The Virginia opossum known as the North American opossum, is the only marsupial found north of Mexico. In the United States, the animal is referred to as a possum, it is a nocturnal animal about the size of a domestic cat. It is a successful opportunist, it is familiar to many North Americans as it is seen near towns, rummaging through garbage cans, can become a nuisance. It is seen as roadkill; the Virginia opossum is the original animal named "opossum". The word comes from Algonquian wapathemwa meaning "white animal". Colloquially, the Virginia opossum is called "possum"; the name opossum is applied more to any of the other marsupials of the families Didelphidae and Caenolestidae. The generic name is derived from Ancient Greek: di, "two", delphus, "womb"; the possums of Australia, whose name is derived from a similarity to the opossums of the Americas, are marsupials, but of the order Diprotodontia. The Virginia opossum is known in Mexico as tlacuache and tlacuachi, from the Nahuatl word tlacuatzin.
The Virginia opossum is found throughout Central America and North America east of the Rockies from Costa Rica to southern Ontario and is expanding its range northward and northeasterly at a significant pace. Its pre-European settlement range was as far north as Maryland; the clearing of dense forests in these areas and further north by settlers allowed the opossum to move northward. Since 1900 it has expanded its range to include most of New England. Areas such as Rhode Island and Waterloo Region and Simcoe County in southern Ontario had sightings of opossums in the 1960s but now have them likely due to global warming causing winters to be warmer with less snow; some people speculate the expansion into Ontario occurred by opossums accidentally being transferred across the St. Lawrence, Detroit and St. Clair Rivers by motor vehicles or trains they may have climbed upon; as the opossum is not adapted to colder winters or heavy snow, its population may be reduced if a colder winter with heavier snow occurs in a particular northern region.
Its ancestors evolved in South America, but invaded North America in the Great American Interchange, after the formation of the Isthmus of Panama about 3 million years ago. The Virginia opossum was not native to the west coast of the United States, it was intentionally introduced into the West during the Great Depression as a source of food, now occupies much of the Pacific coast. Its range has been expanding northward into British Columbia, Canada. A sizable population exists in Utah, feeding on human animal food in the cities; these animals nest under houses to survive the long winters. Virginia opossums can vary in size, with larger specimens found to the north of the opossum's range and smaller specimens in the tropics, they measure 13–37 in long from their snout to the base of the tail, with the tail adding another 8.5–19 in. Weight for males ranges for females from 11 ounces to 8.2 lb. They are one of the world's most variably sized mammals, since a large male from northern North America weighs about 20 times as much as a small female from the tropics.
Their coats are a dull grayish brown, other than on their faces. Opossums have long, prehensile tails, which can be used to grab branches and carry small objects, they have hairless ears and a long, flat nose. Opossums have 50 teeth, more than any other North American land mammal, opposable, clawless thumbs on their rear limbs; the dental formula of an opossum is 5134/4134. No other mammal in North America has more than 6 upper incisors, but the Virginia opossum has 10. Opossums have 13 nipples, arranged in a circle of 12 with one in the middle. For such a widespread and successful species, the Virginia opossum has one of the lowest encephalization quotients of any marsupial, its brain is one-fifth the size of a raccoon's. Virginia opossum tracks show five finger-like toes in both the fore and hind prints; the hind tracks are unusual and distinctive due to the opossum's opposable thumb, which prints at an angle of 90° or greater to the other fingers. Individual adult tracks measure 1.9 in long by 2.0 in wide for the fore prints and 2.5 in long by 2.3 in wide for the hind prints.
Opossums have hind except on the two thumbs. In a soft medium, such as the mud in this photograph, the foot pads show; the tracks in the photograph were made. The four aligned toes on the hind print show the approximate direction of travel. In a pacing gait, the limbs on one side of the body are moved just prior to moving both limbs on the other side of the body; this is illustrated in the pacing diagram, which explains why the left-fore and right-hind tracks are found together. However, if the opossum were not walking
Placentalia is one of the three extant subdivisions of the class of animals Mammalia. The Placentals are distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a late stage of development, it is somewhat of a misnomer since marsupials nourish their fetuses via a placenta. Placental mammals are anatomically distinguished from other mammals by: a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother; the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. The rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint; the presence of a malleolus at the bottom of the fibula. Analysis of retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals.
There are now thought to be three major subdivisions or lineages of placental mammals: Boreoeutheria and Afrotheria, all of which diverged from common ancestors. The orders of placental mammals in the three groups are: Magnorder Afrotheria Superorder Afroinsectiphilia Order Afrosoricida Order Macroscelidea Order Tubulidentata Superorder Paenungulata Order Hyracoidea Mirorder Tethytheria Order Proboscidea Order Sirenia Magnorder Boreoeutheria Superorder Euarchontoglires Grandorder Gliriformes Mirorder Glires Order Lagomorpha Order Rodentia Grandorder Euarchonta Order Scandentia Mirorder Primatomorpha Order Dermoptera Order Primates Superorder Laurasiatheria Order Eulipotyphla Order Chiroptera Order Cetartiodactyla Order Perissodactyla Mirorder Ferae Order Pholidota Order Carnivora Magnorder Xenarthra Order Cingulata Order Pilosa The exact relationships among these three lineages is a subject of debate, three different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals.
These hypotheses are Atlantogenata and Exafroplacentalia. Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on the type of DNA and varying interpretations of paleogeographic data. Cladogram based on Amrine-Madsen, H. et al. and Asher, R. J. et al. True placental mammals arose from stem-group members of the clade Eutheria, which had existed since at least the Middle Jurassic period, about 170 MYA); these early eutherians were nocturnal insect eaters, with adaptations for life in trees. True placentals may have originated in the Late Cretaceous around 90 MYA, but the earliest undisputed fossils are from the early Paleocene, 66 MYA, following the Cretaceous–Paleogene extinction event; the species Protungulatum donnae was thought to be a stem-ungulate known 1 meter above the Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event and Purgatorius considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.
The rapid appearance of placentals after the mass extinction at the end of the Cretaceous suggests that the group had originated and undergone an initial diversification in the Late Cretaceous, as suggested by molecular clocks. The lineages leading to Xenarthra and Afrotheria originated around 90 MYA, Boreoeutheria underwent an initial diversification around 70-80 MYA, producing the lineages that would lead to modern primates, insectivores and carnivorans. However, modern members of the placental orders originated in the Paleogene around 66 to 23 MYA, following the Cretaceous–Paleogene extinction event; the evolution of crown orders such modern primates and carnivores appears to be part of an adaptive radiation that took place as mammals evolved to take advantage of ecological niches that were left open when most dinosaurs and other animals disappeared following the Chicxulub asteroid impact. As they occupied new niches, mammals increased in body size, began to take over the large herbivore and large carnivore niches, left open by the decimation of the dinosaurs.
Mammals exploited niches that the dinosaurs had never touched: for example, bats evolved flight and echolocation, allowing them to be effective nocturnal, aerial insectivores.
In biology, extinction is the termination of an organism or of a group of organisms a species. The moment of extinction is considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point; because a species' potential range may be large, determining this moment is difficult, is done retrospectively. This difficulty leads to phenomena 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 over five billion species, that lived on Earth are estimated to have died out. Estimates on the number of Earth's 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. In 2016, scientists reported that 1 trillion species are estimated to be on Earth with only one-thousandth of one percent described. Through evolution, species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition.
The relationship between animals and their ecological niches has been established. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive with no morphological change for hundreds of millions of years. Mass extinctions are rare events. Only 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 2018 report indicated that the phylogenetic diversity of 300 mammalian species erased during the human era since the Late Pleistocene would require 5 to 7 million years to recover. A dagger symbol placed next to the name of a species or other taxon indicates its status as extinct. A species is extinct. Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation.
A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, sparse distribution over a large range, a lack of individuals of both sexes, or other reasons. Pinpointing the extinction of a species requires a clear definition of that species. If it is to be declared extinct, the species in question must be uniquely distinguishable from any ancestor or daughter species, from any other related species. Extinction of a species plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge. In ecology, extinction is used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but may still exist elsewhere; 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.
An important aspect of extinction is human attempts to preserve critically endangered species. These are reflected by the creation of the conservation status "extinct in the wild". Species listed under this status by the International Union for Conservation of Nature are not known to have any living specimens in the wild, are maintained only in zoos or other artificial environments; some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will be restored to the wild. When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of planned breeding programs; the extinction of one species' wild population can have knock-on effects, causing further extinctions. These are called "chains of extinction"; this is common with extinction of keystone species. A 2018 study indicated that the 6th mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore 2.5 billion years of unique mammal diversity to what it was before the human era.
Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction. The old taxon vanishes, transformed into a successor, or split into more than one. Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey. However, as fossil species leave no genetic material behind, one cannot say whether Hyracotherium evolved into more modern horse species or evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups; the coelacanth, a fish related to lungfish and tetrapods, was consi
Thomas Henry Huxley
Thomas Henry Huxley was an English biologist and anthropologist specialising in comparative anatomy. He is known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution; the stories regarding Huxley's famous debate in 1860 with Samuel Wilberforce were a key moment in the wider acceptance of evolution and in his own career, although historians think that the surviving story of the debate is a fabrication. Huxley had been planning to leave Oxford on the previous day, after an encounter with Robert Chambers, the author of Vestiges, he changed his mind and decided to join the debate. Wilberforce was coached by Richard Owen, against whom Huxley debated about whether humans were related to apes. Huxley was slow to accept some of Darwin's ideas, such as gradualism, was undecided about natural selection, but despite this he was wholehearted in his public support of Darwin. Instrumental in developing scientific education in Britain, he fought against the more extreme versions of religious tradition.
Coining the term in 1869, Huxley elaborated on "agnosticism" in 1889 to frame the nature of claims in terms of what is knowable and what is not. Huxley statesAgnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle... the fundamental axiom of modern science... In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration... In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable. Use of that term has continued to the present day. Much of Huxley's agnosticism is influenced by Kantian views on human perception and the ability to rely on rational evidence rather than belief systems. Huxley had little formal schooling and was self-taught, he became the finest comparative anatomist of the 19th century. He worked on invertebrates, clarifying relationships between groups little understood, he worked on vertebrates on the relationship between apes and humans.
After comparing Archaeopteryx with Compsognathus, he concluded that birds evolved from small carnivorous dinosaurs, a theory accepted today. The tendency has been for this fine anatomical work to be overshadowed by his energetic and controversial activity in favour of evolution, by his extensive public work on scientific education, both of which had significant effects on society in Britain and elsewhere. Huxley’s 1893 Romanes Lecture, “Evolution and Ethics” is exceedingly influential in China. Thomas Henry Huxley was born in Ealing, a village in Middlesex, he was the second youngest of eight children of Rachel Withers. Like some other British scientists of the nineteenth century such as Alfred Russel Wallace, Huxley was brought up in a literate middle-class family which had fallen on hard times, his father was a mathematics teacher at Ealing School until it closed, putting the family into financial difficulties. As a result, Thomas left school after only two years of formal schooling. Huxley's parents were Anglicans, although it was against organized religion Huxley sympathized with the town's Nonconformist.
Despite this unenviable start, Huxley was determined to educate himself. He became one of the great autodidacts of the nineteenth century. At first he read Thomas Carlyle, James Hutton's Geology, Hamilton's Logic. In his teens he taught himself German becoming fluent and used by Charles Darwin as a translator of scientific material in German, he learned Latin, enough Greek to read Aristotle in the original. On, as a young adult, he made himself an expert, first on invertebrates, on vertebrates, all self-taught, he was skilled in drawing and did many of the illustrations for his publications on marine invertebrates. In his debates and writing on science and religion his grasp of theology was better than most of his clerical opponents. Huxley, a boy who left school at ten, became one of the most knowledgeable men in Britain, he was apprenticed for short periods to several medical practitioners: at 13 to his brother-in-law John Cooke in Coventry, who passed him on to Thomas Chandler, notable for his experiments using mesmerism for medical purposes.
Chandler's practice was in London's Rotherhithe amidst the squalor endured by the Dickensian poor. Here Thomas would have seen poverty and rampant disease at its worst. Next, another brother-in-law took him on: his eldest sister's husband. Now 16, Huxley entered Sydenham College, a cut-price anatomy school whose founder, Marshall Hall, discovered the reflex arc. All this time Huxley continued his programme of reading, which more than made up for his lack of formal schooling. A year buoyed by excellent results and a silver medal prize in the Apothecaries' yearly competition, Huxley was admitted to study at Charing Cross Hospital, where he obtained a small scholarship. At Charing Cross, he was taught by Thomas Wharton Jones, Professor of Ophthalmic Medicine and Surgery at University College London. Jones had been Robert Knox's assistant when Knox bought cadavers from Hare; the young Wharton Jones, who acted as go-between, was exonerated of crime, but thought it best to leave Scotland. He was a fine teacher, up-to-date in physiology and an ophthalmic surgeon.
In 1845, under Wharton Jones' guidance, Huxley published his first scientific paper demonstrating the existence of a hitherto unrecognised layer in the inner sheath of hairs, a layer, known sin
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma
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