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, 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 be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating every known aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral and pelvic fins evolved into legs.
In the oceans, primitive sharks became more numerous than in the Late Ordovician. The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common; the Late Devonian extinction which started about 375 million years ago affected marine life, killing off all placodermi, 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, the early formation of the small continent of Euramerica in between; the period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was resolved by the definition of the Devonian period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough.
Murchison and Sedgwick named the period they proposed as the Devonian System. 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, to the beginning of the Carboniferous 358.9 Mya. In nineteenth-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. 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, the latter three of which are placed in the Devonian; the Devonian has erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid but in fact the climate in the Devonian differed during its epochs and between geographic regions.
For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, North America, China, but Africa and South America had a warm temperate climate. 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 DevonianThe 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, was followed by the Emsian, which lasted until the Middle Devonian began, 393.3± 2.7 million years ago. During this time, the first ammonoids appeared. Ammonoids during this time period differed little from their nautiloid counterparts; these ammonoids belong to the order Agoniatitida, which in epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida.
This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic era. Middle DevonianThe Middle Devonian comprised two subdivisions: first the Eifelian, which gave way to the Givetian 387.7± 2.7 million years ago. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments due to drastic environmental changes and due to the increasing competition and diversity of jawed fishes; the shallow, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs, the ability to crawl out of the water and onto the land for short periods of time. Late DevonianFinally, 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 fossil record in the ensuing Famennian subdivisi
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are uncertain; the name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich. The Oligocene is followed by the Miocene Epoch; the Oligocene is the final epoch of the Paleogene Period. The Oligocene is considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, a regression of tropical broad leaf forests to the equatorial belt; the start of the Oligocene is marked by a notable extinction event called the Grande Coupure. By contrast, the Oligocene–Miocene boundary is not set at an identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the cooler Miocene.
Oligocene faunal stages from youngest to oldest are: The Paleogene Period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2 °C, 400,000-year temperature depression leads the 2 °C, seven-million-year stepwise climate change 33.5 Ma. The stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55 m decrease in sea level with a related temperature depression; the 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma. A deep 400,000-year glaciated Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island. During this epoch, the continents continued to drift toward their present positions. Antarctica became more isolated and developed an ice cap. Mountain building in western North America continued, the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea.
A brief marine incursion marks the early Oligocene in Europe. Marine fossils from the Oligocene are rare in North America. There appears to have been a land bridge in the early Oligocene between North America and Europe, since the faunas of the two regions are similar. Sometime during the Oligocene, South America was detached from Antarctica and drifted north towards North America, it allowed the Antarctic Circumpolar Current to flow cooling the Antarctic continent. Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced by temperate deciduous forests. Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts; however at the end of the period, grass was not quite common enough for modern savannas. In North America, subtropical species dominated with cashews and lychee trees present, temperate trees such as roses and pines were common; the legumes spread, while sedges and ferns continued their ascent.
More open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleocene epoch 30 million years earlier. Marine faunas became modern, as did terrestrial vertebrate fauna on the northern continents; this was more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as equids, rhinos and camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded; the first felid, originated in Asia during the late Oligocene and spread to Europe. South America was isolated from the other continents and evolved a quite distinct fauna during the Oligocene; the South American continent became home to strange animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchians, terror birds, carnivorous metatheres, like the borhyaenids remained the dominant predators. Brontotheres died out in the Earliest Oligocene, creodonts died out outside Africa and the Middle East at the end of the period.
Multituberculates, an ancient lineage of primitive mammals that originated back in the Jurassic became extinct in the Oligocene, aside from the gondwanatheres. The Oligocene was home to a wide variety of strange mammals. A good example of this would be the White River Fauna of central North America, which were a semiarid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium, running rhinoceratoids, three-toed equids, nimravids and early canids like Hesperocyon. Merycoidodonts, an endemic American group, were diverse during this time. In Asia during the Oligocene, a group of running rhinoceratoids gave rise to the indricotheres, like Paraceratherium, which were the largest land mammals to walk the Earth; the marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene; the fossil record of marine mammals is a little spotty during this time, not as well known as the Eocene o
Canada is a country in the northern part of North America. Its ten provinces and three territories extend from the Atlantic to the Pacific and northward into the Arctic Ocean, covering 9.98 million square kilometres, making it the world's second-largest country by total area. Canada's southern border with the United States is the world's longest bi-national land border, its capital is Ottawa, its three largest metropolitan areas are Toronto and Vancouver. As a whole, Canada is sparsely populated, the majority of its land area being dominated by forest and tundra, its population is urbanized, with over 80 percent of its inhabitants concentrated in large and medium-sized cities, many near the southern border. Canada's climate varies across its vast area, ranging from arctic weather in the north, to hot summers in the southern regions, with four distinct seasons. Various indigenous peoples have inhabited what is now Canada for thousands of years prior to European colonization. Beginning in the 16th century and French expeditions explored, settled, along the Atlantic coast.
As a consequence of various armed conflicts, France ceded nearly all of its colonies in North America in 1763. In 1867, with the union of three British North American colonies through Confederation, Canada was formed as a federal dominion of four provinces; this began an accretion of provinces and territories and a process of increasing autonomy from the United Kingdom. This widening autonomy was highlighted by the Statute of Westminster of 1931 and culminated in the Canada Act of 1982, which severed the vestiges of legal dependence on the British parliament. Canada is a parliamentary democracy and a constitutional monarchy in the Westminster tradition, with Elizabeth II as its queen and a prime minister who serves as the chair of the federal cabinet and head of government; the country is a realm within the Commonwealth of Nations, a member of the Francophonie and bilingual at the federal level. It ranks among the highest in international measurements of government transparency, civil liberties, quality of life, economic freedom, education.
It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many other countries. Canada's long and complex relationship with the United States has had a significant impact on its economy and culture. A developed country, Canada has the sixteenth-highest nominal per capita income globally as well as the twelfth-highest ranking in the Human Development Index, its advanced economy is the tenth-largest in the world, relying chiefly upon its abundant natural resources and well-developed international trade networks. Canada is part of several major international and intergovernmental institutions or groupings including the United Nations, the North Atlantic Treaty Organization, the G7, the Group of Ten, the G20, the North American Free Trade Agreement and the Asia-Pacific Economic Cooperation forum. While a variety of theories have been postulated for the etymological origins of Canada, the name is now accepted as coming from the St. Lawrence Iroquoian word kanata, meaning "village" or "settlement".
In 1535, indigenous inhabitants of the present-day Quebec City region used the word to direct French explorer Jacques Cartier to the village of Stadacona. Cartier used the word Canada to refer not only to that particular village but to the entire area subject to Donnacona. From the 16th to the early 18th century "Canada" referred to the part of New France that lay along the Saint Lawrence River. In 1791, the area became two British colonies called Upper Canada and Lower Canada collectively named the Canadas. Upon Confederation in 1867, Canada was adopted as the legal name for the new country at the London Conference, the word Dominion was conferred as the country's title. By the 1950s, the term Dominion of Canada was no longer used by the United Kingdom, which considered Canada a "Realm of the Commonwealth"; the government of Louis St. Laurent ended the practice of using'Dominion' in the Statutes of Canada in 1951. In 1982, the passage of the Canada Act, bringing the Constitution of Canada under Canadian control, referred only to Canada, that year the name of the national holiday was changed from Dominion Day to Canada Day.
The term Dominion was used to distinguish the federal government from the provinces, though after the Second World War the term federal had replaced dominion. Indigenous peoples in present-day Canada include the First Nations, Métis, the last being a mixed-blood people who originated in the mid-17th century when First Nations and Inuit people married European settlers; the term "Aboriginal" as a collective noun is a specific term of art used in some legal documents, including the Constitution Act 1982. The first inhabitants of North America are hypothesized to have migrated from Siberia by way of the Bering land bridge and arrived at least 14,000 years ago; the Paleo-Indian archeological sites at Old Crow Flats and Bluefish Caves are two of the oldest sites of human habitation in Canada. The characteristics of Canadian indigenous societies included permanent settlements, complex societal hierarchies, trading networks; some of these cultures had collapsed by the time European explorers arrived in the late 15th and early 16th centuries and have only been discovered through archeological investigations.
The indigenous population at the time of the first European settlements is estimated to have been between 200,000
The Tetrapodomorpha are a clade of vertebrates consisting of tetrapods and their closest sarcopterygian relatives that are more related to living tetrapods than to living lungfish. Advanced forms transitional between fish and the early labyrinthodonts, such as Tiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish, half-tetrapods, in appearance and limb morphology; the Tetrapodomorpha contains the crown group tetrapods and several groups of early stem tetrapods, which includes several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda are the Stem Tetrapoda, a paraphyletic unit encompassing the fish to tetrapod transition. Among the characters defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa. Another key trait is choana. Most fish have two pairs of nostrils, one on either side of the head for incoming water and another pair for outgoing water.
Early tetrapodomorphs such as Kenichthys had excurrent nostrils that had migrated to the edge of the mouth. In tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana. Tetrapodomorph fossils are known from the early Devonian onwards, include Osteolepis, Panderichthys and Tungsenia. After Benton, 2004. Subclass SarcopterygiiInfraclass TetrapodomorphaOrder RhizodontidaFamily Sauripteridae Family RhizodontidaeSuperorder Osteolepidida Family Canowindridae Family Thysanolepidae Family TristichopteridaeOrder Osteolepiformes Family Osteolepidae Family MegalichthyidaeClade Elpistostegalia Clade StegocephaliaFamily Elpistostegidae Family Whatcheeriidae Family Colosteidae Superfamily Baphetoidea Superclass TetrapodaOther clades include the Eotetrapodiformes. Older taxons that include late stem tetrapods and early tetrapods are the Labyrinthodontia and Ichthyostegalia; the cladogram is based on a phylogenetic analysis of 46 taxa using 204 characters by B. Swartz in 2012.
Mikko Haaramo. "Tetrapodomorpha - Terrestrial vertebrate-like sarcopterygians". Archived from the original on 12 May 2006. Retrieved 6 April 2006. P. E. Ahlberg & Z. Johanson. "Osteolepiforms and the ancestry of tetrapods". Nature. 395: 792–794. Bibcode:1998Natur.395..792A. Doi:10.1038/27421. Michel Laurin, Marc Girondot & Armand de Ricqlès. "Early tetrapod evolution". TREE. 15
Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. In archaeology, absolute dating is based on the physical and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates. Techniques include tree rings in timbers, radiocarbon dating of wood or bones, trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.
In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from young to systems such as uranium–lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth. Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments. One of the most used and well-known absolute dating techniques is carbon-14 dating, used to date organic remains.
This is a radiometric technique. Cosmic radiation entering the earth’s atmosphere produces carbon-14, plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as predators eat other animals. With death, the uptake of carbon-14 stops, it takes 5,730 years for half the carbon-14 to change to nitrogen. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact; the short half-life of carbon-14, 5,730 years, makes dating reliable only up to about 50,000 years. The technique cannot pinpoint the date of an archeological site better than historic records, but is effective for precise dates when calibrated with other dating techniques such as tree-ring dating. An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem.
It is possible in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating; the development of accelerator mass spectrometry dating, which allows a date to be obtained from a small sample, has been useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most used is potassium–argon dating. Potassium-40 is a radioactive isotope of potassium that decays into argon-40; the half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.
Argon, a noble gas, is not incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K–Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing dates items to the last time they were heated; this technique is based on the principle. This process frees electrons within minerals. Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light; this light can be measured to determine the last time. Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
Because of these and other factors, Thermoluminescence is at the most about 15% accurate. It cannot be used to date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to s
Groenlandaspis is an extinct genus of arthrodire from the Late Devonian. Fossils of the different species are found late Devonian strata in all continents except eastern Asia; the generic name commemorates the fact that the first specimens of the type species were found in Greenland. As with all other arthrodires, Groenlandaspis had a joint in the back of its head with its thoracic armor, allowing for its head to be thrown back, increasing its gape. However, as its head is somewhat compressed in comparison with many other arthrodires, as the dorsal side came to a low, pyramid-like peak, it is believed that Groenlandaspis could not throw its head back far, it was a small fish, only 7.5 centimetres in length, lived on the ocean floor, where it is thought to have fed on either small prey, or detritus.
The Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is called the Age of Reptiles and the Age of Conifers; the Mesozoic is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic and succeeded by the Cenozoic. The era is subdivided into three major periods: the Triassic and Cretaceous, which are further subdivided into a number of epochs and stages; the era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic and evolutionary activity; the era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between cooling periods. Overall, the Earth was hotter than it is today.
Dinosaurs first appeared in the Mid-Triassic, became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic; the first mammals appeared during the Mesozoic, but would remain small—less than 15 kg —until the Cenozoic. The flowering plants arose in the Triassic or Jurassic and came to prominence in the late Cretaceous when they replaced the conifers and other gymnosperms as the dominant trees; the phrase "Age of Reptiles" was introduced by the 19th century paleontologist Gideon Mantell who viewed it as dominated by diapsids such as Iguanodon, Megalosaurus and Pterodactylus. Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζῷον meaning "animal" or "living being"; the name "Mesozoic" was proposed in 1840 by the British geologist John Phillips. Following the Paleozoic, the Mesozoic extended 186 million years, from 251.902 to 66 million years ago when the Cenozoic Era began.
This time frame is separated into three geologic periods. From oldest to youngest: Triassic Jurassic Cretaceous The lower boundary of the Mesozoic is set by the Permian–Triassic extinction event, during which 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct, it is known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper boundary of the Mesozoic is set at the Cretaceous–Paleogene extinction event, which may have been caused by an asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are believed to have contributed to the Cretaceous–Paleogene extinction event. 50% of all genera became extinct, including all of the non-avian dinosaurs. The Triassic ranges from 252 million to 201 million years ago, preceding the Jurassic Period; the period is bracketed between the Permian–Triassic extinction event and the Triassic–Jurassic extinction event, two of the "big five", it is divided into three major epochs: Early and Late Triassic.
The Early Triassic, about 252 to 247 million years ago, was dominated by deserts in the interior of the Pangaea supercontinent. The Earth had just witnessed a massive die-off in which 95% of all life became extinct, the most common vertebrate life on land were lystrosaurus and euparkeria along with many other creatures that managed to survive the Permian extinction. Temnospondyls would be the dominant predator for much of the Triassic; the Middle Triassic, from 247 to 237 million years ago, featured the beginnings of the breakup of Pangaea and the opening of the Tethys Sea. Ecosystems had recovered from the Permian extinction. Algae, sponge and crustaceans all had recovered, new aquatic reptiles evolved, such as ichthyosaurs and nothosaurs. On land, pine forests flourished, as did groups of insects like mosquitoes and fruit flies. Reptiles began to get bigger and bigger, the first crocodilians and dinosaurs evolved, which sparked competition with the large amphibians that had ruled the freshwater world mammal-like reptiles on land.
Following the bloom of the Middle Triassic, the Late Triassic, from 237 to 201 million years ago, featured frequent heat spells and moderate precipitation. The recent warming led to a boom of dinosaurian evolution on land as those one began to separate from each other, as well as first pterosaurs. During the Late Triassic, some advanced cynodonts gave rise to the first Mammaliaformes. All this climatic change, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which many archosaurs, most synapsids, all large amphibians became extinct, as well as 34% of marine life, in the Earth's fourth mass extinction event; the cause is debatable. The Jurassic ranges from 200 million years to 145 million years ago and features three major epochs: The Early Jurassic, the Middle Jurassic, the L