Mediterranean monk seal
The Mediterranean monk seal is a monk seal belonging to the family Phocidae. As of 2015, it is estimated that fewer than 700 individuals survive in three or four isolated subpopulations in the Mediterranean, in the Aegean Sea, the archipelago of Madeira and the Cabo Blanco area in the northeastern Atlantic Ocean, it is believed to be the world's rarest pinniped species. This species of seal grows from 80 centimetres long at birth up to an average of 2.4 metres as adults. Males weigh an average of 320 kilograms and females weigh 300 kilograms, with overall weight ranging from 240–400 kilograms, they are thought to live up to 45 years old. The monk seals' pups are about 1 metre long and weigh around 15–18 kilograms, their skin being covered by 1–1.5 centimeter-long, dark brown to black hair. On their bellies, there is a white stripe, which differs in shape between the two sexes. In females the stripe is rectangular in shape whereas in males it is butterfly shaped; this hair is replaced after six to eight weeks by the usual short hair adults carry.
Pregnant Mediterranean monk seals use inaccessible undersea caves while giving birth, though historical descriptions show they used open beaches until the 18th century. There are eight pairs of teeth in both jaws. Believed to have the shortest hair of any pinniped, the Mediterranean monk seal fur is black or brown to dark grey, with a paler belly, close to white in males; the snout is short broad and flat, with pronounced, long nostrils that face upward, unlike their Hawaiian relative, which tend to have more forward nostrils. The flippers are short, with small slender claws. Monk seals have two pairs of retractable abdominal teats, unlike most other pinnipeds. Little is known of this seal's reproduction. Scientists have suggested that they are polygynous, with males being territorial where they mate with females. Although there is no breeding season since births take place year round, there is a peak in October and November; this is the time when caves are prone to wash out due to high surf or storm surge, which causes high mortality rates among monk seal pups at the key Cabo Blanco colony.
According to the IUCN species factsheet, "pup survival is low. Survival of pups born from September to January is 29%; this low survival rate is associated with mortality caused by severe storms, high swells and tides, but impoverished genetic variability and inbreeding may be involved. Pups born during the rest of the year had a survival rate of 71%". In 2008, lactation was reported in an open beach, the first such record since 1945, which could suggest the seal could begin feeling safe to return to open beaches for breeding purposes in Cabo Blanco. Pups make first contact with the water two weeks after their birth and are weaned at around 18 weeks of age. Most individuals are believed to reach maturity at four years of age; the gestation period lasts close to a year. However, it is believed to be common among monk seals of the Cabo Blanco colony to have a gestation period lasting longer than a year. Mediterranean monk seals are diurnal and feed on a variety of fish and mollusks octopus and eels, up to 3 kg per day.
They are known to forage at depths up to 250 meters, with an average depth varying between specimens. Monk seals prefer hunting in wide-open spaces, they are successful bottom-feeding hunters. The habitat of this pinniped has changed over the years. In ancient times, up until the 20th century, Mediterranean monk seals had been known to congregate, give birth, seek refuge on open beaches. In more recent times, they have left their former habitat and now only use sea caves for these activities; these caves are inaccessible to humans. Their caves have underwater entries and their caves are positioned along remote or rugged coastlines. Scientists have confirmed this is a recent adaptation, most due to the rapid increase in human population and industry, which have caused increased disturbance by humans and the destruction of the species' natural habitat; because of these seals' shy nature and sensitivity to human disturbance, they have adapted to try to avoid contact with humans within the last century, even earlier.
The coastal caves are, dangerous for newborns, are causes of major mortality among pups when sea storms hit the caves. This earless seal's former range extended throughout the Northwest Atlantic Africa and Black Sea, including all offshore islands of the Mediterranean, into the Atlantic and its islands: Canary, Ilhas Desertas, Porto Santo... as far west as the Azores. Vagrants could be found as far south as Gambia and the Cape Verde islands, as far north as continental Portugal and Atlantic France. Several causes provoked a dramatic population decrease over time: on one hand, commercial hunting and, during the 20th century, eradication by fishermen, who used to consider it a pest due to the damage the seal causes to fishing nets when it preys on fish caught in them; some seals have survived in the S
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
An ocean is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean; these are, in descending order by area, the Pacific, Indian and Arctic Oceans. The word "ocean" is used interchangeably with "sea" in American English. Speaking, a sea is a body of water or enclosed by land, though "the sea" refers to the oceans. Saline water covers 361,000,000 km2 and is customarily divided into several principal oceans and smaller seas, with the ocean covering 71% of Earth's surface and 90% of the Earth's biosphere; the ocean contains 97% of Earth's water, oceanographers have stated that less than 5% of the World Ocean has been explored. The total volume is 1.35 billion cubic kilometers with an average depth of nearly 3,700 meters. As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, influences climate and weather patterns; the World Ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger over two million.
The origin of Earth's oceans is unknown. Extraterrestrial oceans may be composed of water or other compounds; the only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories and Venus are theorized to have had large water oceans; the Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of natural satellites; the Solar System's giant planets are thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone.
Ocean planets are a hypothetical type of planet with a surface covered with liquid. The word ocean comes from the figure in classical antiquity, the elder of the Titans in classical Greek mythology, believed by the ancient Greeks and Romans to be the divine personification of the sea, an enormous river encircling the world; the concept of Ōkeanós has an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases. Though described as several separate oceans, the global, interconnected body of salt water is sometimes referred to as the World Ocean or global ocean; the concept of a continuous body of water with free interchange among its parts is of fundamental importance to oceanography. The major oceanic divisions – listed below in descending order of area and volume – are defined in part by the continents, various archipelagos, other criteria.
Oceans are fringed by smaller, adjoining bodies of water such as seas, bays and straits. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system, part of every ocean and the longest mountain range in the world; the continuous mountain range is 65,000 km long. The total mass of the hydrosphere is about 1.4 quintillion metric tons, about 0.023% of Earth's total mass. Less than 3% is freshwater; the area of the World Ocean is about 361.9 million square kilometers, which covers about 70.9% of Earth's surface, its volume is 1.335 billion cubic kilometers. This can be thought of as a cube of water with an edge length of 1,101 kilometers, its average depth is about 3,688 meters, its maximum depth is 10,994 meters at the Mariana Trench. Nearly half of the world's marine waters are over 3,000 meters deep; the vast expanses of deep ocean cover about 66% of Earth's surface. This does not include seas not connected to the World Ocean, such as the Caspian Sea; the bluish ocean color is a composite of several contributing agents.
Prominent contributors include dissolved organic chlorophyll. Mariners and other seafarers have reported that the ocean emits a visible glow which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow, it is most caused by bioluminescence. Oceanographers divide the ocean into different vertical zones defined by physical and biological conditions; the pelagic zone includes all open ocean regions, can be divided into further regions categorized by depth and light abundance. The photic zone includes the oceans from the surface to a depth of
The leopard seal referred to as the sea leopard, is the second largest species of seal in the Antarctic. Its only natural predators are the killer whale, it feeds on a wide range of prey including cephalopods, other pinnipeds, krill and fish. It is the only species in the genus Hydrurga, its closest relatives are the Ross seal, the crabeater seal and the Weddell seal, which together are known as the tribe of lobodontini seals. The name hydrurga means "water worker" and leptonyx is the Greek for "small clawed". French zoologist Henri Marie Ducrotay de Blainville described the leopard seal in 1820; the leopard seal has a distinctively muscular body shape, when compared to other seals. This species of seal is known for its massive reptilian-like head and jaws that allow it to be one of the top predators in its environment. A notable key feature of leopard seals is its counter-shaded coat. A counter-shaded coat is. Leopard seals have a silver to dark gray blended coat that makes up its distinctive "leopard" coloration with a spotted pattern, whereas the ventral side of the coat is paler in color—ranging from white to light gray.
Females are larger than the males. The overall length of this seal is 2.4–3.5 m and weight is from 200 to 600 kilograms. They are about the same length as the northern walrus, but less than half the weight. Another notable characteristic of leopard seals are their short clear whiskers that are used to sense their environment. Leopard seals have an enormous mouth relative to their body size; the front teeth are sharp like those of other carnivores, but their molars lock together in a way that allows them to sieve krill from the water, in the manner of the crabeater seal. Since leopard seals are "true" seals, they do not have external pinnae, or ears, but they do have an internal ear canal that leads to an external opening, their hearing in air is similar to that of a human, but scientists have noted that leopard seals use their ears in conjunction with their whiskers to track prey under water. Leopard seals are pagophilic, "ice-loving" seals, which inhabit the Antarctic pack ice between 50˚S and 80˚S.
Sightings of vagrant leopard seals have been recorded on the coasts of Australia, New Zealand, South America, South Africa. In August 2018 an individual was sighted in Geraldton on the west coast of Australia. Higher densities of leopard seals are seen in the Western Antarctic than in other regions. Most leopard seals remain within the pack ice throughout the year and remain solitary during most of their lives with the exception of a mother and her newborn pup; these matrilineal groups can move further north in the austral winter to sub-antarctic islands and the coastlines of the southern continents to provide care for their pups. While solitary animals may appear in areas of lower latitudes, females breed there; some researchers believe. Lone male leopard seals hunt other marine mammals and penguins in the packed ice of antarctic waters; the estimated population of this species ranges from 220,000 to 440,000 individuals, which puts leopard seals at "least concern". Although with an abundance of leopard seals in the antarctic, they are difficult to survey by traditional visual techniques because they spend long periods of time vocalizing under the water during the austral spring and summer when visual surveys are carried out.
This trait of vocalizing underwater for long periods has made them available to acoustic surveys, allowing researchers to gather most of what is known about them. Leopard seals are vocal underwater during the austral summer; the male seals produce loud calls for many hours each day. While singing the seal hangs upside down and rocks from side to side under the water, their back is bent, the neck and cranial thoracic region is inflated and as they call their chest pulses. The male calls can be split into two categories: vocalizing and silencing, in which vocalizing is when they are making noises underwater, silencing noted as the breathing period at the air surface. Adult male leopard seals have only a few stylized calls, some are like bird or cricket-like trills yet others are low haunting moans. However, scientists have identified five distinctive sounds that male leopard seals make, which include: the high double trill, medium single trill, low descending trill, low double trill, a hoot with a single low trill.
These cadence of calls are believed to be a part of a long range acoustic display for territorial purposes, or the attraction of a potential mate. The leopard seals have age-related differences in their calling patterns, just like birds. Where the younger male seals have many different types of variable calls – the adult male seals have only a few stylized calls; each male leopard seal produces these individual calls, can arrange their few call types into individually distinctive sequences. The acoustic behavior of the leopard seal is believed to be linked to their breeding behaviour. In male seals, vocalizing coincides with the timing of their breeding season, which falls between November and the first week of January. Conversely, a female leopard seal can attribute calls to their environment as well. Since leopard seals live in an area difficult for humans to survive in, not much is known on their reproduction and breeding habits. However, it is known that their breeding system is polygynous, meaning that males mate with multiple femal
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
Monk seals are earless seals of the tribe Monachini. They are the only earless seals found in tropical climates; the two genera of monk seals and Neomonachus, comprise three species: the Mediterranean monk seal, Monachus monachus. The two surviving species in imminent danger of extinction. All three monk seal species were classified in genus Monachus until 2014, when the Caribbean and Hawaiian species were placed into a new genus, Neomonachus. Monk seals are agile, they have a flat snout with nostrils on the top. Monk seals are polygynous, group together in harems, they feed on bony fish and cephalopods, but they are opportunistic. The skin is covered in small hairs, which are black in males and brown or dark gray in females. Monk seals are found in the Hawaiian archipelago, certain areas in the Mediterranean Sea, in the tropical areas of the west Atlantic Ocean. All species experienced overhunting by sealers; the Hawaiian monk seal experienced population drops in the 19th century and during World War II, the Caribbean monk seal was exploited since the 1500s until the 1850s, when populations were too low to hunt commercially.
The Mediterranean monk seal has experienced commercial hunting since the Middle Ages and eradication by fishermen. Monk seals have developed a fear of humans, may abandon beaches due to human presence. Around 1,700 monk seals remain. Monk seals are earless seals of the tribe Monachini; the tribe was first conceived by Victor Blanchard Scheffer in his 1958 book Seals, Sea Lions, Walruses: A Review of the Pinnipedia. The two genera of monk seals and Neomonachus, comprise three species: the Mediterranean monk seal, the Hawaiian monk seal, the Caribbean monk seal, which became extinct in the 20th century. All three monk seal species were classified in genus Monachus until 2014, when comparison of the species' mitochondrial cytochrome b DNA sequences led biologists to place the Caribbean and Hawaiian species in a new genus, Neomonachus. Fossils of the Mediterranean and Caribbean species are known from the Pleistocene; the time of divergence between the Hawaiian and Caribbean species, 3.7 million years ago, corresponds to the closing of the Central American Seaway by the formation of the Isthmus of Panama.
The divergence between Mediterranean seals and the New World clade was dated to 6.3 Mya ago. The Hawaiian monk seal, as the name suggests, lives in the Hawaiian archipelago. Monk seals migrated to Hawaii between 4–11 Mya through an open-water passage between North and South America called the Central American Seaway; the Isthmus of Panama closed the seaway 3 Mya. The species may have evolved in the Pacific or Atlantic, but in either case, came to Hawaii long before the first Polynesians; when monk seals are not hunting or eating, they bask on the beaches. The habitat of the Mediterranean monk seal has changed over the years. Prior to the 20th century, they had been known to congregate, give birth, seek refuge on open beaches. Since sealing had ended, they have left their former habitat and now only use sea caves for such behavior. More than not, these caves are rather inaccessible to humans due to underwater entries, because the caves are along remote or rugged coastlines. Scientists have confirmed this is a recent adaptation, most due to the rapid increase in human population and industry, which have caused increased disturbance by humans and the destruction of the species' natural habitat.
Because of these seals' shy nature and sensitivity to human disturbance, they have adapted to try to avoid contact with humans within the last century, even earlier. The coastal caves are, dangerous for newborns, are causes of major mortality among pups when sea storms hit the caves. Caribbean monk seals were found in warm temperate and tropical waters of the Caribbean Sea, Gulf of Mexico, the west Atlantic Ocean, they preferred to haul out at sites on isolated and secluded atolls and islands, but visited the mainland coasts and deeper waters offshore. This species may have fed in shallow reefs. Monk seals are part of the family Phocidae, the members of which are characterized by their lack of external ears, the inability to rotate the hind flippers under the body, shed their hair and the outer layer of their skin in an annual molt. Monk seals as a whole vary minutely in size, with all adults measuring on average 8 feet and 500 pounds, they exhibit sexual dimorphism, in that the males are larger than females, with the exception of the Hawaiian monk seal, where females are larger.
Its white belly, gray coat, slender physique distinguish it from the harbor seal, another earless seal. Much like elephant seals, they shed the outer layer of their skin in an annual molt; the Mediterranean monk seal has a short and flat snout, with pronounced, long nostrils that face upwards. The flippers are short, with small, slender claws; the monk seal’s physique is ideally suited for hunting its prey: fish, octopus and squid in deep-water coral beds. The fur coats of males is black, brown or dark gray in females. Pups are about 3.3 feet long and weigh around 33–40 pounds, their skin
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