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The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period (about 135 mya) to the beginning of the Paleocene epoch of the Tertiary period (65 mya). The end of the Cretaceous also defines the boundary between the Mesozoic and Cenozoic eras.

As with other older geologic periods, the rock beds that define the Cretaceous are well identified, but the exact dates of the period's start and end are uncertain by a few million years. No great extinction or burst of diversity separated the Cretaceous from the Jurassic. However, the end of the period is most sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 64.3 mya. This bolide collision is probably responsible for the major, extensively-studied Cretaceous-Tertiary extinction eventThe Cretaceous-Tertiary (KT) extinction event also known as the KT boundary (from German: , was a period of extremely frequent extinction of species, about 65. 5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of th. The Cretaceous (from LatinAlternative meanings: See Latin (disambiguation Latin was the language originally spoken in the region around Rome called Latium. It gained great importance as the formal language of the Roman Empire. All Romance languages are descended from Latin, and ma creta, for chalk) was named for the extensive beds of chalk ( calcium carbonateProperties General Name Calcium carbonate Chemical formula Ca CO Appearance White solid Physical Formula weight 100. 1 amu Melting point Liquifies under high pressure at 1612 K (1339 °C) Boiling point decomposes at 1172 K (899 °C) Density 2. 7 ×103 kg/ m3 deposited by the shells of marine invertebrateInvertebrate is a term coined by Chevalier de Lamarck to describe any animal without a backbone or vertebra, like insects, squids and worms. He divided them into two groups, the Insecta and the Vermes. However, the invertebrates are not a coherent group os) found in the upper Cretaceous of BritainThe word Britain is used to refer to the United Kingdom (UK): i. the United Kingdom of Great Britain and Northern Ireland (from 1927), the United Kingdom of Great Britain and Ireland ( 1801- 1927) or the United Kingdom of Great Britain ( 1707- 1801). and adjacent continental EuropeFor the band of the same name, see Europe (band . Europe is a continent forming the westermost part of the Eurasian supercontinent. Europe is bounded to the north by the Arctic Ocean, to the west by the Atlantic Ocean, to the south by the Mediterranean Se.

The Cretaceous is usually separated into lower and upper subdivisions. The faunal stageFaunal stages are a subdivision of geologic time used primarily by paleontologists who study fossils rather than by geologists who study rock formations. Typically, a faunal stage will consist of a series of rocks that contain similar fossils. There wills from youngest to oldest are:

During the Cretaceous, the late Paleozoic - early Mesozoic supercontinent of Pangea completed its breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened and South America drifted westwards, Gondwana itself broke up as Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached). Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Within the continents, a broad shallow sea advanced across central North America (the Western Interior Seaway) and then started to recede, leaving thick marine deposits sandwiched between coal beds.

Other important Cretaceous exposures occur in Europe and China. In the area that is now India, massive lava beds called the Deccan Traps were laid down in the very late Cretaceous and early Paleocene. Climates were warm, and even polar regions had no permanent ice.

On land, plants became quite modern, although the now-ubiquitous grasses did not evolve until the end of the period. Flowering plants were widespread, their evolution aided by the appearance of bees. Conifers thrived, as they do today. The first representatives of many modern trees -- figs, sycamores, and magnolias for example -- appear in the Cretaceous.

On land, mammals were a small and still relatively minor component of the fauna. The fauna was dominated by reptiles, especially dinosaurs. Pterosaurs were common in the skies and in marine environments (particularly in the early and middle Cretaceous), although on land they faced competition from the adaptive radiation of birds.

The insects began to diversify, and the oldest known ants, termites and butterflies appeared. Aphids, grasshoppers, and gall wasp s appeared. Another important insect to evolve was the eusocial bee, which was integral to the ecology and evolution of flowering plants.

In the seas, rays, modern sharks and bony fish became common. Marine reptiles (including mosasaurs and plesiosaurs) and globotruncanid foraminiferids thrived. Baculites, a straight-shelled form of ammonite, flourished in the seas. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. The Cretaceous also saw the first radiation of the diatoms in the oceans (freshwater diatoms did not appear until the Miocene).

In the extinction event that defines end of the Cretaceous, a significant number of marine forms disappeared. These include most shelled cephalopods (all ammonites, most nautilids), all belemnites, and reef-forming rudist molluscs), as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most famous of the Cretaceous extinctions. Dinosaurs that were unique to the period (such as Tyrannosaurus rex and Ankylosaurus) were wiped out. The last of the pterosaurs went extinct and the vast majority of birds did as well, including the enantiornithines and Hesperornithiformes.