Cretaceous

The Cretaceous is a geologic period that lasted from about 143.1 to 66 Ma. It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 million years, it is the ninth and longest geological period of the entire Phanerozoic. The name is derived from the Latin wikt:creta#Latin, 'chalk', which is abundant in deposits from the latter half of the period. It is usually abbreviated K, for its German translation Kreide.
The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land. The world was largely ice-free, although there is some evidence of brief periods of glaciation during the cooler first half, and forests extended to the poles.
Many of the dominant taxonomic groups present in modern times can be ultimately traced back to origins in the Cretaceous. During this time, new groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials, with the earliest crown group birds appearing towards the end of the Cretaceous. Teleost fish, the most diverse group of modern vertebrates, continued to diversify during the Cretaceous with the appearance of their most diverse subgroup Acanthomorpha during this period. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.
The Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out, widely thought to have been caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.

Etymology and history

The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from the Latin creta, meaning chalk. The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822. Alcide d'Orbigny in 1840 divided the French Cretaceous into five étages : the Neocomian, Aptian, Albian, Turonian, and Senonian, later adding the Urgonian between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.

Geology

Subdivisions

The Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature, the Cretaceous is sometimes divided into three series: Neocomian, Gallic and Senonian. A subdivision into 12 stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.
From youngest to oldest, the subdivisions of the Cretaceous Period are:

Epoch	Age/Stage	Start	Definition	Etymology
Epoch	Age/Stage		Definition	Etymology
Paleocene	Danian	66
Late Cretaceous	Maastrichtian	72.2 ± 0.2	top: iridium anomaly at the Cretaceous–Paleogene boundary base:first occurrence of Pachydiscus neubergicus	Maastricht Formation, Maastricht, Netherlands
Late Cretaceous	Campanian	83.6 ± 0.2	base: last occurrence of Marsupites testudinarius	Champagne, France
Late Cretaceous	Santonian	85.7 ± 0.2	base: first occurrence of Cladoceramus undulatoplicatus	Saintes, France
Late Cretaceous	Coniacian	89.8 ± 0.3	base: first occurrence of Cremnoceramus rotundatus	Cognac, France
Late Cretaceous	Turonian	93.9 ± 0.2	base: first occurrence of Watinoceras devonense	Tours, France
Late Cretaceous	Cenomanian	100.5 ± 0.1	base: first occurrence of Rotalipora globotruncanoides	Cenomanum; Le Mans, France
Early Cretaceous	Albian	113.2 ± 0.3	base: first occurrence of Praediscosphaera columnata	Aube, France
Early Cretaceous	Aptian	121.4 ± 0.6	base: magnetic anomaly M0r	Apt, France
Early Cretaceous	Barremian	125.77 ± 1.5	base: first occurrence of Spitidiscus hugii and S. vandeckii	Barrême, France
Early Cretaceous	Hauterivian	132.6 ± 0.6	base: first occurrence of Acanthodiscus	Hauterive, Switzerland
Early Cretaceous	Valanginian	137.05 ± 0.2	base: first occurrence of Calpionellites darderi	Valangin, Switzerland
Early Cretaceous	Berriasian	143.1 ±0.6	base: first occurrence of Berriasella jacobi ; first occurrence of Calpionella alpina	Berrias, France

Boundaries

The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined Global Boundary Stratotype Section and Point. Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and the lack of any chemostratigraphic events, such as isotope excursions that could be used to define or correlate a boundary. Calpionellids, an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary. In particular, the first appearance Calpionella alpina, coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous. The working definition for the boundary has often been placed as the first appearance of the ammonite Strambergella jacobi, formerly placed in the genus Berriasella, but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of C. alpina. The boundary is officially considered by the International Commission on Stratigraphy to be approximately 145 Ma, but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 Ma.
The upper boundary of the Cretaceous is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and extending into the Gulf of Mexico. This layer has been dated at 66.043 Ma.
At the end of the Cretaceous, the impact of a large body with the Earth may have been the punctuation mark at the end of a progressive decline in biodiversity during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as the K–Pg boundary. Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant ecological niches.
Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked solar energy. As is the case today, photosynthesizing organisms, such as phytoplankton and land plants, formed the primary part of the food chain in the late Cretaceous, and all else that depended on them suffered, as well. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top predators, such as Tyrannosaurus rex, also perished. Yet only three major groups of tetrapods disappeared completely; the non-avian dinosaurs, the plesiosaurs and the pterosaurs. The other Cretaceous groups that did not survive into the Cenozoic the ichthyosaurs, last remaining temnospondyls, and nonmammalian were already extinct millions of years before the event occurred.
Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails, and mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, ammonites are thought to have been the principal food of mosasaurs, a group of giant marine lizards related to snakes that became extinct at the boundary.
Omnivores, insectivores, and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds that survived the extinction fed on insects, larvae, worms, and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on detritus.
In stream communities, few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.
The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavorable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.