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 extinction event, brought on by an asteroid impact and an ensuing impact winter, killed off 75% of life on Earth, most famously the non-avian dinosaurs. The thermal maximum was a major climatic event wherein nearly 10,000 gigatons of carbon was released into the atmosphere, causing a spike in global temperatures and ocean acidification.

The Paleocene was characterized by tropical and subtropical forests with low species richness in regards to plant life, populated by small mammals, birds, and reptiles which were rapidly evolving to take advantage of the recently-emptied Earth. The extinction event caused a faunal turnover of species, with previously abundant species being replaced by previously uncommon species. Further, some reached enormous sizes, such as the mammal Barylambda at 650 kg (1,430 lb), the flightless bird Gastornis at 2 m (6 ft 7 in) standing, and the snake Titanboa at 13 m (43 ft) in length. The poles featured a temperate climate and also supported forests. In the seas, ray-finned fish rose to dominate open ocean and reef ecosystems.

Etymology[edit]

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 can be dropped, not just one.[4]

Boundaries and subdivisions[edit]

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 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 (6.2 to 9.3 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 ends with the Paleocene–Eocene thermal maximum, a period of intense warming and ocean acidification brought about by the release of 10,000 gigatons of carbon into the atmosphere over the course of about 5,000 years, which led to a mass extinction in benthic foraminifera–planktonic creatures which are used as bioindicators of the health of a marine ecosystem. This event happened around 55.8 mya, and was one of the most significant periods of global change during the Cenozoic.[10][11][12]

Paleogeography[edit]

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

In many ways, the Paleocene continued geological processes that had begun during the Late Cretaceous. During the Paleocene, the continents continued to drift toward their present positions. The supercontinent Laurasia had not yet fully separated into three continents–Europe and Greenland were still connected, North America and Asia were still intermittently joined by the land bridge Beringia, but Greenland and North America were beginning to separate.[13]

North and South America remained separated by equatorial seas (they joined during the Neogene). The components of the former southern supercontinent Gondwanaland continued to drift apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa was heading north towards Europe, slowly closing the Tethys Ocean, and India began its migration to Asia which would later lead to the formation of the Himalayas upon collision.[13]

The Laramide orogeny, which began in the late Cretaceous, continued to uplift the Rocky Mountains; it ended at the end of the Paleocene.[14] 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.[15]

Antarctica was still connected to South America and Australia. 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, and Antarctica would not begin to freeze until the Oligocene.[16]

Climate[edit]

A graph showing the global average temperature throughout the Cenozoic

The effects of the meteor impact 66 mya, such as the impact winter, were likely fleeting, and, in regards to climate, conditions reverted to normal without any lasting impact in a short time frame.[17] The freezing temperatures probably reversed after 3 years[18] and returned to normal within decades,[19] sulfuric acid aerosols causing acid rain probably dissipated after 10 years,[20] and dust from the impact blocking out sunlight and inhibiting photosynthesis would have lasted up to a year[21] though potential global wildfires raging for several years would have released more particulates into the atmosphere.[22]

In general, the Paleocene climate was, much like in the Cretaceous, tropical or subtropical,[13][23][24][25] and the poles were temperate[26] and ice free[27] with an average global temperature of roughly 24–25 °C (75–77 °F).[28] For comparison, the average global temperature for the period between 1951 and 1980 was 14 °C (57 °F).[29] 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.[30]

At the Paleocene–Eocene thermal maximum, the marine average temperature rose by some 5 to 8°C (9 to 14°F) due to an increase in carbon ejected into the atmosphere, and this carbon also interfered with the carbon cycle and caused ocean acidification for the next 200,000 years.[31][32]

Flora[edit]

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).[33][34]

In general, the forests of the Paleocene were species-poor, and diversity did not fully recover until the end of the Paleocene.[23][35] 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.[35] Though there was not a major die-off of plant genera over the boundary,[36] 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.[37] The "disaster plants" that refilled the emptied landscape crowded out many Cretaceous plants, and resultantly, many went extinct by the middle-Paleocene.[23]

Restoration of Patagonia during the Danian

Nonetheless, the warm temperatures supported, worldwide, tropical and subtropical forests mainly populated by conifers, and, nearer to the poles, broad-leaved dicots.[27] The extinction of dinosaurs and megaherbivores may have allowed the forests to grow quite dense. The warm Paleocene climate, much like in the Cretaceous, allowed for polar forests; 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.[26] In Patagonia, the landscape supported tropical rainforests, cloud rainforests, mangrove forests, swamp forests, savannas, and sclerophyllous forests.[27]

Flowering plants (angiosperms), first seen in the Cretaceous, 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.[38] 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.[39]

Fauna[edit]

After the extinction event, every land animal over 25 kg (55 lb) vanished, leaving open several niches at the beginning of this epoch.[40]

Mammals[edit]

Restoration of the herbivorous late Paleocene pantodont Barylambda, which could have weighed up to 650 kg (1,430 lb)[41]

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.[42] 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.[43] 

Following the extinction event, mammals very quickly diversified and filled the empty niches.[44][45] Modern mammals are subdivided into therians (placentals and marsupials) and monotremes. The first placentals and marsupials evolved in the Paleocene, such as the first elephant Eritherium;[46] forerunners of carnivorans, the Miacoidea;[47] and the North American marsupial Peradectes.[48] The only monotreme known from the Paleocene is Obdurodon sudamericanum.[49] 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.[50][51] Large carnivores include the wolf-like Mesonychia, such as Ankalagon[52] and Sinonyx.[53]

However, in general, Paleocene mammals retained their 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.[13] 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.[54]

At the end of the Paleocene, more modern and more successful mammal groups appeared in the fossil record–such as even-toed ungulates, odd-toed ungulates, primates, and hyaenodonts[55]and mammal size greatly increased during the cooling trend following the thermal maximum.[56] It is thought their appearance in the fossil record coincides with the thermal maximum which caused the forests of the Paleocene to be replaced by grasslands, and grass, being harder to digest than leaves, caused an increase in herbivore size, which led to an increase in predator size.[57][58]

Birds[edit]

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,[59] 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.[60] The fossil record also records early owls such as the large Berruornis from France,[61] and the smaller Ogygoptynx from the United States.[62]

Conversely, almost all archaic birds (any bird outside Neornithes) went extinct during the extinction event; however, the archaic Qinornis is recorded in the Paleocene.[60] 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.[63]

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.[64] 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.[40][65] 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.[66]

Reptiles[edit]

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,[67] and a hadrosaur femur from the San Juan Basin dated to 64.5 mya,[68] but such stray late forms may be zombie taxon that were washed out and moved to younger sediments.[69]

Model of Titanboa eating a crocodile at the Smithsonian

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.[70] Further, the late Paleocene snake Titanoboa grew to over 13 m (43 ft) long, the longest snake ever recorded.[71]

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.[72] One example of a Paleocene crocodile is Borealosuchus, which averaged 3.7 m (12 ft) in length at the Wannagan Creek site.[73] 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.[74]

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.[75] Turtles were not greatly affected by the extinction event, and around 80% of species survived.[76] In Colombia, a 60 million year old turtle with a 1.7 m (5 ft 7 in) carapace, Carbonemys, was discovered.[77]

Sea life[edit]

Otodus obliquus shark tooth

Though nearly 90% of all calcifying plankton species perished, some species surpassed previous abundance levels within 500,000 years of the extinction event. Likewise, 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.[78] 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.[17] 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.[79]

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 almost immediately following the extinction event. 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, filling vacant, open-ocean predatory niches as well as spreading out into recovering, hard-coral-dominated 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 end of the Paleocene, percomorphs evolved into vastly varying creatures such as early tunas, pufferfish, swordfish, reef fish, anglerfish, and so forth.[80][81]

Conversely, sharks and rays appear to have been unable to exploit the vacant niches, and recovered the same pre-extinction abundance.[78][82] 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.[83] The first megatoothed shark, Otodus obliquus–the ancestor of the giant megalodon–is recorded from the Paleocene.[84]

See also[edit]

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