The Cretaceous period followed the Jurassic period on the geological time scale; it was the last period in the Mesozoic era. The period lasted from approximately 145 to 65 million years ago, or 80 million years total. The geologic time scale is not exact, and the dates of the Cretaceous period on various scales vary by about 5 to 10 million years. Most of the large Jurassic sauropods, stegosaurs, and theropods disappeared in the early part of this period, but were replaced by an incredibly large array of new dinosaur groups. These included the horned types, duck-billed, and armored sauropods, and new types of theropod carnivores.

In a roundabout way, the Cretaceous period got its name from the type of rock deposited along the northern shores of the Tethys Sea in a band running from what is now Ireland and Britain to the Middle East. This rock—formed from the metamorphosed deposits of the tiny limestone skeletons of diatoms—is known as chalk. The Latin for chalk is creta, so the period was named the Cretaceous. “Cretaceous” was first used to describe such a rock found in France. In 1822, Jean-Baptiste-Julien Omalius d’Halloy (1783–1875) used the name “Terrain Cretacé” to describe the strata and associated units of chalk (craie in French) found in that country. Since these same strata were also present across the English Channel, English geologists began calling them the Cretaceous System.