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Plotosaurus ("swimmer lizard") is an extinct genus of large which lived during the Upper Cretaceous (Maastrichtian) in what is now North America. The taxon was initially described by Berkeley paleontologist Charles Lewis Camp in 1942 from several more or less complete fossil specimens discovered in California, USA. Originally named as Kolposaurus (meaning "bay lizard"), it was changed to Plotosaurus in 1951 when Camp discovered that the name had already been assigned to a type of nothosaur. Two species were historically assigned to the genus, but since 2008, only P. bennisoni has been recognized as valid.
Unlike other mosasaurids, Plotosaurus possesses a morphology converging with those of Ichthyosauria, suggesting a much more advanced swimming adaptation than some of its close relatives. It is these same adaptations that have led to its classification in a rather derived position within this group of marine reptiles. Although only known from North America, fragmentary specimens known from Japan that could possibly belong to the genus suggest that it may have had a wider distribution.
The fossil skeletons came under the study of the UC Berkeley's Museum of Paleontology director Charles Lewis Camp, who published his research in 1942. In it, he recognized that they represent a new genus of mosasaur. Camp particularly noticed the highly derived aquatic adaptations that were far more specialized than other mosasaurs, calling it "the most advanced genus yet described in the family Mosasauridae." He named this genus Kolposaurus, a portmanteau of the Ancient Greek words κόλπος ( kólpos, "bay") and σαῦρος ( saûros, "lizard"), and the type species Kolposaurus bennisoni in honor of Bennison with UCMP 32778 as its holotype. Camp (1942) also identified a second species which he named Kolposaurus tuckeri after Tucker. Its holotype was the same skeleton discovered by its namesake (UCMP 33913), differentiated from K. bennisoni by its larger size and more numerous pygal vertebrae. The Caltech skeletons were identified as K. tuckeri, albeit tentatively for CIT 2750. If its assignment was correct, this would have provided additional information to differentiate the species from K. bennisoni, namely in its less derived skull morphology as inferred by smaller and shorter nostrils, the frontal bone not being as extended backwards, less numerous Pterygoid bone, the Quadrate bone being taller than it is wide, and a smaller Parietal eye; different counts of vertebrae bearing certain processes; and a larger interclavicle in proportion to the skull. In 1951, Camp discovered that the name Kolposaurus was preoccupied by a nothosaur, and so renamed the genus to Plotosaurus as a portmanteau of the Ancient Greek πλώτης ( plôtes, "swimmer") and σαῦρος. A 2008 study by paleontologists Johan Lindgren, Michael Caldwell, and John Jagt redescribed Plotosaurus based on a reexamination of the specimens described by Camp (1942) and new fossils uncovered after it. They found that much of the traits thought to differentiate between the two species were actually shared, and the remaining distinct features were likely the result of intraspecific variation. This rendered P. tuckeri a junior synonym of P. bennisoni, making the genus Monotypic taxon.
All formal fossils of Plotosaurus are actually known from California only. However, fossils that could possibly belong to the genus have been discovered elsewhere in the world. In 2016, a fragmentary lower jaw discovered in the Quiriquina Formation, near , Chile, was identified by and colleagues as coming from a Plotosaurus-like mosasaur. However, in 2019, Paulina Jiménez-Huidobro and her colleagues discovered that this attribution was erroneous because the fossil did not correspond to the genus diagnostics, the authors reidentifying it as possibly coming instead from a Halisaurus. In 2021, Taichi Kato and colleagues described two tail vertebrae from two separate localities in Ibaraki Prefecture, Japan, one from the Hiraiso Formation and the other from the lower Isoai Formation. The vertebrae are referred to as cf. Plotosaurus Species because of their similar measurements to the California specimens.
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