Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Hypsibema missouriensis
Remains of "Hypsibema missouriensis" were first discovered in Bollinger County, Missouri by members of the Chronister family while they were digging a cistern, and were subsequently collected by Stewart, later nicknamed "Dinosaur Dan." In 1942, Stewart, of the Missouri Geological Survey, had been examining clay near Glen Allen when he came upon a boy who led him to the family at work digging. According to Stewart, property owner Lulu Chronister had found several "unusual" bones while digging and had saved them. They had been found about deep in the Chronisters' well, which had an overall depth of , "imbedded in a black plastic clay." Stewart reported his discovery to the Smithsonian Institution, which bought the remains—thirteen vertebrae of a dinosaur's tail—from Chronister for US$50, which was later used to purchase a cow. Two other bones, of unknown type, were also recovered from the site, while one additional vertebrae had been given by Lulu Chronister to a friend. At the Smithsonian, the bones were analyzed but the species from which they originated was incorrectly identified.