The Western Interior Seaway, was a large inland sea that started to expand in the early Cretaceous period, though geological evidence suggests it started to expand in the late Jurassic period. It existed at its fullest extent from the mid-late Cretaceous period. At its greatest extent it was wide, long, and had a depth maximum depth of .
Various different species of fish existed in the Western Interior Seaway, throughout its 30 million time frame. Examples of which Class Genus Extent Location Chondrichthyes Cretolamna Cretaceous-Miocene Kansas Chondrichthyes Cretoxyrhina Cenomanian-Campanian Kansas Chondrichthyes Odontaspis Cretaceous Delaware Chondrichthyes Ptychodus Albian-Maastrichtian Kansas Chondrichthyes Scapanorhynchus Cretaceous-Miocene Worldwide Chondrichthyes Squalicorax Cenomanian-Campanian Wyoming Osteichthyes Micropycnodon Santonian Colorado Actinopterygii Bananogmius Santonian-Maastrichtian South Dakota Actinopterygii Belonostomus Cenomanian-Masstrichtian Montana Actinopterygii Cimolichthys Cenomanian-Masstrichtian Canada Actinopterygii Elopopsis Cenomanian-Campanian Colorado Actinopterygii Enchodus Cretaceous-Paleogene Colorado Actinopterygii Gyrodus Jurassic-Cretaceous Canada Actinopterygii Leptecodon Santonian Kansas Actinopterygii Pachyrhizodus Cenomanian-Maastrichtian South Dakota Actinopterygii Protosphyraena Coniacian-Maastrichtian Kansas Actinopterygii Saurodon Coniacian-Santonian Tennessee Actinopterygii Stratodus Santonian-Maastrichtian Alabama Actinopterygii Xiphactinus Albian-Maastrichtian Colorado
References
Nicholls, Elizabeth L., and Russell, Anthony P. "Paleobiogeography of the Cretaceous Western Interior Seaway of North America: the vertebrate evidence." Palaeogeography, Palaeoclimatology, Palaeoecology 79(1990):149-169 Spencer, Lucas G., and Sullivan M. Robert. Late Cretaceous vertebrates from the Western Interior. Albuquerque: New Mexico Museum of Natural History & Science, 2006. Bulletin 35 http://econtent.unm.edu/cdm/ref/collection/bulletins/id/710