Bycatch, in the fishing industry, is a fish or other marine species, caught unintentionally while catching certain target species and target sizes of fish, crabs etc. Bycatch is either of a different species, the wrong sex, or is undersized or juvenile individuals of the target species; the term "bycatch" is sometimes used for untargeted catch in other forms of animal harvesting or collecting. In 1997, the Organisation for Economic Co-operation and Development defined bycatch as "total fishing mortality, excluding that accounted directly by the retained catch of target species". Bycatch is a mechanism of overfishing for unintentional catch; the average annual bycatch rate of pinnipeds and cetaceans in the U. S. from 1990 to 1999 was estimated at 6215 animals with a standard error of 448. The fisherman bycatch issue originated due to the "mortality of dolphins in tuna nets in the 1960s". There are at least four different ways the word "bycatch" is used in fisheries: Catch, retained and sold but, not the target species for the fishery Species/sizes/sexes of fish which fishermen discard Non-target fish, whether retained and sold or discarded Unwanted invertebrate species, such as echinoderms and non-commercial crustaceans, various vulnerable species groups, including seabirds, sea turtles, marine mammals and elasmobranchs.
Additionally, the term "deliberate bycatch" is used to refer to bycatch as a source of illegal wildlife trade in several areas throughout world. Given the popularity of recreational fishing throughout the world, a small local study in the US in 2013 suggested that discards may be an important unmonitored source of fish mortality; the highest rates of incidental catch of non-target species are associated with tropical shrimp trawling. In 1997, the Food and Agriculture Organization of the United Nations documented the estimated bycatch and discard levels from shrimp fisheries around the world, they found discard rates as high as 20:1 with a world average of 5.7:1. Shrimp trawl fisheries catch 2% of the world total catch of all fish by weight, but produce more than one-third of the world total bycatch. American shrimp trawlers produce bycatch ratios between 3:1 and 15:1. Trawl nets in general, shrimp trawls in particular, have been identified as sources of mortality for cetacean and finfish species.
When bycatch is discarded, it is dead or dying. Tropical shrimp trawlers make trips of several months without coming to port. A typical haul may last 4 hours. Just before it is pulled on board the net is washed by zigzagging at full speed; the contents are dumped on deck and are sorted. An average of 5.7:1 means. In tropical inshore waters the bycatch consists of small fish; the shrimps are stored on-board. Recent sampling in the South Atlantic rock shrimp fishery found 166 species of finfish, 37 crustacean species, 29 other species of invertebrate among the bycatch in the trawls. Another sampling of the same fishery over a two-year period found that rock shrimp amounted to only 10% of total catch weight. Iridescent swimming crab, dusky flounder, inshore lizardfish, brown shrimp, longspine swimming crabs, other bycatch made up the rest. Despite the use of bycatch reduction devices, the shrimp fishery in the Gulf of Mexico removes about 25–45 million red snapper annually as bycatch, nearly one half the amount taken in directed recreational and commercial snapper fisheries.
Cetaceans, such as dolphins and whales, can be affected by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in frequency. In some fisheries, cetaceans are captured as bycatch but retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries. One example of bycatch is dolphins caught in tuna nets; as dolphins are mammals and do not have gills they may drown. This bycatch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packagings with disclaimers such as "dolphin friendly" to reassure buyers. However, "dolphin friendly" does not mean that dolphins were not killed in the production of a particular tin of tuna, but that the fleet which caught the tuna did not target a feeding pod of dolphins, but relied on other methods to spot tuna schools; the by-catch of the Caspian Seal may be recognized as the one of the biggest entanglements of pinnipeds as by-catch in the world Of the 21 albatross species recognised by IUCN on their Red List, 19 are threatened, the other two are near threatened.
Two species are considered critically endangered: the Amsterdam albatross and the Chatham albatross. One of the main threats is commercial long-line fishing, because the albatrosses and other seabirds which feed on offal are attracted to the set bait, after which they become hooked on the lines and drown. An estimated 100,000 albatross per year are killed in this fashion. Unregulated pirate fisheries exacerbate the problem. Sea turtles critically endangered, have been killed in large numbers in shrimp trawl nets. Estimates indicate that thousands of Kemp's ridley, loggerhead and leatherback sea turtles are caught in shrimp trawl fisheries in the Gulf of Mexico and the US Atlantic annually The speed and length of the trawl method is significant because, “for a tow duration of less than 10 minutes, the mortality rate for sea turtles is less than one percent, whereas for tows greater than six
Portunus was the ancient Roman god of keys, doors and ports. He may have protected the warehouses where grain was stored, but became associated with ports because of folk associations between porta "gate, door" and portus "harbor", the "gateway" to the sea, or because of an expansion in the meaning of portus. Portunus became conflated with the Greek Palaemon. Portunus' festival, celebrated on August 17, the sixteenth day before the Kalends of September, was the Portunalia, a minor occasion in the Roman year. On this day, keys were thrown into a fire for good luck in a solemn and lugubrious manner, his attribute was a key and his main temple in the city of Rome, the Temple of Portunus, was to be found in the Forum Boarium. Portunus appears to be related to the god Janus, with whom he shares many characters and the symbol of the key, he too was represented as a two headed being, with each head facing opposite directions, on coins and as figurehead of ships. He was considered to be "deus portuum portarumque praeses" The relationship between the two gods is underlined by the fact that the date chosen for the dedication of the rebuilt temple of Janus in the Forum Holitorium by emperor Tiberius is the day of the Portunalia, August 17.
Linguist Giuliano Bonfante has speculated, on the grounds of his cult and of the meaning of his name, that Portunus should be a archaic deity and might date back to an era when Latins lived in dwellings built on pilings. He argues that in Latin the words porta and portus share their etymology from the same IE root meaning ford, wading point. Portunus' flamen, the flamen Portunalis, was one of the flamines minores and performed the ritual of oiling the spear on the statue of god Quirinus, with an ointment prepared for this purpose and stored in a small vase. Marcus Terentius Varro, De Lingua Latina vi.19. Chisholm, Hugh, ed.. "Portunus". Encyclopædia Britannica. Cambridge University Press. William Smith, 1875. A Dictionary of Greek and Roman Antiquities: "Portumnalia"
Crabs are decapod crustaceans of the infraorder Brachyura, which have a short projecting "tail" entirely hidden under the thorax. They live in all the world's oceans, in fresh water, on land, are covered with a thick exoskeleton and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, crab lice – are not true crabs. Crabs are covered with a thick exoskeleton, composed of mineralized chitin, armed with a single pair of chelae. Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres. About 850 species of crab are terrestrial or semi-terrestrial species, they were thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World. The earliest unambiguous crab fossils date from the Jurassic, although Carboniferous Imocaris, known only from its carapace, may be a primitive crab.
The radiation of crabs in the Cretaceous and afterward may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, crabs' main predators. Crabs show marked sexual dimorphism. Males have larger claws, a tendency, pronounced in the fiddler crabs of the genus Uca. In fiddler crabs, males have one claw, enlarged and, used for communication for attracting a mate. Another conspicuous difference is the form of the pleon; this is. Crabs attract a mate through chemical, acoustic, or vibratory means. Pheromones are used by most aquatic crabs, while terrestrial and semiterrestrial crabs use visual signals, such as fiddler crab males waving their large claws to attract females; the vast number of brachyuran crabs have mate belly-to-belly. For many aquatic species, mating takes place just after the female is still soft. Females can store the sperm for a long time before using it to fertilise their eggs; when fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material.
In this location, they are protected during embryonic development. Females carrying eggs are called "berried"; when development is complete, the female releases the newly hatched larvae into the water, where they are part of the plankton. The release is timed with the tides; the free-swimming tiny zoea larvae can take advantage of water currents. They have a spine, which reduces the rate of predation by larger animals; the zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk. Each species has a particular number of zoeal stages, separated by moults, before they change into a megalopa stage, which resembles an adult crab, except for having the abdomen sticking out behind. After one more moult, the crab is a juvenile, living on the bottom rather than floating in the water; this last moult, from megalopa to juvenile, is critical, it must take place in a habitat, suitable for the juvenile to survive. Most species of terrestrial crabs must migrate down to the ocean to release their larvae.
After living for a short time as larvae in the ocean, the juveniles must do this migration in reverse. In many tropical areas with land crabs, these migrations result in considerable roadkill of migrating crabs. Once crabs have become juveniles, they will still have to keep moulting many more times to become adults, they are covered with a hard shell. The moult cycle is coordinated by hormones; when preparing for moult, the old shell is softened and eroded away, while the rudimentary beginnings of a new shell form under it. At the time of moulting, the crab takes in a lot of water to expand and crack open the old shell at a line of weakness along the back edge of the carapace; the crab must extract all of itself – including its legs, mouthparts and the lining of the front and back of the digestive tract – from the old shell. This is a difficult process that takes many hours, if a crab gets stuck, it will die. After freeing itself from the old shell, the crab is soft and hides until its new shell has hardened.
While the new shell is still soft, the crab can expand it to make room for future growth. Crabs walk sideways, because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles; some crabs, notably the Portunidae and Matutidae, are capable of swimming, the Portunidae so as their last pair of walking legs is flattened into swimming paddles. Crabs are active animals with complex behaviour patterns, they can communicate by waving their pincers. Crabs tend to be aggressive towards one another, males fight to gain access to females. On rocky seashores, where nearly all caves and crevices
The Decapoda or decapods are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, lobsters and shrimp. Most decapods are scavengers; the order is estimated to contain nearly 15,000 species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp and Anomura including hermit crabs, porcelain crabs, squat lobsters making up the bulk of the remainder; the earliest fossil decapod is the Devonian Palaeopalaemon. Decapods can have as many as 38 appendages, arranged in one pair per body segment; as the name Decapoda implies, ten of these appendages are considered legs. They are the pereiopods, found on the last five thoracic segments. In many decapods, one pair of these "legs" has enlarged pincers, called chelae, with the legs being called chelipeds. In front of the pereiopods are three pairs of maxillipeds which function as feeding appendages; the head has five pairs of appendages, including mouthparts and antennules.
There are five more pairs of appendages on the abdomen. They are called pleopods. There is one final pair called uropods, with the telson, form the tail fan. Classification within the order Decapoda depends on the structure of the gills and legs, the way in which the larvae develop, giving rise to two suborders: Dendrobranchiata and Pleocyemata; the Dendrobranchiata consist of prawns, including many species colloquially referred to as "shrimp", such as the "white shrimp", Litopenaeus setiferus. The Pleocyemata include the remaining groups, including "true shrimp"; those groups which walk rather than swim form a clade called Reptantia. This classification to the level of superfamilies follows De al.. Order Decapoda Latreille, 1802 Suborder Dendrobranchiata Bate, 1888 Penaeoidea Rafinesque, 1815 Sergestoidea Dana, 1852 Suborder Pleocyemata Burkenroad, 1963 Infraorder Stenopodidea Bate, 1888 Infraorder Caridea Dana, 1852 Procaridoidea Chace & Manning, 1972 Galatheacaridoidea Vereshchaka, 1997 Pasiphaeoidea Dana, 1852 Oplophoroidea Dana, 1852 Atyoidea De Haan, 1849 Bresilioidea Calman, 1896 Nematocarcinoidea Smith, 1884 Psalidopodoidea Wood-Mason, 1874 Stylodactyloidea Bate, 1888 Campylonotoidea Sollaud, 1913 Palaemonoidea Rafinesque, 1815 Alpheoidea Rafinesque, 1815 Processoidea Ortmann, 1896 Pandaloidea Haworth, 1825 Physetocaridoidea Chace, 1940 Crangonoidea Haworth, 1825 Infraorder Astacidea Latreille, 1802 Enoplometopoidea de Saint Laurent, 1988 Nephropoidea Dana, 1852 Astacoidea Latreille, 1802 Parastacoidea Huxley, 1879 Infraorder Glypheidea Winckler, 1882 Glypheoidea Winckler, 1882 Infraorder Axiidea de Saint Laurent, 1979b Infraorder Gebiidea de Saint Laurent, 1979 Infraorder Achelata Scholtz & Richter, 1995 Infraorder Polychelida Scholtz & Richter, 1995 Infraorder Anomura MacLeay, 1838 Aegloidea Dana, 1852 Galatheoidea Samouelle, 1819 Hippoidea Latreille, 1825a Chirostyloidea Ortmann, 1892 Lithodoidea Samouelle, 1819 Lomisoidea Bouvier, 1895 Paguroidea Latreille, 1802 Infraorder Brachyura Linnaeus, 1758 Section Dromiacea De Haan, 1833 Dromioidea De Haan, 1833 Homolodromioidea Alcock, 1900 Homoloidea De Haan, 1839 Section Raninoida De Haan, 1839 Section Cyclodorippoida Ortmann, 1892 Section Eubrachyura de Saint Laurent, 1980 Subsection Heterotremata Guinot, 1977 Aethroidea Dana, 1851 Bellioidea Dana, 1852 Bythograeoidea Williams, 1980 Calappoidea De Haan, 1833 Cancroidea Latreille, 1802 Carpilioidea Ortmann, 1893 Cheiragonoidea Ortmann, 1893 Corystoidea Samouelle, 1819 Dairoidea Serène, 1965 Dorippoidea MacLeay, 1838 Eriphioidea MacLeay, 1838 Gecarcinucoidea Rathbun, 1904 Goneplacoidea MacLeay, 1838 Hexapodoidea Miers, 1886 Leucosioidea Samouelle, 1819 Majoidea Samouelle, 1819 Orithyioidea Dana, 1852c Palicoidea Bouvier, 1898 Parthenopoidea MacLeay, Pilumnoidea Samouelle, 1819 Portunoidea Rafinesque, 1815 Potamoidea Ortmann, 1896 Pseudothelphusoidea Ortmann, 1893 Pseudozioidea Alcock, 1898 Retroplumoidea Gill, 1894 Trapezioidea Miers, 1886 Trichodactyloidea H. Milne-Edwards, 1853 Xanthoidea MacLeay, 1838 Subsection Thoracotremata Guinot, 1977 Cryptochiroidea Paul'son, 1875 Grapsoidea MacLeay, 1838 Ocypodoidea Rafinesque, 1815 Pinnotheroidea De Haan, 1833 List of Atlantic decapod species Phylogeny of Malacostraca Data related to Decapoda at Wikispecies Decapod Crustacea "Tree of Life" page at the Natural History Museum of Los Angeles County Decapoda at Curlie
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
In zoological nomenclature, a type species is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e. the species that contains the biological type specimen. A similar concept is used for suprageneric groups called a type genus. In botanical nomenclature, these terms have no formal standing under the code of nomenclature, but are sometimes borrowed from zoological nomenclature. In botany, the type of a genus name is a specimen, the type of a species name; the species name that has that type can be referred to as the type of the genus name. Names of genus and family ranks, the various subdivisions of those ranks, some higher-rank names based on genus names, have such types. In bacteriology, a type species is assigned for each genus; every named genus or subgenus in zoology, whether or not recognized as valid, is theoretically associated with a type species. In practice, there is a backlog of untypified names defined in older publications when it was not required to specify a type.
A type species is both a concept and a practical system, used in the classification and nomenclature of animals. The "type species" represents the reference species and thus "definition" for a particular genus name. Whenever a taxon containing multiple species must be divided into more than one genus, the type species automatically assigns the name of the original taxon to one of the resulting new taxa, the one that includes the type species; the term "type species" is regulated in zoological nomenclature by article 42.3 of the International Code of Zoological Nomenclature, which defines a type species as the name-bearing type of the name of a genus or subgenus. In the Glossary, type species is defined as The nominal species, the name-bearing type of a nominal genus or subgenus; the type species permanently attaches a formal name to a genus by providing just one species within that genus to which the genus name is permanently linked. The species name in turn is fixed, to a type specimen. For example, the type species for the land snail genus Monacha is Helix cartusiana, the name under which the species was first described, known as Monacha cartusiana when placed in the genus Monacha.
That genus is placed within the family Hygromiidae. The type genus for that family is the genus Hygromia; the concept of the type species in zoology was introduced by Pierre André Latreille. The International Code of Zoological Nomenclature states that the original name of the type species should always be cited, it gives an example in Article 67.1. Astacus marinus Fabricius, 1775 was designated as the type species of the genus Homarus, thus giving it the name Homarus marinus. However, the type species of Homarus should always be cited using its original name, i.e. Astacus marinus Fabricius, 1775. Although the International Code of Nomenclature for algae and plants does not contain the same explicit statement, examples make it clear that the original name is used, so that the "type species" of a genus name need not have a name within that genus, thus in Article 10, Ex. 3, the type of the genus name Elodes is quoted as the type of the species name Hypericum aegypticum, not as the type of the species name Elodes aegyptica.
Glossary of scientific naming Genetypes – genetic sequence data from type specimens. Holotype Paratype Principle of Typification Type Type genus
The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, named after Cambria, the Latinised name for Wales, where rocks from this age were first studied; the Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons of the geologic time scale, it spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period, about 541 million years ago, when hard-shelled creatures first appeared in abundance. Little is known about the Precambrian, despite it making up seven-eighths of the Earth's history, what is known has been discovered from the 1960s onwards; the Precambrian fossil record is poorer than that of the succeeding Phanerozoic, fossils from the Precambrian are of limited biostratigraphic use. This is because many Precambrian rocks have been metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain buried beneath Phanerozoic strata.
It is thought that the Earth coalesced from material in orbit around the Sun at 4,543 Ma, may have been struck by a large planetesimal shortly after it formed, splitting off material that formed the Moon. A stable crust was in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma; the term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time. "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. As of 2010, the United States Geological Survey considers the term informal, lacking a stratigraphic rank. A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area.
However, there is evidence. There is a solid record of bacterial life throughout the remainder of the Precambrian. Excluding a few contested reports of much older forms from North America and India, the first complex multicellular life forms seem to have appeared at 1500 Ma, in the Mesoproterozoic era of the Proterozoic eon. Fossil evidence from the Ediacaran period of such complex life comes from the Lantian formation, at least 580 million years ago. A diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma; these are referred to as Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic eon. By the middle of the following Cambrian period, a diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa; the increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals and other microbes formed prokaryotic mats that covered terrestrial areas.
Tracks from an animal with leg like appendages have been found in what was mud 551 million years ago. Evidence of the details of plate motions and other tectonic activity in the Precambrian has been poorly preserved, it is believed that small proto-continents existed prior to 4280 Ma, that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth"; the atmosphere of the early Earth is not well understood. Most geologists believe it was composed of nitrogen, carbon dioxide, other inert gases, was lacking in free oxygen. There is, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism.
This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have combined with other elements in Earth's crust iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed real dates to be assigned to specific formations and features; the Precambrian is divided into