[[Image:Modern-marine-reptiles-001.jpg|thumb|Extant Cenozoic marine reptiles:
[[Saltwater crocodile]] (top left)
[[Sea turtle]] (top right)
[[Marine iguana]] (bottom left)
[[Sea snake]] (bottom right)]]
Marine reptiles are which have become secondarily adaptation for an aquatic animal or semiaquatic life in a marine ecosystem environment. Only about 100 of the 12,000 Extant taxon reptile species and subspecies are classed as marine reptiles, including , , and saltwater crocodiles.
The earliest marine reptile was Mesosaurus (not to be confused with Mosasaurus), which arose in the Permian period of the Paleozoic era.[Williston SW (1914) Water Reptiles of the Past and Present University of Chicago Press (reprint 2002). ] a classification now cladistically obsolete), , , , , and . Most marine reptile groups became extinct at the end of the Cretaceous period, but some still existed during the Cenozoic, most importantly the sea turtles. Other Cenozoic marine reptiles included the Bothremydidae,
Some marine reptiles, such as ichthyosaurs, plesiosaurs, Metriorhynchidae 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 on land.
Extant groups
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: there are seven extant species of sea turtles, which live mostly along the tropical and subtropical coastlines, though some do migrate long distances and have been known to travel as far north as Scandinavia. Sea turtles are largely solitary animals, though some do form large, though often loosely connected groups during nesting season. Although only seven turtle species are truly marine, many more dwell in brackish waters.
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Sea snakes: the most abundant of the marine reptiles, there are over 60 different species of sea snakes. They inhabit the tropical and subtropical waters of the Indian Ocean and Pacific Ocean, though very limited reports of sightings suggest they may be extending into the Atlantic Ocean. Sea snakes are venomous and their bites have been known to be fatal, though generally they only bite when provoked and often inject only a very small, non-fatal quantity of venom. Sea snakes are distinguished from terrestrial snakes by a vertically flattened tail.
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Marine iguana: marine iguanas live only on the Galápagos Islands and are not fully adapted to marine life. Although they feed exclusively on marine plants and spend a good deal of their time in the water, they do nest on land and need to bask in the sun to reach their ideal body temperature; they are thus also subject to terrestrial predators.
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Saltwater and American crocodiles: none of the extant species of crocodiles are truly marine; however, the saltwater crocodile ( Crocodylus porosus) does display adaptations to saltwater inhabitation and dwells in the brackish waters of Southeast Asia and Australia. Saltwater crocodiles dispose of excess salt in their bodies through specialized . These are the largest species of crocodile, also making them the largest reptiles. They can grow up to six meters in length.
Extinct groups
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Ichthyosauria were marine reptiles with a dolphin-like body shape that flourished during the Mesozoic era.
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were a diverse group of aquatic reptiles adapted for flipper-based aquatic locomotion. This group included the , , and .
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were a group of large, aquatic (relatives of modern-day lizards and snakes) which became the dominant marine predators towards the end of the Cretaceous period.
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were a group of aquatic or semiaquatic marine reptiles of uncertain placement within the diapsid clade. One subgroup, the , are known for their unusual downturned snouts and crushing dentition.
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were quite possibly the first fully aquatic reptilian animals and the only group known from the Paleozoic, though they are technically not true reptiles in the sense of not being part of the diapsid group. Taxonomic placement either puts these Permian animals resembling long-snouted lizards with newt-like tails either as members of Parareptilia or as a type of Synapsid.
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were marine that flourished during the Jurassic period.
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Tanystropheus and related aquatic that lived during the Triassic period.
Adaptation to the marine environment
Conservation
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.
See also
