Plesiosauroidea (;
Greek:
plēsios/πλησιος 'near' or 'close to' and
sauros/σαυρος 'lizard') is an
extinct clade of
carnivorous plesiosaur marine reptiles. They have the longest neck to body ratio of any reptile.
Plesiosauroids are known from the
Jurassic and
Cretaceous periods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.