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
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
The coyote, Canis latrans, is a canine native to North America. It is smaller than its close relative, the gray wolf, smaller than the related eastern wolf and red wolf, it fills much of the same ecological niche as the golden jackal does in Eurasia, though it is larger and more predatory, is sometimes called the American jackal by zoologists. The coyote is listed as least concern by the International Union for Conservation of Nature due to its wide distribution and abundance throughout North America, southwards through Mexico, into Central America; the species is able to adapt to and expand into environments modified by humans. It is enlarging its range, with coyotes moving into urban areas in the Eastern U. S. and was sighted in eastern Panama for the first time in 2013. As of 2005, 19 coyote subspecies are recognized; the average male weighs the average female 7 to 18 kg. Their fur color is predominantly light gray and red or fulvous interspersed with black and white, though it varies somewhat with geography.
It is flexible in social organization, living either in a family unit or in loosely knit packs of unrelated individuals. It has a varied diet consisting of animal meat, including deer, hares, birds, amphibians and invertebrates, though it may eat fruits and vegetables on occasion, its characteristic vocalization is a howl made by solitary individuals. Humans are the coyote's greatest threat, followed by gray wolves. In spite of this, coyotes sometimes mate with gray, eastern, or red wolves, producing "coywolf" hybrids. In the northeastern United States and eastern Canada, the eastern coyote is the result of various historical and recent matings with various types of wolves. Genetic studies show that most North American wolves contain some level of coyote DNA; the coyote is a prominent character in Native American folklore in the Southwestern United States and Mexico depicted as a trickster that alternately assumes the form of an actual coyote or a man. As with other trickster figures, the coyote uses deception and humor to rebel against social conventions.
The animal was respected in Mesoamerican cosmology as a symbol of military might. After the European colonization of the Americas, it was reviled in Anglo-American culture as a cowardly and untrustworthy animal. Unlike wolves, which have undergone an improvement of their public image, attitudes towards the coyote remain negative. Coyote males average 8 to 20 kg in weight, while females average 7 to 18 kg, though size varies geographically. Northern subspecies, which average 18 kg, tend to grow larger than the southern subspecies of Mexico, which average 11.5 kg. Body length ranges on average from 1.0 to 1.35 m, tail length 40 cm, with females being shorter in both body length and height. The largest coyote on record was a male killed near Afton, Wyoming, on November 19, 1937, which measured 1.5 m from nose to tail, weighed 34 kg. Scent glands are a bluish-black color; the color and texture of the coyote's fur varies somewhat geographically. The hair's predominant color is light gray and red or fulvous, interspersed around the body with black and white.
Coyotes living at high elevations tend to have more black and gray shades than their desert-dwelling counterparts, which are more fulvous or whitish-gray. The coyote's fur consists of soft underfur and long, coarse guard hairs; the fur of northern subspecies is longer and denser than in southern forms, with the fur of some Mexican and Central American forms being hispid. Adult coyotes have a sable coat color, dark neonatal coat color, bushy tail with an active supracaudal gland, a white facial mask. Albinism is rare in coyotes; the coyote is smaller than the gray wolf, but has longer ears and a larger braincase, as well as a thinner frame and muzzle. The scent glands are the same color, its fur color variation is much less varied than that of a wolf. The coyote carries its tail downwards when running or walking, rather than horizontally as the wolf does. Coyote tracks can be distinguished from those of dogs by less rounded shape. Unlike dogs, the upper canines of coyotes extend past the mental foramina.
At the time of the European colonization of the Americas, coyotes were confined to open plains and arid regions of the western half of the continent. In early post-Columbian historical records, distinguishing between coyotes and wolves is difficult. One record from 1750 in Kaskaskia, written by a local priest, noted that the "wolves" encountered there were smaller and less daring than European wolves. Another account from the early 1800s in Edwards County mentioned wolves howling at night, though these were coyotes; this species was encountered several times during the Lewis and Clark Expedition, though it was well known to European traders on the upper Missouri. Lewis, writing on 5 May 1805, in northeastern Montana, described the coyote in these terms: The small woolf or burrowing dog of the prairies are the inhabitants invariably of the open plains.
Foxes are small-to-medium-sized, omnivorous mammals belonging to several genera of the family Canidae. Foxes have a flattened skull, upright triangular ears, a pointed upturned snout, a long bushy tail. Twelve species belong to the monophyletic "true foxes" group of genus Vulpes. Another 25 current or extinct species are always or sometimes called foxes. Foxes live on every continent except Antarctica. By far the most common and widespread species of fox is the red fox with about 47 recognized subspecies; the global distribution of foxes, together with their widespread reputation for cunning, has contributed to their prominence in popular culture and folklore in many societies around the world. The hunting of foxes with packs of hounds, long an established pursuit in Europe in the British Isles, was exported by European settlers to various parts of the New World; the word fox comes from Old English. This in turn derives from Proto-Indo-European *puḱ-, meaning ’thick-haired. Male foxes are known as dogs, tods or reynards, females as vixens, young as cubs, pups, or kits, though the latter name is not to be confused with a distinct species called kit foxes.
Vixen is one of few words in modern English that retains the Middle English southern dialect "v" pronunciation instead of "f". A group of foxes is referred to leash, or earth. Within the Canidae, the results of DNA analysis shows several phylogenetic divisions: The fox-like canids, which include the kit fox, red fox, Cape fox, Arctic fox, fennec fox; the wolf-like canids, including the dog, gray wolf, red wolf, eastern wolf, golden jackal, Ethiopian wolf, black-backed jackal, side-striped jackal and African wild dog. The South American canids, including hoary fox, crab-eating fox and maned wolf. Various monotypic taxa, including the bat-eared fox, gray fox, raccoon dog. Foxes are smaller than some other members of the family Canidae such as wolves and jackals, while they may be larger than some within the family, such as Raccoon dogs. In the largest species, the red fox, males weigh on average between 4.1 and 8.7 kg, while the smallest species, the fennec fox, weighs just 0.7 to 1.6 kg. Fox-like features include a triangular face, pointed ears, an elongated rostrum, a bushy tail.
Foxes are digitigrade, thus, walk on their toes. Unlike most members of the family Canidae, foxes have retractable claws. Fox vibrissae, or whiskers, are black; the whiskers on the muzzle, mystaciae vibrissae, average 100–110 mm long, while the whiskers everywhere else on the head average to be shorter in length. Whiskers are on the forelimbs and average 40 mm long, pointing downward and backward. Other physical characteristics vary according to adaptive significance. Fox species differ in fur color and density. Coat colors range from pearly white to black and white to black flecked with white or grey on the underside. Fennec foxes, for example, have short fur to aid in keeping the body cool. Arctic foxes, on the other hand, have tiny ears and short limbs as well as thick, insulating fur, which aid in keeping the body warm. Red foxes, by contrast, have a typical auburn pelt, the tail ending with white marking. A fox's coat color and texture may vary due to the change in seasons. To get rid of the dense winter coat, foxes moult once a year around April.
Coat color may change as the individual ages. A fox's dentition, like all other canids, is I 3/3, C 1/1, PM 4/4, M 3/2 = 42. Foxes have pronounced carnassial pairs, characteristic of a carnivore; these pairs consist of the upper premolar and the lower first molar, work together to shear tough material like flesh. Foxes' canines are pronounced characteristic of a carnivore, are excellent in gripping prey. In the wild, the typical lifespan of a fox is one to three years, although individuals may live up to ten years. Unlike many canids, foxes are not always pack animals, they live in small family groups, but some are known to be solitary. Foxes are omnivores; the diet of foxes is made up of invertebrates such as insects, small vertebrates such as reptiles and birds, can include eggs and plants. Many species are generalist predators. Most species of fox consume around 1 kg of food every day. Foxes cache excess food, burying it for consumption under leaves, snow, or soil. Foxes tend to use a pouncing technique where they crouch down to camouflage themselves in the terrain using their hind legs, leap up with great force to land on top of their targeted prey.
Using their pronounced canine te
The Bengal fox known as the Indian fox, is a fox endemic to the Indian subcontinent and is found from the Himalayan foothills and Terai of Nepal through southern India and from southern and eastern Pakistan to eastern India and southeastern Bangladesh. Vulpes bengalensis is a small fox with an elongated muzzle, pointed ears, a bushy tail about 50 to 60% of the length of the head and body, its dorsal pelage is variable, but grayish and paler ventrally. It is more daintily built than Vulpes vulpes; the tail is bushy with a prominent black tip. Back of ears are dark brown with black margin, its rhinarium is naked and the lips are black, with small black hair patches on upper part of nuzzle in front of eyes. The ears have the same colour as the nape or maybe darker, but not having a dark patch as in V. vulpes. Extensive variation in coat colour exists across populations and seasonally within populations, but varies from grey to pale brown; the head and body length is 18 in, with a 10 in long tail. Typical weight is 5 to 9 pounds.
The genus Vulpes can be separated from Canis and Cuon in the Indian region by the flat forehead between the postorbital processes and not inflated by air cells. The processes themselves are concave with a raised anterior edge; the canine teeth are longer. The species is found throughout much of the Indian subcontinent with the exception of the wet forests and the extreme arid zone; the distribution is bounded by the Indus River valley. The preferred habitat is short open scrub or thorn forest, they appear to avoid steep terrain, tall grassland. Indian foxes were considered to be habitat generalists, but recent studies have shown a strong preference for semiarid, short grassland habitats at multiple scales. Bengal foxes are crepuscular in their habits. During the heat of the day, they hide in subterranean dens that they dig; the dens are complex with multiple chambers and escape routes. They are sometimes seen basking at a vantage point around sunset. In captivity, the lifespan is about 6 to 8 years.
The Bengal fox feeds on rodents, crabs, insects, small birds, fruits. Scats of young pups appeared to show that they fed on rodents but are opportunistic feeders. Foxes make a wide range of vocalizations. A chattering cry is the most common call, they growl, whine and bark. The Bengal fox does not appear to have latrine behaviour, a feature seen in some social canids, in which all members defecate at specific spots, they can be heard howling in the night in groups. The Bengal fox forms pair bonds that may last a lifetime, but extra-pair copulations are known to occur. Throughout most of its range, the mating season starts in autumn and after a gestation period around 50–60 days, two to four pups are born in a den. Both parents participate in pup-rearing; the pups are weaned about 3–4 months after emerging from the den. Pup mortality is high during the first few months. Pups may sometimes be nursed by multiple females. During the day, they tend to rest except in summer when they rest in dens. Lack of habitat protection is the greatest threat to the Indian fox.
For example, in southern India, less than 2% of potential Indian fox habitat is covered under the existing protected area network of the states of Karnataka and Andhra Pradesh. Hunting for its skin and flesh, as well as conversion of its grassland habitat to agriculture and bio-fuel plantations, have affected its population density. In addition, its body parts are used in traditional medicine, in some areas it is eaten, they are hunted by the narikuruva tribes of southern India. In Karnataka, they are captured in rituals conducted during Sankranthi. Another major threat is disease such as canine distemper virus and rabies, which spills over from the large unvaccinated populations of free-ranging dogs found throughout their range. IUCN/SSC Canid Specialist Group: Bengal Fox BBC - Science & Nature - Wildfacts - Indian fox, Bengal fox Status of Indian Fox
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated