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 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
Dwarf musk deer
The dwarf musk deer or Chinese forest musk deer is an artiodactyl native to southern and central China and northernmost Vietnam. On June 14, 1976, China entered. Four subspecies are recognized: M. b. berezovskii M. b. bijiangensis M. b. caobangis M. b. yanguiensis
In biology, a type is a particular specimen of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralize the defining features of that particular taxon. In older usage, a type was a taxon rather than a specimen. A taxon is a scientifically named grouping of organisms with other like organisms, a set that includes some organisms and excludes others, based on a detailed published description and on the provision of type material, available to scientists for examination in a major museum research collection, or similar institution. According to a precise set of rules laid down in the International Code of Zoological Nomenclature and the International Code of Nomenclature for algae and plants, the scientific name of every taxon is always based on one particular specimen, or in some cases specimens. Types are of great significance to biologists to taxonomists. Types are physical specimens that are kept in a museum or herbarium research collection, but failing that, an image of an individual of that taxon has sometimes been designated as a type.
Describing species and appointing type specimens is part of scientific nomenclature and alpha taxonomy. When identifying material, a scientist attempts to apply a taxon name to a specimen or group of specimens based on his or her understanding of the relevant taxa, based on having read the type description, preferably based on an examination of all the type material of all of the relevant taxa. If there is more than one named type that all appear to be the same taxon the oldest name takes precedence, is considered to be the correct name of the material in hand. If on the other hand the taxon appears never to have been named at all the scientist or another qualified expert picks a type specimen and publishes a new name and an official description; this process is crucial to the science of biological taxonomy. People's ideas of how living things should be grouped shift over time. How do we know that what we call "Canis lupus" is the same thing, or the same thing, as what they will be calling "Canis lupus" in 200 years' time?
It is possible to check this because there is a particular wolf specimen preserved in Sweden and everyone who uses that name – no matter what else they may mean by it – will include that particular specimen. Depending on the nomenclature code applied to the organism in question, a type can be a specimen, a culture, an illustration, or a description; some codes consider a subordinate taxon to be the type, but under the botanical code the type is always a specimen or illustration. For example, in the research collection of the Natural History Museum in London, there is a bird specimen numbered 18184.108.40.206. This is a specimen of a kind of bird known as the spotted harrier, which bears the scientific name Circus assimilis; this particular specimen is the holotype for that species. That species was named and described by Jardine and Selby in 1828, the holotype was placed in the museum collection so that other scientists might refer to it as necessary. Note that at least for type specimens there is no requirement for a "typical" individual to be used.
Genera and families those established by early taxonomists, tend to be named after species that are more "typical" for them, but here too this is not always the case and due to changes in systematics cannot be. Hence, the term name-bearing type or onomatophore is sometimes used, to denote the fact that biological types do not define "typical" individuals or taxa, but rather fix a scientific name to a specific operational taxonomic unit. Type specimens are theoretically allowed to be aberrant or deformed individuals or color variations, though this is chosen to be the case, as it makes it hard to determine to which population the individual belonged; the usage of the term type is somewhat complicated by different uses in botany and zoology. In the PhyloCode, type-based definitions are replaced by phylogenetic definitions. In some older taxonomic works the word "type" has sometimes been used differently; the meaning was similar in the first Laws of Botanical Nomenclature, but has a meaning closer to the term taxon in some other works: Ce seul caractère permet de distinguer ce type de toutes les autres espèces de la section.
… Après avoir étudié ces diverses formes, j'en arrivai à les considérer comme appartenant à un seul et même type spécifique. Translation: This single character permits distinguish this type from all other species of the section... After studying the diverse forms, I came to consider them as belonging to the one and the same specific type. In botanical nomenclature, a type, "is that element to which the name of a taxon is permanently attached." In botany a type is either an illustration. A specimen is a real plant and kept safe, "curated", in a herbarium. Examples of where an illustration may serve as a type include: A detailed drawing, etc. depicting the plant, from the early days of plant taxonomy. A dried plant was difficult to transport and hard to keep safe for the future. Skilled botanical artists were sometimes employed by a botanist to make a faithful and detailed illustration; some such illustrations have become the best record a
Siberian musk deer
The Siberian musk deer is a musk deer found in the mountain forests of Northeast Asia. It is most common in the taiga of southern Siberia, but is found in parts of Mongolia, Inner Mongolia and the Korean peninsula, their small shape allows them to hide from predators through tiny openings in the rocky terrain and allow them to run exceptionally fast from their predators. Although bearing fangs, Siberian musk deer are herbivores with their main source of nutrients being lichens. Due to the severe amount of poaching for its musk gland, the deer population is continuing to decrease, it is expected. However, efforts from each sighted countries are beginning to reintroduce the musk deer's population. Siberia, North Mongolia, North China and Korea - M. m. moschiferus Russian Far East - M. m. turovi Verkhoyansk Ridge - M. m. arcticus Sakhalin - M. m. sachalinensis Korea - M. m. parvipes It takes a year for the Siberian musk deer to reach maturity with an average deer to live at least 10 – 14 years. During breeding season, male deer will grow tusks instead of antlers.
These tusks are used to attract females. Tusks that are longer and stronger creates a more intimidating stance and becomes more attractive to females as the offspring of that male are to become healthier and fit. Once the male and the female deer have procreated, the females will become pregnant lasting over 6 months and can give birth to 1-3 offspring between the months of May through June. Musk will mark their territory warning trespassing deers not to cross the boundary; when marking their territories, musk deer gather fallen branches, tree trunks, as well as plant stems and place them in a circle. While placing the various branches around the circle, the deer will do an olfactory examination and turn the back of its body towards the marked territories. Other ways the Siberian Musk Deer will mark its territory is by defecating in marked territories or unclaimed territories. Most Siberian musk deer are nocturnal inhabiting the mountainous taiga and found in shrub-covered slopes where foods are abundant.
The rocky location provides crevices and crags for the musk deer to hide from many predators, such as lynx and wolverines. Musk deer have a preference for digestible nutritious foods that are both rich in protein and low in fiber. During periods of winter, musk deer can survive in poorer food quality ranging in foods that are low in proteins but are high in energy and can be digested; the majority of their diet consists of lichens, pine needles and tree barks. During the winter, 99% of musk deer's diet are lichens. Siberian Musk deer have a preference for digestible nutritious foods, it is nocturnal, migrates only over short distances. It prefers altitudes of more than 2600 m. Adults are small, weighing 7–17 kg; the Siberian musk deer is classified as Vulnerable by the IUCN. It is hunted for its musk gland. Only a few tens of grams can be extracted from an adult male, it is possible to remove the gland without killing the deer, but this is done. In 2016, the Korean company Sooam Biotech was reported to be attempting to clone the Siberian musk deer to help conserve the species.
The most striking characteristics of the Siberian musk deer are kangaroo-like face. Males grow the teeth for display instead of antlers. A distinct subspecies roams the island of Sakhalin. World population: 230,000 Declining Russian Federation, Sakhalin population: 600-500 Declining Russian Federation, the Eastern Siberian population: 27,000-30,000 Declining Russian Federation, Far Eastern population: 150,000 Declining Mongolia: 44,000 Declining China: unknown Declining Democratic People's Republic of Korea: unknown Declining Republic of Korea: unknown Declining Siberian musk deer preputial gland secretions are constituted of free fatty acids and phenols and steroids. Cholestanol, androsterone, Δ4-3α-hydroxy-17-ketoandrostene, 5β,3α-hydroxy-17-ketoandrostane, 5α,3β,17α-dihydroxyandrostane, 5β,3α,17β-dihydroxyandrostane and 5β,3α,17α-dihydroxyandrostane can be isolated from the steroid fraction. 3-Methylpentadecanone was not identified among the secretion lipids. The decline of the Siberian Musk Deer's population began in China where most of the deer population was abundant.
Most notably in the Sichuan plains, the musk production was accounted for 80% of the domestic trade in the 1950s. New sightings of musk deer was spotted in the upper northeast Asia and Russia. After the 1980s, the production begins to decline due to hunting for their musk glands; the cycle of over-harvesting the deer's musk continued until the exploitation reduced the musk deer's population. Another threat comes from the habitat loss by deforestation. For a long period, China cut more of its forest. 200million cm3 of China's forest recourses were cut down in the past 25 years in order to harvest the timber stock in trade for commerce. Deforestation is a severe threat to the musk deer's long term survival because the deer can only live in a few areas; the Siberian musk deer is considered vulnerable, but is declining to endangerment. In Russia, the Siberian Musk Deer is protected as Very Rare under part 7.1 of the Law of the Mongolian Animal Kingdom and under the 1995 Mongolian Hunting Law. The musk deers are protected under the National Parks which accounts for 13% of the Siberian Musk Deer population.
In China, at the international level, trading mus
The water deer is a small deer superficially more similar to a musk deer than a true deer. Native to China and Korea, there are two subspecies: the Chinese water deer and the Korean water deer. Despite its lack of antlers and certain other anatomical anomalies—including a pair of prominent tusks, it is classified as a cervid, its unique anatomical characteristics have caused it to be classified in its own genus as well as its own subfamily. However, studies of mitochondrial control region and cytochrome b DNA sequences placed it near Capreolus within an Old World section of the subfamily Capreolinae, its prominent tusks, similar to those of musk deer, have led to both being colloquially named vampire deer in English-speaking areas to which they have been imported. The species is listed as Vulnerable by the IUCN; the genus name Hydropotes derives from the two ancient greek words ὕδωρ, meaning "water", ποτής, meaning "the fact of drinking", refers to the preference of this cervid for rivers and swamps.
The etymology of the species name corresponds to the Latin word inermis meaning unarmed, defenceless — itself constructed from the prefix in- meaning without and the stem arma meaning defensive arms, armor —, refers to the fact that the water deer is antlerless. Water deer are indigenous to the lower reaches of the Yangtze River, coastal Jiangsu province, islands of Zhejiang of east-central China, in Korea, where the demilitarized zone has provided a protected habitat for a large number. In China, water deer are found in Zhejiang, Hubei, Anhui, Fujian, Jiangxi and Guangxi, they are now extinct in western China. In Korea, water deer are found nationwide. Water deer inhabit the land alongside rivers, where they are protected from sight by the tall reeds and rushes, they are seen on mountains, swamps and open cultivated fields. Water deer are proficient swimmers, can swim several miles to reach remote river islands. Chinese Water Deer are now located in United Kingdom and Argentina, some in the United States.
Despite a listing of ‘Vulnerable’ by the International Union for Conservation of Nature, in South Korea the animal is thriving due to the extinction of natural predators such as Korean tigers and leopards. Since 1994, Korean water deer have been designated as “harmful wildlife”, a term given by the Ministry of Environment to wild creatures that can cause harm to humans or to their property. Certain local governments offer bounties from 30,000 won to 50,000 won during the farming season. However, the hunting of water deer is not restricted to the warm season, as 18 hunting grounds are in operation this winter. Chinese water deer were first introduced into Great Britain in the 1870s; the animals were kept in the London Zoo until 1896, when Herbrand Russell oversaw their transferral to Woburn Abbey, Bedfordshire. More of the animals were added to the herd over the next three decades. In 1929 and 1930, 32 deer were transferred from Woburn to Whipsnade in Bedfordshire, released into the park; the current population of Chinese water deer at Whipsnade is estimated to be more than 600, while the population at Woburn is in excess of 250.
The majority of the current population of Chinese water deer in Britain derives from escapees, with the remainder being descended from a number of deliberate releases. Most of these animals still reside close to Woburn Abbey, it appears that the deer's strong preference for a particular habitat – tall reed and grass areas in rich alluvial deltas - has restricted its potential to colonize further afield. The main area of distribution is from Woburn, east into Cambridgeshire, Norfolk and North Essex, south towards Whipsnade. There have been small colonies reported in other areas; the British Deer Society coordinated a survey of wild deer in the United Kingdom between 2005 and 2007 and noted the Chinese water deer as "notably increasing its range" since the last census in 2000. A small population existed in France originating from animals that had escaped an enclosure in 1960 in western France; the population was reinforced in 1965 and 1970 and the species has been protected since 1974. Despite efforts to locate the animals with the help of local hunters, there have been no sightings since 2000, the population is assumed to be extinct.
The species is not native, but a few small deer farms in the southeastern United States have bred water deer. The water deer has narrow pectoral and pelvic girdles, long legs, a long neck; the powerful hind legs are longer than the front legs, so that the haunches are carried higher than the shoulders. They run with rabbit-like jumps. In the groin of each leg is an inguinal gland used for scent marking; the short tail is no more than 5–10 cm / 1.9–3.8 in. in length and is invisible, except when it is held raised by the male during the rut. The ears are short and rounded, both sexes lack antlers; the coat is an overall golden brown color, may be interspersed with black hairs, while the undersides are white. The tapered face is reddish brown or gray in color, the chin and upper throat are cream colored; the hair is longest on rump. In the fall, the summer coat is replaced by a thicker, coarse-haired winter coat that varies from light brown to grayish brown. Neither the head nor the tail poles are well differentiated as in gregarious deer.
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