Phanerozoic/Cretaceous period

The Cretaceous period is the third and final period in the Mesozoic Era. It began 145.5 million years ago after the Jurassic Period and ended 65.5 million years ago, before the  Paleogene Period of the Cenozoic Era. The word Cretaceous comes from Latin word for "chalk bearing" which is named for the white chalk cliffs of Dover England.

Epochs
The Cretaceous period is composed of the Early Cretaceous Epoch and the Late Cretaceous Epoch.

Early Cretaceous
145.5 MA to 99.6 MA

The Cretaceous period started where the Jurassic period left off. Before the start of the Jurassic period, Gondwana and Laurasia started breaking up. During the Early Cretaceous period, the two continents continued to break up.

In terms of life, ichthyosaurs started to decline in this Epoch. Ichthyosaurs, Greek for "fish lizard", were fish-like marine reptiles. They flourished in the Mesozoic Era. Where the ichthyosaurs started declining, psittacosaurs, spinosaurs and coelurosaurs started appearing.



Late Cretaceous
99.6 MA to 65.5 MA

The chalky sediments - as nowadays exposed in the white cliffs of Dover - formed during this Epoch.

At the end of the Cretaceous, the climate started to cool and a mass extinction occurred.

Climate
The climate was hot and humid during most of the Cretaceous Period. Fossils from the Cretaceous indicate the presence of tropical and subtropical plants in locations far away from the equator. Some plants existed in Antarctica and what is now Alaska. As the continents separated, water was able to circulate in the equatorial and mid-latitudes. The circulation of seawater distributed the warmth between the separating continents and around the globe. There were no glaciers, which contributed to a much higher sea level with shallow seas covering extensive parts of the continents.

The climate started to cool at the end of the Cretaceous Period.

Continental Configuration
During the Cretaceous Period the lithospheric plates started to transform into the present configuration. Laurasia and Gondwanaland continued to break up, widening the Atlantic Ocean. South America and Africa completely separated by the Late Cretaceous. Australia remained connected to Antarctica. India moved northward toward Asia.

There was continued mountain building in the western North American continent. The Sevier orogeny started during the Nevadan but continued to the Cretaceous. Associated with the Sevier orogeny were large bodies of Magma created from subduction. These bodies cooled to form the Siera Nevada Batholiths. Another mountain-building event was the Laramide Orogeny which resulted from the collision and subduction of the Farallon plate. The angle of subduction lowered, pushing mountain-building farther inland. The Rocky Mountains were created from the Laramide Orogeny. The Laramide Orogeny continued into the Paleogene Period.

Most recognizable continents were covered in a shallow sea. The North American continent developed a depression when the Farallon continent had previously collided with it. This was a foreland basin. The consequence was a shallow epicontinental sea along the middle of North America, stretching North and South. This area of water is known today as the Western Interior Seaway. There are facies that show a pattern of fining upward, followed by a coarsening upward, reflecting a transgression followed by a regression. In many parts of the world, due to the high sea level, marine limestones are abundant. Because of the high sea level, chalk deposited on the continents. One example of this is the Dover cliffs of southern England.

Fauna


Coccolithophores became abundant during the Late Cretaceous. Coccolithophores are single-celled marine plants that live in large numbers throughout the upper layers of the ocean. They are characterized by a plate or shell called coccoliths that are made of calcium carbonate. The accumulation of coccoliths in this period formed large deposits of chalk in places such as the White Cliffs of Dover. This chalk probably developed when the seas were high, and because of the amount of calcium carbonate in the coccoliths. The name "Cretaceous" comes from this deposition of chalk.

Fossils indicate the presence of many dinosaurs, fish, reptiles and amphibians. New species of pterosaurs developed. Pterosaurs were flying reptiles that had wings made of a membrane of skins stretched between fingers. Many predatory fish also emerged from this period, including teleost fish and sharks. Teleost fish were ray-finned fish that preyed on ammonoids. Reptiles were also marine predators. One example of this is the mosasaur, which were marine reptiles that were probably ancestors to modern snakes. Pleisosaurs also existed in this period and were long-necked marine reptiles with fins.

There was a decline in epifaunal sea-floor animals such as brachiopods and crinoids, probably because of an increase in predators in the Cretaceous seas. This brought about an increase in infaunal organisms (burrowing and swimming molluscs, echinoderms).

Flora
Flowering plants appeared, also known as angiosperms. They replaced gymnosperms and were spurned by the diversification of insects and bees.

Also in this period, conifers became dominant gymnosperms.

Extinction Event


At the end of the Cretaceous Period, a mass extinction event occured. Previously called the KT mass extinction, now known as the K-Pg mass extiction, this event was observed in the geologic record by a thin band - the K-Pg boundary. Many plants and animal species died out in this short amount of time. Non-flying dinosaur fossils are found below the K-Pg boundary, meaning many dinosaurs died out during this extinction. Also affected were ammonoids, marine reptiles (plesiosaurs and mosasaurs), rudists, belemnites and more. There was a drastic reductions in coccolithophores, planktonic foraminifera and radiolarians.

Several theories try to explain this event. The most prevalent theory being a single or multiple meteorites hitting the earth, and/or increased volcanic activity. Plants that depended on solar energy for photosynthesis died out. As a result, herbivores that depended on those plants died out and predators that depended on the herbivores died out. Some scientists suggest that a drastic change in climate or sea level caused the mass extinction.