Mosasaurs are only known from the Late Cretaceous. They were top predators of the Western Interior Seaway and evolved into a diversity of forms during their ~25 million years of existence. They ranged in size from less than 5 meters (16ft) (Clidastes) to 14 meters (45ft) (Tylosaurus).
Everhart, M. J., 2005. Oceans of Kansas: A Natural History of the Western Interior Sea. Indiana University Press, Bloomington. 322 pp.