Cretaceous

The Cretaceous (IPA) is a geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the Mesozoic Era, and it is also the longest. At about 77.1 million years, it is the ninth and longest geological period of the whole Phanerozoic. Its name comes from the Latin word for 'chalk', which is common in rocks from the later part of the period. It is usually shortened to K, after its German name, Kreide.

The Cretaceous had a fairly warm climate. Sea levels were high, which created many shallow inland seas. These oceans and seas were home to marine reptiles, ammonites and rudists, which are now extinct. On land, dinosaurs continued to be the largest animals. The world was mostly free of ice, although there is some evidence of brief cold periods during the cooler first half. Forests grew all the way to the poles.

Many of the groups of living things that are common today can be traced back to the Cretaceous. New groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials (Eutheria and Metatheria, respectively). The earliest crown group birds appeared towards the end of the period. Teleost fish, which are the most varied group of modern vertebrates, kept growing in variety, and their most varied subgroup, Acanthomorpha, appeared during this time. In the Early Cretaceous, flowering plants appeared and quickly spread. By the end of the period, they had become the main plants across the Earth, while many earlier gymnosperm groups became less common and died out.

The Cretaceous ended, along with the Mesozoic, with the Cretaceous–Paleogene extinction event. This was a large mass extinction in which many groups of living things died out, including non-bird dinosaurs, pterosaurs, and large marine reptiles. It is widely thought to have been caused by a large asteroid hitting the Earth, which formed the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is marked by a thin layer of rock called the Cretaceous–Paleogene boundary (K–Pg boundary), which is linked to the mass extinction. It lies between the Mesozoic and Cenozoic Eras.

Etymology and history

Belgian geologist Jean d'Omalius d'Halloy first described the Cretaceous as its own time period in 1822. He called it the Terrain Crétacé. He studied strata in the Paris Basin.

The period got its name from thick layers of chalk. Chalk is a rock made of calcium carbonate. It was left behind by the shells of tiny sea animals called invertebrates, mainly coccoliths. Chalk layers like these were found in the upper Cretaceous of Western Europe. The name comes from the Latin word creta, which means "chalk."

In 1822, Conybeare and Phillips divided the Cretaceous into two parts. In 1840, Alcide d'Orbigny split the French Cretaceous into five stages, called étages. These were the Neocomian, Aptian, Albian, Turonian, and Senonian. Later, he added two more stages. The Urgonian came between the Neocomian and Aptian. The Cenomanian came between the Albian and Turonian.

Geology

See also: Cretaceous–Paleogene extinction event

See also: Category:Cretaceous System of Europe

See also: Category:Cretaceous System of North America

The Cretaceous is split into Early and Late Cretaceous epochs. It has 12 stages. Its upper edge is marked by a layer of iridium found all over the world. This layer is linked to the Chicxulub impact crater and is about 66 million years old. An impact may have ended a long decline in biodiversity. Many plant and animal species died out. Many dinosaurs died out too, and ammonites and mosasaurs disappeared. Some animals that ate insects, worms, and dead plant and animal matter survived, and crocodilians also lived through the change.

The sea level was high and the climate was warm. Warm, shallow seas covered large areas of the land. The period was named after the chalk deposits of Europe, such as the white cliffs of Dover. Chalk is made of tiny skeletons from coccolithophores, a kind of algae that grew well in Cretaceous seas. Half of the world's petroleum reserves formed at this time. These deposits are an important source of oil and gas.

Paleogeography

During the Cretaceous, the supercontinent Pangaea broke apart into the continents we know today. The continents were in very different places than they are now. It had formed in the late Paleozoic and early Mesozoic. As the Atlantic Ocean grew wider, mountains kept forming along the edges of North American Cordillera. This mountain building had started in the Jurassic. It included the Nevadan orogeny, followed by the Sevier and Laramide orogenies. Gondwana began to break up in the Jurassic. This happened much faster during the Cretaceous, and it was almost finished by the end of the period. South America, Antarctica, and Australia moved away from Africa. India and Madagascar stayed joined until about 80 million years ago. This created the new South Atlantic and Indian Oceans. The breaking apart pushed up huge chains of mountains under the sea. These raised eustatic sea levels around the world. North of Africa, the Tethys Sea kept getting narrower.

During most of the Late Cretaceous, the Western Interior Seaway, a large inland sea, split North America in two. Laramidia was to the west, and Appalachia was to the east. Later in the period, this sea pulled back. It left thick sea deposits between layers of coal. Clues from ancient clams suggest that a shallow sea split Africa in half during the Coniacian and Santonian. Another shallow waterway ran between present-day Norway and Greenland. At the peak of the transgression, when seas spread over land, about one-third of Earth's land was underwater. The Cretaceous is especially famous for its chalk. More chalk formed in this period than in any other period of the Phanerozoic. Activity along the Mid-ocean ridge added calcium to the oceans. This made the element easier for calcareous nanoplankton to use. These carbonates and other sedimentary deposits make the Cretaceous rock record especially detailed. Famous formations include the rich sea fossils of Kansas's Smoky Hill Chalk and the land animal fossils of the Hell Creek Formation. Other important rocks from this time are found in Europe, such as the Weald. They are also found in China, such as the Yixian Formation. In what is now India, huge lava beds called the Deccan Traps erupted in the very late Cretaceous and early Paleocene.

Climate

Further information: Cool tropics paradox

Palynological studies show that the Cretaceous climate had three main phases: a warm and dry phase, a warm and wet phase, and a cool and dry phase. The period started with cooling from the Tithonian, the last age of the Jurassic. This cooling continued into the Berriasian, the first age of the Cretaceous. Snow was common in areas far from the equator. After that, temperatures rose again during several warm times. These included the Weissert Thermal Excursion in the middle Valanginian, the Faraoni Thermal Excursion in the middle Hauterivian, and the Hauptblatterton Thermal Event in the early Barremian. Later, the Aptian-Albian Cold Snap brought a short cold period. Then temperatures rose quickly. The hot Mid-Cretaceous Hothouse lasted from the early Albian until the early Campanian, with several periods of very high heat. The warmest of these was the Cenomanian-Turonian Thermal Maximum. During this time, the temperature gradient from the equator to the poles was weak. This slowed global winds, which meant less upwelling and calmer oceans. As a result, widespread black shale formed, and anoxic events happened often. The poles were so warm that ectothermic reptiles could live there. Starting in the Santonian, the climate cooled. This was caused by falling CO₂ levels, and it led into a cooler period. Conditions in the Western Interior Seaway changed little. In west Texas, average yearly temperatures were between 21 and 23 °C during parts of the Maastrichtian. The cool period lasted into the Late Palaeocene.

Large amounts of magma pushed sea levels up, so shallow seas covered much of the continents. The magma was linked to mantle plumes or extensional tectonics. The Tethys Sea also helped warm the global climate. Plant fossils that like warm weather have been found as far north as Alaska and Greenland. Dinosaur fossils have been found within 15 degrees of the Cretaceous south pole. Evidence of rainforests near the pole suggests the Antarctic was warm during the Turonian Age. High temperatures fueled storms, hurricanes, and wildfires that damaged trees. Along with arthropod damage, this made trees produce large amounts of resin during the Cretaceous Resinous Interval, which lasted from 125 to 75 million years ago.

Flora

Today, flowering plants (angiosperms) make up about 90% of living plant species. Before flowering plants became common, during the Jurassic and Early Cretaceous, most land plants were gymnosperms, such as cycads, conifers, ginkgophytes, gnetophytes, and their close relatives. The extinct Bennettitales were also common. Other plants included pteridosperms, or "seed ferns," which were extinct seed plants with fern-like leaves. These included Corystospermaceae and Caytoniales. Scientists are not sure where flowering plants came from. However, clues from genes suggest they are not closely related to any living group of gymnosperms.

The earliest widely accepted evidence of flowering plants is pollen grains with a single groove. They are from the late Valanginian, about 134 million years ago, and were found in Israel and Italy. At first, these grains were rare. A molecular clock suggests that crown-group angiosperms appeared earlier, but this does not fit well with the fossil pollen record. Some of the oldest plant fossils include Montsechia from Las Hoyas in Spain and Archaefructus from the Yixian Formation in China. Flowering plants spread quickly during the middle Cretaceous. By the end of the period, they had become the most common group of land plants, as conifers declined. The oldest known grasses date from the Albian. Ferns in the order Polypodiales also began to diversify during this time.

Terrestrial fauna

On land, mammals were generally small but were an important part of the fauna. In some places, cimolodont multituberculates were more common than dinosaurs. True marsupials and placentals did not exist until the very end. Other early mammal groups, including metatherians and eutherians, had already spread widely. Some were meat-eaters (Deltatheroida), some hunted for food in water (Stagodontidae), and some ate plants (Schowalteria, Zhelestidae). "Archaic" groups like eutriconodonts were common in the Early Cretaceous. By the Late Cretaceous, dryolestoids were the main mammals in South America. The apex predators were archosaurian reptiles, especially dinosaurs, which were at their most varied. Early relatives of modern birds also spread across every continent, even into cold polar regions. Pterosaurs were common in the early and middle Cretaceous but declined for reasons that are not well understood. They were once thought to have lost out to early birds, but scientists no longer think this is a good explanation, based on adaptive radiation. By the end of the period, only three highly specialized families remained: Pteranodontidae, Nyctosauridae, and Azhdarchidae.

The Liaoning lagerstätte (Yixian Formation) in China is an important site with many preserved remains of small dinosaurs, birds, and mammals. It gives a glimpse of life in the Early Cretaceous. The coelurosaur dinosaurs found there belong to the group Maniraptora. This group includes modern birds and their closest relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids, and other avialans. Fossils from this site are known for hair-like feathers. Insects also became more varied. The oldest known ants, termites, and some lepidopterans, relatives of butterflies and moths, appeared, along with Aphids, grasshoppers, and gall wasps. Rhynchocephalians, which today include only the tuatara, disappeared from North America and Europe after the Early Cretaceous. They were gone from North Africa and northern South America by the early Late Cretaceous. Scientists are not sure why they declined, though it has often been linked to competition with advanced lizards and mammals. Choristoderes were freshwater aquatic reptiles that first appeared in the Jurassic. They became very varied in Asia during the Early Cretaceous, including long-necked forms such as Hyphalosaurus and the gharial-like Neochoristodera. In the Late Cretaceous, Champsosaurus spread widely across western North America. Choristoderans were also able to reach the warm Arctic.

Marine fauna

In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs, which lived in the early and middle Cretaceous. They died out during the late Cretaceous Cenomanian-Turonian anoxic event. Plesiosaurs lived through the whole period, and mosasaurs appeared in the Late Cretaceous. Sea turtles from the Cheloniidae and Panchelonioidea groups lived during this time and survived the extinction event. Today, Panchelonioidea is represented by just one species, the leatherback sea turtle. The Hesperornithiformes were birds that could not fly. They dove underwater and swam like grebes.

Baculites, an ammonite with a straight shell, grew well in the seas, along with reef-building rudist clams. Inoceramids were another well-known group of Cretaceous clams. Globotruncanid foraminifera and echinoderms, such as sea urchins and starfish (sea stars), also thrived. Ostracods were very common. Thylacocephala, a group of crustaceans, died out in the Late Cretaceous. The first diatoms with siliceous shells, instead of calcareous ones, appeared in the oceans during this period. Freshwater diatoms did not appear until the Miocene. The Cretaceous was also an important time for bioerosion, which is the making of holes and scrapings in rocks, hardgrounds and shells.