The earliest marine reptiles arose in the Permian period during the Paleozoic era. During the Mesozoic era, many groups of reptiles became adapted to life in the seas, including such familiar clades as the ichthyosaurs, plesiosaurs (these two orders were once thought united in the group "Enaliosauria," a classification now cladistically obsolete), mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians. After the mass extinction at the end of the Cretaceous period, marine reptiles were less numerous, but there was still a high variety of species in the early Cenozoic, such as "true" sea turtles, bothremydids, palaeophiid snakes, a few choristoderes such as Simoedosaurus and dyrosaurid crocodylomorphs. Various types of marine gavialid crocodilians remained widespread as recently as the Late Miocene.
Currently, of the approximately 12,000 extant reptile species and sub-species, only about 100 are classed as marine reptiles: extant marine reptiles include marine iguanas, sea snakes, sea turtles and saltwater crocodiles.
Some marine reptiles, such as ichthyosaurs, plesiosaurs, metriorhynchid thalattosuchians, and mosasaurs became so well adapted to a marine lifestyle that they were incapable of venturing onto land and gave birth in the water. Others, such as sea turtles and saltwater crocodiles, return to shore to lay their eggs. Some marine reptiles also occasionally rest and bask on land.
Most species of marine reptiles are considered endangered to some degree. All but one species of sea turtles are endangered due to destruction of nesting habitats on coastal lands, exploitation, and marine fishing; many species of sea snakes are threatened or endangered due to commercial exploitation (sale of skins) and pollution especially in Asia; marine iguanas are threatened due to their very limited habitation range. Saltwater crocodiles are at low risk for extinction.
Alexandronectes is a genus of elasmosaurid plesiosaur, a type of long-necked marine reptile, that lived in the oceans of Late Cretaceous New Zealand. It contains one species, A. zealandiensis. Fossils of Alexandronectes were found in the Conway Formation of Canterbury.Atopodentatus
Atopodentatus is an extinct genus of marine reptile, possibly basal sauropterygian, known from the early Middle Triassic (Pelsonian substage, Anisian stage) of Luoping County, Yunnan Province, southwestern China. It contains a single species, Atopodentatus unicus. It is thought to have lived between 247 and 240 million years ago, during the Middle Triassic period, about six million years after the Permian extinction. Atopodentatus was an herbivorous marine reptile, although marine reptiles are usually omnivores or carnivores.A near complete skeleton along with a left lateral portion of the skull were discovered near Daaozi village, Yunnan, China. The scientific name derives from the peculiar zipper-shaped morphology of the holotype specimen's jaws and unique dentition. However, two fossil skulls discovered in 2016 indicate that the holotype skull was badly damaged, and that the living animal actually had a hammer-shaped head with shovel-like jaws.Besanosaurus
Besanosaurus (meaning "Besano [Lombardy, N. Italy] lizard") is a genus of large ichthyosaur (a marine reptile, not a dinosaur) that lived during the middle Triassic period, approximately 235 million years ago. This marine reptile came from its own family Besanosauridae and was named by Dal Sasso and Pinna in 1996. The type of species is Besanosaurus leptorhynchus meaning "long-beaked reptile from Besano."Borealonectes
Borealonectes is a genus of rhomaleosaurid pliosauroid, a type of plesiosaur. Its fossils were found in the Callovian-age (Middle Jurassic, about 165-161 million years ago) Hiccles Cove Formation of Melville Island, Canada, one of the islands in the Canadian Arctic Archipelago. It is based on a skull, neck vertebrae, and the right forelimb of one individual. Named in 2008 by Sato and Wu, Borealonectes is one of the few plesiosaurs known from the Jurassic of North America, and the first marine reptile from the Canadian Arctic with a well-preserved skull. The type species is B. russelli.Californosaurus
Californosaurus perrini ('Perrin's California lizard') was an ichthyosaur, an extinct marine reptile, from the Lower Hosselkus Limestone (Carnian, Late Triassic) of California.Contectopalatus
Contectopalatus was a primitive ichthyosaur, an extinct fish-like marine reptile from the Middle Triassic of Germany. It was originally named Ichthyosaurus atavus (Quenstedt, 1851/52), and later Mixosaurus atavus (Quenstedt 1852). It was recognised as a valid genus by Maisch and Matzke in 1998, though other authorities argue that it is synonymous with Mixosaurus. It was 5 metres long.Cyamodontoidea
Cyamodontoidea is an extinct superfamily of placodont marine reptiles from the Triassic period. It is one of the two main groups of placodonts, the other being Placodontoidea. Cyamodontoids are distinguished from placodontoids by their large shells, formed from fused bony plates called osteoderms and superficially resembling the shells of turtles. Cyamodontoids also have distinctive skulls with narrow, often toothless jaws and wide, flaring temporal regions behind the eyes. Two large temporal openings are positioned at the top of the back of the skull, an arrangement that is known as the euryapsid condition and seen throughout Sauropterygia, the marine reptile group to which placodonts belong. Cyamodontoids are also distinguished by their large crushing teeth, which grow from the palatine bones on the roof of the mouth.Liopleurodon
Liopleurodon (; meaning 'smooth-sided teeth') is a genus of large, carnivorous marine reptile belonging to the Pliosauroidea, a clade of short-necked plesiosaurs. The two species of Liopleurodon lived during the Callovian stage of the Middle Jurassic Period (c. 160 to 155 mya). It was the apex predator of the Middle to Late Jurassic seas that covered Europe. The largest species, L. ferox, is estimated to have grown up to 6.4 metres (21 ft) in length.The name "Liopleurodon" (meaning "smooth-sided tooth") derives from Ancient Greek words: λεῖος leios, "smooth"; πλευρά pleurá, "side" or "rib"; and ὀδόν odṓn, "tooth".Makhaira rossica
Makhaira rossica is an extinct genus and species of a marine reptile belonging to the family Pliosauridae. It lived during the Cretaceous period (Hauterivian epoch, about 130 million years ago), and its fossils have been found in Russia.Mauisaurus
Mauisaurus ("Māui lizard") is a genus of plesiosaur that lived during the Late Cretaceous period around 77 million years ago in what is now New Zealand. It was the largest plesiosaur, and perhaps the largest marine reptile in New Zealand waters at the time. Numerous specimens have been attributed to this genus in the past, but a 2017 paper restricts Mauisaurus to the lectotype and declares it dubious.Neusticosaurus
Neusticosaurus (sometimes misspelled Neuticosaurus) ("swimming lizard"), is an extinct genus of marine reptile belonging to the nothosaur order, from Italy, Switzerland and Germany. At 18 cm (8 in) long, it was one of the smallest nothosaurs. Neusticosaurus probably fed on small fish.Notochelone
Notochelone is an extinct genus of sea turtle, which existed about 100 million years ago. The species was first described by Richard Lydekker in 1889. It was the most common marine reptile living in the inlands of the sea around Queensland, Australia. It was a small turtle, and was about the same size as the modern green turtle. Analytical studies have indicated that the creatures frequently ate benthic molluscs.Pararcus
Pararcus is an extinct genus of placodont marine reptile from the Middle Triassic of the Netherlands. The genus is monotypic and the type species is Pararcus diepenbroeki. Pararcus is known from a holotype skeleton about 1.35 metres (4.4 ft) long from the Lower Muschelkalk of Winterswijk.Saurosphargis
Saurosphargis is an extinct genus of a basal marine reptile, saurosphargid, known from the Middle Triassic (Anisian age) of southwestern Poland and eastern Netherlands. It contains a single species, Saurosphargis volzi.Shastasaurus
Shastasaurus ("Mt. Shasta lizard") is an extinct genus of ichthyosaur from the middle and late Triassic, and is the largest known marine reptile. Specimens have been found in the United States, Canada, and China.Shonisaurus
Shonisaurus is a genus of ichthyosaur. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period, about 215 million years ago.Sinopliosaurus
Sinopliosaurus is a genus of pliosauroid plesiosaur, a type of short-necked marine reptile, from the Early Cretaceous of People's Republic of China. One species, "S." fusuiensis, was later shown to be based on teeth from a spinosaurid theropod dinosaur.Thaisaurus
Thaisaurus is an extinct genus of ichthyopterygian marine reptile that lived during the Early Triassic. Fossils have been found in Thailand.Woolungasaurus
Woolungasaurus glendowerensis ('Glendower's Woolunga lizard', named after an Aboriginal mythical reptile, Persson 1960) is a plesiosaur, an extinct marine reptile, belonging to the Elasmosauridae.
The type species Woolungasaurus glendowerensis, named by Per Ove Persson in 1960, is known from a partial skeleton, holotype QM F6890, (forty-six vertebrae, ribs, forearms, shoulder girdle and part of the rear limbs) unearthed from the Wallumbilla Formation (Albian, Lower Cretaceous) of the Richmond District, Queensland, Australia. The specific name refers to Glendower Station. Another find of undetermined species, consisting of twelve vertebrae, was unearthed from the Maree Formation (Cretaceous, of uncertain age) of Neales River, near Lake Eyre, South Australia. In addition a skull from Yamborra Creek, near Maxwelltown, Queensland, described by Persson in 1982 has been referred to Woolungasaurus.
Woolungasaurus appears to have been a typical elasmosaur, with forty sharp teeth and an estimated length of about 9.5 metres. Persson (1982) believes it to be closely related to the North American elasmosaur Hydralmosaurus.
The genus was referred to Styxosaurus by Sven Sachs in 2004.