The first ichthyosaur was discovered in 1811 by an eleven or twelve year old girl in the cliffs around Lyme Regis in southwestern England. Mary Anning, of whom it was written "She sells sea shells by the seashore," grew up to become renowned as the world's first professional fossil collector, and made a lifelong career of collecting and selling fossils from her shop in Lyme Regis. She also discovered the first plesiosaur in 1821. (The first dinosaur, Iguanodon, meaning "iguana tooth", wasn't discovered until 1822, and the term 'dinosaur' wasn't coined until 1841.)
Slightly less menacing than ichthyosaurs, unless you were dinner, were the plesiosaurs. Once described as "snakes threaded through the bodies of turtles," plesiosaurs had dorso-ventrally flattened, turtle-shaped bodies with long, paddle-like flippers at the four "corners," short, pointed tails, and long, snake-like necks. Their relatively small heads were nonetheless armed with a battery of sharp teeth. They could conceivably have come ashore to lay eggs, much like modern sea turtles do, although nothing is actually known about their breeding habits. The largest plesiosaur, Elasmosaurus, reached a length of 46 feet (14 m), over half of which was neck. Its neck contained 71 vertebrae. Plesiosaurs lived throughout the Jurassic and Cretaceous periods.
Pliosaurs, thought to be related to plesiosaurs, were more streamlined, had shorter necks, larger heads, and longer jaws. Kronosaurus, the largest pliosaur, from the Early Cretaceous of Australia, was 42 feet (12.8 m) long, of which 9 feet (3 m) were its skull and jaws, which were considerably larger, and presumably more powerful than the largest dinosaur, Tyrannosaurus rex. Like their cousins the plesiosaurs, pliosaurs survived to the end of the Cretaceous Period.
Ichthyosaurs, plesiosaurs, and pliosaurs were so supremely adapted to their watery environment that their skeletons show few or no affinities with other reptilian lineages. They are thought to be descended from some form of diapsid reptile, but their taxonomic position and relationship with errestrial reptiles remains unclear. Not so, however, with the mosasaurs. They were giant sea-going monitor lizards, and are placed securely within the Squamata, the order which includes lizards and snakes.
Vaguely crocodilian in form, mosasaurs had large, flat tails and paddle-shaped limbs, which undoubtedly made them powerful swimmers. The largest, at about 33 feet (10 m), had around 50 vertebrae in its body, with another 50 in its long, oar-like tail. With their huge heads and long, powerful jaws, mosasaurs were, along with the pliosaurs, the top predators of the Late Cretaceous seas. At least some of them fed on ammonites, as demonstrated by mosasaur toothmarks on ammonite fossils.
All of these marine reptiles became extinct, along with the dinosaurs and ammonites, at the end of the Cretaceous Period, 65 to 67 million years ago. Which is fortunate, as it leaves nothing in our oceans today more ferocious than giant man-eating sharks. Curiously, no Mesozoic marine reptile seems to have ever adapted to feed on plankton, as modern whales and basking sharks do. It is also curious that no reptile since the end of the Cretaceous has taken to an exclusively aquatic, marine habitat. (Marine turtles were already known during Cretaceous times.
McGowan, Dinosaurs, Spitfires, and Sea Dragons. 1991, Harvard University Press, Cambridge.
Dixon, Dougal, Barry Cox, R.J.G. Savage, and Brian Gardiner. The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. 1988, Macmillan Publishing Company, New York.