In June 1936, a high school student from Gustine named Allan Bennison found three vertebrae next to a hadrosaur fossil in shale hills near Patterson, two of which he donated to UC Berkeley as UCMP 32943.

This would turn fruitful, as in 1937 he discovered a partial skeleton from grey sandstone hills near Pacheco Pass during a survey of Late Cretaceous beds in the Diablo Range.

Field expeditions of the California Institute of Technology in Moreno Formation outcrops north of Coalinga between 1938 and 1940 uncovered three additional partial skeletons.

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 teeth, the quadrate being taller than it is wide, and a smaller pineal foramen; different counts of vertebrae bearing certain processes; and a larger interclavicle in proportion to the skull.

[3] 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 πλωτώ (plôtô, "swimmer") and σαῦρος.

[5] 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.

Some of the tail vertebrae are fused together; unlike in most mosasaurs this is not a pathological condition but another aquatic adaptation that allowed Plotosaurus to achieve a more efficient swimming design.