Cretaceous-Tertiary extinction event

The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago (Ma) at the end of the Maastrichtian age of the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

period. It was a large-scale mass extinction

Extinction event

An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

of animal and plant species in a geologically short period of time. Widely known as the K–T extinction event, it is associated with a geological signature known as the K–T boundary

K–T boundary

The K–T boundary is a geological signature, usually a thin band, dated to 65.5 ± 0.3 Ma ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...

, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous

Cretaceous

Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary

Tertiary

The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

Period (a historical term for the period of time now covered by the Paleogene

Paleogene

The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...

and Neogene

Neogene

The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...

periods). The event marks the end of the Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

Era and the beginning of the Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

Era. With "Tertiary" being discouraged as a formal time or rock unit by the International Commission on Stratigraphy

International Commission on Stratigraphy

The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...

, the K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers.

Non-avian

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

fossils are found only below the K–T boundary, indicating that non-avian dinosaurs became extinct during the boundary event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaur

Mosasaur

Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s, plesiosaur

Plesiosaur

Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s, pterosaur

Pterosaur

Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...

s and many species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

of plants and invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s also became extinct. Mammalian clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s passed through the boundary with few extinctions, and evolutionary radiation

Evolutionary radiation

An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...

from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms.

Scientists theorize

Scientific method

Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...

that the K–T extinctions were caused by one or more catastrophic events, such as massive asteroid impact

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

(s) (like the Chicxulub impact

Chicxulub Crater

The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...

), or increased volcanic activity

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

. Several impact craters and massive volcanic activity, such as that in the Deccan traps

Deccan Traps

The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than thick and cover an area of and a volume of...

, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, leading to a massive disruption in Earth's ecology

Ecology

Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

. Many researchers believe the extinction was more gradual, resulting from a combination of the events above and others including sea level and climate changes.

Extinction patterns

Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clade

Clade

s. Species that depended on photosynthesis

Photosynthesis

declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the Earth's surface. This plant extinction caused a major reshuffling of the dominant plant groups. Photosynthesizing organisms, including phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

and land plants, formed the foundation of the food chain

Food chain

A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

in the late Cretaceous as they do today. Evidence suggests that herbivorous animals died out when the plants on which they depended for food became scarce. Consequently, top predators such as Tyrannosaurus rex
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

also perished.

Coccolithophorids and molluscs (including ammonite

Ammonite

Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...

s, rudists, freshwater snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s and mussel

Mussel

The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater 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 oval.The...

s, and those organisms whose food chain

Food chain

included these shell builders) became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaur

Mosasaur

Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s, a group of giant marine reptiles that became extinct at the boundary.

Omnivores, insectivores and carrion

Carrion

Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...

-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

Carnivore

A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

mammals. Mammals and birds that survived the extinction fed on insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, worm

Worm

The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

s, and snails, which in turn fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus

Detritus (biology)

In biology, detritus is non-living particulate organic material . It typically includes the bodies or fragments of dead organisms as well as fecal material. Detritus is typically colonized by communities of microorganisms which act to decompose the material...

(non-living organic material).

In stream

Stream

A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...

communities

Biocoenosis

A biocoenosis , coined by Karl Möbius in 1877, describes the interacting organisms living together in a habitat . This term is rarely used in English, as this concept has not been popularized in Anglophone countries...

few animal groups became extinct because stream communities rely less directly on food from living plants and more on detritus that washes in from land, buffering 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

Pelagic zone

Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...

than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production

Primary production

400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...

from living phytoplankton while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.

The largest air-breathing survivors of the event, crocodyliforms and champsosaurs

Choristodera

Choristodera is an order of semi-aquatic diapsid reptiles which ranged from the Middle Jurassic, or possibly Late Triassic, to at least the early Miocene. Choristoderes have been found in North America, Asia, and Europe. The most common fossils are typically found from the Late Cretaceous to the...

, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, 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.

After the K–T event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niche

Ecological niche

In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

Microbiota

The K–T boundary represents one of the most dramatic turnovers in the fossil record for various calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

nanoplankton that formed the calcium

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

deposits that gave the Cretaceous its name. The turnover in this group is clearly marked at the species level. Statistical analysis of marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in speciation

Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

. The K–T boundary record of dinoflagellate

Dinoflagellate

The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...

s is not as well-understood, mainly because only microbial cyst

Microbial cyst

A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed down and...

s provide a fossil record, and not all dinoflagellate species have cyst-forming stages, thereby likely causing diversity to be underestimated. Recent studies indicate that there were no major shifts in dinoflagellates through the boundary layer.

Radiolaria have left a geological record since at least the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

times, and their mineral fossil skeletons can be tracked across the K–T boundary. There is no evidence of mass extinction of these organisms, and there is support for high productivity of these species in Southern high latitudes

Antarctic Circle

The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. For 2011, it is the parallel of latitude that runs south of the Equator.-Description:...

as a result of cooling temperatures in the early Paleocene. Approximately 46% of diatom

Diatom

Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

species survived the transition from the Cretaceous to the Upper Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

. This suggests a significant turnover in species, but not a catastrophic extinction of diatoms, across the K–T boundary.

The occurrence of plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic foraminifera

Foraminifera

The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

across the K–T boundary has been studied since the 1930s. Research spurred by the possibility of an impact event at the K–T boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary. However, there is debate ongoing between groups that believe the evidence indicates substantial extinction of these species at the K–T boundary, and those who believe the evidence supports multiple extinctions and expansions through the boundary.

Numerous species of benthic foraminifera became extinct during the K–Pg event, presumably because they depend on organic debris for nutrients, since the biomass

Biomass (ecology)

Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

in the ocean is thought to have decreased. However, as the marine microbiota recovered, it is thought that increased speciation of benthic foraminifera resulted from the increase in food sources. Phytoplankton recovery in the early Paleocene provided the food source to support large benthic foraminiferal assemblages, which are mainly detritus-feeding. Ultimate recovery of the benthic populations occurred over several stages lasting several hundred thousand years into the early Paleocene.

Marine invertebrates

There is variability in the fossil record as to the extinction rate of marine invertebrates

Marine invertebrates

Marine invertebrates are animals that inhabit a marine environment and are invertebrates, lacking a vertebral column. In order to protect themselves, they may have evolved a shell or a hard exoskeleton, but this is not always the case....

across the K–T boundary. The apparent rate is influenced by the lack of fossil records rather than actual extinction.

Ostracodes, a class of small crustaceans that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracode diversity was lower in the Paleocene than any other time in the Tertiary. However, current research cannot ascertain whether the extinctions occurred prior to or during the boundary interval itself.

Approximately 60% of late-Cretaceous Scleractinia

Scleractinia

Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

coral

Coral

Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

genera failed to cross the K–T boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow tropical waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the photic zone

Photic zone

The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

) areas of the ocean were less impacted by the K–T boundary. Colonial coral species rely upon symbiosis

Symbiosis

Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

with photosynthetic algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, which collapsed due to the events surrounding the K–T boundary. However, the use of data from coral fossils to support K–Pg extinction and subsequent Paleocene recovery must be weighed against the changes that occurred in coral ecosystems through the K–T boundary.

The numbers of cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

, echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

, and bivalve genera exhibited significant diminution after the K–T boundary. Most species of brachiopods, a small phylum

Phylum

In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

of marine invertebrates, survived the K–Pg event and diversified during the early Paleocene.

Except for nautiloid

Nautiloid

Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...

s (represented by the modern order Nautilida

Nautilida

The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...

) and coleoids (which had already diverged

Genetic divergence

Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes through time, often after the populations have become reproductively isolated for some period of time...

into modern octopodes

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

, squid

Squid

Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

s, and cuttlefish

Cuttlefish

Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....

) all other species of the mollusca

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

n class Cephalopoda became extinct at the K–T boundary. These included the ecologically significant belemnoids

Belemnoidea

Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles...

, as well as the ammonoids, a group of highly diverse, numerous, and widely distributed shelled cephalopods. Researchers have pointed out that the reproductive strategy of the surviving nautiloids, which rely upon few and larger eggs, played a role in outsurviving their ammonoid counterparts through the extinction event. The ammonoids utilized a planktonic strategy of reproduction (numerous eggs and planktonic larvae), which would have been devastated by the K–T boundary event. Additional research has shown that subsequent to this elimination of ammonoids from the global biota, nautiloids began an evolutionary radiation into shell shapes and complexities theretofore known only from ammonoids.

Approximately 35% of echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

genera became extinct at the K–T boundary, although taxa

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

that thrived in low-latitude, shallow-water environments during late Cretaceous had the highest extinction rate. Mid-latitude, deep-water echinoderms were much less affected at the K–T boundary. The pattern of extinction points to habitat loss, specifically the drowning of carbonate platform

Carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonous calcareous deposits . Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism...

s, the shallow-water reefs in existence at that time, by the extinction event.

Other invertebrate groups, including rudists

Rudists

Rudists are a group of box, tube or ring shaped marine heterodont bivalves that arose during the Jurassic, and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean.- Shell description :...

(reef-building clams) and inoceramids

Inoceramus

Inoceramus is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus Pteria....

(giant relatives of modern scallops), also became extinct at the K–T boundary.

Fish

There are substantial fossil records of jawed

Gnathostomata

Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...

fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

es across the K–T boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. Within cartilaginous fish

Chondrichthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

, approximately 80% of the shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, rays

Rajiformes

Rajiformes is one of the four orders of batoids, flattened cartilaginous fishes related to sharks.Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body. The undulatory pectoral fin motion...

, and skate

Skate

Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera. There are two subfamilies, Rajinae and Arhynchobatinae ....

s families survived the extinction event, and more than 90% of teleost fish

Teleostei

Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

(bony fish) families survived. There is evidence of a mass kill of bony fishes at a fossil site immediately above the K–T boundary layer on Seymour Island

Seymour Island

Seymour Island is an island in the chain of 16 major islands around the tip of the Graham Land on the Antarctic Peninsula. Graham Land is closer to continental land mass than any other part of that Antarctica. It lies within the section of the island chain that resides off the west side of the...

near Antarctica, apparently precipitated by the K–Pg event. However, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.

Terrestrial invertebrates

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

damage to the fossilized leaves of flowering plant

Flowering plant

The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

s from fourteen sites in North America were used as a proxy for insect diversity across the K–T boundary and analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. However, during the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after the extinction event.

Terrestrial plants

There is overwhelming evidence of global disruption of plant communities at the K–T boundary. However, there were important regional differences in plant succession. In North America, the data suggest massive devastation and mass extinction of plants at the K–T boundary sections, although there were substantial megafloral changes before the boundary.

In high southern hemisphere latitudes, such as New Zealand and Antarctica the mass die-off of flora caused no significant turnover in species, but dramatic and short-term changes in the relative abundance of plant groups. In North America, approximately 57% of plant species became extinct. The Paleocene recovery of plants began with recolonizations by fern species, represented as a fern spike

Fern spike

In paleontology, a fern spike is the occurrence of abundant fern spores in the fossil record, usually immediately after an extinction event...

in the geologic record; this same type of fern recolonization was observed after the 1980 Mount St. Helens eruption.

Due to the wholesale destruction of plants at the K–T boundary there was a proliferation of saprotrophic organisms such as fungi

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

that do not require photosynthesis

Photosynthesis

and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and there was plenty of organic matter to feed on. Once the atmosphere cleared, photosynthetic organisms like ferns and other plants returned. Polyploidy

Polyploidy

Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...

appears to have enhanced the ability of flowering plants to survive the extinction, probably because the additional copies of the genome such plants possessed allowed them to more readily adapt to the rapidly changing environmental conditions which followed the impact.

Amphibians

There is no evidence of K–T boundary mass extinctions of amphibian

Amphibian

Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, and there is strong evidence that most amphibians survived the event relatively unscathed. Several in-depth studies of salamander

Salamander

Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

genera in fossil beds in Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

show that six of seven genera were unchanged after the event.

Frog

Frog

Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

species appear to have survived into the Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

with few species becoming extinct. However, the fossil record for frog families and genera is uneven. An extensive survey of three genera of frogs in Montana show that they were unaffected by the K–Pg event and survived apparently unchanged. The data show little or no evidence for extinction of amphibian families that bracket the K–Pg event. Amphibian survival resulted from the clade's ability to seek shelter in water or to build burrows in sediments, soil, wood, or beneath rocks.

Non-archosaur reptiles

The two living non-archosaur

Archosaur

Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

ian reptile taxa, testudines (turtles) and lepidosaurs (snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s, lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s, and worm lizards

Amphisbaenia

The Amphisbaenia are a usually legless suborder of squamates closely related to lizards and snakes. As many species possess a pink body coloration and scales arranged in rings, they have a superficial resemblance to earthworms. They are very poorly understood, due to their burrowing lifestyle...

), along with choristoderes (semi-aquatic archosauromorphs which died out in the early Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

), survived through the K–T boundary. Over 80% of Cretaceous turtle species passed through the K–T boundary. Additionally, all six turtle families in existence at the end of the Cretaceous survived into the Tertiary and are represented by current species.

Living lepidosaurs include Rhynchocephalia (tuatara

Tuatara

The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...

s) and Squamata

Squamata

Squamata, or the scaled reptiles, is the largest recent order of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the...

. The Rhynchocephalia were a widespread and relatively successful group of lepidosaurs in the early Mesozoic, but began to decline by the mid-Cretaceous. They are represented today by a single genus located exclusively in New Zealand

Biodiversity of New Zealand

The biodiversity of New Zealand, a large Pacific archipelago, is one of the most unusual on Earth, due to its long isolation from other continental landmasses. Its affinities are derived from Gondwana, from which it separated 82 million years ago, New Caledonia and Lord Howe Island, both of which...

.

The order Squamata, which is represented today by lizards, snakes, and amphisbaenia, radiated into various ecological niches during the Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

and were successful throughout the Cretaceous. They survived through the K–T boundary and are currently the most successful and diverse group of living reptiles with more than 6,000 extant species. No known family of terrestrial squamates became extinct at the boundary, and fossil evidence indicates they did not suffer any significant decline in numbers. Their small size, adaptable metabolism, and ability to move to more favorable habitats were key factors in their survivability during the late Cretaceous and early Paleocene.

Non-archosaurian marine reptiles including mosasaur

Mosasaur

Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s and plesiosaur

Plesiosaur

Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s, giant aquatic reptiles that were the top marine predators, became extinct by the end of the Cretaceous.

Archosaurs

The archosaur

Archosaur

clade includes two living orders, crocodilia

Crocodilia

Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

ns (of which Alligatoridae

Alligatoridae

Alligatoridae is a family of crocodylians that includes alligators and caimans.- True alligators :Alligators proper occur in the fluvial deposits of the age of the Upper Chalk in Europe, where they did not die out until the Pliocene age. The true alligators are now restricted to two species, A...

, Crocodylidae and Gavialidae

Gavialidae

Gavialidae is a family of reptiles within the order Crocodilia. Gavialidae consists of only one surviving species, the gharial , which is native to India. Many extinct species are also known...

are the only surviving families) and dinosaurs (of which birds are the sole surviving members), along with the extinct non-avian dinosaur

Dinosaur

s and pterosaur

Pterosaur

Crocodyliforms

Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to the K–T boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodyliforms inhabited freshwater and terrestrial environments, except for the Dyrosauridae

Dyrosauridae

Dyrosauridae is a family of extinct neosuchian crocodyliforms that lived from the Late Cretaceous to the Eocene. Fossils of this group have been found in almost every continent, specifically Africa, Asia, Europe, North America and South America....

which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–T boundary, the only apparent trend being that no large crocodiles survived. Crocodyliform survivability across the boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments like modern marine crocodile juveniles, which would have helped them survive where other marine reptiles became extinct; freshwater environments were not as strongly affected by the K–Pg event as marine environments.

Pterosaurs

Only one family of pterosaurs, Azhdarchidae

Azhdarchidae

Azhdarchidae is a family of pterosaurs known primarily from the late Cretaceous Period, though an isolated vertebrae apparently from an azhdarchid is known from the early Cretaceous as well...

, was definitely present in the Maastrichtian, and it became extinct at the K–T boundary. These large pterosaurs were the last representatives of a declining group that contained 10 families during the mid-Cretaceous. Smaller pterosaurs became extinct prior to the Maastrichtian during a period that saw a decline in smaller animal species while larger species became more prevalent. While this was occurring, modern birds

Modern birds

Modern birds are the most recent common ancestor of all living birds and all its descendants.Modern birds are characterised by feathers, a beak with no teeth , the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton...

were undergoing diversification and replacing archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches.

Birds

Most paleontologists regard birds as the only surviving dinosaurs (see Origin of birds). However, all non-neornithean birds became extinct, including flourishing groups like enantiornithines

Enantiornithes

Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...

and hesperornithiforms

Hesperornithes

Hesperornithes is an extinct and highly specialized clade of Cretaceous toothed birds. Hesperornithine birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all...

. Several analyses of bird fossils show divergence of species prior to the K–T boundary, and that duck, chicken and ratite

Ratite

A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...

bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing a range of different species provides definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. None of them are known to survive into the Paleogene, and their persistence into the latest Maastrichtian therefore provides strong evidence for a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact. A small fraction of the Cretaceous bird species survived the impact, giving rise to today's birds. So far, only a single bird species, which has not been named, has been confidently identified from both above and below the K-Pg boundary (it is present in the Maastrichtian Hell Creek Formation

Hell Creek Formation

The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...

and Danian Fort Union Formation

Fort Union Formation

The Fort Union Formation is a geologic unit containing sandstones, shales, and coal beds in Wyoming, Montana, and parts of adjacent states. In the Powder River Basin, it contains important economic deposits of coal, uranium, and coalbed methane....

. The only bird group which can confidently be said to have survived the K–T boundary are the Neornithines (though one Paleogene species, Qinornis paleocenica, may represent a surviving non-neornithine bird). Neornithines may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of neornithines can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K–T boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs.

Non-avian dinosaurs

Excluding a few controversial claims, scientists agree that all non-avian dinosaurs became extinct at the K–T boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. Since there is no evidence that late Maastrichtian nonavian dinosaurs could burrow, swim or dive, they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–T boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food as both herbivorous dinosaurs would have found plant material scarce, and carnivores would have quickly found prey to be in short supply.
The growing consensus about the endothermy of dinosaurs (see dinosaur physiology) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic ("cold-blooded") crocodiles have very limited needs for food (they can survive several months without eating) while endothermic ("warm-blooded") animals of similar size need much more food in order to sustain their faster metabolism. Thus, under the circumstances of food chain disruption previously mentioned, non-avian dinosaurs died
while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch.

Whether the extinction occurred gradually or very suddenly is debatable, as both views have support in the fossil record. A study of 29 fossil sites in Catalan Pyrenees

Pyrenees

The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...

of Europe in 2010 support that dinosaurs there had great diversity until the proposed asteroid impact. Others have interpreted the fossil bearing rocks along Red Deer River

Red Deer River

The Red Deer River is a river in Alberta, Canada. It is a major tributary of the South Saskatchewan River.Red Deer River has a total length of and a drainage area of...

in Alberta, Canada, as supporting a gradual extinction of non-avian dinosaurs; during the last 10 million years of the Cretaceous layers there, the number of dinosaur species seems to have decreased from about 45 to about 12. Other scientists have pointed out the same.

Several researchers have argued for Paleocene dinosaurs

Paleocene dinosaurs

Paleocene dinosaurs describe families or genera of non-avian dinosaurs that may have survived the Cretaceous–Tertiary extinction event 65.5 million years ago...

. These arguments are based on the discovery of dinosaur remains in the Hell Creek Formation up to 1.3 metre above and 40,000 years later than the K–T boundary. Pollen samples recovered near a fossilized hadrosaur femur

Femur

The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

recovered in the Ojo Alamo Sandstone at the San Juan River indicate that the animal lived during the Tertiary, approximately 64.5 Ma (about 1 million years after the K–Pg event). If their existence past the K–T boundary can be confirmed, these hadrosaurids would be considered a Dead Clade Walking

Dead Clade Walking

The phrase Dead Clade Walking refers to the fact that some clades of organisms which survived mass extinctions, either become extinct a few million years after the mass extinction or fail to recover in numbers and diversity.-Origin of phrase:...

. Current research indicates that these fossils were eroded from their original locations and then re-buried in much later sediments (reworked).

Mammals

All major Cretaceous mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

ian lineages, including monotreme

Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

s (egg-laying mammals), multituberculates

Multituberculata

The Multituberculata were a group of rodent-like mammals that existed for approximately one hundred and twenty million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene. At least 200 species are...

, marsupial

Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s and placentals

Eutheria

Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...

, dryolestoidea

Dryolestoidea

Dryolestoidea is an extinct clade of Mesozoic mammals that only contains two orders. It has been suggested that this group contained the ancestors of modern therian mammals. They are mostly represented by teeth, fragmented dentaries and parts of the rostrum. The Jurassic forms retained a...

ns, and gondwanatheres

Gondwanatheria

Gondwanatheria is an extinct group of mammals that lived during the Upper Cretaceous through the Eocene in the Southern Hemisphere, including Antarctica...

survived the K–Pg event, although they suffered losses. In particular, marsupials largely disappeared from North America, and the Asian deltatheroida

Deltatheroida

Deltatheroida is an extinct group of basal metatherians that lived in the Cretaceous and were closely related to marsupials. Their fossils are restricted to Central Asia and North America...

ns, primitive relatives of extant marsupials, became extinct. In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven marsupial species are not found above the boundary.

Mammalian species began diversifying approximately 30 million years prior to the K–T boundary. Diversification of mammals stalled across the boundary.
Current research indicates that mammals did not explosively diversify across the K–T boundary, despite the environment niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–T boundary, including Chiroptera (bats) and Cetartiodactyla

Cetartiodactyla

Cetartiodactyla is the clade in which whales and even-toed ungulates have currently been placed. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla reflects the idea that whales evolved within the artiodactyls...

(a diverse group that today includes whales and dolphins and even-toed ungulate

Even-toed ungulate

The even-toed ungulates are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates such as horses....

s), although recent research concludes that only marsupial

Marsupial

orders diversified after the K–T boundary.

K–T boundary mammalian species were generally small, comparable in size to rat

Rat

Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s; this small size would have helped them to find shelter in protected environments. In addition, it is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–T boundary environmental stresses.

North American fossils

In North American terrestrial sequences, the extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian palynomorph

Palynomorph

Palynomorph is the geological term used to describe a particle of a size between five and 500 micrometres, found in rock deposits and composed of organic material such as chitin, pseudochitin and sporopollenin...

record and the post-boundary fern spike.

At present the most informative sequence of dinosaur-bearing rocks in the world from the K–T boundary is found in western North America, particularly the late Maastrichtian

Maastrichtian

The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

-age Hell Creek Formation

Hell Creek Formation

The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...

of Montana, US. This formation, when compared with the older (approximately 75 Ma) Judith River

Judith River Formation

The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the upper Cretaceous, between 80 and 75 million years ago, corresponding to the "Judithian" land vertebrate age...

/Dinosaur Park Formation

Dinosaur Park Formation

The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

s (from Montana and Alberta, Canada, respectively) provides information on the changes in dinosaur populations over the last 10 million years of the Cretaceous. These fossil beds are geographically limited, covering only part of one continent.

The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of several major clades: Tyrannosaurus
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

, Ankylosaurus
Ankylosaurus
Ankylosaurus is a genus of ankylosaurid dinosaur, containing one species, A. magniventris...

, Pachycephalosaurus
Pachycephalosaurus
Pachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few...

, Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

and Torosaurus
Torosaurus
Torosaurus is a genus of ceratopsid dinosaur that lived during the late Cretaceous period , between 70 and 65 million years ago. It possessed one of the largest skulls of any known land animal. The frilled skull reached in length...

, which suggests food was plentiful immediately prior to the extinction.

In addition to rich dinosaur fossils, there are also plant fossils that illustrate the reduction in plant species across the K–T boundary. In the sediments below the K–T boundary the dominant plant remains are angiosperm pollen grains, but the actual boundary layer contains little pollen and is dominated by fern spores. Normal pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by modern volcanic eruptions, where the recovery is led by ferns which are later replaced by larger angiosperm plants.

Marine fossils

The mass extinction of marine plankton appears to have been abrupt and right at the K–T boundary. Ammonite

Ammonite

genera became extinct at or near the K–T boundary; however, there was a smaller and slower extinction of ammonite genera prior to the boundary that was associated with a late Cretaceous marine regression. The gradual extinction of most inoceramid bivalves began well before the K–T boundary, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Further analysis shows that several processes were in progress in the late Cretaceous seas and partially overlapped in time, then ended with the abrupt mass extinction.

Duration

The length of time taken for the extinction to occur is a controversial issue, because some theories about the extinction's causes require a rapid extinction over a relatively short period (from a few years to a few thousand years) while others require longer periods. The issue is difficult to resolve because of the Signor-Lipps effect

Signor-Lipps effect

The Signor–Lipps effect is a paleontological principle proposed by Philip W. Signor and Jere H. Lipps which states that, since the fossil record of organisms is never complete, neither the first nor the last organism in a given taxon will be recorded as a fossil.One famous example is the...

; that is, the fossil record is so incomplete that most extinct species probably died out long after the most recent fossil that has been found. Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K–Pg extinction to a few million years after it.

Causes of extinctions

There have been several hypotheses regarding the cause of the K–T boundary which led to the massive extinction. These hypotheses have centered on either impact events or increased volcanism; some include elements of both. The current overall consensus among paleontologists based on accumulated evidence is that the primary cause for the extinction was an asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

impact

Impact event

which occurred about 65.5 Ma, severely disrupting the Earth's biosphere

Biosphere

The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

Impact event

In 1980, a team of researchers consisting of Nobel prize

Nobel Prize

The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

–winning physicist Luis Alvarez, his son geologist Walter Alvarez

Walter Alvarez

Walter Alvarez is a professor in the Earth and Planetary Science department at the University of California, Berkeley. He is most widely known for the theory that dinosaurs were killed by an asteroid impact, developed in collaboration with his father, Nobel Prize winning physicist Luis...

, and chemists Frank Asaro

Frank Asaro

Frank Asaro is an Emeritus Senior Scientist at the Lawrence Berkeley National Laboratory associated with the University of California at Berkeley...

and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous–Tertiary boundary contain a concentration

Concentration

In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

of iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

many times greater than normal (30 times and 130 times background in the two sections originally studied). Iridium is extremely rare in the earth's crust

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

because it is a siderophile element, and therefore most of it travelled with the iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

as it sank into the earth's core during planetary differentiation

Planetary differentiation

In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

. As iridium remains abundant in most asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, the Alvarez team suggested that an asteroid struck the earth at the time of the K–T boundary. There were other earlier speculations on the possibility of an impact event

Impact event

, but this was the first evidence uncovered.

Such an impact would have inhibited photosynthesis by generating a dust cloud, which would block sunlight for a year or less, and by injecting sulfuric acid

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

aerosol

Aerosol

Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

s into the stratosphere

Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

, which would reduce sunlight reaching the Earth's surface by 10–20%. It would take at least ten years for those aerosols to dissipate, which would account for the extinction of plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s and phytoplankton

Phytoplankton

, and of organisms dependent on them (including predatory animals as well as herbivores). Small creatures whose food chains were based on detritus

Detritus

Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...

would have a reasonable chance of survival. The consequences of reentry of ejecta into Earth's atmosphere would include a brief (hours long) but intense pulse of infrared radiation

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

, killing exposed organisms. Global firestorm

Firestorm

A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most commonly a natural phenomenon, created during some of the largest bushfires, forest fires, and wildfires...

s may have resulted from the heat pulse and the fall back to Earth of incendiary fragments from the blast. The high levels during the late Cretaceous would have supported intense combustion. The level of atmospheric plummeted in the early Tertiary Period. If widespread fires occurred, they would have increased the content of the atmosphere and caused a temporary greenhouse effect

Greenhouse effect

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

once the dust cloud settled, and this would have exterminated the most vulnerable organisms that survived the period immediately after the impact.

The impact may also have produced acid rain

Acid rain

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

, depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor, lasting for approximately 12 years. The acidity was neutralized

Buffer solution

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...

by the environment, and the survival of animals vulnerable to acid rain effects (such as frog

Frog

Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s) indicate this was not a major contributor to extinction. Impact theories can only explain very rapid extinctions, since the dust clouds and possible sulfuric aerosols would wash out of the atmosphere in a fairly short time—possibly under ten years.

Subsequent research identified the Chicxulub Crater buried under Chicxulub

Chicxulub, Yucatán

Chicxulub is a town, and surrounding municipality of the same name, in the Mexican state of Yucatán.At the census of 2005, the town had a population of 5,052 people....

on the coast of Yucatán

Yucatán

Yucatán officially Estado Libre y Soberano de Yucatán is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 106 municipalities and its capital city is Mérida....

, Mexico as the impact crater which matched the Alvarez hypothesis dating. Identified in 1990 based on the work of Glen Penfield done in 1978, this crater is oval, with an average diameter of about 180 kilometres (111.8 mi), about the size calculated by the Alvarez team. The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed in the ocean and would have caused megatsunami

Megatsunami

Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis...

s, for which evidence has been found in several locations in the Caribbean and eastern United States—marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact. The asteroid landed in a bed of gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

(calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

), which would have produced a vast sulfur dioxide aerosol

Aerosol

. This would have further reduced the sunlight reaching the Earth's surface and then precipitated as acid rain, killing vegetation, plankton and organisms which build shells from calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

(coccolithophore

Coccolithophore

Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...

s and molluscs

Mollusca

). In February 2008, a team of researchers used seismic images of the crater to determine that the impactor landed in deeper water than was previously assumed. They argued that this would have resulted in increased sulfate aerosols in the atmosphere, which could have made the impact deadlier by altering climate and by generating acid rain.

Most paleontologists now agree that an asteroid did hit the Earth about 65 Ma ago, but there is an ongoing dispute whether the impact was the sole cause of the extinctions. There is evidence that there was an interval of about 300 ka from the impact to the mass extinction. In 1997, paleontologist Sankar Chatterjee

Sankar Chatterjee

Sankar Chatterjee is a paleontologist, and is the Paul W. Horn Professor of Geosciences at Texas Tech University and Curator of Paleontology at the Museum of Texas Tech University. He earned his Ph. D. from the University of Calcutta in 1970 and was a Post-doctoral Fellow at the Smithsonian...

drew attention to the proposed and much larger 600 km (372.8 mi) Shiva crater

Shiva crater

The Shiva crater is a sea floor structure located beneath the Indian Ocean, west of Mumbai, India. It was named by the paleontologist Sankar Chatterjee after Shiva, the Hindu god of destruction and renewal....

and the possibility of a multiple-impact scenario.

In 2007, a hypothesis was put forth that argued the impactor that killed the dinosaurs 65 Ma belonged to the Baptistina family

Baptistina family

The Baptistina family is an asteroid family that was likely produced by the breakup of an asteroid across 80 million years ago following an impact with a smaller body. The largest presumed remnant of this parent asteroid is 298 Baptistina....

of asteroids. Concerns have been raised regarding the reputed link, in part because very few solid observational constraints exist of the asteroid or family. Indeed, it was recently discovered that 298 Baptistina does not share the same chemical signature as the source of the K–Pg impact. Although this finding may make the link between the Baptistina family and K–Pg impactor more difficult to substantiate, it does not preclude the possibility. A 2011 WISE

Wide-field Infrared Survey Explorer

Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...

study of reflected light from the asteroids of the family estimated the break-up at 80 Ma, giving it insufficient time to shift orbits and impact the Earth by 65 Ma.

In March 2010 an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a 10 – space rock hurtled into earth at Chicxulub

Chicxulub, Yucatán

Chicxulub is a town, and surrounding municipality of the same name, in the Mexican state of Yucatán.At the census of 2005, the town had a population of 5,052 people....

on Mexico's Yucatan Peninsula

Yucatán Peninsula

The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...

. The collision would have released the same energy as 100 TtTNT, over a billion times the energy of the bombs dropped on Nagasaki and Hiroshima).

Deccan Traps

Before 2000, arguments that the Deccan Traps

Deccan Traps

flood basalt

Flood basalt

A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...

s caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 Ma and lasted for over 2 million years. The most recent evidence shows that the traps erupted over 800,000 years spanning the K–T boundary, and therefore may be responsible for the extinction and the delayed biotic recovery thereafter.

The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulfuric aerosols into the air which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the greenhouse effect

Greenhouse effect

when the dust and aerosols cleared from the atmosphere.

In the years when the Deccan Traps hypothesis was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data

Signor-Lipps effect

. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.

Multiple impact event

Several other craters also appear to have been formed about the time of the K–T boundary. This suggests the possibility of near simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the Shoemaker-Levy 9

Comet Shoemaker-Levy 9

Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...

cometary impact with Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. In addition to the 180 km (111.8 mi) Chicxulub Crater

Chicxulub Crater

The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...

, there is the 24 km (14.9 mi) Boltysh crater

Boltysh crater

The Boltysh Crater is an impact crater in the Kirovohrad Oblast province of Ukraine. The crater is in diameter and its age of 65.17 ± 0.64 million years, based on argon dating techniques, is within error of that of Chicxulub Crater in Mexico, and the K–T boundary...

in Ukraine

Ukraine

Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...

, the 20 km (12.4 mi) Silverpit crater

Silverpit crater

Silverpit crater is a buried sub-sea structure under the North Sea off the coast of the United Kingdom. The crater-like form, named after the Silver Pit — a nearby sea-floor valley recognized by generations of fishermen — was discovered during the routine analysis of seismic data collected during...

, a suspected impact crater in the North Sea

North Sea

In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

, and the controversial and much larger 600 km (372.8 mi) Shiva crater

Shiva crater

. Any other craters that might have formed in the Tethys Ocean

Tethys Ocean

The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...

would have been obscured by tectonic events like the relentless northward drift of Africa and India.

Maastrichtian sea-level regression

There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic

Mesozoic

era. In some Maastrichtian

Maastrichtian

stage

Faunal stage

In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...

rock layers from various parts of the world, the later layers are terrestrial; earlier layers represent shorelines and the earliest layers represent seabeds. These layers do not show the tilting and distortion associated with mountain building

Orogeny

Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

, therefore, the likeliest explanation is a "regression", that is, a drop in sea level. There is no direct evidence for the cause of the regression, but the explanation which is currently accepted as the most likely is that the mid-ocean ridges

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

became less active and therefore sank under their own weight.

A severe regression would have greatly reduced the continental shelf

Continental shelf

The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

area, which is the most species-rich part of the sea, and therefore could have been enough to cause a marine mass extinction. However research concludes that this change would have been insufficient to cause the observed level of ammonite extinction. The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth's albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

and therefore increasing global temperatures.

Marine regression also resulted in the loss of epeiric sea

Epeiric Sea

An epeiric sea is a shallow sea that extends over part of a continent.Epeiric seas are usually associated with the marine transgressions of the geologic past, which have variously been due to either global eustatic sea level changes, local tectonic deformation, or both, and are occasionally...

s, such as the Western Interior Seaway

Western Interior Seaway

The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves, Laramidia and Appalachia, during most of the mid- and late-Cretaceous Period...

of North America. The loss of these seas greatly altered habitats, removing coastal plain

Coastal plain

A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...

s that ten million years before had been host to diverse communities such as are found in rocks of the Dinosaur Park Formation

Dinosaur Park Formation

. Another consequence was an expansion of freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

environments, since continental runoff now had longer distances to travel before reaching oceans. While this change was favorable to freshwater

Freshwater

vertebrates, those that prefer marine

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

environments, such as shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, suffered.

Multiple causes

In a review article, J. David Archibald and David E. Fastovsky discussed a scenario combining three major postulated causes: volcanism, marine regression

Marine regression

Marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land....

, and extraterrestrial impact. In this scenario, terrestrial and marine communities were stressed by the changes in and loss of habitats. Dinosaurs, as the largest vertebrates, were the first to be affected by environmental changes, and their diversity declined. At the same time, particulate materials from volcanism cooled and dried areas of the globe. Then, an impact event occurred, causing collapses in photosynthesis-based food chains, both in the already-stressed terrestrial food chains and in the marine food chains. The major difference between this hypothesis and the single-cause hypotheses is that its proponents view the suggested single causes as either not sufficient in strength to cause the extinctions or not likely to produce the taxonomic pattern of the extinction.

External links

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Extinction patterns

Microbiota

Marine invertebrates

Fish

Terrestrial invertebrates

Terrestrial plants

Amphibians

Non-archosaur reptiles

Archosaurs

Crocodyliforms

Pterosaurs

Birds

Non-avian dinosaurs

Mammals

North American fossils

Marine fossils

Duration

Causes of extinctions

Impact event

Deccan Traps

Multiple impact event

Maastrichtian sea-level regression

Multiple causes

See also

External links