. The earliest described specimens of
Plotosaurus were discovered in the early 20th century from
Moreno Formation deposits along the
San Joaquin Valley,
California. The first was a pair of caudal vertebrae collected during 1918 or 1920 by an
Oakland resident named Herman G. Walker while exploring the
Panoche Hills. They were donated to the
University of California, Berkeley's
Museum of Paleontology under the catalog UCMP 36050. 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. Bennison was inspired to geology by his science teacher M. Merrill Thompson and continued to study the stratigraphy of surrounding hills. 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. Bennison notified Thompson, and the two brought UC Berkeley paleontologists
Samuel Paul Welles, Curtis J. Hesse, Owen J. Poe, and Thompson's students to excavate the find. The fossil consisted of a complete skull, eighteen articulated vertebrae, an
interclavicle, four ribs, and rib fragments. It was curated to UC Berkeley's museum as UCMP 32778. In August of the same year, a second skeleton was collected by a joint UC Berkeley-
California State University, Fresno party while excavating an
elasmosaur fossil in the Panoche Hills around forty miles southeast of Bennison's skeleton. This skeleton, first found by Fresno State professor William M. Tucker, was far larger than Bennison's skeleton and consisted of an articulated series of fifty-four dorsal, pygal, and caudal vertebrae. It was sent to UC Berkeley as UCMP 33913. Field expeditions of the
California Institute of Technology in Moreno Formation outcrops north of
Coalinga between 1938 and 1940 uncovered three additional partial skeletons. The most complete, CIT 2750, consisted of a large skull, thirty-nine front vertebrae, a shoulder girdle, and front paddles, while the other two (CIT 2751 and 2755) preserved the mosasaurs' tails. '' 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 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. 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. 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 sp. because of their similar measurements to the California specimens. == Description ==