Which epoch was the Cretaceous?
Cretaceous
The Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian (lower/early), Gallic (middle) and Senonian (upper/late). A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.
Prehistory of the United States
During the Late Cretaceous the Gulf of Mexico had expanded through North America until reaching Alaska. Up to half of the continent's modern surface area may have been submerged by this sea. This is called the Western Interior Seaway. It covered the majority of states like Texas, New Mexico, Oklahoma, Colorado, Kansas, Nebraska, the Dakotas, and Wyoming. The seafloor was smooth and probably never submerged by more than 600 feet of water. A great diversity of invertebrates flourished both on the bottom and in the water column. Examples include ammonites, giant clams, crinoids, rudists, and squid. Vertebrate life included bony fishes, mosasaurs, plesiosaurs, sharks, and turtles. Areas now occupied by the plains of the east coast from New Jersey southward as well as the Gulf coast region were covered in seawater during the Cretaceous. Sea levels reached their highest level in all of geologic history during the Cretaceous, although they declined before the period ended. Scleractinian corals were more diverse during the Cretaceous than they are today. Back then coral reefs formed along the Gulf coast. Rudist bivalves also constructed reefs in the Gulf coast region. Another bivalve, Exogyra, was so common its fossils are found in almost every Cretaceous marine deposit. During the Cretaceous, the dominant group of living fishes, the teleosts, first achieved ascendency over their holostean forebears. The aquatic toothed bird Hesperornis is the only known Cretaceous bird whose remains are found with any frequency in North America.