The Plesiosauria (Greek: πλησίος, plesios, meaning “near to” and Sauria) or plesiosaurs are an order or clade of Mesozoic marine reptiles (marine Sauropsida), belonging to the Sauropterygia.
Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution.
Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835. The first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821. Since then, more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy, relationships and way of life.
Plesiosaurs had a broad flat body and a short tail. Their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis. The flippers made a flying movement through the water. Plesiosaurs breathed air, and bore live young; there are indications that they were warm-blooded.
Plesiosaurs showed two main morphological types. Some species, with the “plesiosauromorph” build, had (sometimes extremely) long necks and small heads; these were relatively slow and caught small sea animals. Other species, some of them reaching a length of up to seventeen metres, had the “pliosauromorph” build with a short neck and a large head; these were apex predators, fast hunters of large prey. The two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea. Modern research, however, indicates that several “long-necked” groups might have had some short-necked members or vice versa. Therefore, the purely descriptive terms “plesiosauromorph” and “pliosauromorph” have been introduced, which do not imply a direct relationship. “Plesiosauroidea” and “Pliosauroidea” today have a more limited meaning. The term “plesiosaur” is properly used to refer to the Plesiosauria as a whole, but informally it is sometimes meant to indicate only the long-necked forms, the old Plesiosauroidea.
In general, plesiosaurians varied in adult length from between 1.5 metres (5 ft) to about 15 metres (49 ft). The group thus contained some of the largest marine apex predators in the fossil record, roughly equalling the longest ichthyosaurs, mosasaurids, sharks and toothed whales in size. Some plesiosaurian remains, such as a 2.9 metres (10 ft) long set of highly reconstructed and fragmentary lower jaws preserved in the Oxford University Museum and referable to Pliosaurus rossicus (previously referred to Stretosaurus and Liopleurodon), indicated a length of 17 metres (56 ft). However, it was recently argued that its size cannot be currently determined due to their being poorly reconstructed and a length of thirteen metres is more likely. MCZ 1285, a specimen currently referable to Kronosaurus queenslandicus, from the Early Cretaceous of Australia, was estimated to have a skull length of 2.85 m (9 ft).
The typical plesiosaur had a broad, flat, body and a short tail. Plesiosaurs retained their ancestral two pairs of limbs, which had evolved into large flippers. Plesiosaurs were related to the earlier Nothosauridae, that had a more crocodile-like body. The flipper arrangement is unusual for aquatic animals in that probably all four limbs were used to propel the animal through the water by up-and-down movements. The tail was most likely only used for helping in directional control. This contrasts to the ichthyosaurs and the later mosasaurs, in which the tail provided the main propulsion.
To power the flippers, the shoulder girdle and the pelvis had been greatly modified, developing into broad bone plates at the underside of the body, which served as an attachment surface for large muscle groups, able to pull the limbs downwards. In the shoulder, the coracoid had become the largest element covering the major part of the breast. The scapula was much smaller, forming the outer front edge of the trunk. To the middle, it continued into a clavicle and finally a small interclavicular bone. As with most tetrapods, the shoulder joint was formed by the scapula and coracoid. In the pelvis, the bone plate was formed by the ischium at the rear and the larger pubic bone in front of it. The ilium, which in land vertebrates bears the weight of the hindlimb, had become a small element at the rear, no longer attached to either the pubic bone or the thighbone. The hip joint was formed by the ischium and the pubic bone. The pectoral and pelvic plates were connected by a plastron, a bone cage formed by the paired belly ribs that each had a middle and an outer section. This arrangement immobilised the entire trunk.
To become flippers, the limbs had changed considerably. The limbs were very large, each about as long as the trunk. The forelimbs and hindlimbs strongly resembled each other. The humerus in the upper arm, and the thighbone in the upper leg, had become large flat bones, expanded at their outer ends. The elbow joints and the knee joints were no longer functional: the lower arm and the lower leg could not flex in relation to the upper limb elements, but formed a flat continuation of them. All outer bones had become flat supporting elements of the flippers, tightly connected to each other and hardly able to rotate, flex, extend or spread. This was true of the ulna, radius, metacarpals and fingers, as well of the tibia, fibula, metatarsals and toes. Furthermore, in order to elongate the flippers, the number of phalanges had increased, up to eighteen in a row, a phenomenon called hyperphalangy. The flippers were not perfectly flat, but had a lightly convexly curved top profile, like an airfoil, to be able to “fly” through the water.