The K–Pg extinction event was global, rapid, and selective, eliminating a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species became extinct. The event appears to have affected all of the continents at the same time. Non-avian
dinosaurs, for example, are seen in the
Maastrichtian of North America,
Europe, Asia,
Africa, South America, and
Antarctica, but are not found from the Cenozoic era anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as
New Mexico,
Alaska,
China, and
New Zealand. Despite the event's severity, there was significant variability in the rate of extinction between and within different
clades. Species that depended on
photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the
solar energy reaching the ground. This plant extinction caused a major reshuffling of the dominant plant groups.
Omnivores,
insectivores, and
carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. Neither strictly
herbivorous nor strictly
carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on
insects,
worms, and
snails, which in turn fed on
detritus (dead plant and animal matter). In
stream and
lake ecosystems, few animal groups became extinct, including large forms like
crocodyliforms and
champsosaurs, because such ecosystems rely less directly on food from living plants, and more on detritus washed in from the land, protecting them from this extinction. Modern crocodilians can live as scavengers and survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous period. Similar, but more complex, patterns have been found in the oceans. Extinction was more severe among animals living in the
water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on
primary production from living
phytoplankton, while animals on the
ocean floor always or sometimes feed on detritus. Consequently,
coccolithophores—vital to the open ocean ecosystem during the late Cretaceous—were nearly eradicated; however, researchers have theorized that surviving
mixotrophic coccolithophores, capable of movement and ingestion of prey particles in addition to photosynthesis, were critical to restoring the algal food web over time.
Coccolithophorids and
mollusks (including
ammonites,
rudists,
freshwater snails, and
mussels), and those organisms whose
food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that
ammonites were the principal prey of
mosasaurs, a group of giant marine
reptiles that became extinct during the K-Pg event. The K–Pg extinction had a profound effect on the
evolution of life on Earth. The elimination of dominant Cretaceous groups allowed other organisms to take their place, causing a remarkable amount of
species diversification during the Paleogene Period. Despite the massive loss of life inferred to have occurred during the extinction, and a number of geologic formations worldwide that span the boundary, only a few fossil sites contain direct evidence of the mass mortality that occurred exactly at the K-Pg boundary. These include the
Tanis site of the
Hell Creek Formation in
North Dakota, USA, which contains a high number of well-preserved fossils that appear to have been buried in a catastrophic flood event likely caused by the impact. Another important site is the
Hornerstown Formation in
New Jersey, USA, which has prominent layer at the K-Pg boundary known as the Main Fossiliferous Layer (MFL). It contains a
mass accumulation of disarticulated vertebrate remains, likely deposited by a catastrophic impact‑related flood.
Microbiota The
K–Pg boundary represents one of the most dramatic turnovers in the
fossil record for
nannoplankton that formed the
calcium deposits for which the Cretaceous is named. The turnover in this group is clearly marked at the species level. Statistical analysis of
marine 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. Major spatial differences existed in calcareous nannoplankton diversity patterns; in the Southern Hemisphere, the extinction was less severe and recovery occurred much faster than in the Northern Hemisphere. Following the extinction, survivor communities dominated for several hundred thousand years. The North Pacific acted as a diversity hotspot from which later nannoplankton communities radiated as they replaced survivor faunas across the globe. The K–Pg boundary record of
dinoflagellates is not so well understood, mainly because only
microbial cysts provide a fossil record, and not all dinoflagellate species have cyst-forming stages, which likely causes diversity to be underestimated. There were blooms of the taxa
Thoracosphaera operculata and
Braarudosphaera bigelowii at the boundary.
Radiolaria have left a geological record since at least the
Ordovician times, and their mineral fossil skeletons can be tracked across the K–Pg 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 as a result of cooling temperatures in the early
Paleocene. The occurrence of
planktonic
foraminifera across the K–Pg boundary has been studied since the 1930s. Research spurred by the possibility of an impact event at the K–Pg boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary; and those who think the evidence supports a gradual extinction through the boundary. There is strong evidence that local conditions heavily influenced diversity changes in planktonic foraminifera. Low and mid-latitude communities of planktonic foraminifera experienced high extinction rates, while high latitude faunas were relatively unaffected. Numerous species of
benthic foraminifera became extinct during the event, presumably because they depend on organic debris for nutrients, while
biomass in the ocean is thought to have decreased. 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 '', an
ammonite from the
Owl Creek Formation (Upper Cretaceous), in Owl Creek near
Ripley, Mississippi. There is significant variation in the fossil record as to the extinction rate of
marine invertebrates across the K–Pg boundary. The apparent rate is influenced by a lack of fossil records, rather than by extinctions. Ostracods that were heavily sexually selected were more vulnerable to extinction, and ostracod sexual dimorphism was significantly rarer following the mass extinction. Among
decapods, extinction patterns were highly heterogeneous and cannot be neatly attributed to any particular factor. Decapods that inhabited the Western Interior Seaway were especially hard-hit, while other regions of the world's oceans were refugia that increased chances of survival into the Palaeocene. Among retroplumid crabs, the genus
Costacopluma was a notable survivor. Approximately 60% of late-Cretaceous
scleractinian coral genera failed to cross the K–Pg 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) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon
symbiosis with photosynthetic
algae, which collapsed due to the events surrounding the K–Pg boundary, but 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–Pg boundary. with the gradual extinction of most inoceramid bivalves beginning well before the K–Pg boundary. Deposit feeders were the most common bivalves in the catastrophe's aftermath. Abundance was not a factor that affected whether a bivalve taxon went extinct, according to evidence from North America. Veneroid bivalves developed deeper burrowing habitats as the recovery from the crisis ensued. bivalves from the Late Cretaceous of the Omani Mountains, United Arab Emirates. Scale bar is 10 mm. Except for
nautiloids (represented by the modern order
Nautilida) and
coleoids (which had already
diverged into modern
octopodes,
squids, and
cuttlefish) all other species of the
molluscan class Cephalopoda became extinct at the K–Pg boundary. These included the ecologically significant
belemnoids, as well as the
ammonoids, a group of highly diverse, numerous, and widely distributed shelled cephalopods. Ammonite genera became extinct at or near the K–Pg boundary; there was a smaller and slower extinction of ammonite genera prior to the boundary associated with a late Cretaceous marine regression, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Approximately 35% of echinoderm genera became extinct at the K–Pg boundary, although
taxa that thrived in low-latitude, shallow-water environments during the late Cretaceous had the highest extinction rate. Mid-latitude, deep-water echinoderms were much less affected at the K–Pg boundary. The pattern of extinction points to habitat loss, specifically the drowning of
carbonate platforms, the shallow-water reefs in existence at that time, by the extinction event. Atelostomatans were affected by the
Lilliput effect.
Terrestrial invertebrates Insect damage to the fossilized leaves of
flowering plants from fourteen sites in North America was used as a proxy for insect diversity across the K–Pg 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. During the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after the extinction event. Studies of the size of the
ichnotaxon Naktodemasis bowni, produced by either cicada nymphs or beetle larvae, over the course of the K-Pg transition show that the
Lilliput effect occurred in terrestrial invertebrates thanks to the extinction event. The extinction event produced major changes in Paleogene insect communities. Many groups of ants were present in the Cretaceous, but in the Eocene ants became dominant and diverse, with larger colonies. Butterflies diversified as well, perhaps to take the place of leaf-eating insects wiped out by the extinction. The advanced mound-building termites,
Termitidae, also appear to have risen in importance.
Fish There are fossil records of
jawed fishes across the K–Pg boundary, which provide good evidence of extinction patterns of these classes of marine vertebrates. While the deep-sea realm was able to remain seemingly unaffected, there was an equal loss between the open marine
apex predators and the
durophagous feeders on the continental shelf. Within
cartilaginous fish, approximately 7 out of the 41 families of
neoselachians (modern
sharks, skates, and rays) disappeared after this event and
batoids (skates and rays) lost nearly all the identifiable species, while more than 90% of
teleost fish (bony fish) families survived. In the Maastrichtian age, 28
shark families and 13 batoid families thrived, of which 25 and 9, respectively, survived the K–T boundary event. Forty-seven of all neoselachian genera cross the K–T boundary, with 85% being sharks. Batoids display with 15%, a comparably low survival rate. Among elasmobranchs, those species that inhabited higher latitudes and lived pelagic lifestyles were more likely to survive, whereas epibenthic lifestyles and durophagy were strongly associated with the likelihood of perishing during the extinction event. There is evidence of a mass extinction of
bony fishes at a fossil site immediately above the K–Pg boundary layer on
Seymour Island near
Antarctica, apparently precipitated by the K–Pg extinction event; the marine and freshwater environments of fishes mitigated the environmental effects of the extinction event. although acanthomorphs diversified rapidly after the extinction. Teleost fish diversified explosively after the mass extinction, filling the niches left vacant by the extinction. Groups appearing in the Paleocene and Eocene epochs include billfish, tunas, eels, and flatfish. Yet there are several species of Maastrichtian amphibian, not included as part of this study, which are unknown from the Paleocene. These include the frog
Theatonius lancensis and the
albanerpetontid Albanerpeton galaktion; therefore, some amphibians do seem to have become extinct at the boundary. The relatively low levels of extinction seen among amphibians probably reflect the low extinction rates seen in freshwater animals.
Reptiles Choristoderes The
choristoderes (a group of semi-aquatic diapsids of uncertain position) survived across the K–Pg boundary The gharial-like choristodere genus
Champsosaurus palatal teeth suggest that there were dietary changes among the various species across the K–Pg event.
Turtles More than 80% of Cretaceous
turtle species passed through the K–Pg boundary. All six turtle families in existence at the end of the Cretaceous survived into the
Paleogene and are represented by living species. Analysis of turtle survivorship in the Hell Creek Formation shows a minimum of 75% of turtle species survived. Following the extinction event, turtle diversity exceeded pre-extinction levels in the Danian of North America, although in South America it remained diminished. European turtles likewise recovered rapidly following the mass extinction.
Lepidosauria The
rhynchocephalians, which were a globally distributed and diverse group of lepidosaurians during the early
Mesozoic, had begun to decline by the mid-Cretaceous, although they remained successful in the Late Cretaceous of southern
South America. They are represented today by a single species, the
tuatara (
Sphenodon punctatus) found in
New Zealand. Outside of New Zealand, one rhynchocephalian is known to have crossed the K-Pg boundary,
Kawasphenodon peligrensis, known from the earliest Paleocene (Danian) of Patagonia. The order
Squamata comprising lizards and snakes first diversified during the Jurassic and continued to diversify throughout the Cretaceous. They are currently the most successful and diverse group of living reptiles, with more than 10,000 extant species. The only major group of terrestrial lizards to go extinct at the end of the Cretaceous were the
polyglyphanodontians, a diverse group of mainly herbivorous lizards known predominantly from the Northern Hemisphere. The
mosasaurs, a diverse group of large predatory marine reptiles, also became extinct. Fossil evidence indicates that squamates generally suffered very heavy losses in the K–Pg event, only recovering 10 million years after it. The extinction of Cretaceous lizards and snakes may have led to the evolution of modern groups such as iguanas, monitor lizards, and boas. Pan-Gekkotans weathered the extinction event well, with multiple lineages likely surviving.
Marine reptiles ∆44/42Ca values indicate that prior to the mass extinction, marine reptiles at the top of food webs were feeding on only one source of calcium, suggesting their populations exhibited heightened vulnerability to extinctions at the terminus of the Cretaceous. Along with the aforementioned mosasaurs,
plesiosaurs, represented by the families
Elasmosauridae and
Polycotylidae, became extinct during the event. The
ichthyosaurs had disappeared from fossil record tens of millions of years prior to the K-Pg extinction event.
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–Pg 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, which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–Pg boundary, the only apparent trend being that no large crocodiles survived. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments as do modern marine
crocodile juveniles, which would have helped them survive where other
marine reptiles became extinct; freshwater environments were not so strongly affected by the K–Pg extinction event as marine environments were. Among the terrestrial clade
Notosuchia, only the family
Sebecidae survived; the exact reasons for this pattern are not known. Sebecids were large terrestrial predators, are known from the Eocene of Europe, and would survive in South America into the Miocene. Tethysuchians radiated explosively after the extinction event.
Pterosaurs Two families of pterosaurs,
Azhdarchidae and
Nyctosauridae, were definitely present in the Maastrichtian, and they likely became extinct at the K–Pg boundary. Several other pterosaur lineages may have been present during the Maastrichtian, such as the
ornithocheirids,
pteranodontids, a possible
tapejarid, a possible
thalassodromid and a basal toothed taxon of uncertain affinities, though they are represented by fragmentary remains that are difficult to assign to any given group. While this was occurring, modern birds were undergoing diversification; traditionally it was thought that they replaced archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches, but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis, and small pterosaurs were present in the Late Cretaceous. At least some niches previously held by birds were reclaimed by pterosaurs prior to the K–Pg event.
Non-avian dinosaurs '' was among the dinosaurs living on Earth before the extinction. Scientists agree that all non-avian dinosaurs became extinct at the K–Pg boundary. There is no evidence that late Maastrichtian non-avian dinosaurs could burrow, swim, or dive, which suggests they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–Pg boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food, as herbivorous dinosaurs would have found plant material scarce and carnivores would have quickly found prey in short supply. Prolonged cold is unlikely to have been a reason for the extinction of non-avian dinosaurs given the adaptations of many dinosaurs to cold environments. Whether the extinction occurred gradually or suddenly has been debated, with both views having support from the fossil record. Interpretations of the dinosaur fossil record have pointed to both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous. It may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. A highly informative sequence of dinosaur-bearing rocks from the K–Pg boundary is found in western North America, particularly the late Maastrichtian-age
Hell Creek Formation of
Montana. Comparison with the older
Judith River Formation (Montana) and
Dinosaur Park Formation (
Alberta), which both date from approximately 75 Ma, 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. A study of 29 fossil sites in Catalan
Pyrenees of Europe in 2010 supports the view that dinosaurs there had great diversity until the asteroid impact, with more than 100 living species. Pollen samples recovered near a fossilized
hadrosaur femur recovered in the
Ojo Alamo Sandstone at the
San Juan River in Colorado, indicate the hadrosaur lived during the Cenozoic, approximately (about 1 million years after the K–Pg extinction event). If their existence past the K–Pg boundary can be confirmed, these hadrosaurids would be considered a
dead clade walking. The scientific consensus is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as
reworked fossils).
Birds Most
paleontologists regard birds as the only surviving dinosaurs (see
Origin of birds). It is thought that all non-avian
theropods became extinct, including then-flourishing groups such as
enantiornithines and
hesperornithiforms. Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken, and
ratite bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing a range of different species provide definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. The absence of these birds in the Paleogene is evidence that a mass extinction of archaic birds took place there, Only a small fraction of ground and water-dwelling Cretaceous bird species survived the impact, giving rise to today's birds. The only bird group known for certain to have survived the K–Pg boundary is the
Aves. The open niche space and relative scarcity of predators following the K-Pg extinction allowed for adaptive radiation of various avian groups.
Ratites, for example, rapidly diversified in the early Paleogene and are believed to have convergently developed flightlessness at least three to six times, often fulfilling the niche space for large herbivores once occupied by non-avian dinosaurs.
Mammals Mammalian species began diversifying approximately 30 million years prior to the K–Pg boundary. Diversification of mammals stalled across the boundary. All major Late Cretaceous mammalian lineages, including
monotremes (egg-laying mammals),
multituberculates,
metatherians (which includes modern marsupials),
eutherians (which includes modern placentals),
meridiolestidans, and
gondwanatheres survived the K–Pg extinction event, although they suffered losses. In particular, metatherians largely disappeared from North America, and the Asian
deltatheroidans became extinct (aside from the lineage leading to
Gurbanodelta). In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven metatherians species are not found above the boundary. A recent study indicates that metatherians suffered the heaviest losses at the K–Pg event, followed by multituberculates, while eutherians recovered the quickest. K–Pg boundary mammalian species were generally small, comparable in size to
rats; this small size would have helped them find shelter in protected environments. 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–Pg boundary environmental stresses. Some research indicates that mammals did not explosively diversify across the K–Pg boundary, despite the ecological niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera (
bats) and Cetartiodactyla (a diverse group that today includes
whales and dolphins and
even-toed ungulates), Also significant, within the mammalian genera, new species were approximately 9.1% larger after the K–Pg boundary. After about 700,000 years, some mammals had reached 50 kilos (110 pounds), a 100-fold increase over the weight of those which survived the extinction. It is thought that body sizes of placental mammalian survivors
evolutionarily increased first, allowing them to fill niches after the extinctions, with
brain sizes increasing later in the
Eocene.
Terrestrial plants Plant fossils illustrate the reduction in plant species across the K–Pg boundary. There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary. In North America, approximately 57% of plant species became extinct. 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. European flora was also less affected, most likely due to its distance from the site of the Chicxulub impact. In northern Alaska and the Anadyr-Koryak region of Russia, the flora was minimally impacted. Another line of evidence of a major floral extinction is that the divergence rate of subviral pathogens (viroids) of angiosperms sharply decreased, which indicates an enormous reduction in the number of flowering plants. However, phylogenetic evidence shows no mass angiosperm extinction. Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of
saprotrophic organisms, such as
fungi, that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and plenty of organic matter to feed on was present. Once the atmosphere cleared photosynthetic organisms returned – initially ferns and other ground-level plants. In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a
fern spike in the geologic record; this same pattern of fern recolonization was observed after the
1980 Mount St. Helens eruption. Just two species of fern appear to have dominated the landscape for centuries after the event. In the sediments below the K–Pg boundary the dominant plant remains are
angiosperm pollen grains, but the boundary layer contains little pollen and is dominated by fern spores. More usual 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. In North American terrestrial sequences, the extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian
pollen record and the post-boundary fern spike. Beyond extinction impacts, the event also caused more general changes of flora such as giving rise to
neotropical rainforest
biomes like the
Amazonia, replacing species composition and structure of local forests
during ~6 million years of recovery to former levels of plant
diversity.
Fungi While it appears that many fungi were wiped out at the K-Pg boundary, there is some evidence that some fungal species thrived in the years after the extinction event. Microfossils from that period indicate a great increase in fungal spores, long before the resumption of plentiful fern spores in the recovery after the impact. Monoporisporites and
hypha are almost exclusive microfossils for a short span during and after the iridium boundary. These
saprophytes would not need sunlight, allowing them to survive during a period when the atmosphere was likely clogged with dust and sulfur aerosols. == Dating ==