Plesiosaurus, the Long-Necked Marine Reptile

ROGER HARRIS / Getty Images

As you may already have surmised from its name, Plesiosaurus is the eponymous member of the family of marine reptiles known as plesiosaurs, which were characterized by their sleek bodies, wide flippers, and relatively small heads set at the end of long necks. These Mesozoic reptiles were once famously described as looking like "a snake threaded through the shell of a turtle," although it was quickly established that they didn't have shells and were only distantly related to modern testudines.

Plesiosaurs were closely related to, but distinct from, pliosaurs, contemporary marine reptiles possessing thicker torsos, shorter necks, and longer heads. The eponymous member of the pliosaur family was – you guessed it – Pliosaurus. Like all marine reptiles, Plesiosaurus was not technically a dinosaur, having evolved from different antecedents in the reptile family tree.

There's a lot we still don't know about Plesiosaurus, which, like many "name brand" prehistoric reptiles, is much less well understood than the family to which it gave its name. (For a terrestrial parallel, think of the enigmatic Hadrosaurus and the well-known family of dinosaurs to which it belonged, the hadrosaurs, or duck-billed dinosaurs). Discovered very early in paleontological history by the pioneering English fossil hunter Mary Anning in 1823, Plesiosaurus created a sensation back in the early 19th century. Scientists at the time didn't quite know what to make of this 15-foot-long, 120-million-year-old beast. However, Plesiosaurus wasn't the first marine reptile to be discovered in England; that honor belongs to the distantly related Ichthyosaurus.

The Lifestyle of Plesiosaurus

Plesiosaurs in general, and Plesiosaurus in particular, weren't the most accomplished swimmers, ​since they lacked the hydrodynamic builds of their bigger, meaner and more streamlined cousins, the pliosaurs. To date, it's unknown whether Plesiosaurus and its ilk lumbered onto dry land to lay their eggs or gave birth to live young while still swimming (though the latter is the increasingly favored possibility). We do know, however, that plesiosaurs went extinct along with the dinosaurs 65 million years ago, and have not left any living descendants. (Why is this important? Well, many otherwise well-meaning people insist that the putative Loch Ness Monster is actually a plesiosaur that survived extinction!)

The heyday of the plesiosaurs and pliosaurs was the middle-to-late Mesozoic Era, especially the late Jurassic and early Cretaceous period; by the end of the Mesozoic Era, these marine reptiles had been widely supplanted by even more vicious mosasaurs, which likewise succumbed to the K/T Extinction 65 million years ago. The big fish/bigger fish template applies throughout evolutionary history; an argument has been made that mosasaurs partly went extinct because of the increasing diversity and dominance of sharks, the best-equipped marine predators yet evolved by Mother Nature.


Plesiosaurus (Greek for "almost lizard"); pronounced PLEH-see-oh-SORE-us


Oceans worldwide

Historical Period:

Early-Middle Jurassic (135-120 million years ago)

Size and Weight:

About 15 feet long and 1,000 pounds


Fish and mollusks

Distinguishing Characteristics:

Long neck; tapered body; blunt flippers; small head with sharp teeth


mla apa chicago
Your Citation
Strauss, Bob. "Plesiosaurus, the Long-Necked Marine Reptile." ThoughtCo, Sep. 8, 2021, Strauss, Bob. (2021, September 8). Plesiosaurus, the Long-Necked Marine Reptile. Retrieved from Strauss, Bob. "Plesiosaurus, the Long-Necked Marine Reptile." ThoughtCo. (accessed June 6, 2023).