Of all the reptiles that crawled, stomped, swam and flew in the Mesozoic era, plesiosaurs have a unique distinction: practically no one insists that tyrannosaurs still walk 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 aquatic reptiles that paddled their way across the oceans, lakes, rivers and swamps of the Jurassic and Cretaceous periods. Confusingly, the label "plesiosaur" also encompasses the pliosaurs ("Pliocene lizards"), which had 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 much more dangerous than great white sharks!
Plesiosaur Evolution
Despite their aquatic lifestyle, it's important to realize that plesiosaurs were reptiles, and not fish--meaning they had to breathe air just like their dinosaur cousins. What this implies, of course, is that these marine reptiles evolved from terrestrial ancestors of the Triassic period, although paleontologists disagree about the exact lineage (as always, it's possible that the basic plesiosaur body plan developed more than once, in an example of convergent evolution). Some paleontologists think the earliest aquatic ancestors of the plesiosaurs were the nothosaurs, typified by Nothosaurus.
As is often the case with Mother 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 sea serpent 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 25 tons). However, not all Cretaceous pliosaurs reached enormous sizes: for example, Dolichorhynchops was a 17-foot-long runt (and may have subsisted on soft-bellied squids rather than more robust fare).
Plesiosaur Behavior
Just as plesiosaurs and pliosaurs (with some notable exceptions) differed in body plan, they also differed in 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. Today, most experts say plesiosaurs' heads and necks weren't strong or flexible enough to be used this way; underwater, though, they would have been agile enough to snap up any slow-moving fish, shellfish or squids in the immediate vicinity.
As the above paragraph implies, plesiosaurs were far from the fastest aquatic reptiles of prehistoric times (in a head-to-head match, most genuses of plesiosaur would likely have been outflippered by most genuses of ichthyosaur, the slightly earlier "fish lizards" that evolved hydrodynamic, tuna-like shapes). It's believed that one of the developments that doomed plesiosaurs in the later Cretaceous was the evolution of faster fish, not to mention the appearance of more agile aquatic reptiles like mosasaurs.
As a general rule, the pliosaurs of the late Jurassic and Cretaceous were bigger, stronger, and just plain meaner than the average long-necked plesiosaur. Genuses like Kronosaurus and Cryptoclidus attained sizes comparable to sperm whales, except that these predators were equipped with numerous, sharp teeth rather than filters for feeding on plankton. Whereas most plesiosaurs are believed to have subsisted on fish, plesiosaurs (like modern-day sharks) probably fed on anything and everything that ventured their way, ranging from fish to squids to other aquatic reptiles.
Next Page: Plesiosaur Fossils, Plesiosaur Cryptozoology, and a List of Plesiosaur Genuses
