Identification of 8640 specimens of bryozoan, mollusk, decapod, and vertebrate fossils from the Paleocene Clayton Formation in southern Illinois has resulted in recognition of 44 species-level taxa of which 40 have been identified to genus or species. The fauna is less diverse than other Clayton Formation faunas which may be a result of its having formed in an area of reduced salinity at the head of the Mississippi Embayment. One bryozoan, seven mollusk, and one shark species comprise 93% of the individuals. Thirty-nine of the taxa identified have been known previously from occurrences in Cretaceous rocks of the Atlantic and Gulf Coastal Plains and the Upper Midwest documenting a high percentage of survivors in this fauna. The area, although connected to the Gulf of Mexico, may have served as a refugium.
Bulletin of the Mizunami Fossil Museum
Cope, Kerri H; Utgaard, John E.; Masters, John M.; Feldmann, Rodney (2005). The Fauna of the Clayton Formation (Paleocene, Danian) of Southern Illinois: A Case of K/P Survivorship and Danian Recovery. Bulletin of the Mizunami Fossil Museum(32) 97-108. Retrieved from https://oaks.kent.edu/geolpubs/142