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What was the Cretaceous period and how did it get its name?
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 dHalloy (1783-1875) used the name Terrain Cretace 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.
What are the major divisions of the Cretaceous?
Scientists divide the Cretaceous into two epochs: the Early Cretaceous (also called the Lower Cretaceous or, less formally, lower or early Cretaceous), from approximately 144 to 89 million years ago; and the Late Cretaceous (also called the Upper Cretaceous, or upper or late Cretaceous), from approximately 89 to 65 million years ago. Each of these main epochs is broken up into smaller ages. The following chart gives the European nomenclature for each age:
Why did the dinosaurs thrive and diversify in the Cretaceous?
Although there is no clear answer to this question, paleontologists know that a revolution in life occurred during this time period. This took place as many types of modern flora and fauna made their first appearances. Some scientists theorize that it was the development, and eventual dominance, of a new group of plants, the angiosperms (flowering plants), in harmony with the development of new groups of insects that provided fresh sources of food for the dinosaurs. All these new food sources allowed the dinosaurs to continue to dominate throughout the Cretaceous period.
By the Cretaceous period, continental drift had moved the continents to nearly familiar positions: 1) North America, 2) Eurasia, 3) South America, 4) Africa, 5) the Indian subcontinent, 6) Antarctica, and 7) Australia (map based on a U.S. Geological Survey illustration).
How did Laurasia and Gondwana (Gondwanaland) change during the Cretaceous period?
During the Cretaceous period, both Laurasia and Gondwana fragmented into smaller landmasses and separated from each other. The whole motif of the Cretaceous was change change from the ancient topography to the more familiar forms we see today.
In the Early Cretaceous period, Laurasia began to break up due to the action of an extension of the Mid-Atlantic Ridge, with North America and Greenland separating from Eurasia. Rifting occurred in Gondwana, with South America and Africa beginning to separate. In the middle of the Cretaceous period, Gondwana had separated into four major landmasses: South America, Africa, the combined India and Madagascar, and the combined Antarctica and Australia.
By the Late Cretaceous, North America and Greenland began to split, as did Australia and Antarctica, and India and Madagascar. The Atlantic Ocean continued to widen, and India and Australia moved northward. By the end of the Cretaceous, the continents began to assume their modern outlines and headed toward their current destinations on the planets surface. This also led to the development and widening of our modern oceans and seas.