Marine reptile

Reptiles that live in the sea:
•Saltwater crocodile(top left)
•Sea turtle(top right)
•Marine iguana(bottom left)
•Sea snake(bottom right)

Marine reptilesarereptileswhich have become secondarilyadaptedfor anaquaticorsemiaquaticlife in amarineenvironment.Only about 100 of the 12,000extantreptilespecies and subspecies are classed as marine reptiles, includingmarine iguanas,sea snakes,sea turtlesandsaltwater crocodiles.

The earliest marine reptile wasMesosaurus(not to be confused withMosasaurus), which arose in thePermianperiod of thePaleozoicera.During theMesozoicera, many groups of reptiles became adapted to life in the seas, including such familiar clades as theichthyosaurs,plesiosaurs(these two orders were once thought united in the group "Enaliosauria",a classification now cladistically obsolete),mosasaurs,nothosaurs,placodonts,sea turtles,thalattosaursandthalattosuchians.Most marine reptile groups became extinct at the end of theCretaceousperiod, but some still existed during the Cenozoic, most importantly thesea turtles.Other Cenozoic marine reptiles included thebothremydids,palaeophiidsnakes, a fewchoristoderessuch asSimoedosaurusanddyrosauridcrocodylomorphs. Various types of marinegavialidcrocodilians remained widespread as recently as the Late Miocene.

Some marine reptiles, such as ichthyosaurs, plesiosaurs,metriorhynchidthalattosuchians, 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 andbaskon land.

Extant (living) varieties

Hawksbill sea turtle(Eretmochelys imbricata)

• Sea turtles: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.
• 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 theIndianandPacific oceans,though very limited reports of sightings suggest they may be extending into theAtlantic 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.
• Marine iguana:marine iguanas live only on theGalápagos Islandsand are not fully adapted to marine life. Although they feed exclusively onmarine plantsand 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.
• SaltwaterandAmerican 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 specializedsalt glands.These are the largest species of crocodile, also making them the largestreptiles.They can grow up to six meters in length.American crocodiles (Crocodylus acutus) similarly prefer brackish over freshwater habitats.

Extinct groups

Fossil ofOphthalmosaurus icenius,a species of ichthyosaur

• Ichthyosaurswere marine reptiles with a dolphin-like body shape that flourished during the Mesozoic era.
• Sauropterygianswere a diverse group of aquatic reptiles adapted for flipper-based aquatic locomotion. This group included theplesiosaurs,nothosaurs,andplacodonts.
• Mosasaurswere a group of large, aquaticsquamates(relatives of modern-day lizards and snakes) which became the dominant marine predators towards the end of the Cretaceous period.
• Thalattosaurswere a group of aquatic or semiaquatic marine reptiles of uncertain placement within thediapsidclade. One subgroup, thethalattosauroids,are known for their unusual downturned snouts and crushing dentition.
• Mesosaurswere quite possibly the first fully aquatic reptilian animals and the only group known from thePaleozoic,though they are technically not true reptiles in the sense of not being part of thediapsidgroup. Taxonomic placement either puts these Permian animals resembling long-snouted lizards with newt-like tails either as members ofParareptiliaor as a type ofSynapsid.
• Thalattosuchianswere marinecrocodylomorphsthat flourished during the Jurassic era.

Adaptation to the marine environment

Main article:Physiology of underwater diving § Aquatic reptiles

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

• List of marine reptiles
• Reptiles portal