The Smoky Hill Chalk Member of the Niobrara Chalk was deposited in the Western Interior Sea during a span of about five million years extending from the upper Coniacian through the lower Campanian. Coincidentally, this period also encompasses much of the early evolution and radiation of the family Mosasauridae. Thousands of mosasaur specimens have been collected from the Smoky Hill Chalk Member of western Kansas since the first Yale College Scientific Expedition in 1870. Early workers viewed the entire Smoky Hill Chalk Member as the “Pteranodon beds” and were not able to provide accurate stratigraphic information. Even though mosasaurs are represented by numerous complete and well preserved specimens, the ranges of the various species could not be defined further without better stratigraphic data. In 1898, S. W. Williston first described the general distribution of mosasaurs within the Rudistes and Hesperornis beds of the Smoky Hill Chalk Member. Seventy years later, D. A. Russell grouped mosasaur species into upper and lower zones in the chalk and in 1990, J. D. Stewart incorporated D. E. Hattin's stratigraphic data into his biostratigraphic zones and refined the occurrences of mosasaur species to a higher degree than had been done previously. B. A. Schumacher in 1993, and M. A. Sheldon in 1996, reviewed existing collections and provided further definition to mosasaur biostratigraphy. New specimens of Tylosaurus proriger, and Clidastes liodontus with accurate stratigraphic information reported by M. J. Everhart and co-workers in 1997 further expanded and refined the temporal distribution of these species within the Smoky Hill Chalk Member.