Xiphactinus lived during the Late Cretaceous (80 million years ago) of western Kansas. The prominent cut-water spine of the pectoral fin gives this large bony fish (teleost) its name. The yellow chalk containing its bones is composed of billions of microscopic fossil plankton (coccoliths). The entire ancient Niobrara sea bottom of Kansas and the central United States was made of the carbonate shells, or tests, of such plankton during the Cretaceous Period. Cretaceous is Latin for “chalky”.