Plesiosaurswere a diverse and highly successful group of marine reptiles that made up the order Plesiosauria. They were entirely or almost entirely aquatic, and thrived throughout the Jurassic and Cretaceous periods. Plesiosaurs became extinct during the Cretaceous-Paleogene extinction event, which was also responsible for the extinction of non-avian dinosaurs, pterosaurs, and most marine reptiles.
The order Plesiosauria was comprised of two superfamilies, Plesiosauroidea and Pliosauroidea.
- Superfamily Plesiosauroidea
- Superfamily Pliosauroidea
Plesiosaurs were medium to large-sized marine reptiles. The smallest, around 1.8 meters (6 feet) long and 70 kilograms in weight, belong to the dubious genus "Plesiosaurus". The largest plesiosaur species are the elasmosaurid Mauisaurus haasti and the pliosaurid genus Pliosaurus macromerus, which both may have weighed between 20 and 30 tons and attained 20 and 11 metres long, respectively. Elasmosaurus platyurus was also very large, growing to 14 meters long and possibly exceeding 12 tons.
Plesiosaurs possessed broad, sometimes slightly flattened bodies with oar-shaped flippers. As with all vertebrates, each flipper possessed five fingers. Both plesiosauroids and pliosauroids had relatively short tails that appear to have been used as rudders rather than for propulsion. Unlike mosasaurs, plesiosaurs probably relied solely on their flippers for propulsion.
The necks of pliosauroids were comparatively short and heavily muscled, while plesiosauroids' necks were both relatively and absolutely longer. In plesiosaurs such as Elasmosaurus spp. the neck may have been more than 50% of the animal's total length and contained over 70 cervical vertebrae. Contrary to popular belief, the necks of plesiosauroids would have been relatively stiff and not held above the body like the neck of a sauropod.