Plotosaurus
Plotosaurus ("swimmer lizard") is an extinct genus of mosasaurs who lived during the Upper Cretaceous (Maastrichtian) in what is now North America. Only one species is recognized, P. bennisoni, described by Berkeley paleontologist Charles Lewis Camp in 1942 from fossils discovered in California. Originally named 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. Unlike other mosasaurids, Plotosaurus possesses a morphology converging with those of ichthyosaurs, suggesting a much more advanced swimming adaptation than some of its close relatives. Discovery and naming 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