Paleocene

The Paleocene ( /ˈpæliəˌsiːn, ˈpæ-, -lioʊ-/[2]) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era. As with many geologic periods, the strata that define the epoch's beginning and end are well identified, but the exact ages remain uncertain.

The Paleocene Epoch is bracketed by two major events in Earth's history. It started with the mass extinction event at the end of the Cretaceous, known as the Cretaceous–Paleogene (K–Pg) boundary. This was a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora. The die-off of the dinosaurs left unfilled ecological niches worldwide. The Paleocene ended with the Paleocene–Eocene Thermal Maximum, a geologically brief (~0.2 million year) interval characterized by extreme changes in climate and carbon cycling.

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]

Etymology

The name "Paleocene" is a combination of "palaeo-", from Ancient Greek παλαιός (palaios), meaning "old", and Eocene, as it refers to strata originally considered part of the lower Eocene. (It is not, as sometimes said, a combination of "palaeo-" and Ancient Greek καινός [kainos], meaning "new".)[3]

Boundaries and subdivisions

The K–Pg boundary that marks the separation between Cretaceous and Paleocene is visible in the geological record of much of the Earth by a discontinuity in the fossil fauna and high iridium levels. There is also fossil evidence of abrupt changes in flora and fauna. There is some evidence that a substantial but very short-lived climatic change may have happened in the very early decades of the Paleocene. There are several theories about the cause of the K–Pg extinction event, with most evidence supporting the impact of a 10 km diameter asteroid forming the buried Chicxulub crater on the coast of Yucatan, Mexico.

The end of the Paleocene (≈55.8 Ma) was also marked by a time of major change, one of the most significant periods of global change during the Cenozoic.[4] The Paleocene–Eocene Thermal Maximum upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and a major turnover in mammals on land.

The Paleocene is divided into three stages, the Danian, the Selandian and the Thanetian, as shown in the table above. Additionally, the Paleocene is divided into six Mammal Paleogene zones.

Climate

The early Paleocene was cooler and drier than the preceding Cretaceous, though temperatures rose sharply during the Paleocene–Eocene Thermal Maximum. The climate became warm and humid worldwide towards the Eocene boundary, with subtropical vegetation growing in Greenland and Patagonia, crocodilians swimming off the coast of Greenland, and early primates evolving in the tropical palm forests of northern Wyoming.[5] The Earth's poles were cool and temperate; North America, Europe, Australia and southern South America were warm and temperate; equatorial areas had tropical climates; and north and south of the equatorial areas, climates were hot and arid,[6] not dissimilar to today's global desert belts around 30 degrees northern and southern latitude.

Paleogeography

In many ways, the Paleocene continued processes that had begun during the late Cretaceous Period. During the Paleocene, the continents continued to drift toward their present positions. Supercontinent Laurasia had not yet separated into three continents - Europe and Greenland were still connected, North America and Asia were still intermittently joined by a land bridge, while Greenland and North America were beginning to separate.[7] The Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, which ended in the succeeding epoch.

South and North America remained separated by equatorial seas (they joined during the Neogene); the components of the former southern supercontinent Gondwanaland continued to split 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 that would lead to a tectonic collision and the formation of the Himalayas.

The inland seas in North America (Western Interior Seaway) and Europe had receded by the beginning of the Paleocene, making way for new land-based flora and fauna.

Oceans

Warm seas circulated throughout the world, including the poles. The earliest Paleocene featured a low diversity and abundance of marine life, but this trend reversed later in the epoch.[7] Tropical conditions gave rise to abundant marine life, including coral reefs. With the demise of marine reptiles at the end of the Cretaceous, sharks became the top predators. At the end of the Cretaceous, the ammonites and many species of foraminifera became extinct.

Marine fauna also came to resemble modern fauna, with only the marine mammals and the Carcharhinid sharks missing.

Flora

Terrestrial Paleocene strata immediately overlying the K–Pg boundary is in places marked by a "fern spike": a bed especially rich in fern fossils.[8] Ferns are often the first species to colonize areas damaged by forest fires; thus the fern spike may indicate post-Chicxulub crater devastation.[9]

In general, the Paleocene is marked by the development of modern plant species. Cacti and palm trees appeared. Paleocene and later plant fossils are generally attributed to modern genera or to closely related taxa.

The warm temperatures worldwide gave rise to thick tropical, sub-tropical and deciduous forest cover around the globe (the first recognizably modern rainforests) with ice-free polar regions covered with coniferous and deciduous trees.[7] With no large browsing dinosaurs to thin them, Paleocene forests were probably denser than those of the Cretaceous.[10]

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.

Fauna

Mammals

Titanoides
Life restoration of Titanoides

Mammals had first appeared in the Late Triassic, evolving from advanced cynodonts, and developed alongside the dinosaurs, exploiting ecological niches untouched by the larger and more famous Mesozoic animals: in the insect-rich forest underbrush and high up in the trees. These smaller mammals (as well as birds, reptiles, amphibians, and insects) survived the mass extinction at the end of the Cretaceous which wiped out the non-avian dinosaurs, and mammals diversified and spread throughout the world.

While early mammals were small nocturnal animals that mostly ate soft plant material and small animals such as insects, the demise of the non-avian dinosaurs and the beginning of the Paleocene saw mammals growing bigger and occupying a wider variety of ecological niches. Ten million years after the death of the non-avian dinosaurs, the world was filled with rodent-like mammals, medium-sized mammals scavenging in forests, and large herbivorous and carnivorous mammals hunting other mammals, birds, and reptiles.

Fossil evidence from the Paleocene is scarce, and there is relatively little known about mammals of the time. Because of their small size (constant until late in the epoch) early mammal bones are not well preserved in the fossil record, and most of what we know comes from fossil teeth (a much tougher substance), and only a few skeletons.[7]

The brain to body mass ratios of these archaic mammals were quite low.[11]

Mammals of the Paleocene include:

Reptiles

Asiatosuchus
Section of an Asiatosuchus jaw

Because of the climatic conditions of the Paleocene, reptiles were more widely distributed over the globe than at present. Among the sub-tropical reptiles found in North America during this epoch are champsosaurs (fully aquatic reptiles), crocodilia, soft-shelled turtles, palaeophid snakes, varanid lizards, and Protochelydra zangerli (similar to modern snapping turtles).

Examples of champsosaurs of the Paleocene include Champsosaurus gigas, the largest champsosaur ever discovered. This creature was unusual among Paleocene non-squamate reptiles in that C. gigas became larger than its known Mesozoic ancestors: C. gigas is more than twice the length of the largest Cretaceous specimens (3 meters versus 1.5 meters). Another genus, Simoedosaurus, was similarly large; it appears rather suddenly in the fossil record, as its closest relatives occurred in the Early Cretaceous. Reptiles as a whole decreased in size after the K–Pg event. Champsosaurs declined towards the end of the Paleocene and became extinct during the Miocene.

Wannaganosuchus
Wannaganosuchus, a crocodilian from the Paleocene.

Examples of Paleocene crocodylians are Borealosuchus (formerly Leidyosuchus) formidabilis, the apex predator and the largest animal of the Wannagan Creek fauna, and the alligatorid Wannaganosuchus.

Non-avian dinosaurs may have survived to some extent into the early Danian stage of the Paleocene Epoch circa 64.5 Mya. The controversial evidence for such is a hadrosaur leg bone found from Paleocene strata in New Mexico;[13] but such stray late forms may be derived fossils.[14]

Several species of snakes, such as Titanoboa and Gigantophis, grew to over 6 meters long.[15]

Birds

Birds began to re-diversify during the epoch, occupying new niches. Genetic studies suggest that nearly all modern bird clades can trace their origin to this epoch, with Neornithes having undergone an extremely fast, "star-like" radiation of species in the early Palaeocene in response to the vacancy of niches left by the KT event.[16]

Large flightless birds have been found in late Paleocene deposits, including the omnivorous Gastornis in Europe and carnivorous terror birds in South America, the latter of which survived until the Pleistocene.

In the late Paleocene, early owl types appeared, such as Ogygoptynx in the United States and Berruornis in France.

References

  1. ^ "ICS - Chart/Time Scale". www.stratigraphy.org.
  2. ^ Jones, Daniel (2003) [1917], Peter Roach; James Hartmann; Jane Setter (eds.), English Pronouncing Dictionary, Cambridge: Cambridge University Press, ISBN 3-12-539683-2
  3. ^ T. C. R. Pulvertaft (1999). ""Paleocene" or "Palaeocene"" (PDF). Bulletin of the Geological Society of Denmark. 46: 52.CS1 maint: Uses authors parameter (link)
  4. ^ Gavin A. Schmidt and Drew T. Shindell (2003). "Atmospheric composition, radiative forcing, and climate change as a consequence of a massive methane release from gas hydrates" (PDF). Paleoceanography. 18 (1). Bibcode:2003PalOc..18.1004S. doi:10.1029/2002PA000757.CS1 maint: Uses authors parameter (link)
  5. ^ "Science Notes 2003:". Scicom.ucsc.edu. Retrieved 2012-08-28.
  6. ^ "Paleocene Climate". PaleoMap Project. Retrieved 2012-08-28.
  7. ^ a b c d Hooker, J.J., "Tertiary to Present: Paleocene", pp. 459-465, Vol. 5. of Selley, Richard C., L. Robin McCocks, and Ian R. Plimer, Encyclopedia of Geology, Oxford: Elsevier Limited, 2005. ISBN 0-12-636380-3
  8. ^ Vajda, Vivi. "Global Disruption of Vegetation at the Cretaceous-Tertiary Boundary – A Comparison Between the Northern and Southern Hemisphere Palynological Signals". Gsa.confex.com. Retrieved 2006-07-15.
  9. ^ Bigelow, Phillip. "The K–T boundary In The Hell Creek Formation". Scn.org. Archived from the original on 2006-07-12. Retrieved 2006-07-15.
  10. ^ Stephen Jay Gould, ed., The Book of Life (New York: W.W. Norton & Company, 1993), p. 182.
  11. ^ Kazlev, M. Alan (2002) "The Paleocene". Palaeos Cenozoic. Retrieved April 3, 2013.
  12. ^ Musser, A. M. (2003). "Review of the monotreme fossil record and comparison of palaeontological and molecular data". Comparative Biochemistry and Physiology A. 136: 927–942. doi:10.1016/s1095-6433(03)00275-7. Retrieved April 3, 2013.
  13. ^ Fassett, JE, Lucas, SG, Zielinski, RA, and Budahn, JR (2001). "Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone, San Juan Basin, New Mexico and Colorado, USA" (PDF). Catastrophic Events and Mass Extinctions, Lunar and Planetary Contribution. 1053: 45–46. Retrieved 2007-05-18.CS1 maint: Multiple names: authors list (link)
  14. ^ Sullivan, RM (2003). "No Paleocene dinosaurs in the San Juan Basin, New Mexico". Geological Society of America Abstracts with Programs. 35 (5): 15. Retrieved 2007-07-02.
  15. ^ Rage, Jean-Claude; Métais, Grégoire; Bartolini, Annachiara; Brohi, Imdad A.; Lashari, Rafiq A.; Marivaux, Laurent; Merle, Didier; Solangi, Sarfraz H. (2014). "First report of the giant snake Gigantophis (Madtsoiidae) from the Paleocene of Pakistan: Paleobiogeographic implications". Geobios. 47 (3): 147–153. doi:10.1016/j.geobios.2014.03.004.
  16. ^ Linnéa Smeds, Hans Ellegren, The Dynamics of Incomplete lineage sorting across the Ancient Adaptive Radiation of Neoavian Birds

External links

Afroaves

Afroaves is a clade of birds, consisting of the kingfishers and kin (Coraciiformes), woodpeckers and kin (Piciformes), hornbills and kin (Bucerotiformes), trogons (Trogoniformes), cuckoo roller (Leptosomatiformes), mousebirds (Coliiformes), owls (Strigiformes), raptors (Accipitriformes) and New World vultures (Cathartiformes). The most basal clades are predatory, suggesting the last common ancestor of the group was also.

Cladogram of Afroaves relationships based on Prum, R.O. et al. (2015) with some clade names after Yury, T. et al. (2013) and Kimball et al. 2013.

Apodiformes

Traditionally, the bird order Apodiformes contained three living families: the swifts (Apodidae), the treeswifts (Hemiprocnidae), and the hummingbirds (Trochilidae). In the Sibley-Ahlquist taxonomy, this order is raised to a superorder Apodimorphae in which hummingbirds are separated as a new order, Trochiliformes. With nearly 450 species identified to date, they are the most diverse order of birds after the passerines.

Australidelphia

Australidelphia is the superorder that contains roughly three-quarters of all marsupials, including all those native to Australasia and a single species from South America (all other American marsupials are members of the Ameridelphia). Analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has shown that the South American monito del monte's lineage is the most basal of the superorder. The Australian australidelphians form a clade, for which the name Euaustralidelphia ("true Australidelphia") has been proposed (the branching order within this group is yet to be determined). The study also showed that the most basal of all marsupial orders are the other two South American groups (Didelphimorphia and Paucituberculata, with the former probably branching first). This indicates that Australidelphia arose in South America along with the other major divisions of extant marsupials, and likely reached Australia via Antarctica in a single dispersal event after Microbiotheria split off.

Borealosuchus

Borealosuchus (meaning "boreal crocodile") is an extinct genus of crocodylians that lived from the Late Cretaceous to the Eocene in North America. It was named by Chris Brochu in 1997 for several species that had been assigned to Leidyosuchus. The species assigned to it are:

B. sternbergii, the type species, from the Maastrichtian (Late Cretaceous) of Colorado, Montana, North Dakota, South Dakota, and Wyoming; B. acutidentatus, from the Paleocene of Saskatchewan; B. formidabilis, from the Paleocene of North Dakota; B. griffithi, from the Paleocene of Alberta; and B. wilsoni, from the Eocene of Wyoming. B. formidabilis is particularly well-known, represented by the remains of many individuals from the Wannagan Creek site in North Dakota.Borealosuchus was a mid-sized crocodylian; B. acutidentatus reached up to 2.8 metres (9.2 ft) in length with a 36 centimetres (14 in) skull.

Caiman

A caiman is a crocodilian alligatorid belonging to the subfamily Caimaninae, one of two primary lineages within Alligatoridae, the other being alligators.

Caprimulgiformes

The Caprimulgiformes is an order of birds that includes a number of birds with global distribution (except Antarctica). They are generally insectivorous and nocturnal. The order gets its name from the Latin for "goat-milker", an old name based on an erroneous view of the European nightjar's feeding habits.

Carbonemys

Carbonemys cofrinii is an extinct podocnemidid turtle known from the Middle Paleocene Cerrejón Formation of the Cesar-Ranchería Basin in northeastern Colombia. The formation is dated at around 60 to 58 million years ago, starting at about five million years after the KT extinction event.

Cerrejonemys

Cerrejonemys wayuunaiki is an extinct podocnemid turtle which existed in Colombia during the Paleogene period; the Middle to Late Paleocene epoch.

Danian

The Danian is the oldest age or lowest stage of the Paleocene epoch or series, the Paleogene period or system and the Cenozoic era or erathem. The beginning of the Danian age (and the end of the preceding Maastrichtian age) is at the Cretaceous–Paleogene extinction event 66 Ma. The age ended 61.6 Ma, being followed by the Selandian age.

Euarchontoglires

Euarchontoglires (synonymous with Supraprimates) is a clade and a superorder of mammals, the living members of which belong to one of the five following groups: rodents, lagomorphs, treeshrews, colugos and primates.

Glires

Glires (Latin glīrēs, dormice) is a clade (sometimes ranked as a grandorder) consisting of rodents and lagomorphs (rabbits, hares, and pikas). The hypothesis that these form a monophyletic group has been long debated based on morphological evidence. Two morphological studies, published in 2001 and 2003, strongly support the monophyly of Glires. In particular, the 2003 study, reported the discovery of fossil material of basal members of Glires, particularly the genera Mimotona, Gomphos, Heomys, Matutinia, Rhombomylus, and Sinomylus. Their description, in 2005, helped to bridge the gap between more typical rodents and lagomorphs. Data published in 2001, based on nuclear DNA, supported Glires as a sister of Euarchonta to form Euarchontoglires, but some genetic data from both nuclear and mitochondrial DNA have been less supportive. A study, published in 2007, investigating retrotransposon presence/absence data unambiguously supports the Glires hypothesis. Studies published in 2011 and 2015 place Scandentia as a sister clade of the Glires, invalidating Euarchonta as a clade.

Hydromedusa

For the hydrozoan Cnidaria called "Hydromedusa" or "Hydromedusae", see Anthomedusae.

Hydromedusa is a turtle genus in the family Chelidae, commonly known as the South American snake-necked turtles. They are quite closely related to the South American side-necked swamp turtles (Acanthochelys) and the snake-necked turtles of the Australian-Melanesian region (Chelodina), but less closely to the spine-necked river turtles of South America (Podocnemididae) which belong to a more modern lineage of Pleurodira.

Ostrich

The ostriches are a family, Struthionidae, of flightless birds. The two extant species of ostrich are the common ostrich and Somali ostrich, both in the genus Struthio, which also contains several species known from Holocene fossils such as the Asian ostrich. The common ostrich is the more widespread of the two living species, and is the largest living bird species. Other ostriches are also among the largest bird species ever.

Ostriches first appeared during the Miocene epoch, though various Paleocene, Eocene, and Oligocene fossils may also belong to the family. Ostriches are classified in the ratite group of birds, all extant species of which are flightless, including the kiwis, emus, and rheas. Traditionally, the order Struthioniformes contained all the ratites. However, recent genetic analysis has found that the group is not monophyletic, as it is paraphyletic with respect to the tinamous, so the ostriches are classified as the only members of the order.

Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM), alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene Thermal Maximum", was a time period with more than 8 °C warmer global average temperature than today. This climate event began at the time boundary of the Paleogene, between the Paleocene and Eocene geological epochs. The exact age and duration of the event is uncertain but it is estimated to have occurred around 55.5 million years ago.The associated period of massive carbon injection into the atmosphere has been estimated to have lasted no longer than 20,000 years. The entire warm period lasted for about 200,000 years. Global temperatures increased by 5–8 °C. The carbon dioxide was likely released in two pulses, the first lasting less than 2,000 years. Such a repeated carbon release is in line with current global warming. A main difference is that during the Paleocene–Eocene Thermal Maximum, the planet was essentially ice-free. However, the amount of released carbon, according to a recent study, suggests a modest 0.2 gigatonnes per year (at peaks 0.58 gigatonnes); humans today add about 10 gigatonnes per year.The onset of the Paleocene–Eocene Thermal Maximum has been linked to an initial 5 °C temperature rise and to extreme changes in Earth's carbon cycle. The period is marked by a prominent negative excursion in carbon stable isotope (δ13C) records from around the globe; more specifically, there was a large decrease in 13C/12C ratio of marine and terrestrial carbonates and organic carbon.Stratigraphic sections of rock from this period reveal numerous other changes. Fossil records for many organisms show major turnovers. For example, in the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion, planktic foraminifera and calcareous nanofossils all occurred during the beginning stages of PETM. On land, modern mammal orders (including primates) suddenly appear in Europe and in North America. Sediment deposition changed significantly at many outcrops and in many drill cores spanning this time interval.

At least since 1997, the Paleocene–Eocene Thermal Maximum has become a focal point of considerable geoscience research because it probably provides the best past analog by which to understand impacts of global climate warming and of massive carbon input to the ocean and atmosphere, including ocean acidification. Although it is now widely accepted that the PETM represents a "case study" for global warming and massive carbon input to Earth's surface, the cause, details and overall significance of the event remain perplexing.

Paleogene

The Paleogene (; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period. The United States Geological Survey uses the abbreviation PE for the Paleogene, but the more commonly used abbreviation is PG with the PE being used for Paleocene.

This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (55.5/54.8 Mya) was marked by the Paleocene–Eocene Thermal Maximum, one of the most significant periods of global change during the Cenozoic, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals. The terms 'Paleogene System' (formal) and 'lower Tertiary System' (informal) are applied to the rocks deposited during the 'Paleogene Period'. The somewhat confusing terminology seems to be due to attempts to deal with the comparatively fine subdivisions of time possible in the relatively recent geologic past, for which more details are preserved. When the Tertiary Period is divided into two periods instead of directly into five epochs, the periods are more closely comparable to the duration of 'periods' of the preceding Mesozoic and Paleozoic Eras.

Placentalia

Placentalia ("Placentals") is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The Placentals are partly distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a relatively late stage of development. It is somewhat of a misnomer since marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young who are then kept for a period in the mother’s pouch.

Strisores

Strisores is a clade of birds. It includes the living families and orders Caprimulgidae (nightjars, nighthawks and allies), Nyctibiidae (potoos), Steatornithidae (oilbirds), Podargidae (frogmouths), Apodiformes (swifts and hummingbirds), as well as the Aegotheliformes (owlet-nightjars) whose distinctness was only recently realized. The Apodiformes (which include the "Trochiliformes" of the Sibley-Ahlquist taxonomy) and the Aegotheliformes form the Daedalornithes.

Thanetian

The Thanetian is, in the ICS Geologic timescale, the latest age or uppermost stratigraphic stage of the Paleocene Epoch or series. It spans the time between 59.2 and 56 Ma. The Thanetian is preceded by the Selandian age and followed by the Ypresian age (part of the Eocene). The Thanetian is sometimes referred to as the Late Paleocene.

Titanoboa

Titanoboa () is an extinct genus of very large snakes that lived in what is now La Guajira in northeastern Colombia. They could grow up to 12.8 m (42 ft) long and reach a weight of 1,135 kg (2,500 lb).Fossils of Titanoboa have been found in the Cerrejón Formation, and date to around 58 to 60 million years ago. The giant snake lived during the Middle to Late Paleocene epoch, a 10-million-year period immediately following the Cretaceous-Paleogene extinction event.The only known species is Titanoboa cerrejonensis, the largest snake ever discovered, which supplanted the previous record holder, Gigantophis.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon
(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)

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