The Cretaceous Period (135 to 65 million years ago) is the last Period of the Mesozoic Era, the Age of Dinosaurs. Tyrannosaurus rex ruled the Cretaceous, which also witnessed the appearance of horned dinosaurs, duck-billed dinosaurs and coelurosaurs; the latter were feathered (though flightless) dinosaurs, thought to be the ancestors of birds.
A shallow sea bisected North America through most of this Period, stretching from Texas to the Arctic. Marsupials arose early in the Cretaceous, spreading throughout the Southern Continents (which remained attached at that time); South America split from Africa about 100 million years ago and finally broke from the rest of Gondwanaland 75 million years ago, beginning 72 million years of isolation. Madagascar, originally part of Africa, split from India-Antarctica-Australia 100 million years ago and India broke away 15 million years later.
Broadleaf trees first appeared in the Cretaceous and, during the second half of the Period, ants, social bees and snakes evolved. Mammals (placental and marsupial) remained rather small and inconspicuous until the Chicxulub Asteroid struck the Yucatan Peninsula, 65 million years ago, putting an end to the reign of dinosaurs.
Well known Cretaceous rocks of North America include the Pierre Shale of the High Plains, the Niobrara and Smoky Hill Chalks of Kansas-Nebraska, the Dakota Sandstone of the Front Range Hogbacks, the Book Cliffs of Colorado-Utah, the Mancos Shale of the Colorado Plateau and the Cliffhouse Sandstone of Mesa Verde; all of these were deposited within or along the vast Cretaceous Sea.
