Hemoglobin or haemoglobin, abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates as well as the tissues of some invertebrates. Haemoglobin in the blood carries oxygen from the gills to the rest of the body. There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 16 grams of haemoglobin in every 100 ml of blood. In mammals, the protein makes up about 96% of the red blood cells' dry content, around 35% of the total content. Haemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind up to four oxygen molecules. Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide as carbaminohemoglobin, in which CO2 is bound to the heme protein.
The molecule carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen. Haemoglobin is found outside red blood cells and their progenitor lines. Other cells that contain haemoglobin include the A9 dopaminergic neurons in the substantia nigra, alveolar cells, retinal pigment epithelium, mesangial cells in the kidney, endometrial cells, cervical cells and vaginal epithelial cells. In these tissues, haemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism. Haemoglobin and haemoglobin-like molecules are found in many invertebrates and plants. In these organisms, haemoglobins may carry oxygen, or they may act to transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghaemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen-fixing nodules of leguminous plants, before the oxygen can poison the system.
In 1825 J. F. Engelhard discovered that the ratio of iron to protein is identical in the hemoglobins of several species. From the known atomic mass of iron he calculated the molecular mass of hemoglobin to n × 16000, the first determination of a protein's molecular mass; this "hasty conclusion" drew a lot of ridicule at the time from scientists who could not believe that any molecule could be that big. Gilbert Smithson Adair confirmed Engelhard's results in 1925 by measuring the osmotic pressure of hemoglobin solutions; the oxygen-carrying property of hemoglobin was discovered by Hünefeld in 1840. In 1851, German physiologist Otto Funke published a series of articles in which he described growing hemoglobin crystals by successively diluting red blood cells with a solvent such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the resulting protein solution. Hemoglobin's reversible oxygenation was described a few years by Felix Hoppe-Seyler. In 1959, Max Perutz determined the molecular structure of hemoglobin by X-ray crystallography.
This work resulted in his sharing with John Kendrew the 1962 Nobel Prize in Chemistry for their studies of the structures of globular proteins. The role of hemoglobin in the blood was elucidated by French physiologist Claude Bernard; the name hemoglobin is derived from the words heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme group. Each heme group contains one iron atom, that can bind one oxygen molecule through ion-induced dipole forces; the most common type of hemoglobin in mammals contains four such subunits. Hemoglobin consists of protein subunits, these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides; the amino acid sequence of any polypeptide created by a cell is in turn determined by the stretches of DNA called genes. In all proteins, it is the amino acid sequence that determines the protein's chemical properties and function. There is more than one hemoglobin gene: in humans, hemoglobin A is coded for by the genes, HBA1, HBA2, HBB.
The amino acid sequences of the globin proteins in hemoglobins differ between species. These differences grow with evolutionary distance between species. For example, the most common hemoglobin sequences in humans and chimpanzees are nearly identical, differing by only one amino acid in both the alpha and the beta globin protein chains; these differences grow larger between less related species. Within a species, different variants of hemoglobin always exist, although one sequence is a "most common" one in each species. Mutations in the genes for the hemoglobin protein in a species result in hemoglobin variants. Many of these mutant forms of hemoglobin cause no disease; some of these mutant forms of hemoglobin, cause a group of hereditary diseases termed the hemoglobinopathies. The best known hemoglobinopathy is sickle-cell disease, the first human disease whose mechanism was understood at the molecular level. A separate set of diseases called thalassemias involves underproduction of normal and sometimes abnormal hemoglobins, through problems and mutations in globin gene regulation.
All these diseases produce anemia. Variations in hemoglobin amino acid sequences, as with other proteins, may be adaptive. For example, hemoglobin has been found to adapt in different ways to
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
Golden moles are small, insectivorous burrowing mammals endemic to Southern Africa, where their Afrikaans names are gouemolle or kruipmolle. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, other mole-like families, all of which, to various degrees, they resemble as a result of evolutionary convergence. Like most burrowing mammals with similar habits, the Chrysochloridae have short legs with powerful digging claws dense fur that repels dirt and moisture, toughened skin on the head, their eyes are covered with furred skin. The external ears are just tiny openings. In particular, golden moles bear a remarkable resemblance to the marsupial moles of Australia, family Notoryctidae, which they resemble so suggestively that at one time, the marsupial/placental divide not withstanding, some argued that they were related. Considerations that influenced the debate might have included the view that the Chrysochloridae are primitive placentals and the fact that they have many mole-like specializations similar to specializations in marsupial moles.
The rhinarium is a enlarged, dry leathery pad that protects their nostrils while the animal digs. In this respect too, they resemble the marsupial moles; some authors claim their primary sense is of touch, they are sensitive to vibrations which may indicate approaching danger. Note below however, the observations on the malleus in the middle ear; the species range in size from about 8 to about 20 cm. They have muscular shoulders and the forelimbs are radically adapted for digging; the fifth digit is absent and the first and fourth digits are vestigial. The adaptations of the hind feet are less dramatic, they retain all five toes and are webbed as an adaptation to efficient backward shovelling of soil loosened by the front claws. At one time the Chrysochloridae were regarded as primitive. Supporting arguments included: that they were thought to have originated in Gondwana, that they had a low resting metabolic rate, they could switch off thermoregulation when inactive. Like the tenrecs, they possess a cloaca, males lack a scrotum.
However, such points are no longer regarded as suggestive of golden moles as undeveloped "reptilian mammals". Going into a torpor when resting or during cold weather enables them to conserve energy and reduce urgent requirements for food, they have developed efficient kidneys and most species do not need to drink water at all. Most species of Chrysochloridae live exclusively underground in their preferred environments, beneath either grassveld, swamps, deserts, or mountainous terrain. However, Chrysospalax species tend to forage above ground in meadows. Eremitalpa species such as Grant's golden mole live in the sandy Namib desert, where they cannot form tunnels because the sand collapses. Instead during the day, when they must seek shelter, they swim through the loose sand, using their broad claws to paddle, dive down some 50 cm to where it is bearably cool. There they enter a state of torpor, thus conserving energy. At night they emerge to forage on the surface rather than wasting energy shifting sand.
Their main prey are termites that live under isolated grass clumps, they might travel for 6 kilometres a night in search of food. They seek promising clumps by listening for wind-rustled grass-root stresses and termites' head-banging alarm signals, neither of which can be heard above ground, so they stop periodically and dip their heads under the sand to listen. Most other species construct both foraging superficial burrows and deeper permanent burrows for residence. Residential burrows are complex in form, may penetrate as far as a metre below ground and include deep chambers for use as bolt-holes, other chambers as latrines, they compact it into the tunnel walls. They feed on small vertebrates such as lizards or burrowing snakes, they depend on their sense of hearing to locate much of their prey, the cochleas of a number of golden mole species have been found to be long and coiled, which may indicate a greater ecological dependence on low frequency auditory cues than we see in Talpid moles.
Some species have hypertrophied middle ear ossicles, in particular the malleus, adapted towards the detection of seismic vibrations. In this respect there is some apparent convergent evolution to burrowing reptiles in the family Amphisbaenidae. Females give birth to one to three hairless young in a grass-lined nest within the burrow system. Breeding occurs throughout the year; the adults are solitary, their burrowing territory may be aggressively defended from intruders where resources are scarce. Of the 21 species of golden mole, no fewer than 11 are threatened with extinction; the primary causes are sand mining, poor agricultural practices, increasing urbanisation, predation by domestic cats and dogs. The taxonomy of the Chrysochloridae is undergoing a review in the light of new genetic information, they have traditionally been listed with the shrews, hedgehogs and a grab-bag of small, difficult-to-place creatures as part of the order Insectivora. Some authorities retain this classification, at least for the time being.
Others group the golden moles with the tenrecs in a new order, sometimes known as Tenrecomorpha, while others cal
Elephant shrews called jumping shrews or sengis, are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea. Their traditional common English name "elephant shrew" comes from a perceived resemblance between their long noses and the trunk of an elephant, their superficial similarity with shrews in the order Eulipotyphla. However, phylogenetic analysis revealed that elephant shrews are not classified with true shrews, but are in fact more related to elephants than shrews. In 1997 the biologist Jonathan Kingdon proposed that they instead be called "sengis", a term derived from the Bantu languages of Africa, in 1998 they were classified into the new clade Afrotheria, they are distributed across the southern part of Africa, although common nowhere, can be found in any type of habitat, from the Namib Desert to boulder-strewn outcrops in South Africa to thick forest. One species, the North African elephant shrew, remains in the semiarid, mountainous country in the far northwest of the continent.
The creature is one of the fastest small mammals, having been recorded to reach speeds of 28.8 kilometres per hour. Elephant shrews are small, insectivorous mammals resembling rodents or opossums, with scaly tails, long snouts, legs quite long for their size, which are used to move from one place to another like rabbits, they vary in size from 50 to 500 grams. The short-eared elephant shrew has an average size of 150 mm. Although the size of the trunk varies among species, all are able to twist it about in search of food, their lifespans are about two and a half to four years in the wild. They have large canine teeth, high-crowned cheek teeth similar to those of ungulates, their dental formula is 1-220.127.116.11.1.4.2-3 Although diurnal and active, they are difficult to trap and seldom seen. Several species make a series of cleared pathways through the undergrowth and spend their day patrolling them for insect life. If the animal is disturbed, the pathway provides an obstacle-free escape route. Elephant shrews are not social animals, but many live in monogamous pairs, which share and defend their home territory, marked using scent glands.
Rhynchocyon species dig small conical holes in the soil, bandicoot-style, but others may make use of natural crevices, or make leaf nests. Elephant shrews were used in the 1940's to understand the human menstruation cycle; this helped people understand how human reproduction workedShort-eared elephant shrews inhabit the dry steppes and stone deserts of southwestern Africa. They can be found in the Namib Desert, one of the driest regions of the earth. Females drive away other females. Although they live in pairs, the partners do not care much for each other and their sole purpose of associating with the opposite sex is for reproduction. Social behaviors are not common and they have separate nests; the one or two young are well developed at birth. Female elephant shrews undergo a menstrual cycle similar to that of human females and the species is one of the few nonprimate mammals to do so; the elephant shrew mating period lasts for several days. After mating, the pair will return to their solitary habits.
After a gestation period varying from 45 to 60 days, the female will bear litters of one to three young several times a year. The young are born well developed, but remain in the nest for several days before venturing outside. After five days, the young's milk diet is supplemented with mashed insects, which are collected and transported in the cheek pouches of the female; the young slowly start to explore their environment and hunt for insects. After about 15 days, the young will begin the migratory phase of their lives, which lessens their dependency on their mother; the young will establish their own home ranges and will become sexually active within 41–46 days. The thermal characteristics of an elephant shrews with similar body size and distribution are close in most of the classifications, they can maintain homeothermy in different ambient temperatures where most of the species regulate their body temperature at 35° C and neither become hyperthermic but they balance the heat offload by increasing the EWL.
Elephant shrews eat insects, centipedes and earthworms. An elephant shrew uses its nose to find prey and uses its tongue to flick small food into its mouth, much like an anteater. Eating large prey can pose a challenge. Turning its head to one side, it chews pieces off with its cheek teeth, much like a dog chewing a bone; this is a sloppy process, many small pieces of worm drop to the ground. Some elephant shrews feed on small amounts of plant matter new leaves and small fruits. A number of fossil species are known, all from Africa, they were separate from the similar-appearing order Leptictida. A considerable diversification of macroscelids occurred in the Paleogene Era. Some, such as Myohyrax, were so similar to hyraxes that they were included with that group, while others, such as Mylomygale, were rodent-like; these unusual forms all died out by the Pleistocene. Although macroscelids have been classified with many groups on the basis of superficial characteristics, considerable morphological an
Manatees are large aquatic herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species in the order Sirenia: the Amazonian manatee, the West Indian manatee, the West African manatee, they measure up to 4.0 metres long, weigh as much as 590 kilograms, have paddle-like flippers. The etymology of the name is dubious, with connections having been made to Latin "manus", to a word sometimes cited as "manati" used by the Taíno, a pre-Columbian people of the Caribbean, meaning "breast". Manatees are called sea cows, as they are slow plant-eaters and similar to cows on land, they graze on water plants in tropical seas. Manatees are three of the four living species in the order Sirenia; the fourth is the Eastern Hemisphere's dugong. The Sirenia are thought to have evolved from four-legged land mammals more than 60 million years ago, with the closest living relatives being the Proboscidea and Hyracoidea; the Amazonian's hair color is brownish gray, it has thick wrinkled skin with coarse hair, or "whiskers".
Photos are rare. Manatees weigh 400 to 550 kilograms, average 2.8 to 3.0 metres in length, sometimes growing to 4.6 metres and 1,775 kilograms. At birth, baby manatees weigh about 30 kilograms each; the manatee has a large, prehensile upper lip, used to gather food and eat and for social interaction and communication. Manatees have shorter snouts than the dugongs; the lids of manatees' small spaced eyes close in a circular manner. The adults have no incisor or canine teeth, just a set of cheek teeth, which are not differentiated into molars and premolars; these teeth are replaced throughout life, with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, somewhat as elephants' teeth do. At any time, a manatee has no more than six teeth in each jaw of its mouth, its tail is paddle-shaped, is the clearest visible difference between manatees and dugongs. The female manatee has two teats, one under each flipper, a characteristic, used to make early links between the manatee and elephants.
The manatee is unusual among mammals in having just six cervical vertebrae, a number that may be due to mutations in the homeotic genes. All other mammals have other than the two-toed and three-toed sloths. Like the horse, the manatee has a simple stomach, but a large cecum, in which it can digest tough plant matter; the intestines are about 45 meters, unusually long for an animal of the manatee's size. Apart from mothers with their young, or males following a receptive female, manatees are solitary animals. Manatees spend 50% of the day sleeping submerged, surfacing for air at intervals of less than 20 minutes; the remainder of the time is spent grazing in shallow waters at depths of 1–2 metres. The Florida subspecies has been known to live up to 60 years. Manatees swim at about 5 to 8 kilometres per hour. However, they have been known to swim at up to 30 kilometres per hour in short bursts. Manatees are capable of understanding discrimination tasks and show signs of complex associative learning.
They have good long-term memory. They demonstrate discrimination and task-learning abilities similar to dolphins and pinnipeds in acoustic and visual studies. Manatees breed once every two years. Gestation lasts about 12 months and to wean the calf takes a further 12 to 18 months. Manatees emit a wide range of sounds used in communication between cows and their calves, their ears are large internally but the external openings are small, they are located four inches behind each eye. Adults communicate to maintain contact and during sexual and play behaviors. Taste and smell, in addition to sight and touch, may be forms of communication. Manatees eat over 60 different freshwater and saltwater plants. Using their divided upper lip, an adult manatee will eat up to 10%–15% of their body weight per day. Consuming such an amount requires the manatee to graze for up to seven hours a day. To be able to cope with the high levels of cellulose in their plant based diet, manatees utilize hindgut fermentation to help with the digestion process.
Manatees have been known to eat small numbers of fish from nets. Manatees use their flippers to "walk" along the bottom whilst they dig for plants and roots in the substrate; when plants are detected, the flippers are used to scoop the vegetation toward the manatee's lips. The manatee has prehensile lips; the lips use seven muscles to tear at plants. Manatees use front flippers to move the plants into the mouth; the manatee does not have front teeth, behind the lips, on the roof of the mouth, there are dense, ridged pads. These horny ridges, the manatee's lower jaw, tear
The Triassic is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.9 million years ago, to the beginning of the Jurassic Period 201.3 Mya. The Triassic is the shortest period of the Mesozoic Era. Both the start and end of the period are marked by major extinction events. Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth's biosphere impoverished. Therapsids and archosaurs were the chief terrestrial vertebrates during this time. A specialized subgroup of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period; the first true mammals, themselves a specialized subgroup of therapsids evolved during this period, as well as the first flying vertebrates, the pterosaurs, like the dinosaurs, were a specialized subgroup of archosaurs. The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to rift into two separate landmasses, Laurasia to the north and Gondwana to the south.
The global climate during the Triassic was hot and dry, with deserts spanning much of Pangaea's interior. However, the climate became more humid as Pangaea began to drift apart; the end of the period was marked by yet another major mass extinction, the Triassic–Jurassic extinction event, that wiped out many groups and allowed dinosaurs to assume dominance in the Jurassic. The Triassic was named in 1834 by Friedrich von Alberti, after the three distinct rock layers that are found throughout Germany and northwestern Europe—red beds, capped by marine limestone, followed by a series of terrestrial mud- and sandstones—called the "Trias"; the Triassic is separated into Early and Late Triassic Epochs, the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are: During the Triassic all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator and spanning from pole to pole, called Pangaea.
From the east, along the equator, the Tethys sea penetrated Pangaea, causing the Paleo-Tethys Ocean to be closed. In the mid-Triassic a similar sea penetrated along the equator from the west; the remaining shores were surrounded by the world-ocean known as Panthalassa. All the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates; the supercontinent Pangaea was rifting during the Triassic—especially late in that period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangaea, which separated New Jersey from Morocco, are of Late Triassic age. S. these thick sediments comprise the Newark Group. Because a super-continental mass has less shoreline compared to one broken up, Triassic marine deposits are globally rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west, thus Triassic stratigraphy is based on organisms that lived in lagoons and hypersaline environments, such as Estheria crustaceans.
At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea. Africa shared the supercontinent's uniform fauna, dominated by theropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are more common in the south than north; the time boundary separating the Permian and Triassic marks the advent of an extinction event with global impact, although African strata from this time period have not been studied. During the Triassic peneplains are thought to have formed in what is now southern Sweden. Remnants of this peneplain can be traced as a tilted summit accordance in the Swedish West Coast. In northern Norway Triassic peneplains may have been buried in sediments to be re-exposed as coastal plains called strandflats. Dating of illite clay from a strandflat of Bømlo, southern Norway, have shown that landscape there became weathered in Late Triassic times with the landscape also being shaped during that time. At Paleorrota geopark, located in Rio Grande do Sul, the Santa Maria Formation and Caturrita Formations are exposed.
In these formations, one of the earliest dinosaurs, Staurikosaurus, as well as the mammal ancestors Brasilitherium and Brasilodon have been discovered. The Triassic continental interior climate was hot and dry, so that typical deposits are red bed sandstones and evaporites. There is no evidence of glaciation near either pole. Pangaea's large size limited the moderating effect of the global ocean; the strong contrast between the Pangea supercontinent and the global ocean triggered intense cross-equatorial monsoons. The Triassic may have been a dry period, but evidence exists that it was punctuated by several episodes of increased rainfall in tropical and subtropical latitudes of the Tethys Sea and its surrounding land. Sediments and fossils suggestive of a more humid climate are known from the Anisian to Ladinian of the Tethysian domain, from the Carnian and Rhaetian of a larger area that includes the Boreal domain, the North
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll