The Cretaceous period lasted from about 143.1 to 66 million years ago. It is the third and final period of the Mesozoic Era and the longest geological period in the entire Phanerozoic. The name comes from the Latin word creta, meaning "chalk," which was common in deposits from the later part of the period. It is often shortened to "K," based on its German name Kreide.

During the Cretaceous, the climate was warm, leading to high sea levels that formed many shallow inland seas. These seas and oceans were home to now-extinct marine reptiles, ammonites, and rudists. On land, dinosaurs remained the dominant group of animals. The world was mostly free of ice, though there is some evidence of short, cooler periods with brief glaciation. Forests covered areas near the poles.

Many modern groups of living things originated during the Cretaceous. New groups of mammals and birds appeared, including the earliest ancestors of placentals and marsupials (Eutheria and Metatheria). The first true birds appeared toward the end of the period. Teleost fish, the most diverse group of modern vertebrates, continued to evolve, with their most diverse subgroup, Acanthomorpha, emerging during this time. Flowering plants first appeared in the Early Cretaceous and quickly spread, becoming the most common type of plant by the end of the period, while other plant groups, such as gymnosperms, declined.

The Cretaceous period ended with the Cretaceous–Paleogene extinction event, a major extinction that wiped out many species, including non-avian dinosaurs, pterosaurs, and large marine reptiles. This event is believed to have been caused by an asteroid impact that created the Chicxulub crater in the Gulf of Mexico. The end of the Cretaceous is marked by the Cretaceous–Paleogene boundary (K–Pg boundary), a geological layer that separates the Mesozoic and Cenozoic Eras.

Etymology and history

The Cretaceous period was first identified as a separate time in Earth's history by Belgian geologist Jean d'Omalius d'Halloy in 1822. He called it the "Terrain Crétacé" by studying rock layers in the Paris Basin. The name "Cretaceous" comes from the Latin word "creta," which means "chalk." This name reflects the thick layers of chalk found in the upper part of the Cretaceous period in Western Europe. Chalk is made of calcium carbonate from the shells of sea creatures, mainly tiny plankton called coccoliths.

In 1822, Conybeare and Phillips divided the Cretaceous period into two parts. Later, in 1840, Alcide d'Orbigny divided the French Cretaceous into five stages: the Neocomian, Aptian, Albian, Turonian, and Senonian. He later added two more stages: the Urgonian between the Neocomian and Aptian, and the Cenomanian between the Albian and Turonian.

Geology

The Cretaceous Period is divided into two main parts: the Early Cretaceous (Lower Cretaceous) and the Late Cretaceous (Upper Cretaceous). In older scientific writings, it was sometimes split into three parts: the Neocomian (lower/early), the Gallic (middle), and the Senonian (upper/late). Today, scientists worldwide use a system of 12 stages, all based on European rock layers. However, some regions still use different local divisions for the Cretaceous.

From youngest to oldest, the Cretaceous Period is divided into the following parts:

The beginning of the Cretaceous Period has not been clearly defined. The boundary between the Jurassic and Cretaceous periods is the only one without a globally recognized reference point (called a GSSP). This is because the markers used to identify this boundary vary by region, and there are no clear chemical changes in rocks that could help define it. Scientists have studied tiny plankton called calpionellids, which appeared briefly near the end of the Jurassic and beginning of the Cretaceous. The first appearance of a species called Calpionella alpina has been suggested as a possible marker for the start of the Cretaceous. Another marker, the first appearance of the ammonite Strambergella jacobi, has been used but is less reliable because it does not match the timing of C. alpina. The official age of the Jurassic-Cretaceous boundary is about 145 million years ago, though other estimates range as low as 140 million years ago.

The end of the Cretaceous Period is clearly marked by a layer rich in the element iridium, found globally. This layer is linked to the Chicxulub impact crater, located on the Yucatán Peninsula and extending into the Gulf of Mexico. This layer is dated to about 66.043 million years ago.

At the end of the Cretaceous, a massive object may have struck Earth, worsening a decline in biodiversity during the Maastrichtian age. This event caused the extinction of about 75% of Earth’s plant and animal species, creating a sharp boundary known as the K–Pg boundary (formerly called the K–T boundary). It took Earth a long time to recover from this event, even though many ecological niches were available.

The K–Pg extinction event affected different groups of life at different rates. Species that relied on photosynthesis, such as phytoplankton and land plants, declined or disappeared because sunlight was blocked by particles in the atmosphere. Animals that depended on plants and plankton, like herbivores, also died out, which led to the extinction of top predators such as Tyrannosaurus rex. However, only three major groups of tetrapods (four-legged animals) disappeared completely: non-avian dinosaurs, plesiosaurs, and pterosaurs. Other groups, like ichthyosaurs, temnospondyls, and nonmammalian cynodonts, had already gone extinct millions of years earlier.

Many marine organisms, such as coccolithophorids, molluscs (including ammonites, rudists, snails, and mussels), and animals that relied on these creatures, became extinct or suffered heavy losses. For example, ammonites were likely the main food source for mosasaurs, giant marine lizards that also went extinct.

Omnivores, insectivores, and scavengers survived the extinction event, possibly because they had access to food sources like insects, larvae, worms, and snails that fed on dead matter. At the end of the Cretaceous, there were no purely herbivorous or carnivorous mammals. Surviving mammals and birds ate detritus (dead organic material), which helped them survive the collapse of plant-based food chains.

In stream communities, few animals went extinct because they relied more on detritus than on living plants. Similar patterns were seen in the oceans, where animals in the water column (not attached to the seafloor) suffered more than those on or in the seafloor, which could switch to eating detritus.

Large air-breathing animals like crocodilians and champsosaurs survived because they were semiaquatic and could eat detritus. Modern crocodiles can live as scavengers, survive long without food, and hibernate. These traits likely helped their ancestors survive the end of the Cretaceous.

During the Cretaceous, high sea levels and warm climates covered large parts of continents with shallow, warm seas, creating habitats for marine life. The period is named for the widespread chalk deposits in Europe, though in many regions, Cretaceous rocks are marine limestone, a type of rock formed under warm, shallow marine conditions. The high sea level allowed for extensive sedimentation, and Cretaceous rocks are found in many areas worldwide.

Chalk is a rock type common in the Cretaceous. It is made up of coccoliths, tiny calcite skeletons from coccolithophores, a type of algae that thrived in Cretaceous seas.

In the middle Cretaceous, slow deep-sea currents led to oxygen-poor (anoxic) conditions in the ocean. This allowed organic matter to remain undecomposed, forming layers of dark, anoxic shale. These shales are a major source of oil and gas, such as the Mancos Shale in North America and deposits in the Persian Gulf and Gulf of Mexico.

In northwestern Europe, Upper Cretaceous chalk deposits form the Chalk Group, which includes the white cliffs of Dover in England and similar cliffs in France. The Chalk Group is found in England, northern France, the Netherlands, northern Germany, Denmark, and beneath parts of the North Sea. Chalk is loosely packed in many places and contains fossils like sea urchins, belemnites, ammonites, and sea reptiles such as Mosasaurus.

In southern Europe, the Cretaceous is typically a marine system made of hard limestone.

Paleogeography

During the Cretaceous Period, the supercontinent Pangaea, which had begun to break apart in the late Paleozoic and early Mesozoic, fully separated into the continents we see today. However, these continents were positioned differently than they are now. As the Atlantic Ocean expanded, mountain-building events that started during the Jurassic Period continued in the North American Cordillera. These events followed the Nevadan orogeny and included the Sevier and Laramide orogenies.

Gondwana had started to break apart during the Jurassic, but this process sped up during the Cretaceous and was mostly complete by the end of the period. South America, Antarctica, and Australia separated from Africa, though India and Madagascar remained connected until about 80 million years ago. This movement created the South Atlantic and Indian Oceans. The splitting of continents raised underwater mountain ranges, which caused global sea levels to rise. To the north of Africa, the Tethys Sea became narrower. During much of the Late Cretaceous, North America was divided by the Western Interior Seaway, a large inland sea that separated the western landmass, Laramidia, from the eastern landmass, Appalachia. By the end of the period, the seaway had disappeared, leaving thick marine deposits between coal layers. Fossil patterns suggest that Africa was split by a shallow sea during the Coniacian and Santonian stages, connecting the Tethys Sea to the South Atlantic through the Sahara and Central Africa, which were underwater at the time. Another shallow seaway linked what is now Norway and Greenland, connecting the Tethys Sea to the Arctic Ocean and allowing marine life to move between the two regions. At the height of the Cretaceous transgression, one-third of Earth’s current land area was covered by water.

The Cretaceous Period is well known for its chalk deposits, which formed more extensively during this time than in any other period of the Phanerozoic. Increased activity at mid-ocean ridges allowed more seawater to circulate through these ridges, raising calcium levels in the oceans. This made the oceans more saturated with calcium and provided more available calcium for calcareous nanoplankton. These widespread carbonate deposits and other sediments have created a detailed rock record from the Cretaceous. Notable examples in North America include the marine fossils found in Kansas’s Smoky Hill Chalk Member and the terrestrial fossils of the Hell Creek Formation. Other important Cretaceous rock layers are found in Europe, such as the Weald, and in China, such as the Yixian Formation. In what is now India, massive lava flows called the Deccan Traps erupted toward the end of the Cretaceous and into the early Paleocene.

Climate

Evidence from pollen shows the Cretaceous climate had three main phases: a warm-dry phase from the Berriasian to the Barremian, a warm-wet phase from the Aptian to the Santonian, and a cool-dry phase from the Campanian to the Maastrichtian. Like in the Cenozoic era, the 400,000-year eccentricity cycle was the most important orbital pattern affecting the movement of carbon between different areas and influencing global climate. The location of the Intertropical Convergence Zone (ITCZ) was similar to its position today.

The cooling trend that began in the Tithonian period of the Jurassic continued into the Berriasian, the first age of the Cretaceous. The North Atlantic seaway opened, allowing cool water from the Boreal Ocean to flow into the Tethys. Evidence suggests snow was common in high-latitude areas during this time, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was limited to high-latitude mountains, though seasonal snow may have occurred farther from the poles. After the Berriasian, temperatures rose again, with several thermal events, such as the middle Valanginian Weissert Thermal Excursion (WTX), caused by activity from the Paraná-Etendeka Large Igneous Province. This was followed by the middle Hauterivian Faraoni Thermal Excursion (FTX) and the early Barremian Hauptblatterton Thermal Event (HTE). The HTE marked the end of the Tithonian-early Barremian Cool Interval (TEBCI). During this interval, precession was the main orbital factor driving environmental changes in the Vocontian Basin. Much of the TEBCI saw a monsoonal climate in northern Gondwana. A shallow thermocline existed in the mid-latitude Tethys. The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI). This warm period coincided with volcanism from the Manihiki and Ontong Java Plateau and the Selli Event. Early Aptian tropical sea surface temperatures (SSTs) were 27–32 °C, based on TEX 86 measurements from the equatorial Pacific. During the Aptian, Milankovitch cycles influenced the occurrence of anoxic events by affecting the hydrological cycle and land runoff. The early Aptian also had hyperarid events in mid-latitude Asia. The BAWI was followed by the Aptian-Albian Cold Snap (AACS), which began around 118 million years ago. A short, minor ice age may have occurred during this time, as shown by glacial dropstones in the western Tethys and cold-water calcareous nannofossils expanding into lower latitudes. The AACS is linked to an arid period in the Iberian Peninsula.

Temperatures rose sharply after the AACS ended around 111 million years ago with the Paquier/Urbino Thermal Maximum, leading to the Mid-Cretaceous Hothouse (MKH), which lasted from the early Albian to the early Campanian. Faster seafloor spreading and increased carbon dioxide in the atmosphere, along with flood basalt activity, likely caused this period of extreme warmth. The MKH included multiple thermal maxima. The Leenhardt Thermal Event (LTE) occurred around 110 million years ago, followed by the l'Arboudeyesse Thermal Event (ATE) a million years later. Then came the Amadeus Thermal Maximum around 106 million years ago, followed by the Petite Verol Thermal Event (PVTE). The Event 6 Thermal Event (EV6) occurred around 102.5 million years ago, followed by the Breistroffer Thermal Maximum around 101 million years ago. The Cenomanian-Turonian Thermal Maximum, around 94 million years ago, was the most extreme hothouse period of the Cretaceous and linked to a sea level highstand. Temperatures cooled slightly afterward but then rose again with the Coniacian Thermal Maximum around 87 million years ago. Atmospheric CO₂ levels may have varied by thousands of ppm during the MKH. Average polar temperatures during the MKH exceeded 14 °C. These hot temperatures created a small temperature difference between the equator and poles, leading to weaker global winds, less ocean upwelling, and more stagnant oceans. This is shown by widespread black shale deposits and frequent anoxic events. Tropical SSTs in the late Albian likely averaged around 30 °C. Seawater was not hypersaline at this time, as even higher temperatures would have been needed. On land, arid zones expanded northward as subtropical high-pressure belts grew. The Cedar Mountain Formation's Soap Wash flora suggests average temperatures between 19 and 26 °C in Utah at the Albian-Cenomanian boundary. Tropical SSTs during the Cenomanian-Turonian Thermal Maximum were at least 30 °C, with some estimates as high as 33–42 °C. An intermediate estimate is about 33–34 °C. Deep ocean temperatures were 15–20 °C warmer than today, with some studies estimating them as 12–20 °C during the MKH. The poles were so warm that ectothermic reptiles lived there.

Starting in the Santonian, near the end of the MKH, global temperatures began to cool, continuing through the Campanian. This cooling, driven by falling atmospheric CO₂ levels, ended the MKH and led to a cooler period called the Late Cretaceous-Early Palaeogene Cool Interval (LKEPCI). Tropical SSTs dropped from around 35 °C in the early Campanian to about 28 °C in the Maastrichtian. Deep ocean temperatures fell to 9–12 °C, though the temperature difference between tropical and polar seas remained small. Conditions in the Western Interior Seaway changed little between the MKH and the LKEPCI. During this cooler period, the ITCZ became narrower, and monsoon strength in East Asia was linked to atmospheric CO₂ levels. Laramidia had a seasonal, monsoonal climate. The Maastrichtian had a chaotic, highly variable climate. Two temperature increases occurred during the Maastrichtian, contradicting the overall cooling trend. Between 70 and 69 million years ago and 66–65 million years ago, isotopic data show higher atmospheric CO₂ levels (1000–1400 ppmV) and average temperatures in west Texas of 21–23 °C (70–73 °F). A doubling of CO₂ levels was linked to a ~0.6 °

Flora

Flowering plants, also called angiosperms, make up about 90% of all living plant species today. Before angiosperms became widespread during the Jurassic and Early Cretaceous periods, the main plant groups were gymnosperms, such as cycads, conifers, ginkgophytes, gnetophytes, and their close relatives, as well as the now-extinct Bennettitales. Other plant groups included pteridosperms, or "seed ferns," which were a collection of extinct seed plants with fern-like leaves, such as Corystospermaceae and Caytoniales. Scientists are not certain about the exact origin of angiosperms, though molecular evidence suggests they are not closely related to any living gymnosperm group.

The earliest widely accepted evidence of flowering plants includes monosulcate (single-grooved) pollen grains from the late Valanginian period (~134 million years ago), found in Israel and Italy, though these were rare at first. Scientific estimates based on molecular data suggest that angiosperms began to diversify during the Late Triassic or Jurassic periods, but these estimates do not match the fossil record, which shows distinct tricolpate to tricolporoidate (three-grooved) pollen from eudicot angiosperms appearing later. Among the oldest known angiosperm macrofossils are Montsechia from the Barremian-aged Las Hoyas beds in Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen, characteristic of eudicots, first appears in the Late Barremian, while the earliest remains of monocots are from the Aptian period. Flowering plants experienced a rapid diversification during the middle Cretaceous, becoming the dominant land plant group by the end of the period, around the same time that conifers and other previously dominant groups declined. The oldest known grass fossils are from the Albian period, with grasses evolving into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are from the Turonian period (~90 million years ago) in New Jersey, with a preserved trunk diameter of 1.8 meters (5.9 feet) and an estimated height of 50 meters (160 feet).

During the Cretaceous period, ferns in the order Polypodiales, which now make up 80% of living fern species, began to diversify.

Terrestrial fauna

During the Cretaceous period, mammals on land were generally small but played an important role in the ecosystem. In some areas, multituberculate mammals were more numerous than dinosaurs. True marsupials and placentals did not exist until the very end of the period. However, other groups of mammals, such as non-marsupial metatherians and non-placental eutherians, had already started to become very diverse. These mammals included carnivores like Deltatheroida, aquatic foragers like Stagodontidae, and herbivores like Schowalteria and Zhelestidae. In the Early Cretaceous, groups such as eutriconodonts were common, but by the Late Cretaceous, multituberculates and therians dominated northern mammalian communities, while dryolestoids were more common in South America.

The top predators during this time were archosaurian reptiles, especially dinosaurs, which were at their most diverse. Birds, which are ancestors of modern-day birds, also diversified and lived on every continent, even in cold polar regions. Pterosaurs were common in the early and middle Cretaceous but declined for unknown reasons. While it was once thought that competition with early birds caused their decline, scientists now believe that the diversification of birds does not explain the pterosaurs’ decline. By the end of the Cretaceous, only three specialized pterosaur families remained: Pteranodontidae, Nyctosauridae, and Azhdarchidae.

The Liaoning lagerstätte (Yixian Formation) in China is a significant site with many preserved remains of small dinosaurs, birds, and mammals. It offers insight into life during the Early Cretaceous. The coelurosaur dinosaurs found there belong to the group Maniraptora, which includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids, and other avialans. Fossils from this site show that these dinosaurs had hair-like feathers.

Insects became more diverse during the Cretaceous. The oldest known ants, termites, and some lepidopterans (like butterflies and moths) appeared. Aphids, grasshoppers, and gall wasps also appeared during this time.

Some notable animals from the Cretaceous include:
– Tyrannosaurus rex, one of the largest land predators of all time, lived during the Late Cretaceous.
– Velociraptor, up to 2 meters long and 0.5 meters high at the hip, had feathers and lived during the Late Cretaceous.
– Triceratops, a well-known genus of the Cretaceous.
– Quetzalcoatlus, an azhdarchid pterosaur and one of the largest flying animals, lived during the Late Cretaceous.
– Confuciusornis, a crow-sized bird from the Early Cretaceous.
– Ichthyornis, a toothed seabird-like ornithuran from the Late Cretaceous.

Rhynchocephalians, which today only include the tuatara, disappeared from North America and Europe after the Early Cretaceous. By the early Late Cretaceous, they were absent from North Africa and northern South America. The reason for their decline is unclear, but it is often linked to competition with advanced lizards and mammals. However, they remained diverse in high-latitude southern South America, where lizards were rare, and their remains outnumbered those of terrestrial lizards by a ratio of 200:1.

Choristoderes, a group of freshwater aquatic reptiles that first appeared in the Jurassic period, became more diverse in Asia during the Early Cretaceous. This was the peak of their diversity, including long-necked species like Hyphalosaurus and the first records of gharial-like Neochoristodera. These reptiles may have evolved in the absence of aquatic crocodile-like reptiles. During the Late Cretaceous, Champsosaurus, a type of neochoristodere, was widespread in western North America. Due to the warm Arctic climate, choristoderes were also able to live in the Arctic during the Late Cretaceous.

Marine fauna

During the Cretaceous period, rays, modern sharks, and teleosts became common in the seas. Marine reptiles included ichthyosaurs in the early and mid-Cretaceous (which became extinct during the late Cretaceous Cenomanian-Turonian anoxic event), plesiosaurs throughout the entire period, and mosasaurs that appeared in the Late Cretaceous. Sea turtles, such as those in the Cheloniidae and Panchelonioidea groups, lived during the period and survived the extinction event. Today, besides Cheloniidae, only one species from Panchelonioidea remains: the leatherback sea turtle. Hesperornithiformes were flightless, marine diving birds that swam like grebes.

Baculites, an ammonite genus with a straight shell, thrived in the seas along with reef-building rudist clams. Inoceramids were also very important among Cretaceous bivalves and helped scientists identify major changes in marine life, such as at the Turonian-Coniacian boundary. Predatory gastropods with drilling habits were widespread. Globotruncanid foraminifera and echinoderms like sea urchins and starfish (sea stars) were common. Ostracods were abundant in Cretaceous marine settings. Ostracod species where males invested a lot in reproduction had the highest extinction and turnover rates. Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. Diatoms first spread in the oceans during the Cretaceous. Freshwater diatoms appeared later, in the Miocene. Calcareous nannoplankton were important parts of marine life and helped scientists track environmental changes.

The Cretaceous period was important for the development of bioerosion, the process of creating holes and scratches in rocks, hardgrounds, and shells.

• A scene from the early Cretaceous: a Woolungasaurus is attacked by a Kronosaurus.
• Tylosaurus was a large mosasaur, a type of carnivorous marine reptile that appeared in the late Cretaceous.
• Strong-swimming and toothed predatory waterbird Hesperornis roamed late Cretacean oceans.
• The ammonite Discoscaphites iris, Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi.
• A plate with Nematonotus sp., Pseudostacus sp., and a partial Dercetis triqueter, found in Hakel, Lebanon.
• Cretoxyrhina, one of the largest Cretaceous sharks, attacking a Pteranodon in the Western Interior Seaway.