Yukon is the smallest and westernmost of Canada's three federal territories. It has the smallest population of any province or territory in Canada, with 35,874 people, although it has the largest city in any of the three territories. Whitehorse is Yukon's only city. Yukon was split from the Northwest Territories in 1898 and was named the Yukon Territory; the federal government's Yukon Act, which received royal assent on March 27, 2002, established Yukon as the territory's official name, though Yukon Territory is still popular in usage and Canada Post continues to use the territory's internationally approved postal abbreviation of YT. Though bilingual, the Yukon government recognizes First Nations languages. At 5,959 m, Yukon's Mount Logan, in Kluane National Park and Reserve, is the highest mountain in Canada and the second-highest on the North American continent. Most of Yukon has a subarctic climate, characterized by brief warm summers; the Arctic Ocean coast has a tundra climate. Notable rivers include the Yukon River, as well as the Pelly, Peel and Tatshenshini rivers.
The territory is named after the longest river in Yukon. The name itself is from a contraction of the words in the Gwich'in phrase chųų gąįį han, which means white water river and refers to "the pale colour" of glacial runoff in the Yukon River. Long before the arrival of Europeans and southern Yukon was populated by First Nations people, the area escaped glaciation. Sites of archeological significance in Yukon hold some of the earliest evidence of the presence of human habitation in North America; the sites safeguard the earliest First Nations of the Yukon. The volcanic eruption of Mount Churchill in 800 AD in what is now the U. S. state of Alaska blanketed southern Yukon with a layer of ash which can still be seen along the Klondike Highway, which forms part of the oral tradition of First Nations peoples in Yukon and further south in Canada. Coastal and inland First Nations had extensive trading networks. European incursions into the area began early in the 19th century with the fur trade, followed by missionaries.
By the 1870s and 1880s gold miners began to arrive. This drove a population increase that justified the establishment of a police force, just in time for the start of the Klondike Gold Rush in 1897; the increased population coming with the gold rush led to the separation of the Yukon district from the Northwest Territories and the formation of the separate Yukon Territory in 1898. The territory is the approximate shape of a right triangle, bordering the U. S. state of Alaska to the west and northwest for 1,210 km along longitude 141° W, the Northwest Territories to the east and British Columbia to the south. Its northern coast is on the Beaufort Sea, its ragged eastern boundary follows the divide between the Yukon Basin and the Mackenzie River drainage basin to the east in the Mackenzie mountains. Most of the territory is in the watershed of the Yukon River; the southern Yukon is dotted with a large number of large and narrow glacier-fed alpine lakes, most of which flow into the Yukon River system.
The larger lakes include Teslin Lake, Atlin Lake, Tagish Lake, Marsh Lake, Lake Laberge, Kusawa Lake and Kluane Lake. Bennett Lake on the Klondike Gold Rush trail is a lake flowing into Nares Lake, with the greater part of its area within Yukon. Canada's highest point, Mount Logan, is in the territory's southwest. Mount Logan and a large part of Yukon's southwest are in Kluane National Park and Reserve, a UNESCO World Heritage Site. Other national parks include Ivvavik National Vuntut National Park in the north. Other watersheds include the Mackenzie River, the Peel Watershed and the Alsek–Tatshenshini, a number of rivers flowing directly into the Beaufort Sea; the two main Yukon rivers flowing into the Mackenzie in the Northwest Territories are the Liard River in the southeast and the Peel River and its tributaries in the northeast. Notable widespread tree species within Yukon are white spruce. Many trees are stunted because of severe climate; the capital, Whitehorse, is the largest city, with about three-quarters of the population.
British Columbia Northwest Territories Alaska, United States While the average winter temperature in Yukon is mild by Canadian arctic standards, no other place in North America gets as cold as Yukon during extreme cold snaps. The temperature has dropped down to −60 °C three times, 1947, 1954, 1968; the most extreme cold snap occurred in February 1947 when the abandoned town of Snag dropped down to −63.0 °C. Unlike most of Canada where the most extreme heat waves occur in July and September, Yukon's extreme heat tends to occur in June and May. Yukon has recorded 36 °C three times; the first time was in June 1969 when Mayo recorded a temperature of 36.1 °C. 14 years this record was beaten when Forty Mile recorded 36 °C in May 1983. The old record was broken 21 years in June 2004 when the Mayo Road weather station, located just northwest of Whitehorse, recorded a temperature of 36.5 °C. The 2016 census reported a Yukon population of 35,874, an increase of 5.8% from 2011. With a land area of 474,712.64 km2, it had a population de
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. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were 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 warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. 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 gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
Bison are large, even-toed ungulates in the genus Bison within the subfamily Bovinae. Two extant and six extinct species are recognised. Of the six extinct species, five became extinct in the Quaternary extinction event. Bison palaeosinensis evolved in the Early Pleistocene in South Asia, was the evolutionary ancestor of B. priscus, the ancestor of all other Bison species. From 2 MYA to 6,000 BC, steppe bison ranged across the mammoth steppe, inhabiting Europe and northern Asia with B. schoetensacki, North America with B. antiquus, B. latifrons, B. occidentalis. The last species to go extinct, B. occidentalis, was succeeded at 3,000 BC by B. bison. Of the two surviving species, the American bison, B. bison, found only in North America, is the more numerous. Although known as a buffalo in the United States and Canada, it is only distantly related to the true buffalo; the North American species is composed of two subspecies, the Plains bison, B. b. bison, the Wood bison, B. b. athabascae, the namesake of Wood Buffalo National Park in Canada.
A third subspecies, the Eastern Bison is no longer considered a valid taxon, being a junior synonym of B. b. bison. References to "Woods Bison" or "Wood Bison" from the eastern United States confusingly refer to this subspecies, not B. b. athabascae, not found in the region. The European bison, B. bonasus, or wisent, is found in Europe and the Caucasus, reintroduced after being extinct in the wild. While all bison species are classified in their own genus, they are sometimes bred with domestic cattle and produce fertile offspring called beefalo or zubron; the American bison and the European bison are the largest surviving terrestrial animals in North America and Europe. They are typical artiodactyl ungulates, are similar in appearance to other bovines such as cattle and true buffalo, they are muscular with shaggy coats of long hair. Adults grow up to 1.8 metres in length for American Bison and up to 2.8 metres in length for European bison. American bison can weigh from 400 kilograms to 900 kg and European bison can weight from 800 kilograms to 1,000 kilograms.
European bison tend to be heavier than American bison. Bison are nomadic grazers and travel in herds; the bulls leave the herds of females at two or three years of age, join a male herd, which are smaller than female herds. Mature bulls travel alone. Towards the end of the summer, for the reproductive season, the sexes commingle. American bison are known for living in the Great Plains, but had a much larger range including much of the eastern United States and parts of Mexico. Both species were hunted close to extinction during the 19th and 20th centuries, but have since rebounded; the American Plains bison is no longer listed as endangered, but this does not mean the species is secure. Genetically pure B. b. bison number only ~20,000, separated into fragmented herds—all of which require active conservation measures. The Wood bison is on the endangered species list in Canada and is listed as threatened in the United States, though there have been numerous attempts by beefalo ranchers to have it removed from the Endangered Species List.
Although superficially similar and behavioural differences exist between the American and European bison. The American species has 15 ribs, while the European bison has 14; the American bison has four lumbar vertebrae. Adult American bison are less slim in have shorter legs. American bison tend to graze more, browse less than their European relatives, their anatomies reflect this behavioural difference. The body of the American bison is hairier, though its tail has less hair than that of the European bison; the horns of the European bison point through the plane of their faces, making them more adept at fighting through the interlocking of horns in the same manner as domestic cattle, unlike the American bison, which favours butting. American bison are more tamed than their European cousins, breed with domestic cattle more readily; the bovine tribe split about 5 to 10 million years ago into the buffalos and a group leading to bison and taurine cattle. Thereafter, the family lineage of bison and taurine cattle does not appear to be a straightforward "tree" structure as is depicted in much evolution, because evidence of interbreeding and crossbreeding is seen between different species and members within this family many millions of years after their ancestors separated into different species.
This crossbreeding was not sufficient to conflate the different species back together, but it has resulted in unexpected relationships between many members of this group, such as yak being related to American bison, when such relationships would otherwise not be apparent. A 2003 study of mitochondrial DNA indicated four distinct maternal lineages in tribe Bovini: Taurine cattle and zebu Wisent American bison and yak and Banteng and gayalHowever, Y chromosome analysis associated wisent and American bison. An ear
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
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
The even-toed ungulates are ungulates - hoofed animals - which bear weight on two of the five toes: their third and fourth toes. The other three toes are either present, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on one of the five toes: the third toe. Another difference between the two is that even-toed ungulates digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do; the aquatic cetaceans evolved from even-toed ungulates, so modern taxonomic classification sometimes combines the Artiodactyla and Cetacea into the Cetartiodactyla. The 220 land-based even-toed ungulate species include pigs, hippopotamuses, llamas, mouse deer, giraffes, sheep and cattle. Many of these are of great dietary and cultural importance to humans; the oldest fossils of even-toed ungulates date back to the early Eocene. Since these findings simultaneously appeared in Europe and North America, it is difficult to determine the origin of artiodactyls.
The fossils are classified as belonging to the family Dichobunidae. These were small animals, some as small as a hare, with a slim build, lanky legs, a long tail, their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals. Two widespread, but now extinct, families of even-toed ungulates were Enteledontidae and Anthracotheriidae. Entelodonts existed from the middle Eocene to the early Miocene in North America, they had a stocky body with short legs and a massive head, characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine build, with an elongated muzzle; this group appeared in the middle Eocene up until the Pliocene, spread throughout Eurasia and North America. Anthracothereres are thought to be the ancestors of hippos, probably led a similar aquatic lifestyle. Hippopotamuses appeared in the late Miocene and occupied Africa and Asia – they never got to the Americas; the camels were, during large parts of the Cenozoic, limited to North America.
Among the North American camels were groups like the short-legged Merycoidodontidae. They first developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia; the North American varieties became extinct around 10,000 years ago. Suina have been around since the Eocene. In the late Eocene or the Oligocene, two families stayed in Africa. South America was settled by even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries and various species of capreoline deer, South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no native species; the classification of artiodactyls was hotly debated because the ocean-dwelling cetaceans evolved from the land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates are more related to the ocean-dwelling cetaceans than to the other even-toed ungulates.
This makes the Artiodactyla as traditionally defined a paraphyletic taxon, since it includes animals descended from a common ancestor, but does not include all of its descendants. Phylogenetic classification only recognizes monophyletic taxa. To address this problem, the traditional order Artiodactyla and infraorder Cetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon. An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon. Order Artiodactyla/Clade CetartiodactylaSuborder Tylopoda Family †Anoplotheriidae? Family †Cainotheriidae Family †Merycoidodontidae Family †Agriochoeridae Family Camelidae: camels and lamoids or llamas Family †Oromerycidae Family †Xiphodontidae Clade Artiofabula Suborder Suina Family Suidae: pigs Family Tayassuidae: peccaries Family †Sanitheriidae Clade Cetruminantia Clade CetancodontamorphaGenus †Andrewsarchus? Family †Entelodontidae Suborder Whippomorpha Family †Raoellidae Superfamily Dichobunoidea – paraphyletic to Cetacea and Raoellidae Family †Dichobunidae Family †Helohyidae Family †Choeropotamidae Family †Cebochoeridae Family †Mixtotheriidae Infraorder Ancodonta Family †Anthracotheriidae – paraphyletic to Hippopotamidae Family Hippopotamidae: hippos Infraorder Cetacea: whales Parvorder †Archaeoceti Family †Pakicetidae Family †Ambulocetidae Family †Remingtonocetidae Family †Basilosauridae Parvorder Mysticeti: baleen whales Superfamily Balaenoidea: right whales Family Balaenidae: greater right whales Family Cetotheriidae: pygmy right whale Superfamily Balaenopteroidea: large baleen whales Family Balaenopteridae: slender-back rorquals and humpback whale Family Eschrichtiidae: gray whale Parvorder Odontoceti: toothed whales Superfamily Delphinoidea: oceanic dolphins and others Family Delphinidae: oceanic t