Sauropterygia

Sauropterygians are characterised by a number of synapomorphies, including having only a single upper temporal fenestra rather than two temporal fenestrae as is ancestral for diapsids, the supratemporal, postparietal, tabular, and lacrimal bones of the skull have been lost, the nares (external nasal opening) is retracted, there is a large retroarticular process on the mandible, the trunk vertebrae lack intercentra, the sacrum is composed of three or more vertebrae, a sternum is absent, the scapula is positioned superficially relative to the clavicle, the scapulocoracoid is divided, the pectoral and thyroid bear fenestration.^[1] Other characters suggested to be ancestral for Sauropterygia include the squamosal bone descends to the lower (ventral) margin of the skull, the quadrate bone of the skull is covered by the squamosal and quadratojugal when the skull is viewed side-on (lateral view), the bones exhibit pachyostosis (high density), a reduced or absent posterior process on the interclavicle, a curved humerus, and intertrochanteric fossa is small but distinct.^[2]

The number of vertebrae is highly variable,^[3] with several lineages of Eosauropterygia repeatedly independently developing a long neck by increasing the overall number of cervical vertebrae,^[4] taken to the extreme in elasmosaurid plesiosaurs, which have up to 76 neck vertebrae, with the long neck ancestral for plesiosaurs becoming secondarily shortened in some plesiosaur lineages like pliosaurs.^[3] By contrast, placodonts have very short necks with only 6 vertebrae.^[1] The anatomy of the bony labyrinth of the inner ear is variable, being crocodilian-like in Triassic sauropterygians, while those of plesiosaurs is generally more sea-turtle-like.^[5]

The vast majority of sauropterygians are generally thought to have relied on using their limbs to propel themselves through the water (paraxial propulsion) as opposed to undulating their bodies as is typical of living reptiles like crocodilians,^[1][3] with their trunk regions being relatively stiff.^[3] While plesiosaurs developed highly specialised paddle-like limbs, the limbs of non-plesiosaur sauropterygians are less specialised. Non-placodont sauropterygians generally have streamlined bodies.^[1] Many placodonts are covered in a layer of dermal armour.^[1] Compared to earlier diapsids, members of Eosauropterygia show a radical transformation of the pectoral girdle, including ventral (towards the underside of the body) expansions of the coracoid and scapula, as well as the development of a strong clavicular arch.^[6] Members of Eosauropterygia are also characterised by slender and curved femurs. They also lack osteoderms which are present in some other sauropterygians.^[2]

Placodonts are thought to have been durophagous, using rounded teeth (including those on the upper and lower jaws, as well as palatal teeth on the roof of the mouth) to crush hard shelled organisms.^[7] Members of Eosauropterygia are thought to have been piscivores and carnivores.^[8] Sauropterygians were generally marine,^[1] though some Jurassic and Cretaceous plesiosaurs are suggested to have inhabited bodies of freshwater.^[9][10] While pleisosaurs were fully aquatic, some non-plesiosaur sauropterygians may have been capable of limited terrestrial locomotion.^[1] Plesiosaurs as well as some other sauropterygians including the pachypleurosaur Keichousaurus and the nothosaurs Lariosaurus and Nothosaurus are thought to have been viviparous, bearing live young. At least some other sauropterygians may have been oviparous (egg-laying).^[11] Plesiosaurs are suggested to to have been endothermic (warm blooded), and other sauropterygians may have had elevated metabolic rates compared to modern ectothermic (cold-blooded) reptiles.^[12]

The earliest sauropterygians appeared about 248-247 million years ago (Ma) during the Spathian towards the end of the Early Triassic, by which time they already showed considerable diversification.^[2] By the Middle Triassic, some sauropterygians had already reached large body sizes, with some nothosaurs belonging to the genus Nothosaurus from this time period reaching a body length of 5–7 metres (16–23 ft).^[13] The Triassic-Jurassic extinction event wiped out all sauropterygians aside from the plesiosaurs, which would continue to diversify until the end of the Mesozoic, when they would be wiped out by the K-Pg extinction event.^[14]