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Styxosaurus was an elasmosaur, a long-necked variety of the marine reptiles called plesiosaurs. These sea monsters grew upwards of 40 feet (12 meters) long. About half that length was neck, a feature that allowed Styxosaurus to sneak up on schools of fish as it slowly cruised the shallow waters of the Late Cretaceous seas.

The long, pointed teeth of Styxosaurus were useful for seizing and holding prey, but they couldn't cut or chew. Instead, the elasmosaur swallowed fish whole. Predators likely included some of the fiercest monsters of the seas, such as the sharp-toothed shark Cretoxyrhina and the giant mosasaur Tylosaurus.

Like all plesiosaurs, Styxosaurus's limbs were two pairs of flipper-like paddles, which it moved in a figure eight motion to fly through the water much as seals and sea lions do today. In some cases hundreds of stones, called gastroliths, have been found associated with the bellies of these sea monsters, suggesting they swallowed the stones to help with digestion or perhaps provide ballast as they swam.