Reptiles that live in the sea.
Marine reptiles are
reptiles which have become secondarily
adapted for an
semiaquatic life in a
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
plesiosaurs (these two orders were once thought united in the group "Enaliosauria,"
 a classification now cladistically obsolete),
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"
palaeophiid snakes, a few
choristoderes such as
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 of are classed as marine reptiles:
extant marine reptiles include
sea snakes, sea turtles and
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.