Eastern grey kangaroo
The eastern grey kangaroo is a marsupial found in southern and eastern Australia, with a population of several million. It is known as the grey kangaroo and the Forester kangaroo. The eastern grey kangaroo is the second largest and heaviest living marsupial, an adult male will commonly weigh around 50 to 66 kg whereas females commonly weigh around 17 to 40 kg. Large males of species are more heavily built and muscled than the lankier Red Kangaroo. One of these, shot in eastern Tasmania weighed 82 kg, the largest known specimen, examined by Lydekker, had a weight of 91 kg and measured 2.92 m along the curves. When the skin of this specimen was measured it had a length of 2.49 m. The eastern grey is easy to recognise, its soft grey coat is distinctive, Red kangaroos, though sometimes grey-blue in colour, have a totally different face than grey kangaroos. Red kangaroos have distinctive markings in black and white beside their muzzles, grey kangaroos do not have these markings, and their eyes seem large and wide open.
Where their ranges overlap, it is more difficult to distinguish between eastern grey and western grey kangaroos, which are closely related. They have a similar body and facial structure, and their noses/muzzles are fully covered with fine hair. The eastern greys colouration is a grey or brownish-grey, with a lighter silver or cream, sometimes nearly white. The western grey is a dusty brown colour, with more contrast especially around the head. Indigenous Australian names include iyirrbir and kucha, the highest ever recorded speed of any kangaroo was 64 kilometres per hour set by a large female eastern grey kangaroo. Although the red is known, the eastern grey is the kangaroo most often encountered in Australia. The eastern grey prefers open grassland areas of bush for daytime shelter. It inhabits coastal areas, sub-tropical forests, mountain forests, like all kangaroos, it is mainly nocturnal and crepuscular, and is mostly seen early in the morning, or as the light starts to fade in the evening.
In the middle of the day, kangaroos rest in the cover of the woodlands and eat there, the eastern grey kangaroo is predominantly a grazer, eating a wide variety of grasses, whereas some other species include significant amounts of shrubs in the diet. Eastern grey kangaroos are gregarious and form open-membership groups, the groups are made up of 2-3 females and their offspring with the same number of males of which one is dominant
The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian,419.2 million years ago, to the beginning of the Carboniferous,358.9 Mya. It is named after Devon, where rocks from this period were first studied, the first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents, by the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods became well-established, Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fish. The first ray-finned and lobe-finned bony fish appeared, while the placodermi began dominating almost every aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral, in the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician.
The first ammonites, species of molluscs, trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida. The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, while the rock beds that define the start and end of the Devonian period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian 419.2 Mya, another common term is Age of the Fishes, referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian and Farlovian stages, in the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common.
The Devonian Period is formally broken into Early and Late subdivisions, the rocks corresponding to those epochs are referred to as belonging to the Lower and Upper parts of the Devonian System. Early Devonian The Early Devonian lasted from 419.2 ±2.8 to 393.3 ±2.5 and began with the Lochkovian stage, which lasted until the Pragian. It spanned from 410.8 ±2.8 to 407.6 ±2.5, and was followed by the Emsian, which lasted until the Middle Devonian began,393. 3±2.7 million years ago. Middle Devonian The Middle Devonian comprised two subdivisions, first the Eifelian, which gave way to the Givetian 387. 7±2.7 million years ago. Late Devonian Finally, the Late Devonian started with the Frasnian,382.7 ±2.8 to 372.2 ±2.5, during which the first forests took shape on land. The first tetrapods appeared in the record in the ensuing Famennian subdivision. This lasted until the end of the Devonian,358. 9±2.5 million years ago, the Devonian was a relatively warm period, and probably lacked any glaciers
The kangaroo is a marsupial from the family Macropodidae. The Australian government estimates that 34.3 million kangaroos lived within the commercial harvest areas of Australia in 2011, as with the terms wallaroo and wallaby, kangaroo refers to a polyphyletic grouping of species. All three refer to members of the taxonomic family and are distinguished according to size. The largest species in the family are called kangaroos and the smallest are generally called wallabies, the term wallaroos refers to species of an intermediate size. There is the tree-kangaroo, another genus of macropod, which inhabits the rainforests of New Guinea, far northeastern Queensland. Kangaroos have large, powerful legs, large feet adapted for leaping, a long muscular tail for balance. Like most marsupials, female kangaroos have a pouch called a marsupium in which joeys complete postnatal development, the large kangaroos have adapted much better than the smaller macropods to land clearing for pastoral agriculture and habitat changes brought to the Australian landscape by humans.
Many of the species are rare and endangered, while kangaroos are relatively plentiful. The kangaroo is important to both Australian culture and the image, and consequently there are numerous popular culture references. Wild kangaroos are shot for meat, leather hides, and to grazing land. Although controversial, kangaroo meat has perceived health benefits for human consumption compared with traditional meats due to the low level of fat on kangaroos, the word kangaroo derives from the Guugu Yimithirr word gangurru, referring to grey kangaroos. Cook first referred to kangaroos in his entry of 4 August. Guugu Yimithirr is the language of the people of the area, a common myth about the kangaroos English name is that kangaroo was a Guugu Yimithirr phrase for I dont understand you. According to this legend and Banks were exploring the area when they happened upon the animal and they asked a nearby local what the creatures were called. The local responded Kangaroo, meaning I dont understand you, which Cook took to be the name of the creature and this myth was debunked in the 1970s by linguist John B.
Haviland in his research with the Guugu Yimithirr people, Kangaroos are often colloquially referred to as roos. Male kangaroos are called bucks, jacks, or old men, females are does, flyers, or jills, the collective noun for kangaroos is a mob, troop, or court. There are four species that are referred to as kangaroos
In biology and ecology, extinction is the end of an organism or of a group of organisms, normally a species. The moment of extinction is considered to be the death of the last individual of the species, although the capacity to breed. Because a species range may be very large, determining this moment is difficult. This difficulty leads to such as Lazarus taxa, where a species presumed extinct abruptly reappears after a period of apparent absence. More than 99 percent of all species, amounting to five billion species. Estimates on the number of Earths current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described, the relationship between animals and their ecological niches has been firmly established. Mass extinctions are relatively rare events, isolated extinctions are quite common, only recently have extinctions been recorded and scientists have become alarmed at the current high rate of extinctions.
Most species that become extinct are never scientifically documented, some scientists estimate that up to half of presently existing plant and animal species may become extinct by 2100. A dagger symbol next to a name is often used to indicate its extinction. A species is extinct when the last existing member dies, Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation. Pinpointing the extinction of a species requires a definition of that species. If it is to be declared extinct, the species in question must be distinguishable from any ancestor or daughter species. Extinction of a plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould. In ecology, extinction is often used informally to refer to local extinction, in which a species ceases to exist in the area of study. This phenomenon is known as extirpation. Local extinctions may be followed by a replacement of the species taken from other locations, species which are not extinct are termed extant.
Those that are extant but threatened by extinction are referred to as threatened or endangered species, currently an important aspect of extinction is human attempts to preserve critically endangered species
The Pliocene Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58 million years BP. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era, the Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch.588 to 1.806 million years ago, and is now included in the Pleistocene. As with other geologic periods, the geological strata that define the start and end are well identified but the exact dates of the start. The boundaries defining the Pliocene are not set at an easily identified worldwide event, the upper boundary was set at the start of the Pleistocene glaciations. The Pliocene was named by Sir Charles Lyell, the name comes from the Greek words πλεῖον and καινός and means roughly continuation of the recent, referring to the essentially modern marine mollusc faunas. H. W. Fowler called the term a regrettable barbarism, in the official timescale of the ICS, the Pliocene is subdivided into two stages. From youngest to oldest they are, Piacenzian Zanclean The Piacenzian is sometimes referred to as the Late Pliocene, in the system of North American Land Mammal Ages include Hemphillian, and Blancan.
The Blancan extends forward into the Pleistocene, South American Land Mammal Ages include Montehermosan and Uquian. In the Paratethys area the Pliocene contains the Dacian and Romanian stages, as usual in stratigraphy, there are many other regional and local subdivisions in use. In Britain the Pliocene is divided into the stages, Waltonian, Pre-Ludhamian, Thurnian, Bramertonian or Antian, Pre-Pastonian or Baventian and Beestonian. The exact correlations between these stages and the ICS stages is still a matter of detail. The formation of an Arctic ice cap is signaled by a shift in oxygen isotope ratios and ice-rafted cobbles in the North Atlantic. Mid-latitude glaciation was probably underway before the end of the epoch, the global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas. Continents continued to drift, moving from positions possibly as far as 250 km from their present locations to positions only 70 km from their current locations, africas collision with Europe formed the Mediterranean Sea, cutting off the remnants of the Tethys Ocean.
The border between the Miocene and the Pliocene is the time of the Messinian salinity crisis, Sea level changes exposed the land-bridge between Alaska and Asia. Pliocene marine rocks are exposed in the Mediterranean, India. Elsewhere, they are exposed largely near shores, the change to a cooler, seasonal climate had considerable impacts on Pliocene vegetation, reducing tropical species worldwide. Deciduous forests proliferated, coniferous forests and tundra covered much of the north, tropical forests were limited to a tight band around the equator, and in addition to dry savannahs, deserts appeared in Asia and Africa
A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head. The most common design for hunting or combat spears since ancient times has incorporated a metal spearhead shaped like a triangle, the heads of fishing spears usually feature barbs or serrated edges. The word spear comes from the Old English spere, from the Proto-Germanic speri, from a Proto-Indo-European root *sper- spear, Spears can be divided into two broad categories, those designed for thrusting in melee combat and those designed for throwing. The spear has been used throughout history both as a hunting and fishing tool and as a weapon. Along with the axe and club, it is one of the earliest and most important tools developed by early humans, as a weapon, it may be wielded with either one hand or two. It was used in every conflict up until the modern era, where even it continues on in the form of the bayonet. Spear manufacture and use is not confined to human beings and it is practiced by the western chimpanzee.
Chimpanzees near Kédougou, Senegal have been observed to create spears by breaking straight limbs off trees, stripping them of their bark and side branches and they used the weapons to hunt galagos sleeping in hollows. Orangutans have used spears to fish, presumably after observing humans fishing in a similar manner, neanderthals were constructing stone spear heads from as early as 300,000 BP and by 250,000 years ago, wooden spears were made with fire-hardened points. From 200,000 BP onwards, Middle Paleolithic humans began to make stone blades with flaked edges which were used as spear heads. These stone heads could be fixed to the shaft by gum or resin or by bindings made of animal sinew. During this period, a clear difference remained between spears designed to be thrown and those designed to be used in hand-to-hand combat, by the Magdalenian period, spear-throwers similar to the atlatl were in use. Spears were one of the most common weapons used in the Stone Age. They may be seen as the ancestor of such weapons as the lance, the pilum, the halberd, the naginata, the glaive, the bill.
Spears may be used as both a projectile and melee weapons, Spears used primarily for thrusting may be used with either one or two hands and tend to have heavier and sturdier designs than those intended exclusively for throwing. From the atlatl dart, the arrow for use with bows eventually developed, one-handed spears featuring socketed metal heads were used in conjunction with a shield by the earliest Bronze Age cultures. They were wielded in either combat or in large troop formations. This tradition continued from the first Mesopotamian cultures, through the various ancient Egyptian dynasties, during this time the spear was used by cavalry
Sir Richard Owen KCB FRMS FRS was an English biologist, comparative anatomist and paleontologist. Despite being a figure, Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils. Owen produced a vast array of work, but is probably best remembered today for coining the word Dinosauria. Owens approach to evolution can be seen as having anticipated the issues that have gained attention with the recent emergence of evolutionary developmental biology. Owen campaigned for the specimens in the British Museum to be given a new home. This resulted in the establishment, in 1881, of the now world-famous Natural History Museum in South Kensington, bill Bryson argues that, by making the Natural History Museum an institution for everyone, Owen transformed our expectations of what museums are for. Owen was feared and even hated by some such as Thomas Henry Huxley. His career was tainted by a number of controversies, many of which involved accusations that he took credit for other peoples work, Owen was born in Lancaster in 1804, one of six children of a West Indian Merchant named Richard Owen.
His mother, Catherine Parrin, was descended from Huguenots and he was educated at Lancaster Royal Grammar School, in 1820, he was apprenticed to a local surgeon and apothecary and, in 1824, he proceeded as a medical student to the University of Edinburgh. He left the university in the year and completed his medical course in St Bartholomews Hospital, London. In July 1835 Owen married Caroline Amelia Clift in St Pancras by whom he had one son, Richard sadly outlived both his wife and only son. After his death, in 1892, he was survived by his three grandchildren and daughter-in-law Emily Owen, to whom he left much of his £33,000 fortune. Upon completing his education, he contemplated the usual professional career and he was induced by Abernethy to accept the position of assistant to William Clift, conservator of the museum of the Royal College of Surgeons. This congenial occupation soon led him to abandon his intention of medical practice, in 1836, Owen was appointed Hunterian professor, in the Royal College of Surgeons and, in 1849, he succeeded Clift as conservator.
He held the office until 1856, when he became superintendent of the natural history department of the British Museum. He retained office until the completion of work, in December,1883. He lived quietly in retirement at Sheen Lodge, Richmond Park and his career was tainted by accusations that he failed to give credit to the work of others and even tried to appropriate it in his own name. This came to a head in 1846, when he was awarded the Royal Medal for a paper he had written on belemnites, Owen had failed to acknowledge that the belemnite had been discovered by Chaning Pearce, an amateur biologist, four years earlier
The Permian is a geologic period and system which spans 46.7 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic Period 252.2 Mya. It is the last period of the Paleozoic Era, the following Triassic Period belongs to the Mesozoic 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 groups of the mammals, lepidosaurs. The world at the time was dominated by two 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, 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 Earths history, in which nearly 90% of marine species and it would take well into the Triassic for life to recover from this catastrophe.
Recovery from the Permian-Triassic extinction event was protracted, on land, 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 Edouard de Verneuil, the region now lies in the Perm Krai of Russia. This could have in part caused the extinctions of marine species at the end of the period by severely reducing shallow coastal areas preferred by many marine organisms. During the Permian, all the Earths major landmasses were collected into a supercontinent known as Pangaea. The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, 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, deserts seem to have been widespread on Pangaea. Such dry conditions favored gymnosperms, plants with seeds enclosed in a cover, over plants such as ferns that disperse spores in a wetter environment.
The first modern trees appeared in the Permian, 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 gradually warmed, drying the continents 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. By the close of the Permian, trilobites and a host of other groups became extinct. Terrestrial life in the Permian included diverse plants, arthropods, the period saw a massive desert covering the interior of Pangaea
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya and its subdivisions, and its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latinised form of Cymru, the Welsh name for Wales, as a result, our understanding of the Cambrian biology surpasses that of some periods. The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia, the seas were relatively warm, and polar ice was absent for much of the period. The United States Federal Geographic Data Committee uses a barred capital C ⟨Є⟩ character similar to the capital letter Ukrainian Ye ⟨Є⟩ to represent the Cambrian Period, the proper Unicode character is U+A792 Ꞓ LATIN CAPITAL LETTER C WITH BAR.
Despite the long recognition of its distinction from younger Ordovician Period rocks and older Supereon Precambrian rocks, the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. Pedum in Namibia and Newfoundland, and possibly, in the western USA, the stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain. The Cambrian Period followed the Ediacaran Period and was followed by the Ordovician Period, the Cambrian is divided into four epochs and ten ages. Currently only two series and five stages are named and have a GSSP, because the international stratigraphic subdivision is not yet complete, many local subdivisions are still widely used. In some of these subdivisions the Cambrian is divided into three epochs with locally differing names – the Early Cambrian, Middle Cambrian and Furongian, rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.
Trilobite zones allow biostratigraphic correlation in the Cambrian, each of the local epochs is divided into several stages. The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago, the lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites. The recognition of small shelly fossils before the first trilobites, and Ediacara biota substantially earlier and this formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago quickly gained favour, though the used to obtain this number are now considered to be unsuitable. A more precise date using modern radiometric dating yield a date of 541 ±0.3 million years ago, most continental land was clustered in the Southern Hemisphere at this time, but was drifting north. Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian, the sea levels fluctuated somewhat, suggesting there were ice ages, associated with pulses of expansion and contraction of a south polar ice cap.
In Baltoscandia a Lower Cambrian transgression transformed large swathes of the Sub-Cambrian peneplain into a epicontinental sea, the Earth was generally cold during the early Cambrian, probably due to the ancient continent of Gondwana covering the South Pole and cutting off polar ocean currents
Mammals are any vertebrates within the class Mammalia, a clade of endothermic amniotes distinguished from reptiles by the possession of a neocortex, three middle ear bones and mammary glands. All female mammals nurse their young with milk, secreted from the mammary glands, Mammals include the largest animals on the planet, the great whales. The basic body type is a quadruped, but some mammals are adapted for life at sea, in the air, in trees. The largest group of mammals, the placentals, have a placenta, Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale. With the exception of the five species of monotreme, all modern mammals give birth to live young, most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents and Soricomorpha, the next three biggest orders, depending on the biological classification scheme used, are the Primates, the Cetartiodactyla, and the Carnivora. Living mammals are divided into the Yinotheria and Theriiformes There are around 5450 species of mammal, in some classifications, extant mammals are divided into two subclasses, the Prototheria, that is, the order Monotremata, and the Theria, or the infraclasses Metatheria and Eutheria.
The marsupials constitute the group of the Metatheria, and include all living metatherians as well as many extinct ones. Much of the changes reflect the advances of cladistic analysis and molecular genetics, findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora. The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade, the early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the line that led to todays reptiles. Some mammals are intelligent, with some possessing large brains, self-awareness, 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, most mammals are polygynous, but some can be monogamous or polyandrous.
They provided, and continue to provide, power for transport and agriculture, as well as commodities such as meat, dairy products, wool. Mammals are hunted or raced for sport, and are used as model organisms in science, Mammals have been depicted in art since Palaeolithic times, and appear in literature, film and religion. Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is accepted, McKenna & Bell and Wilson & Reader provide useful recent compendiums. Though field work gradually made Simpsons classification outdated, it remains the closest thing to a classification of mammals
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. Lapworth recognized that the fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods. It received international sanction in 1960, when it was adopted as a 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, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life, the worlds first true vertebrates, continued to evolve, and 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 during the Ordovician compared with today. The Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event and it lasted for about 42 million years and ended with the Ordovician–Silurian extinction event, about 443.8 Mya which wiped out 60% of marine genera.
The dates given are recent radiometric dates and vary slightly from those found in other sources and 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 highly significant and 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 and this makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected an international system of subdivisions. There exist Baltoscandic, Siberian, North American, 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, the Floian corresponds to the lower Arenig, the Arenig continues until the early Darriwilian, subsuming the Dapingian.
The Llanvirn occupies the rest of the Darriwilian, and terminates with it at the base of the Late Ordovician. The Sandbian represents the first half of the Caradoc, the Caradoc ends in the mid-Katian, 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, the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia
As a crystalline material is heated during measurements the process of thermoluminescence starts. Thermoluminescence emits a light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating, the technique has wide application, and is relatively cheap at some US$300–700 per object, ideally a number of samples are tested. Sediments are more expensive to date, the destruction of a relatively significant amount of sample material is necessary, which can be a limitation in the case of artworks. The heating must have taken the object above 500° C, which covers most ceramics and it will often work well with stones that have been heated by fire. The clay core of bronze sculptures made by lost wax casting can be tested, different materials vary considerably in their suitability for the technique, depending on several factors. Subsequent irradiation, for if an x-ray is taken, can affect accuracy. Ideally this is assessed by measurements made at the precise findspot over a long period, for artworks, it may be sufficient to confirm whether a piece is broadly ancient or modern, and this may be possible even if a precise date cannot be estimated.
These imperfections lead to local humps and dips in the crystalline materials electric potential, where there is a dip, a free electron may be attracted and trapped. The flux of ionizing radiation—both from cosmic radiation and from natural radioactivity—excites electrons from atoms in the lattice into the conduction band where they can move freely. Most excited electrons will recombine with lattice ions, but some will be trapped. Depending on the depth of the traps the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years. In the process of recombining with an ion, they lose energy and emit photons. The amount of light produced is proportional to the number of trapped electrons that have been freed which is in turn proportional to the dose accumulated. In order to relate the signal to the dose that caused it, it is necessary to calibrate the material with known doses of radiation since the density of traps is highly variable.
Thermoluminescence dating presupposes a zeroing event in the history of the material, either heating or exposure to sunlight, therefore, at that point the thermoluminescence signal is zero. As time goes on, the radiation field around the material causes the trapped electrons to accumulate. In the laboratory, the radiation dose can be measured