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Paleocene

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System/
Period
Series/
Epoch
Stage/
Age
Age (Ma)
Neogene Miocene Aquitanian younger
Paleogene Oligocene Chattian 23.03 27.82
Rupelian 27.82 33.9
Eocene Priabonian 33.9 37.8
Bartonian 37.8 41.2
Lutetian 41.2 47.8
Ypresian 47.8 56.0
Paleocene Thanetian 56.0 59.2
Selandian 59.2 61.6
Danian 61.6 66.0
Cretaceous Upper/
Late
Maastrichtian older
Subdivision of the Paleogene Period
according to the ICS, as of 2017.[1]

The Paleocene, ( /ˈpæliəˌsn, ˈpæ-, -li-/[2]) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name derives from the combining of the Ancient Greek palæo- meaning "old" and the Eocene Epoch (which succeeds the Paleocene), translating to "the old part of the Eocene".

The epoch is bracketed by two major events in Earth's history: the K-Pg extinction event and the Paleocene–Eocene thermal maximum. The K-Pg extinction event, brought on by an asteroid impact and an ensuing impact winter, marked the beginning of the Paleocene and killed off 75% of life on Earth, most famously the non-avian dinosaurs. The end of the epoch was marked by the Paleocene–Eocene thermal maximum, which was a major climatic event wherein nearly carbon was released into the atmosphere and ocean systems en masse, causing a spike in global temperatures and ocean acidification.

The Paleocene continued many geological processes initiated in Mesozoic, and the continents continued moving towards their present positions. The Northern Hemisphere continents were still connected via some land bridges as well as the Southern Hemisphere continents, the Rocky Mountains were being uplifted, the Americas had not yet joined, and the Indian Plate had begun its collision with Asia. In the oceans, the thermohaline circulation probably was much different than it is today, with downwellings occurring in the North Pacific rather than the North Atlantic, and water density was mainly controlled by salinity than temperature.

With a global average temperature of about 24–25 °C (75–77 °F), compared to 14 °C (57 °F) in more recent times, the Earth had a greenhouse climate without permanent ice sheets at the poles. As such, there were forests worldwide–including at the poles–with low species richness in regards to plant life, populated by mainly small creatures which were rapidly evolving to take advantage of the recently-emptied Earth. The extinction event caused a floral and faunal turnover of species, with previously abundant species being replaced by previously uncommon species. Though some animals attained enormous size, most remained rather small, and in the by-and-large absence of large herbivores, the forests grew quite dense. In the seas, ray-finned fish rose to dominate open ocean and reef ecosystems.

Etymology

The word "Paleocene" was first used by French paleobotanist and geologist Wilhelm Philipp Schimper in 1874 while describing deposits near Paris (spelled "Paléocène" in his treatise).[3][4] By this time, Scottish geologist Charles Lyell had already divided the Tertiary into the Eocene, Miocene, Pliocene, and New Pliocene (Holocene) epochs in 1833.[4][5] The term "Eocene" is derived from Ancient Greek eo- eos ἠώς meaning "dawn", and -cene kainos καινός meaning "new" or "recent", as the epoch saw the dawn of recent, or modern, life. The term "Paleocene" is an abbreviation of the Ancient Greek palæo- palaios παλαιός meaning "old", and the word "Eocene", and so means "the old part of the Eocene". The term did not come into broad usage until around 1920.[4]

In North America and mainland Europe, the standard spelling is "Paleocene", whereas it is "Palaeocene" in the UK. However, geologist T. C. R. Pulvertaft has argued that the latter spelling is incorrect because this would imply either a translation of "old recent" or a derivation from "pala" and "Eocene", which would be incorrect because the prefix palæo- uses the ligature æ instead of "a" and "e" individually, so only both characters or neither should be dropped, not just one.[4]

Boundaries and subdivisions

Boundaries

K–Pg boundary recorded in rock (the white stripe in the middle)

The Paleocene is the 10 million year time interval directly after the K–Pg extinction event, which ended the Cretaceous Period and the Mesozoic Era, and started the Cenozoic Era. The Paleocene epoch is divided into three ages: the Danian spanning 66 to 61.6 million years ago (mya), the Selandian spanning 61.6 to 59.2 mya, and the Thanetian spanning 59.2 to 56 mya. It is succeeded by the Eocene.[6]

The K–Pg boundary is clearly defined in the fossil record in numerous places around the world by a high-iridium band, as well as discontinuities with fossil flora and fauna. It is generally thought that a 10 to 15 km (6 to 9 mi) wide asteroid impact, forming the Chicxulub Crater in the Yucatán Peninsula in the Gulf of Mexico, caused a cataclysmic event resulting in the extinction of 75% of all life.[7][8][9]

The Paleocene ended with the Paleocene–Eocene thermal maximum, a period of intense warming and ocean acidification brought about by the release of carbon en masse into the atmosphere and ocean systems,[10] which led to a mass extinction of 30–50% of benthic foraminifera–planktonic creatures which are used as bioindicators of the health of a marine ecosystem–the largest in the last 90 million years.[11] This event happened around 55.8 mya, and was one of the most significant periods of global change during the Cenozoic.[10][12][13]

Stratigraphy

In stratigraphy, a stage is a distinct rock stratum ratified by the International Commission on Stratigraphy (ICS) based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary. The decision to split the Paleocene into three stages was made in 1989.[14]

In 1982, the ICS decided to define the Danian as starting with the K-PG boundary, thus ending the practice of including the Danian in the Cretaceous. In 1991, the GSSP was defined as the well-preserved El Haria Formation near El Kef, Tunisia, 36°09′13″N 8°38′55″E / 36.1537°N 8.6486°E / 36.1537; 8.6486, and the proposal was officially published in 2006.[15]

The sea cliffs of Itzurun beach near the town of Zumaia

Itzurun beach by the Basque town of Zumaia, 43°18′02″N 2°15′34″W / 43.3006°N 2.2594°W / 43.3006; -2.2594, was used to define the Selandian and Thanetian, as the area is a basically continuous early Santonian to early Eocene sea cliff outcropping. The Paleocene section is an essentially complete exposed record 165 m (541 ft) thick mainly composed of alternating hemipelagic sediments deposited at a depth of about 1,000 m (3,300 ft). The Danian deposits are sequestered into the Aitzgorri Limestone Formation, and the Selandian and early Thanetian into the Itzurun Formation. The Itzurun Formation is divided into groups A and B corresponding to the two stages respectively. The two stages were ratified in 2008, and this area was chosen because of its completion, low risk of erosion, proximity to the original areas the stages were defined, accessibility, and the protected status of the area due to its geological significance.[14]

The Selandian was first proposed by Danish geologist Alfred Rosenkrantz in 1924 based on a section of fossil-rich glauconitic marls overlain by gray clay which uncomformably overlies Danian chalk and limestone. The area is now subdivided into the Æbelø Formation, Holmehus Formation, and the Østerrende Clay. The beginning of this stage is defined by the end of the 40 million year carbonate rock deposition from an open ocean environment in the North Sea region. However, the Selandian deposits in this area are directly overlain by the Eocene Fur Formation–the Thanetian was not represented here–and this discontinuity in the deposition record is why the GSSP was moved. Today, the beginning of the Selandian is marked by the appearances of the planktonic Fasciculithus tympaniformis, Neochiastozygus perfectus, and Chiasmolithus edentulus, though some other foraminifera are used by various authors.[14]

The Thanetian was first proposed by Swiss geologist Eugène Renevier in 1873 who included the south England Thanet, Woolwich, and Reading formations. However, in 1880, French geologist Gustave Frédéric Dollfus narrowed the definition to just the Thanet Formation. The Thanetian begins a little after the mid-Paleocene biotic event[14]–a short-lived climatic event caused by an increase in methane[16]recorded at Itzurun as a dark 1 m (3.3 ft) interval from a reduction of calcium carbonate. It begins about 29 m (95 ft) above the base of the Selandian, and is marked by the first appearance of Discoaster and a diversification of Heliolithus, though the best correlation is in terms of geochronology. A chron is when a geomagnetic reversal–when the North and South poles switch polarities–occurs. Chron 1 (C1n) is defined as modern day to about 780,000 years ago, and the n denotes "normal" as in the polarity of today, and an r "reverse" for the opposite polarity.[17] The beginning of the Thanetian is best correlated with the C26r/C26n reversal.[14]

Paleogeography

Paleotectonics

The Laramide orogeny was caused by the subduction of oceanic crust under the North American plate

During the Paleocene, the continents continued to drift toward their present positions.[18] The continents of the Northern Hemisphere (the former components of Laurasia) were sometimes connected via land bridges: Beringia (65.5 and 58 mya) between North America and East Asia, the De Geer route (71–63 mya) between Greenland and Scandinavia, the Thulean route (57 and 55.8 mya) between North America and Western Europe via Greenland, and the Turgai route connecting Europe with Asia (which were separated by the Turgai Strait at this time).[19][20]

The Laramide orogeny, which began in the late Cretaceous, continued to uplift the Rocky Mountains; it ended at the end of the Paleocene.[21] Because of this and a drop in sea levels resulting from tectonic activity, the Western Interior Seaway, which had divided the continent of North America for much of the Cretaceous, had receded.[22]

Between about 60.5 and 54.5 mya, there was heightened volcanic activity–the third largest magmatic event in the last 150 million years–in the North Atlantic region creating the North Atlantic Igneous Province.[23][24] The proto-Iceland hotspot is sometimes cited as being responsible for the initial volcanism, though rifting of already thin crust may have also contributed.[24][25][26] This volcanism may have contributed the opening of the North Atlantic Ocean and seafloor spreading, the divergence of the Greenland Plate from the North American Plate,[27] and, climatically, the thermal maximum by dissociating methane clathrate crystals on the seafloor resulting in the mass release of carbon.[23][28]

North and South America remained separated by the Central American Seaway, though a Panama arc had already formed about 73 mya. The Caribbean Large Igneous Province (now the Caribbean Plate), which had formed from the Galápagos hotspot in the Pacific in the latest Cretaceous, was moving eastward as the North American and South American plates were getting pushed in the opposite direction due to the opening of the Atlantic (strike-slip tectonics).[29][30] This motion would eventually uplift the Isthmus of Panama by 2.6 mya. The Caribbean Plate continued moving until about 50 mya when it reached its current position.[31]

The components of the former southern supercontinent Gondwanaland in the Southern Hemisphere continued to drift apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa was heading north towards Europe, and the Indian subcontinent towards Asia, which would eventually close the Tethys Ocean.[18] The Indian Plate and Eurasian Plates began colliding sometime in the Paleocene or Eocene with uplift (and a land connection) beginning in the Miocene about 24–17 mya. There is evidence that some plants and animals could migrate between India and Asia before the collision, possibly via intermediary island arcs.[32]

Paleoceanography

Today, deep water formation–where, in the thermohaline circulation, warm tropical water becomes colder and saltier at the poles and sinks (downwelling)–occurs in two places: the North Atlantic and the Southern Ocean near the Antarctic Peninsula. However, in the Paleocene, the waterways between the Arctic Ocean and the North Atlantic were somewhat restricted, so North Atlantic Deep Water (NADW) and the Atlantic Meridional Overturning Circulation (AMOC)–which circulates cold water from the Arctic towards the equator–had not yet formed, and so deep water formation probably did not occur in the North Atlantic. The Arctic and Atlantic would not be connected by sufficiently deep waters until the early to middle Eocene.[33] There is evidence of deep water formation in the North Pacific to at least a depth of about 2,900 m (9,500 ft). It is possible that the greenhouse climate shifted precipitation patterns, such that the Southern Hemisphere was wetter than the Northern, or the Southern experienced less evaporation than the Northern. In either case, this would have made the Northern more saline than the Southern, creating a density difference and a downwelling in the North Pacific traveling southward.[34] Deep water formation may have also occurred in the South Atlantic.[35]

It is largely unknown how global currents could have affected global temperature. The formation the Northern Component Waters by Greenland in the Eocene–the predecessor of the AMOC–may have caused an intense warming in the North Hemisphere and cooling in the Southern, as well as an increase in deep water temperatures.[33] In the PETM, it is possible deep water formation occurred in saltier tropical waters and moved polewards, which would increase global surface temperatures by warming the poles.[11][36] Also, Antarctica was still connected to South America and Australia, and, because of this, the Antarctic Circumpolar Current–which traps cold water around the continent and prevents warm equatorial water from entering–had not yet formed, preventing Antarctica from freezing and impacting global climate.[37] If oceanic circulation and deep water temperatures were major contributors to the greenhouse climate, warm coastal upwellings at the poles would have inhibited permanent ice cover.[36]

Conversely, it is possible deep water circulation was not a major contributor to the greenhouse climate, as the elevated global deep water temperatures in the Paleocene may have been too warm for thermohaline circulation as it is today to occur, and deep water temperatures more likely change as a response to global temperature change rather than affecting it. The modern thermohaline circulation may have been a result of the cooling trend in the Eocene, rather than it causing the cooling event.[34][36]

In the Arctic, coastal upwelling may have been largely temperature and wind driven. In summer, the land surface temperature was probably higher than oceanic temperature, and the opposite was true in the winter. This annual cycling is consistent with monsoon seasons in Asia, and, based on this, summer surface water would have had an anti-clockwise (with Earth's rotation) flow, and winter a much weaker clockwise flow. Open-ocean upwelling may have also been possible. Given the restricted nature of waterways between various basins in the polar oceans, and a resultant weak natural inflow and outflow from basin to basin, bathymetry–the shape of the ocean floor–would not have been a very major factor in Arctic circulation.[36]

Climate

Average climate

Global average land (above) and deep sea (below) temperatures throughout the Cenozoic

In general, the Paleocene climate was, much like in the Cretaceous, tropical or subtropical,[18][38][39][40] and the poles were temperate[41] and ice free[42] with an average global temperature of roughly 24–25 °C (75–77 °F).[43] For comparison, the average global temperature for the period between 1951 and 1980 was 14 °C (57 °F).[44]

Global deep water temperatures in the Paleocene likely ranged from 8–12 °C (46–54 °F),[34][36] compared to 0–3 °C (32–37 °F) in modern day.[45] Based on the upper limit, average sea surface temperatures at 60°N and S would have been the same as deep sea temperatures, at 30°N and S about 23 °C (73 °F), and at the equator about 28 °C (82 °F),[36] which is comparable to modern day. Until the mid-Miocene, deep sea and surface water temperatures varied synchronously–as one dipped, so did the other–and, as such, there were probably not very defined thermoclines–layers of water of different temperatures which allow for a large difference in deep water and surface temperatures.[46]

The poles probably had a cool temperate climate; northern Antarctica, Australia, the southern tip of South America, what is now the US and Canada, eastern Siberia, and Europe warm temperate; middle South America, southern and northern Africa, South India, Middle America, and China arid; and northern South America, central Africa, North India, middle Siberia, and what is now the Mediterranean Sea tropical.[47]

Climatic events

The effects of the meteor impact 66 mya, such as the impact winter, and the climate across K-Pg boundary were likely fleeting, and, in regards to climate, conditions reverted to normal without any lasting impact in a short time frame.[48] The freezing temperatures probably reversed after 3 years[49] and returned to normal within decades,[50] sulfuric acid aerosols causing acid rain probably dissipated after 10 years,[51] and dust from the impact blocking out sunlight and inhibiting photosynthesis would have lasted up to a year[52] though potential global wildfires raging for several years would have released more particulates into the atmosphere.[53]

The Dan-C2 Event 65.2 mya in the early Danian spanned about 100,000 years, and was characterized by an increase in carbon, particularly in the deep sea. Since the mid-Maastrichtian, more and more carbon had been sequestered in the deep sea, leading to a trend in increasing deep sea temperatures. The Dan-C2 event may represent a release of this carbon.[54] Around 62.2 mya in the late Danian, there was a warming event and evidence of ocean acidification associated with an increase in carbon; at this time, there was major seafloor spreading in the Atlantic and volcanic activity along the southeast margin of Greenland. The Latest Danian Event for Top Chron C27n Event lasted about 200,000 years and resulted in a 1.6–2.8°C change in temperatures throughout the water column. Though the temperature in the latest Danian varies at about the same magnitude, this event coincides with an increase of carbon.[55]

During the mid-Paleocene biotic event (MPBE) around 59 mya (roughly 50,000 years before the Selandian/Thanetian boundary), the temperature spiked probably due to a mass release of methane hydrate into the atmosphere and ocean systems. Carbon was probably output for 10–11,000 years, and the aftereffects likely subsided around 52–53,000 years later.[56] There is also evidence this occurred again 300,000 years later in the early Thanetian dubbed MPBE-2. Respectively, about 83 and 132 gigatons of methane-derived carbon were ejected into the atmosphere, which suggest a 2–3°C (3.6–5.4°F) rise in temperature, and likely caused heightened seasonality and less stable environmental conditions. It may have also caused an increase of grass in some areas.[16]

The Paleocene–Eocene thermal maximum was an approximate 200,000 year long event where the global average temperature rose by some 5 to 8°C (9 to 14°F),[23] and mid-latitude and polar areas may have exceeded modern tropical temperatures of 24–29 °C (75–84 °F).[57] This was due to an ejection of 2,500–4,500 gigatons of carbon into the atmosphere, most likely caused by the perturbation and release of methane clathrate deposits in the North Atlantic from tectonic activity and resulting increase in bottom water temperatures.[23] The duration of carbon output is controversial, but most likely about 2,500 years.[58] This carbon also interfered with the carbon cycle and caused ocean acidification,[59][60] and potentially altered[35] and slowed down ocean currents, the latter leading to the expansion of bottom water oxygen minimum zones (OMZs).[61] In surface water, OMZs could have also been caused from the formation of strong thermoclines preventing oxygen inflow, and higher temperatures equated to higher productivity leading to higher oxygen usurpation.[62] Further, expanding OMZs could have caused the proliferation of sulfate-reducing microorganisms which create highly toxic hydrogen sulfide H2S as a waste product. During the event, the volume of sulfidic water may have been 10–20% of total ocean volume, in comparison to today's 1%. This may have also caused chemocline upwellings along continents and the dispersal of H2S into the atmosphere.[63]

Flora

Recovery

The strata immediately overlaying the K–Pg extinction event is especially rich in fern fossils. Ferns are often the first species to colonize areas damaged by forest fires, so this "fern spike" may mark the recovery of the biosphere following the impact (which caused blazing fires worldwide).[64][65] In the early Paleocene, there was an increase in herb diversity, and either they were pioneer species and re-colonized the recently-emptied landscape, or they evolved as a response to the increased amount of shade provided in a forested landscape.[66] Lycopods, ferns, and angiosperm shrubs may have been important factors of the Paleocene understory.[67]

In general, the forests of the Paleocene were species-poor, and diversity did not fully recover until the end of the Paleocene.[38][68] For example, the floral diversity of what is now the Holarctic region (comprising most of the Northern Hemisphere) was mainly early members of Ginkgo, Metasequoia, Glyptostrobus, MacGinitiea, Platanus, Carya, Ampelopsis, and Cercidiphyllum.[67] However, patterns in plant recovery varied significantly with latitude, climate, and altitude. For example, what is now Castle Rock featured a rich rainforest only 1.4 million years after the event, probably due to a rain shadow effect causing regular monsoon seasons.[68] Conversely, low plant diversity and a lack of specialization in insects in the Colombian Cerrejón Formation, dated to 58 mya, indicates the ecosystem was still recovering from the extinction event 7 million years later.[69]

There was a major die-off of plant species over the boundary; for example, in the Williston Basin of North Dakota, an estimated 1/3 to 3/5 of plant species went extinct.[70] The extinction event ushered in a floral turnover; for example, the once commonplace Araucariaceae conifers were almost fully replaced by Podocarpaceae conifers, and the once rare Cheirolepidiaceae conifers became the dominant trees in Patagonia.[71][67] However, some plant communities, such as those in eastern North America, were already experiencing an extinction event in the late Maastrichtian, particularly in the 1 million years before the extinction event.[66] The "disaster plants" that refilled the emptied landscape crowded out many Cretaceous plants, and resultantly, many went extinct by the middle-Paleocene.[38]

Restoration of Patagonia during the Danian

Nonetheless, the warm, wet climate supported, worldwide, tropical and subtropical forests mainly populated by conifers and broad-leafed trees.[67][42] In Patagonia, the landscape supported tropical rainforests, cloud rainforests, mangrove forests, swamp forests, savannas, and sclerophyllous forests.[42] The extinction of dinosaurs and megaherbivores may have allowed the forests to grow quite dense,[41] and there is little evidence of wide open plains. Plants evolved several techniques to cope with the high density of these rainforests, such as buttressing to better absorb nutrients and compete with other plants, tall height to reach sunlight, larger diaspore in seeds to provide added nutrition on the dark forest floor, and epiphytism where a plant grows on another plant in response to less space on the forest floor.[67] In the Cerrejón Formation, fossil flora belong to the same families as modern day flora–such as palm trees, legumes, aroids, and malvales–indicating the same flora families have characterized South American rainforests since the Paleocene.[69]

Angiosperms

Flowering plants (angiosperms), which had become dominant among forest taxa by the middle Cretaceous 110–90 mya,[72] continued to develop and proliferate, and along with them coevolved the insects that fed on these plants and pollinated them. Predation by insects was especially high during the thermal maximum.[73] A large number of fruit-bearing plants appeared in the Paleocene in particular, probably to take advantage of the newly evolving birds and mammals for seed dispersal,[74] fill recently emptied niches, and an increase in rainfall.[66]

Fossil Platanus fruit from the Canadian Paskapoo Formation

In what is now the Gulf Coast, angiosperm diversity increased slowly in the early Paleocene, and more rapidly in the middle and late Paleocene. This may have been because the effects of the extinction event were still to some extent felt in the early Paleocene, the early Paleocene may not have had as many open niches, early angiosperms may not have been able to evolve at such an accelerated rate as later angiosperms, low diversity equates to lower evolution rates, or there was not much angiosperm migration into the region in the early Paleocene.[66] Over the extinction event, angiosperms had a higher extinction rate than gymnosperms (which include conifers, cycads, and relatives) and pteridophytes (ferns, horsetails, and relatives); zoophilous angiosperms (those that relied on animals for pollination) had a higher rate than anemophilous angiosperms; and evergreen angiosperms had a higher rate than deciduous angiosperms as deciduous plants can become dormant in harsh conditions.[66]

In the Gulf Coast, angiosperms experienced another extinction event during the thermal maximum, which they recovered quickly from in the Eocene from immigrants from the Caribbean and Europe. During this time, the climate became warmer and wetter, and it is possible that angiosperms evolved to become stenotopic by this time, able to inhabit a narrow range of temperature and moisture; or, since the dominant floral ecosystem was a highly integrated and complex closed-canopy rainforest by the middle Paleocene, the plant ecosystems were more vulnerable to climate change.[66] There is some evidence that, in the Gulf Coast, there was an extinction event in the late Paleocene preceding the thermal maximum, which may have been due to the aforementioned vulnerability of complex rainforests, and the ecosystem may have been disrupted by only a small change in climate.[75]

Polar forests

The warm Paleocene climate, much like in the Cretaceous, allowed for diverse polar forests. Whereas precipitation is a major factor in plant diversity nearer the equator, polar plants had to adapt to varying light availability (polar nights and midnight suns) and temperatures. Because of this, plants from both poles independently evolved some similar characteristics, such as broad leaves. Plant diversity at both poles increased over the course of the Paleocene, especially at the end, in tandem with the increasing global temperature.[76]

At the North Pole, woody angiosperms had become the dominant plant, a reversal from the Cretaceous where herbs proliferated. The Iceberg Bay Formation on Ellesmere Island, Nunavut (latitude 7580° N) shows remains of a late Paleocene redwood forest, with the canopy reaching around 32 m (105 ft), and a climate similar to the Pacific Northwest.[41] On the Alaska North Slope, Metasequoia was the dominant conifer. Much of the diversity represented migrants from nearer the equator. Deciduousness was dominant, probably to conserve energy by retroactively shedding leaves and retaining some energy rather than having them die from frostbite.[76]

At the South Pole, due to the increasing isolation of Antarctica, many plant taxa were endemic to the continent instead of migrating down. In fact, Patagonian flora may have originated in Antarctica.[76][77] The climate was much cooler than in the late Cretaceous, though frost probably was not common in at least coastal areas. East Antarctica was likely warm and humid. Because of this, evergreen forests could proliferate as, in the absence of frost and a low probability of leaves dying, it was more energy efficient to retain leaves than to regrow them every year. It is possible the interior of the continent favored deciduous trees, though prevailing continental climates may have produced winters warm enough to support evergreen forests. Like in the Cretaceous, southern beeches, Podocarpaceous conifers, Nothofagus, and Proteaceae angiosperms were proliferous.[76]

Fauna

After the extinction event, every land animal over 25 kg (55 lb) vanished, leaving open several niches at the beginning of this epoch.[78]

Mammals

Restoration of the herbivorous late Paleocene pantodont Barylambda, which could have weighed up to 650 kg (1,430 lb)[79]

Mammals had first appeared in the Late Triassic, and remained small, nocturnal, and largely insectivorous throughout the Mesozoic to avoid competition with dinosaurs (nocturnal bottleneck). Though mammals could sporadically venture out in daytime roughly 10 million years before the extinction event (cathemerality), they only became strictly diurnal (active in the daytime) sometime after. Further, cathermerality may have evolved due to a decline in dinosaurs preceding the extinction event.[80] However, the largest known Mesozoic mammal, Repenomamus robustus, which reached about 1 m (3 ft 3 in) in length and 12–14 kg (26–31 lb) in weight–comparable to the modern day Virginia opossum–may have operated on the same trophic level as some small dinosaurs.[81] 

In general, Paleocene mammals retained this small size until near the end of the epoch, and, consequently, early mammal bones are not well preserved in the fossil record, and most of what we know comes from fossil teeth.[18] Multituberculates, a now-extinct rodent-like group not closely related to any modern mammal, were the most successful group of mammals in the Mesozoic, and they reached peak diversity in the early Paleocene. The early Paleocene Taeniolabis had the most complex dental makeup of any multituberculate, and dental complexity correlates to a broader range in diet. Multituberculates declined in the late Paleocene and went extinct at the end of the Eocene, probably due to competition from newly evolving rodents.[82]

Nonetheless, following the extinction event, mammals very quickly diversified and filled the empty niches.[83][84] Modern mammals are subdivided into therians (placentals and marsupials) and monotremes. The first placentals and marsupials evolved in the Paleocene.[85] Paleocene marsupials include Peradectes,[86] and monotremes Obdurodon sudamericanum[87] and Monotrematum.[88] The epoch featured the rise of many crown placental groups, such as the earliest afrotherian Ocepeia, xenarthran Utaetus, rodent Tribosphenomys and Paramys, early primates the Plesiadapiformes, earliest carnivorans Ravenictis and Pristinictis, possible pangolins Palaeanodonta, possible forerunners of odd-toed ungulates Phenacodontidae, and eulipotyphlans Nyctitheriidae.[89] Though mammals had probably already begun to diversify around 10 to 20 mya before the extinction event, average mammal size increased greatly after the boundary, and a radiation into frugivory and omnivory began, namely with the newly evolving large herbivores such as the Taeniodonta, Tillodonta, Pantodonta, Polydolopimorphia, and the Dinocerata.[85][90] Large carnivores include the wolf-like Mesonychia, such as Ankalagon[91] and Sinonyx.[92]

However, though there was an explosive diversification, the affinities of most Paleocene mammals is unknown, and only primates, carnivorans, and rodents have unambiguous Paleocene origins, resulting in a 10 million year gap in the fossil record of other mammalian crown orders[89] which do not appear until after the PETM and the subsequent retreat of forests. Some attribute this to the idea that mammals did not achieve great size until the proliferation of grasslands, as grass, being harder to digest than leaves, caused an increase in herbivore size, which led to an increase in predator size.[93][94][95][96] The largest order of Paleocene mammals is Condylarthra, which is a wastebasket taxon for miscellaneous bunodont hoofed mammals. Other ambiguous orders include the Leptictida, Cimolesta, and Creodonta. This uncertainty blurs the early evolution of placentals.[89]

Birds

Gastornis restoration

According to DNA studies, modern birds (Neornithes) rapidly diversified following the extinction of the dinosaurs in the Paleocene, and nearly all modern bird lineages can trace their origins to this epoch with the exception of fowl and the paleognaths. This was one of the fastest diversifications of any group,[97] probably fueled by the diversification of fruit-bearing trees and associated insects, and the modern bird groups had likely already diverged within 4 million years of the extinction event. However, the fossil record of birds in the Paleocene is rather poor compared to other groups, limited globally to mainly waterbirds such as the early penguin Waimanu. The earliest arboreal crown group bird known is Tsidiiyazhi abini, a mousebird dating to around 62 mya.[98] The fossil record also records early owls such as the large Berruornis from France,[99] and the smaller Ogygoptynx from the United States.[100]

Conversely, almost all archaic birds (any bird outside Neornithes) went extinct during the extinction event; however, the archaic Qinornis is recorded in the Paleocene.[98] Their extinction may have led to the proliferation of neornithine birds in the Paleocene, and the only known Cretaceous neornithine bird is the waterbird Vegavis, and possibly also the waterbird Teviornis.[101]

In the Mesozoic, birds and pterosaurs exhibited size-related niche partitioning–no known Late Cretaceous bird has a wingspan greater than 2 m (6 ft 7 in) nor exceeded a weight of 5 kg (11 lb), whereas contemporary pterosaurs ranged from 2–10 m (6 ft 7 in–32 ft 10 in), probably to avoid competition. Their extinction allowed flying birds to attain greater size, such as pelagornithids and pelecaniformes.[102] Some bird species reached gigantic proportions, namely on the archipelago-continent of Europe with the flightless bird Gastornis, which was the largest herbivore at 2 m (6 ft 7 in) tall for the largest species, possibly due to lack of competition from newly emerging large mammalian herbivores which were prevalent on the other continents.[78][103] The carnivorous terror birds in South America have a contentious appearance in the Paleocene with Paleopsilopterus, though the first definitive appearance is in the Eocene.[104]

Reptiles

It is generally believed all non-avian dinosaurs went extinct at the K–Pg extinction event 66 mya, though there are a couple controversial claims of Paleocene dinosaurs which would indicate a gradual decline of dinosaurs. Contentious dates include remains from the Hell Creek Formation dated 40,000 years after the boundary,[105] and a hadrosaur femur from the San Juan Basin dated to 64.5 mya,[106] but such stray late forms may be zombie taxon that were washed out and moved to younger sediments.[107]

In the wake of the extinction event, 83% of lizard and snake (squamate) species went extinct, and the diversity did not fully recover until the end of the Paleocene. However, since the only major squamate lineage to disappear in the event was the mosasaurs, and most major squamate groups had evolved by the Cretaceous, the event probably did not greatly affect squamate evolution, and newly evolving squamates did not seemingly branch out into new niches as mammals, that is, Cretaceous and Paleogene squamates filled the same niches. However, there was a faunal turnover of squamates, and groups that were dominant by the Eocene were not as abundant in the Cretaceous, namely the anguids, iguanas, night lizards, pythons, colubrids, boas, and worm lizards.[108] Further, the late Paleocene snake Titanoboa grew to over 13 m (43 ft) long, the longest snake ever recorded.[109]

Freshwater crocodiles and choristoderans were among the aquatic reptiles to have survived the extinction event, probably because freshwater environments were not as impacted as marine ones.[110] One example of a Paleocene crocodile is Borealosuchus, which averaged 3.7 m (12 ft) in length at the Wannagan Creek site.[111] Two choristoderans are known from the Paleocene: Champsosaurus–the largest is the Paleocene C. gigas at 3 m (9.8 ft)–and Simoedosaurus–the largest specimen measuring 5 m (16 ft). Choristodera went extinct in the Miocene.[112]

Turtles experienced a decline in the Campanian (Late Cretaceous) during a cooling event, and recovered during the thermal maximum at the end of the Paleocene.[113] Turtles were not greatly affected by the extinction event, and around 80% of species survived.[114] In Colombia, a 60 million year old turtle with a 1.7 m (5 ft 7 in) carapace, Carbonemys, was discovered.[115]

Insects

The ant Napakimyrma paskapooensis from the Canadian Paskapoo Formation

Insect recovery varied from place to place. For example, it may have taken until the thermal maximum for insect diversity to recover in the western interior of North America, whereas Patagonian insect diversity had recovered by 4 million years after the extinction event. In some areas, such as the Bighorn Basin in Wyoming, there is a dramatic increase in plant predation in the area during the thermal maximum. However, this is probably not indicative of a diversification event in insects due to rising temperatures because plant predation decreases following the thermal maximum. More likely, insects followed their host plant or plants which were expanding into mid-latitude regions during the thermal maximum, and then retreated afterwards.[73][116]

Though there is a gap in the ant fossil record from 78–55 mya,[117] the great fossil diversity of the modern dominant ant subfamilies–Ponerinae, Myrmicinae, Formicinae, and Dolichoderinae–in the Eocene and their rarity in the Cretaceous indicates an explosive diversification of modern ants in the Paleocene. Ponerines probably diversified first, and then myrmicines towards the late Paleocene, and the other two in the Eocene. The former two acted as major hunters of various arthropods, and probably competed with each other for food and nesting grounds in the dense angiosperm leaf litter. Ants are not as successful in gymnosperm leaf litter. With the exception of Ponerinae, ants in these forests were able to expand their diets to seeds and able to form trophobiotic symbiotic relationships with aphids, mealybugs, treehoppers, and various other honeydew secreting insects which were also successful in angiosperm forests. This allowed those subfamilies to invade other biomes, such as the canopy or temperate environments, and achieve a worldwide distribution by the middle Eocene. Some Paleocene genera still exist today, such as Aneuretus.[118] The only known Paleocene ant fossil is the aneuretine Napakimyrma paskapooensis from the 62–56 million year old Paskapoo Formation.[117]

Amphibians

There is little evidence amphibians were affected very much by the extinction event, probably because the freshwater habitats they inhabited were not as greatly impacted as marine environments.[119] In the Hell Creek Formation of eastern Montana, a 1990 study found no extinction in amphibian species across the boundary.[120] Some modern day families have their origins in the Paleocene, such as the true toads.[121]

Sea life

Otodus obliquus shark tooth

Among marine invertebrates, plankton and those with a planktonic stage in their development (meroplankton) were most impacted by the extinction event, and plankton populations crashed. Nearly 90% of all calcifying plankton species perished.[122] This reverberated up and caused a global marine food chain collapse. For a time, it is possible the mass extinction of these creatures–including the once abundant ammonites, Exogyra oysters, and even vertebrates such as mosasaurs–would have provided food for detritovores, but infaunal molluscan filter feeders relying heavily on high plankton populations would have been temporarily replaced by epifaunal brachiopods during the early Paleocene while the ecosystem was recovering.[123] Marine invertebrate diversity may have taken about 7 million years to recover, though this may be a preservation artifact as anything smaller than 5 mm (0.20 in) is unlikely to be fossilized, and body size may have simply decreased across the boundary.[124] A 2019 study found that in Seymour Island, Antarctica, the marine life assemblage consisted primarily of burrowing creatures–such as burrowing clams and snails–for around 320,000 years after the K–Pg extinction event, and it took around a million years for the marine diversity to return to previous levels. Areas closer to the equator may have been more affected.[48] Sand dollars first evolved in the late Paleocene.[125]

In the Cretaceous, the main reef building creatures were the box-like bivalve rudists instead of coral–though a diverse Cretaceous coral assemblage did exist–and rudists had collapsed by the time of the extinction event. Some corals are known to have survived in higher latitudes in the Late Cretaceous and into the Paleogene, and hard coral-dominated reefs may have recovered by 8 million years after the extinction event, though the coral fossil record of this time is rather sparse.[126] Though there was a lack of extensive coral reefs, there were some colonies mainly dominated by zooxanthellate corals in shallow coastal (neritic) areas. Starting in the latest Cretaceous and continuing until the early Eocene, calcareous corals rapidly diversified. Corals probably competed mainly with red and coralline algae for space on the seafloor. Calcified dasycladalean green algae experienced the greatest diversity in their evolutionary history in the Paleocene.[127]

The early Paleocene trumpetfish Eekaulostomus from Palenque, Mexico

However, the small pelagic fish population recovered rather quickly, and there was a low extinction rate for sharks and rays. Overall, only 12% of fishes went extinct.[122] During the Cretaceous, fishes were not very abundant probably due to heightened predation by or competition with ammonites and squid–however, large predatory fish nonetheless existed, such as ichthyodectids, pachycormids, and pachyrhizodontids.[128] Almost immediately following the extinction event, ray-finned fish–today, representing nearly half of all vertebrate life–became much more numerous and increased in size, and rose to dominate the open-oceans. Acanthomorphs–a group of ray-finned fish fish which, today, represent a third of all vertebrate life–experienced a massive diversification following the extinction event, dominating marine ecosystems by the end of the Paleocene, refilling vacant, open-ocean predatory niches as well as spreading out into recovering reef systems. In specific, percomorphs diversified faster than any other vertebrate group at the time, with the exception of birds; Cretaceous percomorphs varied very little in body plan, whereas, by the Eocene, percomorphs evolved into vastly varying creatures[129] such as early scombrids (today, tuna, mackerels, and bonitos),[128] barracudas,[130] jacks,[129] billfish,[131] pipefish,[132] flatfishes,[133] and aulostomoid (trumpetfish and cornetfish).[134][129][135] However, the discovery of the Cretaceous cusk eel Pastorius methenyi indicates that the body plans of at least some percomorphs were already highly variable, perhaps indicating an already diverse array of percomorph body plans before the Paleocene.[136]

Conversely, sharks and rays appear to have been unable to exploit the vacant niches, and recovered the same pre-extinction abundance.[122][137] However, there was a faunal turnover of sharks from mackeral sharks to ground sharks, as ground sharks were more suited to hunting the rapidly diversifying ray-finned fish whereas mackeral sharks target larger prey.[138] The first megatoothed shark, Otodus obliquus–the ancestor of the giant megalodon–is recorded from the Paleocene.[139]

See also

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External links

  • Paleocene Mammals
  • BBC Changing Worlds: Paleocene
  • Maryland Paleocene Fossils
  • Paleos: Paleocene
  • Paleocene Microfossils: 35+ images of Foraminifera
  • Petrified Wood Museum Palaeocene introduction
  • Smithsonian Paleocene Introduction
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