Cretaceous Period

The Cretaceous Period is a geologic period that began 145 million years ago and ended about 65 million years ago. It is between the Jurassic and Paleogene periods. This period is normally abbreviated as K. The Cretaceous was a period with a relatively warm climate and high eustatic sea level. The oceans and seas were populated with marine reptiles, ammonites and rudists; the land was populated by dinosaurs. New groups of mammals and birds as well as flowering plants appeared. The period ended with one of the largest mass extinctions in Earth’s history, the K-T extinction, when many species, including the dinosaurs, pterosaurs, and large marine reptiles, disappeared.

During the Cretaceous the super continent of Pangaea completed its breakup into the present day continents. As the Atlantic Ocean widened, the convergent-margin orogenies continued in the North American Cordillera. The Nevaden orogeny was followed by the Sevier and Laramide. Even though Gondwana was still intact in the beginning of the Cretaceous, it broke up and South America, Antartica, and Australia detached from Africa. India and Madagascar remained attached. The South Atlantic and Indian oceans were newly formed. This tearing apart lifted great undersea mountain chains, raising eustatic sea levels worldwide. The Tethys Sea continued to narrow. Broad, shallow seas advanced across central North America and Europe. These seas then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous one-third of Earth’s present land area was submerged.

The Cretaceous is justly famous for its chalk. More chalk formed in the Cretaceous than in any other period. Possibly because the oceans were enriched in calcium; this made the oceans more saturated, as well as increased the bio-availability of the element for calcareous nanoplankton. These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine.

Flowering plants spread during this period, although they did not become predominant until the Campanian stage near the end of the epoch. Their evolution was aided by the appearance of bees; in fact angiosperms and insects are a good example of coevolution. The first representatives of many leafy trees appeared in the Cretaceous. On land, mammals were a small, minor component of the fauna. Early marsupial mammals evolved early on, with true placentals emerging in the late part of the period. The fauna was dominated by archosaurian reptiles, especially dinosaurs. Pterosaurs were common in the early and middle Cretaceous, but they faced growing competition birds. In the seas; rays, modern sharks, and telosts became common. Marine reptiles included icthyosaurs, plesiosaurs, and mosasaurs.

The Cretaceous–Tertiary extinction event, which occurred approximately 65million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world. The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. The K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.

Here on Universe Today we have a great article detailing the theory that the K-t event was triggered by an asteroid. Here is a link to a page about the Cretaceous period. Astronomy Cast offers a good episode about the tectonic plate activity that was shaping our world at the time.

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