while most Cretaceous pterosaurs were larger. Some isolated specimens indicate exceptions to this rule, and the divisions of size across time may be a partial result of an incomplete fossil record. Anurognathids may have been the smallest pterosaurs, with wingspans of as small as 0.4 metres (1.3 ft), though the age of these individuals remains uncertain. The largest pterosaurs were members of Azhdarchidae such as Hatzegopteryx and Quetzalcoatlus, which could attain estimated wingspans of 10–11 metres (33–36 ft) and weights of 150–250 kilograms (330–550 lb).

Pterosaurs have large skulls compared to other flying vertebrates, the birds and bats. Later pterosaurs had very elongated skulls, sometimes longer than the whole torso. Many bones were fused in adults. The skulls were pierced by multiple large holes: the bony nostrils, eye sockets, the antorbital fenestrae in the snout side and two temporal fenestrae on each rear side. Monofenestratan pterosaurs fused the nasal and antorbital fenestra into a single large nasantorbital fenestra. The back of the head was at first vertical in orientation, but rotated to nearly horizontal later in evolution of some groups. The paired lower jaws were fused at the front into an elongated mandibular symphysis. The lower jaws of the earliest pterosaurs were pierced at the rear by a mandibular fenestra, but this was lost in later species.