Figure 1. Skull of Plotosaurus. Note the mislabeling of the right supratemporal as a squamosal. The posterior frontal processes are atypical for lizards.
Plotosaurus benisoni (originally Kolposaurus, preoccupied; Camp 1942, 1951; Late Cretaceous; 9m) is a large mosasaur (Lepidosauria, Squamata, Scleroglossa) with relatively small flippers and a large tail fin. It enters the LRT as a sister to Tylosaurus. The nasals, tiny in Tylosaurus, are absent in Plotosaurus.
Figure 2. Plotosaurus from Camp 1951 with hypothetical body outline.
References Camp CL 1942. California Mosasaurs. Memoirs of the University of California 13:1-68. Camp CL 1951.Plotosaurus, a new generic name for Kolposaurus Camp, preoccupied. Journal of Paleontology 25:822.