Plotosaurs possessed several adaptations to marine life not seen in other mosasaurs. Compared with their relatives, they had narrower flippers, large tail fins and a streamlined fusiform body shape. These features probably enabled them to be faster swimmers than most other mosasaurs. They also had relatively large eyes for keen eyesight, and impressions found with their fossils suggest that they had scaly skin.
Based on cladistic analysis, plotosaurs are considered to be the most derived branch of mosasaur evolution.
The type species, P. bennisoni, was named for Allan Bennison, a fossil hunter who discovered the first remains in 1937. It was around 9 metres (30 ft) long, and was the first known mosasaur from California (a year previously, Bennison had also discovered the state's first dinosaur, Saurolophus).
A second species, P. tuckeri, was also found in 1937 by Frank Paiva and Professor William M. Tucker. Although, not quite as advanced in aquatic adaptations as P. bennisoni it was about 40% larger, reaching lengths of around 13 metres (43 ft). However, a recent analysis by Lindgren, Caldwell and Jagt (2008) considers P. tuckeri to be a junior synonym.