Reconstruction of the late Cretaceous ammonite, Placenticeras meeki, which flourished 70 million years ago in the shallow seas that covered much of the western interior of North America from New Mexico to Alberta. Fossils measuring up to 80 cm. in diameter are found in the Pierre Shale. This genus has world-wide distribution but like all other ammonites, it became extinct at the end of the Cretaceous along with the dinosaurs. Extinction of numerous plant and animal groups at this time is attributed to profound environmental disturbances from the collision of an asteroid with the earth near the Yucatan Peninsula in the Gulf of Mexico.