Cretaceous | Research & Encyclopedia Articles

Cretaceous

The Cretaceous, or Cretaceous Period, refers to an interval of geologic time in the Mesozoic Era spanning from approximately 144 million to 66 million years ago. It was during this time period that dinosaurs developed, flourished, and became extinct, and flowering plants and modern insects first made their appearance. It is often divided into Early and Late Cretaceous Epochs. The Cretaceous was preceded by the Jurassic Period and followed by the Tertiary period.

The Cretaceous was a time of great change in the earth's landmass as continents separated and migrated on the globe's surface. At the start of the Cretaceous, South America and Africa were still connected and Antarctica was attached to Australia. North America had broken from Africa but was still attached to Western Europe. India was drifting northward toward the Eurasian continent. By the end of the Cretaceous, South America had split from Africa and moved west, and North America had separated from Western Europe opening the expanses of the Atlantic Ocean.

In Early Cretaceous times seas began to encroach on the continental areas and sedimentary rocks including limestone, and marine clays, were deposited over large areas. The advancing seas reached their greatest extent in the Late Cretaceous Epoch and then began to recede. Global temperatures were warmer than today, but during the Late Cretaceous extensive deposition of chalk (calcium carbonate) in deep ocean waters is thought to have lowered atmospheric concentrations of carbon dioxide. Reduced levels of atmospheric carbon dioxide diminished its role as a "greenhouse" gas, and this in turn led to a cooling of the earth's surface.

Flowering plants (Angiosperms) became dominant on land, and insects increased in abundance with them. Reptiles were dominant land animals until the end of the Cretaceous. Mammals and birds were not important. Flying reptiles called pterosaurs were abundant. At the end of the Cretaceous, dinosaurs suddenly became extinct and other reptile groups were decimated. Many hypotheses have been advanced to explain the disappearance of dinosaurs and a dramatic decline in the abundance of other reptiles. One widely accepted theory postulates a catastrophic collision of an asteroid or comet with the Earth that created a mammoth dust cloud. The cloud engulfed the entire atmosphere, greatly decreasing the amount of sunlight reaching the Earth's surface, and reducing or eliminating photosynthesis. Plants, herbivores and animals dependent on them could not survive, and the biology of the earth changed dramatically. This theory is supported by the very abrupt and widespread scope of extinctions occurring at the end of the Cretaceous period. Although other theories have also been proposed to account for these catastrophic events, including alterations of habitat due to the movement of continents, and the appearance of egg-eating reptiles or mammals, none has received the same support as the catastrophic collision hypothesis.