The final geological period of the Mesozoic era. It extended from about 145 million years ago, following the Jurassic, to about 66 million years ago, when it was succeeded by the Palaeogene period. The name of the period is derived from creta (Latin: chalk) and the Cretaceous was characterized by the deposition of large amounts of chalk in western Europe. The Cretaceous was the time of greatest flooding in the Mesozoic. Angiosperm plants made their first appearance on land and in the early Cretaceous Mesozoic reptiles reached their peak. At the end of the period there was a mass extinction of the dinosaurs, flying reptiles, and ammonites, the cause of which may be related to environmental changes resulting from collisions of the earth with large meteorites (see alvarez event; iridium anomaly).

http://www.ucmp.berkeley.edu/mesozoic/cretaceous/cretlife.html Overview of the Cretaceous period: part of the website of the University of California Museum of Paleontology