Plesiosaurs (203–66 MYA) became common during the Jurassic and lasted to the end of the Cretaceous. This diverse group of marine reptiles replaced ichthyosaurs as the top aquatic predator during the Late Jurassic. While starting at 2 meters, some plesiosaurs were massive: up to 20 meters, the size of a sperm whale.
Compared to nothosaurs, plesiosaurs had further adaptations to an aquatic life: 4 paddle-shaped flippers instead of webbed feet, and elongated necks. Like penguins and sea turtles, plesiosaurs swam largely with their forelimbs, but with a difference: the watery vortices created by the front flippers were woven by the back flippers to move more efficiently.
Plesiosaurs were forelimb-dominated swimmers that used their hind limbs mainly for maneuverability and stability. ~ Chinese paleontologist Shiqiu Liu et al
Elasmosaurus (13–14 m; 2 tonnes) was one of the last plesiosaurs. Its neck ran 6–7 meters; the longest of the plesiosaurs. Elasmosaurus had 72 vertebrates in its neck; more than any known animal. With its tiny head on small sea creatures Elasmosaurus fed. How Elasmosaurus managed to get enough to eat is an enduring mystery.