Toward the end of the Cretaceous period, many species of marine organisms became extinct. Land dinosaurs completely disappeared, along with flying reptiles, sea reptiles, and ichthyosaurs. Other land reptiles were little affected. Most species of turtles, crocodilians, lizards, and snakes survived. Amphibians and mammals were only mildly affected.

Overall there was a major, worldwide decrease in the number of species of plants and animals. This drop in the number of species is one of the events that signaled the end of the Cretaceous period and the beginning of the Tertiary period. The transition between the two periods is known as the K/T boundary ("Cretaceous" in German is die Kreidezeit).

Determining whether all the species died out in a few years or over millions of years has proved to be a difficult problem for geologists and pale-ontologists. Attempts to pinpoint the time of the...