Plotosaurus ("swimmer lizard") is an extinct genus of mosasaur from the Upper Cretaceous (Maastrichtian) of Fresno County, California. Originally named Kolposaurus (meaning "bay lizard") by Berkeley paleontologist Charles Lewis Camp in 1942, it was changed to Plotosaurus in 1951 when Camp discovered the name had already been assigned to a type of nothosaur.
Plotosaurs possessed several adaptations to marine life not seen in other mosasaurs. Their narrower flippers, large tail fins and streamlined fusiform body shape probably enabled them to be faster swimmers than most other mosasaurs. They also had relatively large eyes for keen eyesight. 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 meters (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 meters (over 42 ft). However, a recent analysis by Lindgren, Caldwell and Jagt (2008) considers P. tuckeri to be a junior synonym.
- Camp, C.L. 1942. California Mosasaurs. Memoirs of the University of California 13:1-68.
- Camp, C.L. 1951. Plotosaurus, a new generic name for Kolposaurus Camp, preoccupied. Journal of Paleontology 25:822.
- Hilton, R.P. 2003. Dinosaurs and Other Mesozoic Reptiles of California. Berkeley and Los Angeles: University of California Press, 356 pp. ISBN 0-520-23315-8
- Lindgren, J., Caldwell, M.W. and Jagt, J.W.M. 2008. New data on the postcranial anatomy of the California mosasaur Plotosaurus bennisoni (Camp, 1942) (Upper Cretaceous: Maastrichtian), and the taxonomic status of P. tuckeri (Camp, 1942). Journal of Vertebrate Paleontology 28(4):1043-1054.