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140-737: The oldest known sediments are the Eocene sandstones of the Caballas Formation , while the youngest deposits, the fossiliferous Pisco Formation , date to the Early Pleistocene . In relation to present-day, topography the fill of Pisco Basin makes the upper part of the Coastal Cordillera of southern Peru , the coastal plains, the Ica-Nazca Depression and the Andean foothills. The basin

280-520: A paraphyletic group) by therapsids occurred around the Kungurian / Roadian transition, which is often called Olson's extinction (which may be a slow decline over 20 Ma rather than a dramatic, brief event). Another point of view put forward in the Escalation hypothesis predicts that species in ecological niches with more organism-to-organism conflict will be less likely to survive extinctions. This

420-401: A "collection" (such as a time interval) to assess the relative diversity of that collection. Every time a new species (or other taxon ) enters the sample, it brings over all other fossils belonging to that species in the collection (its " share " of the collection). For example, a skewed collection with half its fossils from one species will immediately reach a sample share of 50% if that species

560-466: A "major" extinction event, and the data chosen to measure past diversity. In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five particular geological intervals with excessive diversity loss. They were originally identified as outliers on a general trend of decreasing extinction rates during the Phanerozoic , but as more stringent statistical tests have been applied to

700-425: A Phanerozoic phenomenon, with merely the observable extinction rates appearing low before large complex organisms with hard body parts arose. Extinction occurs at an uneven rate. Based on the fossil record , the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. The Oxygen Catastrophe, which occurred around 2.45 billion years ago in

840-648: A backdrop of decreasing extinction rates through time. Four of these peaks were statistically significant: the Ashgillian ( end-Ordovician ), Late Permian , Norian ( end-Triassic ), and Maastrichtian (end-Cretaceous). The remaining peak was a broad interval of high extinction smeared over the later half of the Devonian , with its apex in the Frasnian stage. Through the 1980s, Raup and Sepkoski continued to elaborate and build upon their extinction and origination data, defining

980-409: A considerable period of time after a mass extinction, and which were reduced to only a few species, are likely to have experienced a rebound effect called the " push of the past ". Darwin was firmly of the opinion that biotic interactions, such as competition for food and space – the 'struggle for existence' – were of considerably greater importance in promoting evolution and extinction than changes in

1120-424: A high-resolution biodiversity curve (the "Sepkoski curve") and successive evolutionary faunas with their own patterns of diversification and extinction. Though these interpretations formed a strong basis for subsequent studies of mass extinctions, Raup and Sepkoski also proposed a more controversial idea in 1984: a 26-million-year periodic pattern to mass extinctions. Two teams of astronomers linked this to

1260-425: A hypothetical brown dwarf in the distant reaches of the solar system, inventing the " Nemesis hypothesis " which has been strongly disputed by other astronomers. Around the same time, Sepkoski began to devise a compendium of marine animal genera , which would allow researchers to explore extinction at a finer taxonomic resolution. He began to publish preliminary results of this in-progress study as early as 1986, in

1400-470: A lack of consensus on Late Triassic chronology For much of the 20th century, the study of mass extinctions was hampered by insufficient data. Mass extinctions, though acknowledged, were considered mysterious exceptions to the prevailing gradualistic view of prehistory, where slow evolutionary trends define faunal changes. The first breakthrough was published in 1980 by a team led by Luis Alvarez , who discovered trace metal evidence for an asteroid impact at

1540-431: A large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes. To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide,

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1680-440: A long-term stress is compounded by a short-term shock. Over the course of the Phanerozoic , individual taxa appear to have become less likely to suffer extinction, which may reflect more robust food webs, as well as fewer extinction-prone species, and other factors such as continental distribution. However, even after accounting for sampling bias, there does appear to be a gradual decrease in extinction and origination rates during

1820-399: A new wave of studies into the dynamics of mass extinctions. These papers utilized the compendium to track origination rates (the rate that new species appear or speciate ) parallel to extinction rates in the context of geological stages or substages. A review and re-analysis of Sepkoski's data by Bambach (2006) identified 18 distinct mass extinction intervals, including 4 large extinctions in

1960-813: A paper which identified 29 extinction intervals of note. By 1992, he also updated his 1982 family compendium, finding minimal changes to the diversity curve despite a decade of new data. In 1996, Sepkoski published another paper which tracked marine genera extinction (in terms of net diversity loss) by stage, similar to his previous work on family extinctions. The paper filtered its sample in three ways: all genera (the entire unfiltered sample size), multiple-interval genera (only those found in more than one stage), and "well-preserved" genera (excluding those from groups with poor or understudied fossil records). Diversity trends in marine animal families were also revised based on his 1992 update. Revived interest in mass extinctions led many other authors to re-evaluate geological events in

2100-503: A role in triggering the ETM2 and ETM3. An enhancement of the biological pump proved effective at sequestering excess carbon during the recovery phases of these hyperthermals. These hyperthermals led to increased perturbations in planktonic and benthic foraminifera , with a higher rate of fluvial sedimentation as a consequence of the warmer temperatures. Unlike the PETM, the lesser hyperthermals of

2240-613: A separate event from the P–T extinction; if so, it would be larger than some of the "Big Five" extinction events.   The End Cretaceous extinction, or the K–Pg extinction (formerly K–T extinction) occurred at the Cretaceous ( Maastrichtian ) – Paleogene ( Danian ) transition. The event was formerly called the Cretaceous-Tertiary or K–T extinction or K–T boundary; it is now officially named

2380-526: A significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions. To test

2520-508: A species' true extinction must occur after its last fossil, and that origination must occur before its first fossil. Thus, species which appear to die out just prior to an abrupt extinction event may instead be a victim of the event, despite an apparent gradual decline looking at the fossil record alone. A model by Foote (2007) found that many geological stages had artificially inflated extinction rates due to Signor-Lipps "backsmearing" from later stages with extinction events. Other biases include

2660-586: A time interval, and sampling time intervals in sequence, can together be combined into equations to predict extinction and origination with less bias. In subsequent papers, Alroy continued to refine his equations to improve lingering issues with precision and unusual samples. McGhee et al. (2013), a paper which primarily focused on ecological effects of mass extinctions, also published new estimates of extinction severity based on Alroy's methods. Many extinctions were significantly more impactful under these new estimates, though some were less prominent. Stanley (2016)

2800-750: A wide variety of climate conditions that includes the warmest climate in the Cenozoic Era , and arguably the warmest time interval since the Permian-Triassic mass extinction and Early Triassic, and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the Paleocene–Eocene Thermal Maximum (PETM) at 56 Ma to a maximum during the Eocene Optimum at around 49 Ma. During this period of time, little to no ice

2940-401: Is also the largest known extinction event for insects . The highly successful marine arthropod, the trilobite , became extinct. The evidence regarding plants is less clear, but new taxa became dominant after the extinction. The "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of early synapsids . The recovery of vertebrates took 30 million years, but

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3080-454: Is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized,

3220-447: Is because the very traits that keep a species numerous and viable under fairly static conditions become a burden once population levels fall among competing organisms during the dynamics of an extinction event. Furthermore, many groups that survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline, and these are often referred to as " Dead Clades Walking ". However, clades that survive for

3360-638: Is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene. Other studies suggest a more modest rise in carbon dioxide levels. The increase in atmospheric carbon dioxide has also been hypothesised to have been driven by increased seafloor spreading rates and metamorphic decarbonation reactions between Australia and Antarctica and increased amounts of volcanism in

3500-556: Is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C . The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event , which may be related to

3640-668: Is renowned for hosting various highly fossiliferous stratigraphic units ; the Pisco Formation has provided a wealth of marine mammals (including sloths), birds, fish and other groups, as have the Chilcatay , Otuma and Paracas Formations . The basin developed in a setting of extensional tectonics from Eocene to the Late Miocene with short-lived episode of basin inversion in the Middle Miocene. Late Pliocene and Pleistocene uplift of

3780-703: Is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 Ma. At the end of the MECO, the MLEC resumed. Cooling and the carbon dioxide drawdown continued through the late Eocene and into the Eocene–Oligocene transition around 34 Ma. The post-MECO cooling brought with it a major aridification trend in Asia, enhanced by retreating seas. A monsoonal climate remained predominant in East Asia. The cooling during

3920-464: Is strong evidence supporting periodicity in a variety of records, and additional evidence in the form of coincident periodic variation in nonbiological geochemical variables such as Strontium isotopes, flood basalts, anoxic events, orogenies, and evaporite deposition. One explanation for this proposed cycle is carbon storage and release by oceanic crust, which exchanges carbon between the atmosphere and mantle. Mass extinctions are thought to result when

4060-451: Is the " Pull of the recent ", the fact that the fossil record (and thus known diversity) generally improves closer to the modern day. This means that biodiversity and abundance for older geological periods may be underestimated from raw data alone. Alroy (2010) attempted to circumvent sample size-related biases in diversity estimates using a method he called " shareholder quorum subsampling" (SQS). In this method, fossils are sampled from

4200-423: Is the first to be sampled. This continues, adding up the sample shares until a "coverage" or " quorum " is reached, referring to a pre-set desired sum of share percentages. At that point, the number of species in the sample are counted. A collection with more species is expected to reach a sample quorum with more species, thus accurately comparing the relative diversity change between two collections without relying on

4340-524: Is the period of time when the Antarctic ice sheet began to rapidly expand. Greenhouse gases, in particular carbon dioxide and methane , played a significant role during the Eocene in controlling the surface temperature. The end of the PETM was met with very large sequestration of carbon dioxide into the forms of methane clathrate , coal , and crude oil at the bottom of the Arctic Ocean , that reduced

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4480-632: Is the second epoch of the Paleogene Period in the modern Cenozoic Era . The name Eocene comes from the Ancient Greek Ἠώς ( Ēṓs , " Dawn ") and καινός ( kainós , "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene

4620-500: The Andean margin contemporary marine transgressions are also known from southern Chile, Patagonia and Colombia. As such the marine transgression is thought to represent a regional phenomenon with the steadily rising central Andes being an exception. Eocene The Eocene ( IPA : / ˈ iː ə s iː n , ˈ iː oʊ -/ EE -ə-seen, EE -oh- ) is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It

4760-579: The Cambrian . These fit Sepkoski's definition of extinction, as short substages with large diversity loss and overall high extinction rates relative to their surroundings. Bambach et al. (2004) considered each of the "Big Five" extinction intervals to have a different pattern in the relationship between origination and extinction trends. Moreover, background extinction rates were broadly variable and could be separated into more severe and less severe time intervals. Background extinctions were least severe relative to

4900-520: The Cambrian explosion , five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and best-known, the Cretaceous–Paleogene extinction event , which occurred approximately 66 Ma (million years ago), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major Phanerozoic mass extinctions, there are numerous lesser ones, and

5040-739: The Laramide Orogeny came to an end in the Eocene, and compression was replaced with crustal extension that ultimately gave rise to the Basin and Range Province . The Kishenehn Basin, around 1.5 km in elevation during the Lutetian, was uplifted to an altitude of 2.5 km by the Priabonian. Huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte . At about 35 Ma, an asteroid impact on

5180-684: The Middle Eocene Climatic Optimum (MECO). At around 41.5 Ma, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. A similar shift in carbon isotopes is known from the Northern Hemisphere in the Scaglia Limestones of Italy. Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. The warming

5320-526: The Paleoproterozoic , is plausible as the first-ever major extinction event. It was perhaps also the worst-ever, in some sense, but with the Earth's ecology just before that time so poorly understood, and the concept of prokaryote genera so different from genera of complex life, that it would be difficult to meaningfully compare it to any of the "Big Five" even if Paleoproterozoic life were better known. Since

5460-406: The amount of oxygen in the Earth's atmosphere more or less doubled. During the warming in the early Eocene between 55 and 52 Ma, there were a series of short-term changes of carbon isotope composition in the ocean. These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of 4–8 °C (7.2–14.4 °F) at the surface of

5600-461: The biodiversity on Earth . Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms . It occurs when the rate of extinction increases with respect to the background extinction rate and the rate of speciation . Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from disagreement as to what constitutes

5740-629: The biosphere rather than the total diversity and abundance of life. For this reason, well-documented extinction events are confined to the Phanerozoic eon – with the sole exception of the Oxygen Catastrophe in the Proterozoic – since before the Phanerozoic, all living organisms were either microbial, or if multicellular then soft-bodied. Perhaps due to the absence of a robust microbial fossil record, mass extinctions might only seem to be mainly

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5880-417: The proxy data . Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to 20 °C (36 °F) colder than

6020-654: The southeast United States . After the Paleocene–Eocene Thermal Maximum, members of the Equoidea arose in North America and Europe, giving rise to some of the earliest equids such as Sifrhippus and basal European equoids such as the palaeothere Hyracotherium . Some of the later equoids were especially species-rich; Palaeotherium , ranging from small to very large in size, is known from as many as 16 species. Established large-sized mammals of

6160-515: The Andes . The marine Oligo - Miocene (25–16 Ma) marine transgression is evidenced by a series of sedimentary strata containing fossils of marine diatoms , Peruchilus snails and Pitar and Cucullaea clams. Oligo-Miocene marine environments in the Pisco Basin range from littoral to shelf . Moquegua Basin southeast of Pisco Basin appear to have been unaffected by the transgression. Within

6300-650: The Azolla Event. This cooling trend at the end of the EECO has also been proposed to have been caused by increased siliceous plankton productivity and marine carbon burial, which also helped draw carbon dioxide out of the atmosphere. Cooling after this event, part of a trend known as the Middle-Late Eocene Cooling (MLEC), continued due to continual decrease in atmospheric carbon dioxide from organic productivity and weathering from mountain building . Many regions of

6440-480: The Cretaceous–Paleogene (or K–Pg) extinction event. About 17% of all families, 50% of all genera and 75% of all species became extinct. In the seas all the ammonites , plesiosaurs and mosasaurs disappeared and the percentage of sessile animals was reduced to about 33%. All non-avian dinosaurs became extinct during that time. The boundary event was severe with a significant amount of variability in

6580-404: The EECO. Relative to present-day values, bottom water temperatures are 10 °C (18 °F) higher according to isotope proxies. With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures. An issue arises, however, when trying to model the Eocene and reproduce the results that are found with

6720-585: The Early Eocene had negligible consequences for terrestrial mammals. These Early Eocene hyperthermals produced a sustained period of extremely hot climate known as the Early Eocene Climatic Optimum (EECO). During the early and middle EECO, the superabundance of the euryhaline dinocyst Homotryblium in New Zealand indicates elevated ocean salinity in the region. One of the unique features of

6860-440: The Earth including the poles. Tropical forests extended across much of modern Africa, South America, Central America, India, South-east Asia and China.  Paratropical forests grew over North America, Europe and Russia, with broad-leafed evergreen and broad-leafed deciduous forests at higher latitudes. Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from

7000-557: The Eocene have been found on Ellesmere Island in the Arctic . Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene also have been found in Greenland and Alaska . Tropical rainforests grew as far north as northern North America and Europe . Palm trees were growing as far north as Alaska and northern Europe during

7140-608: The Eocene include the Uintatherium , Arsinoitherium , and brontotheres , in which the former two, unlike the latter, did not belong to ungulates but groups that became extinct shortly after their establishments. Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like Hyaenodon and Daphoenus (the earliest lineage of a once-successful predatory family known as bear dogs ). Entelodonts meanwhile established themselves as some of

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7280-501: The Eocene is marked by the Paleocene–Eocene Thermal Maximum , a short period of intense warming and ocean acidification brought about by the release of carbon en masse into the atmosphere and ocean systems, which led to a mass extinction of 30–50% of benthic foraminifera (single-celled species which are used as bioindicators of the health of a marine ecosystem)—one of the largest in the Cenozoic. This event happened around 55.8 Ma, and

7420-544: The Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of proxies support the presence of a warmer equable climate being present during this period of time. A few of these proxies include the presence of fossils native to warm climates, such as crocodiles , located in the higher latitudes, the presence in the high latitudes of frost-intolerant flora such as palm trees which cannot survive during sustained freezes, and fossils of snakes found in

7560-491: The Eocene, the continents continued to drift toward their present positions. At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents may have mixed with colder Antarctic waters, distributing the heat around the planet and keeping global temperatures high. When Australia split from the southern continent around 45 Ma, the warm equatorial currents were routed away from Antarctica. An isolated cold water channel developed between

7700-458: The Eocene-Oligocene transition is the timing of the creation of the circulation is uncertain. For Drake Passage , sediments indicate the opening occurred ~41 Ma while tectonics indicate that this occurred ~32 Ma. Solar activity did not change significantly during the greenhouse-icehouse transition across the Eocene-Oligocene boundary. During the early-middle Eocene, forests covered most of

7840-748: The Late Devonian extinction interval ( Givetian , Frasnian, and Famennian stages) to be statistically significant. Regardless, later studies have affirmed the strong ecological impacts of the Kellwasser and Hangenberg Events.   The End Permian extinction or the "Great Dying" occurred at the Permian – Triassic transition. It was the Phanerozoic Eon's largest extinction: 53% of marine families died, 84% of marine genera, about 81% of all marine species and an estimated 70% of terrestrial vertebrate species. This

7980-674: The MECO. Both groups of modern ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like Mesonyx . Early forms of many other modern mammalian orders appeared, including horses (most notably the Eohippus ), bats , proboscidians (elephants), primates, and rodents . Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia , Egypt , and southeast Asia . Marine fauna are best known from South Asia and

8120-663: The Miocene and Pliocene in 1853. After decades of inconsistent usage, the newly formed International Commission on Stratigraphy (ICS), in 1969, standardized stratigraphy based on the prevailing opinions in Europe: the Cenozoic Era subdivided into the Tertiary and Quaternary sub-eras, and the Tertiary subdivided into the Paleogene and Neogene periods. In 1978, the Paleogene was officially defined as

8260-462: The North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered. Another method considered for producing the warm polar temperatures were polar stratospheric clouds . Polar stratospheric clouds are clouds that occur in

8400-415: The PETM event in the sea floor or wetland environments. For contrast, today the carbon dioxide levels are at 400 ppm or 0.04%. During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. Methane has 30 times more of a warming effect than carbon dioxide on a 100-year scale (i.e., methane has a global warming potential of 29.8±11). Most of the methane released to

8540-476: The Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs. In 1989, Tertiary and Quaternary were removed from the time scale due to the arbitrary nature of their boundary, but Quaternary was reinstated in 2009. The Eocene is a dynamic epoch that represents global climatic transitions between two climatic extremes, transitioning from the hot house to the cold house. The beginning of

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8680-430: The Phanerozoic. This may represent the fact that groups with higher turnover rates are more likely to become extinct by chance; or it may be an artefact of taxonomy: families tend to become more speciose, therefore less prone to extinction, over time; and larger taxonomic groups (by definition) appear earlier in geological time. It has also been suggested that the oceans have gradually become more hospitable to life over

8820-561: The Sun, oscillations in the galactic plane, or passage through the Milky Way's spiral arms. However, other authors have concluded that the data on marine mass extinctions do not fit with the idea that mass extinctions are periodic, or that ecosystems gradually build up to a point at which a mass extinction is inevitable. Many of the proposed correlations have been argued to be spurious or lacking statistical significance. Others have argued that there

8960-415: The accumulating data, it has been established that in the current, Phanerozoic Eon, multicellular animal life has experienced at least five major and many minor mass extinctions. The "Big Five" cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of a relatively smooth continuum of extinction events. All of the five in the Phanerozoic Eon were anciently preceded by

9100-405: The actual determined temperature at the poles. This error has been classified as the "equable climate problem". To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below. Due to the nature of water as opposed to land, less temperature variability would be present if

9240-410: The addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data. The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles. With the increased sea surface temperatures and the increased temperature of

9380-422: The amount of polar stratospheric clouds. While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds. It

9520-456: The atmosphere during this period of time would have been from wetlands, swamps, and forests. The atmospheric methane concentration today is 0.000179% or 1.79 ppmv . As a result of the warmer climate and the sea level rise associated with the early Eocene, more wetlands, more forests, and more coal deposits would have been available for methane release. If we compare the early Eocene production of methane to current levels of atmospheric methane,

9660-524: The atmospheric carbon dioxide. This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, and it is hypothesized that the sequestration was mainly due to organic carbon burial and weathering of silicates. For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at

9800-445: The basin may be consequence of the subduction of Nazca Ridge . Sedimentary strata of the basin shows evidence for a series of marine transgressions during the last 50 million years . These marine transgressions occurred in a sequence 41-34 Ma , 31-28 Ma, 25-16 Ma, 15-11 Ma, 10-5 Ma, and 4-2 Ma. The end of most of the marine transgressions is thought to be associated either with global sea level falls or compressional events in

9940-400: The biases inherent to sample size. Alroy also elaborated on three-timer algorithms, which are meant to counteract biases in estimates of extinction and origination rates. A given taxon is a "three-timer" if it can be found before, after, and within a given time interval, and a "two-timer" if it overlaps with a time interval on one side. Counting "three-timers" and "two-timers" on either end of

10080-413: The context of their effects on life. A 1995 paper by Michael Benton tracked extinction and origination rates among both marine and continental (freshwater & terrestrial) families, identifying 22 extinction intervals and no periodic pattern. Overview books by O.H. Walliser (1996) and A. Hallam and P.B. Wignall (1997) summarized the new extinction research of the previous two decades. One chapter in

10220-478: The correlation of extinction and origination rates to diversity. High diversity leads to a persistent increase in extinction rate; low diversity to a persistent increase in origination rate. These presumably ecologically controlled relationships likely amplify smaller perturbations (asteroid impacts, etc.) to produce the global effects observed. A good theory for a particular mass extinction should: It may be necessary to consider combinations of causes. For example,

10360-625: The dawn of recent, or modern, life. Scottish geologist Charles Lyell (ignoring the Quaternary) divided the Tertiary Epoch into the Eocene, Miocene , Pliocene , and New Pliocene ( Holocene ) Periods in 1833. British geologist John Phillips proposed the Cenozoic in 1840 in place of the Tertiary, and Austrian paleontologist Moritz Hörnes introduced the Paleogene for the Eocene and Neogene for

10500-413: The decline into an icehouse climate and the rapid expansion of the Antarctic ice sheet . The transition from a warming climate into a cooling climate began at around 49 Ma. Isotopes of carbon and oxygen indicate a shift to a global cooling climate. The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. One proposed cause of

10640-437: The deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport. While typically seen as a control on ice growth and seasonality,

10780-410: The deep ocean. On top of that, MECO warming caused an increase in the respiration rates of pelagic heterotrophs , leading to a decreased proportion of primary productivity making its way down to the seafloor and causing a corresponding decline in populations of benthic foraminifera. An abrupt decrease in lakewater salinity in western North America occurred during this warming interval. This warming

10920-458: The difficulty in assessing taxa with high turnover rates or restricted occurrences, which cannot be directly assessed due to a lack of fine-scale temporal resolution. Many paleontologists opt to assess diversity trends by randomized sampling and rarefaction of fossil abundances rather than raw temporal range data, in order to account for all of these biases. But that solution is influenced by biases related to sample size. One major bias in particular

11060-411: The early Eocene would have produced triple the amount of methane. The warm temperatures during the early Eocene could have increased methane production rates, and methane that is released into the atmosphere would in turn warm the troposphere, cool the stratosphere, and produce water vapor and carbon dioxide through oxidation. Biogenic production of methane produces carbon dioxide and water vapor along with

11200-888: The early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well. The earliest definitive Eucalyptus fossils were dated from 51.9 Ma, and were found in the Laguna del Hunco deposit in Chubut province in Argentina . Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry, with forests thinning considerably in some areas. The newly evolved grasses were still confined to river banks and lake shores, and had not yet expanded into plains and savannas . The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By

11340-617: The eastern coast of North America formed the Chesapeake Bay impact crater . The Tethys Ocean finally closed with the collision of Africa and Eurasia, while the uplift of the Alps isolated its final remnant, the Mediterranean , and created another shallow sea with island archipelagos to the north. Planktonic foraminifera in the northwestern Peri-Tethys are very similar to those of the Tethys in

11480-492: The effect of reducing the estimated severity of the six sampled mass extinction events. This effect was stronger for mass extinctions which occurred in periods with high rates of background extinction, like the Devonian. Because most diversity and biomass on Earth is microbial , and thus difficult to measure via fossils, extinction events placed on-record are those that affect the easily observed, biologically complex component of

11620-513: The end of the Cretaceous period. The Alvarez hypothesis for the end-Cretaceous extinction gave mass extinctions, and catastrophic explanations, newfound popular and scientific attention. Another landmark study came in 1982, when a paper written by David M. Raup and Jack Sepkoski was published in the journal Science . This paper, originating from a compendium of extinct marine animal families developed by Sepkoski, identified five peaks of marine family extinctions which stand out among

11760-752: The end of the period of pressure. Their statistical analysis of marine extinction rates throughout the Phanerozoic suggested that neither long-term pressure alone nor a catastrophe alone was sufficient to cause a significant increase in the extinction rate. MacLeod (2001) summarized the relationship between mass extinctions and events that are most often cited as causes of mass extinctions, using data from Courtillot, Jaeger & Yang et al. (1996), Hallam (1992) and Grieve & Pesonen (1992): The most commonly suggested causes of mass extinctions are listed below. The formation of large igneous provinces by flood basalt events could have: Flood basalt events occur as pulses of activity punctuated by dormant periods. As

11900-535: The end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America , Africa , India and Australia . Antarctica began the Eocene fringed with a warm temperate to sub-tropical rainforest . Pollen found in Prydz Bay from the Eocene suggest taiga forest existed there. It became much colder as

12040-502: The enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate. The azolla event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Assuming the carbon dioxide concentrations were at 900 ppmv prior to the Azolla Event they would have dropped to 430 ppmv, or 30 ppmv more than they are today, after

12180-466: The enhanced carbon dioxide levels found in the early Eocene. The isolation of the Arctic Ocean, evidenced by euxinia that occurred at this time, led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments on the seabed and effectively sequestered the carbon by locking it out of the atmosphere for good. The ability for the azolla to sequester carbon is exceptional, and

12320-456: The entire Phanerozoic. As data continued to accumulate, some authors began to re-evaluate Sepkoski's sample using methods meant to account for sampling biases . As early as 1982, a paper by Phillip W. Signor and Jere H. Lipps noted that the true sharpness of extinctions was diluted by the incompleteness of the fossil record. This phenomenon, later called the Signor-Lipps effect , notes that

12460-534: The expansion of the ice sheet was the creation of the Antarctic Circumpolar Current . The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic along with creating ocean gyres that result in the upwelling of colder bottom waters. The issue with this hypothesis of the consideration of this being a factor for

12600-473: The extant manatees and dugongs . It is thought that millions of years after the Cretaceous-Paleogene extinction event , brain sizes of mammals now started to increase , "likely driven by a need for greater cognition in increasingly complex environments". Extinction event An extinction event (also known as a mass extinction or biotic crisis ) is a widespread and rapid decrease in

12740-556: The following section. The "Big Five" mass extinctions are bolded. Graphed but not discussed by Sepkoski (1996), considered continuous with the Late Devonian mass extinction At the time considered continuous with the end-Permian mass extinction Includes late Norian time slices Diversity loss of both pulses calculated together Pulses extend over adjacent time slices, calculated separately Considered ecologically significant, but not analyzed directly Excluded due to

12880-429: The former source lists over 60 geological events which could conceivably be considered global extinctions of varying sizes. These texts, and other widely circulated publications in the 1990s, helped to establish the popular image of mass extinctions as a "big five" alongside many smaller extinctions through prehistory. Though Sepkoski died in 1999, his marine genera compendium was formally published in 2002. This prompted

13020-615: The four were given informal early/late substages. Wolfe tentatively deemed the Franklinian as Early Eocene, the Fultonian as Middle Eocene, the Ravenian as Late, and the Kummerian as Early Oligocene. The beginning of the Kummerian was refined by Gregory Retallack et al (2004) as 40 Mya, with a refined end at the Eocene-Oligocene boundary where the younger Angoonian floral stage starts. During

13160-508: The geological record.   The largest extinction was the Kellwasser Event ( Frasnian - Famennian , or F-F, 372 Ma), an extinction event at the end of the Frasnian, about midway through the Late Devonian. This extinction annihilated coral reefs and numerous tropical benthic (seabed-living) animals such as jawless fish, brachiopods , and trilobites . The other major extinction

13300-451: The impact of one or more large bolides in Siberia and in what is now Chesapeake Bay . As with other geologic periods , the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain. The term "Eocene" is derived from Ancient Greek Ἠώς ( Ēṓs ) meaning "Dawn", and καινός kainos meaning "new" or "recent", as the epoch saw

13440-569: The initial stages of the opening of the Drake Passage ~38.5 Ma was not global, as evidenced by an absence of cooling in the North Atlantic. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that

13580-451: The large terrestrial vertebrate niches. The dinosaurs themselves had been beneficiaries of a previous mass extinction, the end-Triassic , which eliminated most of their chief rivals, the crurotarsans . Similarly, within Synapsida , the replacement of taxa that originated in the earliest, Pennsylvanian and Cisuralian evolutionary radiation (often still called " pelycosaurs ", though this is

13720-516: The largest omnivores. The first nimravids , including Dinictis , established themselves as amongst the first feliforms to appear. Their groups became highly successful and continued to live past the Eocene. Basilosaurus is a very well-known Eocene whale , but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in cetaceans occurred. The first sirenians were evolving at this time, and would eventually evolve into

13860-422: The last 500 million years, and thus less vulnerable to mass extinctions, but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable. There is still debate about the causes of all mass extinctions. In general, large extinctions may result when a biosphere under long-term stress undergoes a short-term shock. An underlying mechanism appears to be present in

14000-556: The lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and trap outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II). Methane

14140-607: The lower, the Priabonian Stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene. The Western North American floras of the Eocene were divided into four floral "stages" by Jack Wolfe ( 1968 ) based on work with the Puget Group fossils of King County, Washington . The four stages, Franklinian , Fultonian , Ravenian , and Kummerian covered the Early Eocene through early Oligocene, and three of

14280-430: The marine aspect of the end-Cretaceous extinction appears to have been caused by several processes that partially overlapped in time and may have had different levels of significance in different parts of the world. Arens and West (2006) proposed a "press / pulse" model in which mass extinctions generally require two types of cause: long-term pressure on the eco-system ("press") and a sudden catastrophe ("pulse") towards

14420-434: The mass extinction were global warming , related to volcanism , and anoxia , and not, as considered earlier, cooling and glaciation . However, this is at odds with numerous previous studies, which have indicated global cooling as the primary driver. Most recently, the deposition of volcanic ash has been suggested to be the trigger for reductions in atmospheric carbon dioxide leading to the glaciation and anoxia observed in

14560-610: The maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm , while model simulations suggest a concentration of 1,680 ppm fits best with deep sea, sea surface, and near-surface air temperatures of the time. Other proxies such as pedogenic (soil building) carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years. This large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from

14700-530: The members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat. Rodents were widespread. East Asian rodent faunas declined in diversity when they shifted from ctenodactyloid-dominant to cricetid–dipodid-dominant after

14840-474: The methane, as well as yielding infrared radiation. The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide. At about the beginning of the Eocene Epoch (55.8–33.9 Ma)

14980-557: The middle Lutetian but become completely disparate in the Bartonian, indicating biogeographic separation. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. Eurasia was separated in three different landmasses 50 Ma; Western Europe, Balkanatolia and Asia. About 40 Ma, Balkanatolia and Asia were connected, while Europe

15120-399: The modern mammal orders appear within a brief period during the early Eocene . At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like artiodactyls , perissodactyls , and primates , had features like long, thin legs , feet, and hands capable of grasping, as well as differentiated teeth adapted for chewing. Dwarf forms reigned. All

15260-644: The ocean. Recent analysis of and research into these hyperthermals in the early Eocene has led to hypotheses that the hyperthermals are based on orbital parameters, in particular eccentricity and obliquity. The hyperthermals in the early Eocene, notably the Palaeocene–Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2), and the Eocene Thermal Maximum 3 (ETM3), were analyzed and found that orbital control may have had

15400-426: The old, dominant group and makes way for the new one, a process known as adaptive radiation . For example, mammaliaformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs , but could not compete in the large terrestrial vertebrate niches that dinosaurs monopolized. The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into

15540-400: The ongoing mass extinction caused by human activity is sometimes called the sixth mass extinction . Mass extinctions have sometimes accelerated the evolution of life on Earth . When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the newly dominant group is "superior" to the old but usually because an extinction event eliminates

15680-416: The orbital parameters were theorized as a possible control on continental temperatures and seasonality. Simulating the Eocene by using an ice free planet, eccentricity , obliquity , and precession were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in

15820-558: The origination rate in the middle Ordovician-early Silurian, late Carboniferous-Permian, and Jurassic-recent. This argues that the Late Ordovician, end-Permian, and end-Cretaceous extinctions were statistically significant outliers in biodiversity trends, while the Late Devonian and end-Triassic extinctions occurred in time periods which were already stressed by relatively high extinction and low origination. Computer models run by Foote (2005) determined that abrupt pulses of extinction fit

15960-686: The pattern of prehistoric biodiversity much better than a gradual and continuous background extinction rate with smooth peaks and troughs. This strongly supports the utility of rapid, frequent mass extinctions as a major driver of diversity changes. Pulsed origination events are also supported, though to a lesser degree which is largely dependent on pulsed extinctions. Similarly, Stanley (2007) used extinction and origination data to investigate turnover rates and extinction responses among different evolutionary faunas and taxonomic groups. In contrast to previous authors, his diversity simulations show support for an overall exponential rate of biodiversity growth through

16100-416: The period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra . During the Eocene, plants and marine faunas became quite modern. Many modern bird orders first appeared in the Eocene. The Eocene oceans were warm and teeming with fish and other sea life. The oldest known fossils of most of

16240-474: The physical environment. He expressed this in The Origin of Species : Various authors have suggested that extinction events occurred periodically, every 26 to 30 million years, or that diversity fluctuates episodically about every 62 million years. Various ideas, mostly regarding astronomical influences, attempt to explain the supposed pattern, including the presence of a hypothetical companion star to

16380-438: The polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth

16520-575: The presumed far more extensive mass extinction of microbial life during the Great Oxidation Event (a.k.a. Oxygen Catastrophe) early in the Proterozoic Eon . At the end of the Ediacaran and just before the Cambrian explosion , yet another Proterozoic extinction event (of unknown magnitude) is speculated to have ushered in the Phanerozoic. In May 2020, studies suggested that the causes of

16660-566: The rate of extinction between and among different clades . Mammals , descended from the synapsids , and birds , a side-branch of the theropod dinosaurs, emerged as the two predominant clades of terrestrial tetrapods. Despite the common presentation focusing only on these five events, no measure of extinction shows any definite line separating them from the many other Phanerozoic extinction events that appear only slightly lesser catastrophes; further, using different methods of calculating an extinction's impact can lead to other events featuring in

16800-458: The reduction in carbon dioxide during the warming to cooling transition was the azolla event . With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of azolla , which is a floating aquatic fern, on the Arctic Ocean . The significantly high amounts of carbon dioxide also acted to facilitate azolla blooms across the Arctic Ocean. Compared to current carbon dioxide levels, these azolla grew rapidly in

16940-454: The region. One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the Himalayas ; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that

17080-445: The removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. Another hypothesis still implicates a diminished negative feedback of silicate weathering as a result of continental rocks having become less weatherable during the warm Early and Middle Eocene, allowing volcanically released carbon dioxide to persist in the atmosphere for longer. Yet another explanation hypothesises that MECO warming

17220-588: The rock exposure of Western Europe indicates that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias . Research completed after the seminal 1982 paper (Sepkoski and Raup) has concluded that a sixth mass extinction event due to human activities is currently under way: Extinction events can be tracked by several methods, including geological change, ecological impact, extinction vs. origination ( speciation ) rates, and most commonly diversity loss among taxonomic units. Most early papers used families as

17360-623: The same short time interval. To circumvent this issue, background rates of diversity change (extinction/origination) were estimated for stages or substages without mass extinctions, and then assumed to apply to subsequent stages with mass extinctions. For example, the Santonian and Campanian stages were each used to estimate diversity changes in the Maastrichtian prior to the K-Pg mass extinction. Subtracting background extinctions from extinction tallies had

17500-572: The surface and deep oceans, as inferred from foraminiferal stable oxygen isotope records. The resumption of a long-term gradual cooling trend resulted in a glacial maximum at the late Eocene/early Oligocene boundary. The end of the Eocene was also marked by the Eocene–Oligocene extinction event , also known as the Grande Coupure . The Eocene is conventionally divided into early (56–47.8 Ma), middle (47.8–38 Ma), and late (38–33.9 Ma) subdivisions. The corresponding rocks are referred to as lower, middle, and upper Eocene. The Ypresian Stage constitutes

17640-602: The top five. Fossil records of older events are more difficult to interpret. This is because: It has been suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods. However, statistical analysis shows that this can only account for 50% of the observed pattern, and other evidence such as fungal spikes (geologically rapid increase in fungal abundance) provides reassurance that most widely accepted extinction events are real. A quantification of

17780-399: The tropics that would require much higher average temperatures to sustain them. TEX 86 BAYSPAR measurements indicate extremely high sea surface temperatures of 40 °C (104 °F) to 45 °C (113 °F) at low latitudes, although clumped isotope analyses point to a maximum low latitude sea surface temperature of 36.3 °C (97.3 °F) ± 1.9 °C (35.4 °F) during

17920-524: The two continents. However, modeling results call into question the thermal isolation model for late Eocene cooling, and decreasing carbon dioxide levels in the atmosphere may have been more important. Once the Antarctic region began to cool down, the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north and reinforcing the cooling. The northern supercontinent of Laurasia began to fragment, as Europe , Greenland and North America drifted apart. In western North America,

18060-453: The unit of taxonomy, based on compendiums of marine animal families by Sepkoski (1982, 1992). Later papers by Sepkoski and other authors switched to genera , which are more precise than families and less prone to taxonomic bias or incomplete sampling relative to species. These are several major papers estimating loss or ecological impact from fifteen commonly-discussed extinction events. Different methods used by these papers are described in

18200-487: The vacant niches created the opportunity for archosaurs to become ascendant . In the seas, the percentage of animals that were sessile (unable to move about) dropped from 67% to 50%. The whole late Permian was a difficult time, at least for marine life, even before the P–T boundary extinction. More recent research has indicated that the End-Capitanian extinction event that preceded the "Great Dying" likely constitutes

18340-457: The world became more arid and cold over the course of the stage, such as the Fushun Basin. In East Asia, lake level changes were in sync with global sea level changes over the course of the MLEC. Global cooling continued until there was a major reversal from cooling to warming in the Bartonian. This warming event, signifying a sudden and temporary reversal of the cooling conditions, is known as

18480-528: Was a major step into the icehouse climate. Multiple proxies, such as oxygen isotopes and alkenones , indicate that at the Eocene–Oligocene transition, the atmospheric carbon dioxide concentration had decreased to around 750–800 ppm, approximately twice that of present levels . Along with the decrease of atmospheric carbon dioxide reducing the global temperature, orbital factors in ice creation can be seen with 100,000-year and 400,000-year fluctuations in benthic oxygen isotope records. Another major contribution to

18620-457: Was another paper which attempted to remove two common errors in previous estimates of extinction severity. The first error was the unjustified removal of "singletons", genera unique to only a single time slice. Their removal would mask the influence of groups with high turnover rates or lineages cut short early in their diversification. The second error was the difficulty in distinguishing background extinctions from brief mass extinction events within

18760-470: Was caused by the simultaneous occurrence of minima in both the 400 kyr and 2.4 Myr eccentricity cycles. During the MECO, sea surface temperatures in the Tethys Ocean jumped to 32–36 °C, and Tethyan seawater became more dysoxic. A decline in carbonate accumulation at ocean depths of greater than three kilometres took place synchronously with the peak of the MECO, signifying ocean acidification took place in

18900-465: Was connected 34 Ma. The Fushun Basin contained large, suboxic lakes known as the paleo-Jijuntun Lakes. India collided with Asia , folding to initiate formation of the Himalayas . The incipient subcontinent collided with the Kohistan–Ladakh Arc around 50.2 Ma and with Karakoram around 40.4 Ma, with the final collision between Asia and India occurring ~40 Ma. The Eocene Epoch contained

19040-402: Was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand. The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked

19180-409: Was one of the most significant periods of global change during the Cenozoic. The middle Eocene was characterized by the shift towards a cooler climate at the end of the EECO, around 47.8 Ma, which was briefly interrupted by another warming event called the middle Eocene climatic optimum (MECO). Lasting for about 400,000 years, the MECO was responsible for a globally uniform 4° to 6°C warming of both

19320-401: Was present on Earth with a smaller difference in temperature from the equator to the poles . Because of this the maximum sea level was 150 meters higher than current levels. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene–Oligocene transition at 34 Ma. During this decrease, ice began to reappear at the poles, and the Eocene–Oligocene transition

19460-437: Was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase

19600-506: Was the Hangenberg Event (Devonian-Carboniferous, or D-C, 359 Ma), which brought an end to the Devonian as a whole. This extinction wiped out the armored placoderm fish and nearly led to the extinction of the newly evolved ammonoids . These two closely spaced extinction events collectively eliminated about 19% of all families, 50% of all genera and at least 70% of all species. Sepkoski and Raup (1982) did not initially consider

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