Of all the reptiles that crawled, stomped, swam and flew their way through the Mesozoic era, plesiosaurs and pliosaurs have a unique distinction: practically no one insists that tyrannosaurs still roam the earth, but a vocal minority believes that some species of these "sea serpents" have survived down to the present day. However, this lunatic fringe doesn't include many respected biologists or paleontologists, as we'll see below.
Plesiosaurs (Greek for "almost lizards") were large, long-necked, four-flippered marine reptiles that paddled their way through the oceans, lakes, rivers and swamps of the Jurassic and Cretaceous periods. Confusingly, the name "plesiosaur" also encompasses the pliosaurs ("Pliocene lizards," even though they lived tens of millions of years before), which possessed more hydrodynamic bodies, with bigger heads and shorter necks. Even the biggest plesiosaurs (such as the 40-foot-long Elasmosaurus) were relatively gentle fish-feeders, but the largest pliosaurs (such as Liopleurodon) were every bit as dangerous as a Great White Shark.
Despite their aquatic lifestyles, it's important to realize that plesiosaurs and pliosaurs were reptiles, and not fish--meaning they had to surface frequently to breathe air. What this implies, of course, is that these marine reptiles evolved from a terrestrial ancestor of the early Triassic period, almost certainly an archosaur. (Paleontologists disagree about the exact lineage, and it's possible that the plesiosaur body plan evolved convergently more than once.) Some experts think the earliest marine ancestors of the plesiosaurs were the nothosaurs, typified by the early Triassic Nothosaurus.
As is often the case in nature, the plesiosaurs and pliosaurs of the late Jurassic and Cretaceous periods tended to be bigger than their early Jurassic cousins. One of the earliest known plesiosaurs, Thalassiodracon, was only about six feet long; compare that to the 55-foot length of Mauisaurus, a plesiosaur of the late Cretaceous. Similarly, the early Jurassic pliosaur Rhomaleosaurus was "only" about 20 feet long, while the late Jurassic Liopleurodon attained lengths of 40 feet (and weighed in the neighborhood of 25 tons). However, not all pliosaurs were equally big: for example, the late Cretaceous Dolichorynchops was a 17-foot-long runt (and may have subsisted on soft-bellied squids rather than more robust prehistoric fish).
Just as plesiosaurs and pliosaurs (with some notable exceptions) differed in their basic body plans, they also differed in their behavior. For a long time, paleontologists were puzzled by the extremely long necks of some plesiosaurs, speculating that these reptiles held their heads high above the water (like swans) and dived them down to spear fish. It turns out, though, that the heads and necks of plesiosaurs weren't strong or flexible enough to be used this way, though they certainly would have combined to make an impressive underwater fishing apparatus.
Despite their sleek bodies, plesiosaurs were far from the fastest marine reptiles of the Mesozoic Era (in a head-to-head match, most plesiosaurs would likely have been outflippered by most ichthyosaurs, the slightly earlier "fish lizards" that evolved hydrodynamic, tuna-like shapes). One of the developments that doomed the plesiosaurs of the late Cretaceous period was the evolution of faster, better-adapted fish, not to mention the evolution of more agile marine reptiles like mosasaurs.
As a general rule, the pliosaurs of the late Jurassic and Cretaceous periods were bigger, stronger, and just plain meaner than their long-necked plesiosaur cousins. Genera like Kronosaurus and Cryptoclidus attained sizes comparable to modern grey whales, except that these predators were equipped with numerous, sharp teeth rather than plankton-scooping baleen. Whereas most plesiosaurs subsisted on fish, pliosaurs (like their underwater neighbors, the prehistoric sharks) probably fed on anything and everything that ventured their way, ranging from fish to squids to other marine reptiles.