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The Cretaceous ( ) is a
geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions into which geologist A geologist is a s ...
that lasted from about 145 to 66
million years ago One million (1,000,000), or one thousand thousand, is the natural number In mathematics, the natural numbers are those used for counting (as in "there are ''six'' coins on the table") and total order, ordering (as in "this is the ''third'' l ...
(Mya). It is the third and final period of the
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy ...
Era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Compar ...
, as well as the longest. At around 79 million years, it is the longest geological period of the entire
Phanerozoic The Phanerozoic Eon is the current in the , and the one during which abundant and has existed. It covers million years to the present, and it began with the Period when animals first developed hard shells preserved in the fossil record. The ...
. The name is derived from the Latin
''creta''
''creta''
, "
chalk Chalk is a soft, white, porous Porosity or void fraction is a measure of the (i.e. "empty") spaces in a , and is a of voids over the total volume, between 0 and 1, or as a between 0% and 100%. Strictly speaking, some tests measure the "acce ...

chalk
", which is abundant in 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 Climate is the long-term pattern of weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena ...

climate
, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-
extinct Extinction is the termination of a kind of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by ...
marine reptile (top left) Sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the green sea turtle, loggerhead ...
s,
ammonites Ammonoids are a group of extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Li ...

ammonites
, and
rudists Rudists are a group of extinct box-, tube- or ring-shaped marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% ...
, while
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
s continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s and
birds Birds are a group of s constituting the Aves , characterised by s, toothless beaked jaws, the of eggs, a high rate, a four-chambered , and a strong yet lightweight . Birds live worldwide and range in size from the to the . There ar ...

birds
appeared. During the Early Cretaceous,
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s 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 The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also ...
groups. The Cretaceous (along with the Mesozoic) ended with the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden of three-quarters of the and on , approximately 66 million years ago. With the exception of some species such as ...
, a large mass extinction in which many groups, including non-avian dinosaurs,
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes excep ...
s, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt
Cretaceous–Paleogene boundary Complex Cretaceous-Paleogene clay layer (gray) in the Geulhemmergroeve tunnels near Geulhem, the Netherlands. Finger is on the actual K–Pg boundary. The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) ...
(K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and
Cenozoic The Cenozoic ( ; ) is Earth's current geological era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the ge ...

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 (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is a ...
in the
Paris Basin The Paris Basin is one of the major geological regions of France France (), officially the French Republic (french: link=no, République française), is a List of transcontinental countries, transcontinental country spanning Western Euro ...
and named for the extensive beds of
chalk Chalk is a soft, white, porous Porosity or void fraction is a measure of the (i.e. "empty") spaces in a , and is a of voids over the total volume, between 0 and 1, or as a between 0% and 100%. Strictly speaking, some tests measure the "acce ...

chalk
(
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

calcium carbonate
deposited by the shells of marine
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s, principally
coccoliths Coccoliths are individual plates of calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from ...
), found in the upper Cretaceous of
Western Europe Western Europe is the western region of Europe Europe is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical r ...

Western Europe
. The name Cretaceous was derived from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
''creta'', meaning ''chalk''. The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822.
Alcide d'Orbigny Alcide Charles Victor Marie Dessalines d'Orbigny (6 September 1802 – 30 June 1857) was a French Natural history, naturalist who made major contributions in many areas, including zoology (including malacology), palaeontology, geology, archaeology ...
in 1840 divided the French Cretaceous into five ''étages'' (stages): the
NeocomianIn geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes b ...
, 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 Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...
and
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous The Cretaceous ( ) is a that lasted from about 145 to 66 (Mya). It is the third and final period of the , as well as the longest. At around 79 millio ...
epochs, or Lower and Upper Cretaceous
series Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used i ...
. In older literature, the Cretaceous is sometimes divided into three series:
NeocomianIn geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes b ...
(lower/early), Gallic (middle) and Senonian (upper/late). A subdivision in 12
stages Stage or stages may refer to: Acting * Stage (theatre) In theatre and performing arts, the stage (sometimes referred to as the deck in stagecraft) is a designated space for the performance of theatrical production, productions. The stage s ...
, 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:


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 The 'golden spike' marking the Ediacaran GSSP A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time sca ...
(GSSP). Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and lack of any chemostratigraphic events, such as
isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of ...
excursions (large sudden changes in ratios of isotopes), that could be used to define or correlate a boundary. Calpionellids, an enigmatic group of
planktonic Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocea ...

planktonic
protists A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
with urn-shaped calcitic
tests Test(s), testing, or TEST may refer to: * Test (assessment) A test or examination (exam or evaluation) is an educational assessment intended to measure a test-taker's knowledge, skill, aptitude, physical fitness, or classification in many ...
briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested to represent the most promising candidates for fixing the Jurassic–Cretaceous boundary In particular, the first appearance '' Calpionella alpina,'' co-inciding 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 ''Berriasella'' is a discoidal evolute perisphinctacean ammonite *Belemnoidea—an extinct group of . {{fossil cephalopods, state=collapsed Prehistoric cephalopods by classification Coleoidea ..., and type genus for the neocomitid subfam ...
'', 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 to be approximately 145 million years ago by the
International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigr ...
, but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago. The upper boundary of the Cretaceous is sharply defined, being placed at an
iridium Iridium is a with the Ir and 77. A very hard, brittle, silvery-white of the , iridium is considered to be the second-densest naturally occurring metal (after ) with a density of as defined by experimental . It is the most -resistant meta ...

iridium
-rich layer found worldwide that is believed to be associated with the
Chicxulub impact crater
Chicxulub impact crater
, with its boundaries circumscribing parts of the
Yucatán Peninsula The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula A peninsula ( la, paeninsula from 'almost' and 'island') is a landform A landform is a natural or artificial feature of the solid surface of the Earth ...

Yucatán Peninsula
and into the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin 400px, Diagrammatic cross-section of an ocean basin, showing the various geographic features In hydrology Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning ...

Gulf of Mexico
. This layer has been dated at 66.043 Mya. At the end of the Cretaceous, the impact of a large
body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of animals ...
with the Earth may have been the punctuation mark at the end of a progressive decline in
biodiversity Biodiversity is the biological variety and Genetic variability, variability of life, life on Earth. Biodiversity is a measure of variation at the Genetics, genetic, species, and ecosystem level. Terrestrial biodiversity is usually greater near ...

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 K–Pg boundary (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
s. Despite the severity of K-Pg extinction event, significant variability in the rate of extinction occurred between and within different
clades A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual cont ...
. Species that depended on
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
declined or became extinct as atmospheric particles blocked
solar energy Solar energy is Solar irradiance, radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy including solar water heating, and solar architecture. It ...

solar energy
. As is the case today, photosynthesizing organisms, such as
phytoplankton Phytoplankton () are the autotrophic An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compoun ...

phytoplankton
and land
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, formed the primary part of the
food chain A food chain is a linear network of links in a food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource sy ...

food chain
in the late Cretaceous, and all else that depended on them suffered, as well.
Herbivorous A herbivore is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top
predator Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

predator
s, such as ''
Tyrannosaurus rex ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosau ...

Tyrannosaurus rex
'', also perished. Yet only three major groups of
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s disappeared completely; the nonavian
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
s, the
plesiosaurs The Plesiosauria (; Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 ...

plesiosaurs
and the
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes excep ...
s. The other Cretaceous groups that did not survive into the Cenozoic Era, the
ichthyosaur Ichthyosaurs (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Myce ...
s and last remaining temnospondyls and nonmammalian
cynodont The cynodonts ("dog teeth") (clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all ...

cynodont
s were already extinct millions of years before the event occurred.
Coccolithophorids A coccolithophore (or coccolithophorid, from the adjective) is a unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functio ...

Coccolithophorids
and
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s, including
ammonite *Belemnoidea—an extinct group of . {{fossil cephalopods, state=collapsed Prehistoric cephalopods by classification Coleoidea ...

ammonite
s,
rudist Rudists are a group of extinct box-, tube- or ring-shaped Marine (ocean), marine Heterodonta, heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were ma ...
s,
freshwater snail Freshwater snails are gastropod mollusks which live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs ...
s, and
mussel Mussel () is the used for members of several families of s, from saltwater and habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or ...

mussel
s, as well as organisms whose
food chain A food chain is a linear network of links in a food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource sy ...

food chain
included these shell builders, became extinct or suffered heavy losses. For example,
ammonites Ammonoids are a group of extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Li ...

ammonites
are thought to have been the principal food of
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Ma ...
s, a group of giant marine reptiles that became extinct at the boundary.
Omnivores An omnivore () is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular ...
,
insectivores A Asilidae, robber fly eating a hoverfly File:Myresluger2.jpg, The giant anteater, a large insectivorous mammal An insectivore is a Carnivore, carnivorous plant or animal that eats insects. An alternative term is entomophage, which also refer ...
, and
carrion Carrion (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in re ...

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 A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization allowed the domestication of animals such as chickens, sheep, rabbi ...
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s. Mammals and birds that survived the extinction fed on
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s,
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e,
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limb Limb can refer to: *Limb (anatomy), an appendage of a human or animal *Limb Music, a record label *Limb (album), an ...

worm
s, 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 biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. In
stream A stream is a body of water (Lysefjord) in Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokmål (, ; literally "book tongue") is an official written standard for the No ...

stream
communities A community is a social unit (a group of living things) with commonality such as norms, religion Religion is a social system, social-cultural system of designated religious behaviour, behaviors and practices, morality, morals, beliefs, wor ...
, 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 A water column is a concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind ...
, than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ch ...
from living phytoplankton, while animals living on or in the
ocean floor The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
ocean floor
feed on detritus or can switch to detritus feeding. The largest air-breathing survivors of the event,
crocodilian Crocodilia (or Crocodylia, both ) is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...
s 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.


Geologic formations

The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms. The Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of
marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its na ...

limestone
, a rock type that is formed under warm, shallow marine conditions. Due to the high sea level, there was extensive
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...
for such
sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks In geology Geology (from the Ancient Greek γ ...
. Because of the relatively young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide.
Chalk Chalk is a soft, white, porous Porosity or void fraction is a measure of the (i.e. "empty") spaces in a , and is a of voids over the total volume, between 0 and 1, or as a between 0% and 100%. Strictly speaking, some tests measure the "acce ...

Chalk
is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of
coccolith Coccoliths are individual plates of calcium carbonate formed by coccolithophores (single-celled algae such as '' Emiliania huxleyi'') which are arranged around them in a ''coccosphere''. Formation and composition Coccoliths are formed within t ...
s, microscopically small
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

calcite
skeletons of
coccolithophore A coccolithophore (or coccolithophorid, from the adjective) is a Unicellular organism, unicellular, eukaryotic phytoplankton (alga). They belong either to the kingdom Protista, according to Robert Whittaker (ecologist), Robert Whittaker's Kingdom ...
s, a type of
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
that prospered in the Cretaceous seas. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half of the world's petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic
shale Shale is a fine-grained, clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defi ...

shale
s were formed during this interval, such as the
Mancos Shale The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) Formation (stratigraphy), geologic formation of the Western United States. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures n ...
of western North America. These shales are an important
source rock In petroleum geology Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for (). Sedimentary b ...
for
oil and gas A fossil fuel is a hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and ...
, for example in the subsurface of the North Sea.


Europe

In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the
Chalk Group The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in ...
, which forms the
white cliffs of Dover The White Cliffs of Dover is the region of English coastline The coast, also known as the coastline or seashore, is defined as the area where land meets the sea or ocean, or as a line that forms the boundary between the land and the ocean ...
on the south coast of
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest of England and the Celtic Sea to the southwest. E ...

England
and similar cliffs on the coast. The
group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with ...
is found in England, northern France, the
low countries The term Low Countries, also known as the Low Lands ( nl, de Lage Landen, french: les Pays-Bas) and historically called the Netherlands ( nl, de Nederlanden), Flanders, or Belgica, refers to a coastal lowland region in Northwestern Europe ...
, northern
Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin Berlin (; ) is the Capital city, capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3,769,495 inh ...

Germany
,
Denmark Denmark ( da, Danmark, ) is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that are broadly divided by physical characteristics ( physical geography), hu ...

Denmark
and in the subsurface of the southern part of the
North Sea The North Sea is a sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
. Chalk is not easily consolidated and the Chalk Group still consists of loose sediments in many places. The group also has other
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its na ...

limestone
s and
arenite Arenite (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic ...
s. Among the fossils it contains are
sea urchin Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone — from the intertidal seashore down to . The spherical, ha ...

sea urchin
s,
belemnite Belemnitida (or the belemnite) is an extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that emb ...

belemnite
s,
ammonite *Belemnoidea—an extinct group of . {{fossil cephalopods, state=collapsed Prehistoric cephalopods by classification Coleoidea ...

ammonite
s and sea reptiles such as ''
Mosasaurus ''Mosasaurus'' (; "lizard of the Meuse River") is the type genus of the mosasaurs, an extinct group of aquatic squamate reptiles. It lived from about 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cre ...

Mosasaurus
''. In southern Europe, the Cretaceous is usually a marine system consisting of competent limestone beds or incompetent
marl __NOTOC__ Marl or marlstone is a carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its ...

marl
s. Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European
continental shelf A continental shelf is a portion of a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth descriptio ...

continental shelf
, at the margin of the
Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was an ocean during much of the Era located between the ancient continents of and , before the opening of the and oceans during the Period. Etym ...
.


North America

During the Cretaceous, the present North American continent was isolated from the other continents. In the Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea Inland may refer to: Places Sweden * Inland Fräkne Hundred, a hundred of Bohus ...
started forming. This inland sea separated the elevated areas of
Laramidia Laramidia was an island continent that existed during the Late Cretaceous The Late Cretaceous (100.5–66 Year#SI prefix multipliers, Ma) is the younger of two epoch (geology), epochs into which the Cretaceous geological period is divided in the ge ...
in the west and
Appalachia Appalachia () is a cultural region 's map of native American cultural areas within the territory of the United States (1948) as defined by Melville J. Herskovits influence , homelands of the Celtic languages The Celtic languages ( ...
in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.


Paleogeography

During the Cretaceous, the late-
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Eu ...
-to-early-Mesozoic
supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...
of
Pangaea Pangaea or Pangea () was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology) ...

Pangaea
completed its
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building A mountain is an elevated portion of the Earth's crust, gen ...
breakup into the present-day
continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of th ...

continent
s, although their positions were substantially different at the time. As the widened, the convergent-margin mountain building (
orogenies An orogeny is an event that leads to both structural Deformation (physics), deformation and compositional differentiation of the Earth's lithosphere (Crust (geology), crust and uppermost Mantle (geology), mantle) at Convergent boundary, converge ...
) that had begun during the
Jurassic The Jurassic ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
continued in the
North American Cordillera The North American Cordillera, sometimes also called the Western Cordillera of North America, the Western Cordillera or the Pacific Cordillera, is the North America North America is a continent entirely within the Northern Hemisphere a ...
, as the
Nevadan orogeny#REDIRECT Nevadan orogenyThe Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from th ...
was followed by the Sevier orogeny, Sevier and Laramide orogeny, Laramide orogenies. Gondwana had begun to break up during the Jurassic Period, but its fragmentation accelerated during the Cretaceous and was largely complete by the end of the period. South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other until around 80 million years ago); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising Sea level#Sea level and dry land, eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. During the most of the Late Cretaceous, North America would be divided in two by the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea Inland may refer to: Places Sweden * Inland Fräkne Hundred, a hundred of Bohus ...
, a large interior sea, separating
Laramidia Laramidia was an island continent that existed during the Late Cretaceous The Late Cretaceous (100.5–66 Year#SI prefix multipliers, Ma) is the younger of two epoch (geology), epochs into which the Cretaceous geological period is divided in the ge ...
to the west and
Appalachia Appalachia () is a cultural region 's map of native American cultural areas within the territory of the United States (1948) as defined by Melville J. Herskovits influence , homelands of the Celtic languages The Celtic languages ( ...
to the east, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression (geology), transgression, one-third of Earth's present land area was submerged. The Cretaceous is justly famous for its
chalk Chalk is a soft, white, porous Porosity or void fraction is a measure of the (i.e. "empty") spaces in a , and is a of voids over the total volume, between 0 and 1, or as a between 0% and 100%. Strictly speaking, some tests measure the "acce ...

chalk
; indeed, more chalk formed in the Cretaceous than in any other period in the
Phanerozoic The Phanerozoic Eon is the current in the , and the one during which abundant and has existed. It covers million years to the present, and it began with the Period when animals first developed hard shells preserved in the fossil record. The ...
. Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for Coccolithophores, calcareous nanoplankton. These widespread carbonates and other sedimentary rock, sedimentary deposits make the Cretaceous rock record especially fine. Famous Geologic formation, formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe (e.g., the Weald) and China (the Yixian Formation). In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene.


Climate

The cooling trend of the last epoch of the Jurassic continued into the first age of the Cretaceous. There is evidence that snowfalls were common in the higher latitudes, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was however restricted to high-latitude mountains, though seasonal snow may have existed farther from the poles. Rafting by ice of stones into marine environments occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia. After the end of the first age, however, temperatures increased again, and these conditions were almost constant until the end of the period. The warming may have been due to intense volcanic activity which produced large quantities of carbon dioxide. Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric Carbon dioxide, CO2 pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between . Atmospheric CO2 and temperature relations indicate a doubling of pCO2 was accompanied by a ~0.6 °C increase in temperature. The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics, further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
fossils have been found within 15 degrees of the Cretaceous south pole. It was suggested that there was Antarctica, Antarctic marine glaciation in the Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the result of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S. A very gentle temperature gradient from the equator to the poles meant weaker global winds, which drive the ocean currents, resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black
shale Shale is a fine-grained, clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defi ...

shale
deposition and frequent anoxic events. Sediment cores show that tropical sea surface temperatures may have briefly been as warm as , warmer than at present, and that they averaged around . Meanwhile, deep ocean temperatures were as much as warmer than today's.


Flora

Flowering plants (angiosperms) make up around 90% of living plant species today. Prior to the rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated by
gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also ...
groups, including cycads, Pinophyta, conifers, ginkgophytes, Gnetophyta, gnetophytes and close relatives, as well as the extinct Bennettitales. Other groups of plants included Pteridospermatophyta, pteridosperms or "seed ferns", a collective term to refer to disparate groups of fern-like plants that produce seeds, including groups such as Corystospermaceae and Caytoniales. The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms. The earliest widely accepted evidence of flowering plants are monosulcate (single grooved) pollen grains from the late Valanginian (~ 134 million years ago) found in Israel, and Italy, initially at low abundance. Molecular clock estimates conflict with fossil estimates, suggesting the diversification of Crown group, crown-group angiosperms during the Upper Triassic or Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen of Eudicots, eudicot angiosperms. Among the oldest records of Angiosperm macrofossils are ''Montsechia'' from the Barremian aged La Huérguina Formation, Las Hoyas beds of Spain and ''Archaefructus'' from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian. Flowering plants underwent a rapid radiation beginning during the middle Cretaceous, becoming the dominant group of land plants by the end of the period, coindicent with the decline of previously dominant groups such as conifers. The oldest known fossils of Poaceae, grasses are from the Albian, with the family having diversified into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are known from the Turonian (c. 90 Ma) of New Jersey, with the trunk having a preserved diameter of and an estimated height of . During the Cretaceous, Polypodiales ferns, which make up 80% of living fern species, would also begin to diversify.


Terrestrial fauna

On land,
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s were generally small sized, but a very relevant component of the Fauna (animals), fauna, with cimolodont multituberculates outnumbering dinosaurs in some sites. Neither true marsupials nor placentals existed until the very end, but a variety of non-marsupial metatherians and non-placental eutherians had already begun to diversify greatly, ranging as carnivores (Deltatheroida), aquatic foragers (Stagodontidae) and herbivores (''Schowalteria'', Zhelestidae). Various "archaic" groups like eutriconodonts were common in the Early Cretaceous, but by the Late Cretaceous northern mammalian faunas were dominated by multituberculates and therians, with dryolestoids dominating South America. The apex predators were archosaurian reptiles, especially
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) a ...

dinosaur
s, which were at their most diverse stage. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons (once thought to be due to competition with early birds, but now it is understood avian adaptive radiation is not consistent with pterosaur decline), and by the end of the period only two highly specialized Family (biology), families remained. The Liaoning lagerstätte (Yixian Formation) in China is a treasure chest of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous. The coelurosaur dinosaurs found there represent types of the group Maniraptora, which includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids along with other avialans. Fossils of these dinosaurs from the Liaoning lagerstätte are notable for the presence of hair-like feathers. Insects diversified during the Cretaceous, and the oldest known ants, termites and some lepidopterans, akin to Butterfly, butterflies and moths, appeared. Aphids, grasshoppers and gall wasps appeared. File:Tyrannosaurus-rex-Profile-steveoc86.png, ''
Tyrannosaurus rex ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosau ...

Tyrannosaurus rex
'', one of the largest land predators of all time, lived during the Late Cretaceous File: Velociraptor Restoration.png, Up to 2 m long and 0.5 m high at the hip, ''Velociraptor'' was feathered and roamed the Late Cretaceous File: Triceratops by Tom Patker.png, ''Triceratops'', one of the most recognizable genera of the Cretaceous File:Quetzalcoatlus07.jpg, The azhdarchid ''Quetzalcoatlus'', one of the largest animals to ever fly, lived during the Late Cretaceous File:Confuciusornis sanctus mmartyniuk.png, ''Confuciusornis'', a genus of crow-sized birds from the Early Cretaceous File:Ichthyornis restoration.jpeg, ''Ichthyornis'' was a toothed, seabird-like ornithuran from the Late Cretaceous


Rhynchocephalians

Rhynchocephalia, Rhynchocephalians (which today only includes the Tuatara) disappeared from North America and Europe after the Early Cretaceous, and were absent from North Africa and northern South America by the early
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous The Cretaceous ( ) is a that lasted from about 145 to 66 (Mya). It is the third and final period of the , as well as the longest. At around 79 millio ...
. The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals. They appear to have remained diverse in high-latitude southern South America during the Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards 200:1.


Choristodera

Choristodera, Choristoderes, a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as ''Hyphalosaurus'' and the first records of the gharial-like Neochoristodera, which appear to have evolved in the regional absence of aquatic Neosuchia, neosuchian crocodyliformes. During the Late Cretaceous the neochoristodere ''Champsosaurus'' was widely distributed across western North America.


Marine fauna

In the seas, batoidea, rays, modern sharks and teleosts became common. Marine reptiles included
ichthyosaur Ichthyosaurs (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Myce ...
s in the early and mid-Cretaceous (becoming extinct during the late Cretaceous Cenomanian-Turonian anoxic event), plesiosaurs throughout the entire period, and
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Ma ...
s appearing in the Late Cretaceous. ''Baculites'', an
ammonite *Belemnoidea—an extinct group of . {{fossil cephalopods, state=collapsed Prehistoric cephalopods by classification Coleoidea ...

ammonite
genus with a straight shell, flourished in the seas along with reef-building
rudist Rudists are a group of extinct box-, tube- or ring-shaped Marine (ocean), marine Heterodonta, heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were ma ...
clams. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera and echinoderms such as sea urchins and Asteroidea, starfish (sea stars) thrived. The first radiation of the diatoms (generally silicon dioxide, siliceous shelled, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene. The Cretaceous was also an important interval in the evolution of bioerosion, the production of borings and scrapings in rocks, hardgrounds and shells. File:Kronosaurus hunt1DB.jpg, A scene from the early Cretaceous: a ''Woolungasaurus'' is attacked by a ''Kronosaurus''. File:Tylosaurus pembinensis 1DB.jpg, ''Tylosaurus'' was a large
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Ma ...
, carnivorous marine reptiles that emerged in the late Cretaceous. File:Hesperornis BW.jpg, Strong-swimming and toothed predatory waterbird ''Hesperornis'' roamed late Cretacean oceans. File:DiscoscaphitesirisCretaceous.jpg, The
ammonite *Belemnoidea—an extinct group of . {{fossil cephalopods, state=collapsed Prehistoric cephalopods by classification Coleoidea ...

ammonite
''Discoscaphites iris'', Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi File:The fossils from Cretaceous age found in Lebanon.jpg, A plate with ''Nematonotus sp.'', ''Pseudostacus sp.'' and a partial ''Dercetis triqueter'', found in Hakel, Lebanon File:Cretoxyrhina attacking Pteranodon.png, ''Cretoxyrhina'', one of the largest Cretaceous sharks, attacking a ''Pteranodon'' in the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea Inland may refer to: Places Sweden * Inland Fräkne Hundred, a hundred of Bohus ...


See also

* Mesozoic, Mesozoic Era * Cretaceous–Paleogene extinction event, Cretaceous-Paleogene extinction * Chalk Formation * Cretaceous Thermal Maximum * List of fossil sites (with link directory) * South Polar region of the Cretaceous


References


Citations


Bibliography

* * * * * —detailed coverage of various aspects of the evolutionary history of the insects. * * *


External links


UCMP Berkeley Cretaceous pageCretaceous Microfossils: 180+ images of Foraminifera
* {{Authority control Cretaceous, Geological periods