Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene 'extinction event, formerly known as the Cretaceous-Tertiary extinction event', was a major mass extinction of three-quarters of the plant and animal species on Earth which occurred approximately 66 million years ago. The event caused the extinction of all non-avian dinosaurs and most other tetrapods weighing more than, with the exception of some ectothermic species such as sea turtles and crocodilians. It marked the end of the Cretaceous period, and with it the Mesozoic era, while heralding the beginning of the current geological era, the Cenozoic Era. In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary or K–T boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows unusually high levels of the metal iridium, which is more common in asteroids than in the Earth's crust.
As originally proposed in 1980 by a team of scientists led by Luis Alvarez and his son Walter, it is now generally thought that the K–Pg extinction resulted from the impact of a massive asteroid wide, creating the Chicxulub impact crater and devastating the global environment 66 million years ago, primarily through a lingering impact winter which halted photosynthesis in plants and plankton. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the Chicxulub crater in the Gulf of Mexico's Yucatán Peninsula in the early 1990s. The timing of the ejecta layer, together with the match between fossil‑record ecological patterns and modeled environmental disruptions, supports the conclusion that the Chicxulub impact triggered the mass extinction. A 2016 drilling project into the Chicxulub peak ring confirmed that the peak ring contained granite ejected within minutes from deep in the Earth, but contained hardly any gypsum, the usual sulfate-containing sea floor rock in the region. The gypsum would have vaporized and dispersed as an aerosol into the atmosphere, causing longer-term effects on the climate and food chain. In October 2019, researchers proposed a mechanism of the mass extinction, arguing that the Chicxulub asteroid impact event rapidly acidified the oceans and produced long-lasting effects on the climate.
Other proposed causal or contributing factors to the extinction have included the Deccan Traps and other volcanic eruptions, climate change, and sea level change. However, in January 2020, scientists reported that climate-modeling of the mass extinction event favored the asteroid impact and not volcanism.
A wide range of terrestrial species perished in the K–Pg mass extinction, the best-known being the non-avian dinosaurs, along with many mammals, birds, lizards, insects, plants, and all of the pterosaurs. The K–Pg mass extinction also killed off plesiosaurs and mosasaurs and devastated teleost fish, sharks, mollusks, and many species of plankton in the oceans. It is estimated that 75% or more of all animal and marine species on Earth vanished. However, the extinction also provided evolutionary opportunities. In its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the following Paleogene Period, evolving new forms such as horses, whales, bats, and primates. The surviving group of dinosaurs were avians, a few species of ground and water fowl, which radiated into all modern species of birds. Among the other groups, teleost fish and perhaps lizards also radiated into their modern species.
Extinction patterns
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. Nevertheless, high latitudes appear to have been less strongly affected than low latitudes.
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.
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. The impact's blockage of sunlight devastated marine food webs which relied on photosynthetic plankton, and “the oceans may have very well returned to a single-celled state that the world had not seen in over half a billion years.” 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, 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. After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches. Evidence from the Salamanca Formation suggests that biotic recovery was more rapid in the Southern Hemisphere than in the Northern Hemisphere.
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. 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 various calcareous 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. Recent studies indicate that there were no major shifts in dinoflagellates through the boundary layer. 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. Approximately 46% of diatom species survived the transition from the Cretaceous to the Upper Paleocene, a significant turnover in species but not a catastrophic extinction.
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; there is ongoing debate between groups which think the evidence indicates substantial extinction of these species at the K–Pg 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. In some areas, such as Texas, benthic foraminifera show no sign of any major extinction event, however. 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.