Arctocephalus forsteri, the Australasian fur seal, South Australian fur seal, New Zealand fur seal, Antipodean fur seal, or long-nosed fur seal, is a species of fur seal found around southern Australia and New Zealand. The name New Zealand fur seal is used by English speakers in New Zealand; as of 2014, the common name long-nosed fur seal has been proposed for the population of seals inhabiting Australia. Although the Australian and New Zealand populations show some genetic differences, their morphologies are similar, thus they remain classed as a single species. After the arrival of humans in New Zealand, after the arrival of Europeans in Australia and New Zealand, hunting reduced the population near to extinction. Males have been reported as large as 160 kg. Males can be 2 metres long. Females are between 30–50 kg on average, can be as long as 1.5 metres. Pups are 3.3–3.9 kg on average, between 40 and 55 cm long. At 290 days old males are about 14.1 kg, females are about 12.6 kg. They have external ears and hind flippers that rotate forward, which visibly distinguish them from other seals.
They have a pointy nose with long pale whiskers. The fur seals are covered by two layers of fur; the coat is grey-brown on their back, lighter on their belly. Some have white tips on longer upper hairs. So called "Upland Seals" once found on Antipodes Islands and Macquarie Island have been claimed as a distinct subspecies with thicker furs by scientists although it is unclear whether these seals were genetically distinct; the species occurs in New Zealand. It is found in the coastal waters and on the offshore islands of southern Australia, from the south-west corner of Western Australia to just east of Kangaroo Island in South Australia, in southern Tasmania and the subantarctic Macquarie Island. Small populations are forming in Bass Strait and coastal waters of Victoria and southern New South Wales. Before the arrival of humans in New Zealand, the species bred around all the New Zealand mainland and its subantarctic islands. There are now established and expanding colonies around the entire South Island, on Stewart Island and all of the New Zealand subantarctic islands.
There are newly established breeding colonies on the North Island. The species can "porpoise" out of the water when travelling at sea, they can dive longer than any other fur seal. Females can dive for about 9 minutes and to a depth of about 312 metres, can dive deeper and longer in autumn and winter. Males can dive for about 15 minutes to a depth of about 380 metres. On average, the species only dives for 1–2 minutes; when they dive for food they dive deeper during the day but shallower at night, because during the day their prey migrates to deeper depths and migrates back up during the night. Lactating females alter their dive patterns in order to care for their young. Dives are shorter, from around 9 minutes down to 5 minutes. Several longer trips may be taken at first to find patches of prey; the shorter dives utilise these patches. Due to the differences in diving pattern between males and females, there is little inter-sexual competition for food sources. Males forage over continental shelf breaks in deeper water, while females utilise the continental shelf as foraging grounds.
It is believed that differences in diving abilities and depths could be the cause of some sexual dimorphism between males and females. Diving behaviour by the pups begins in the months leading up to weaning, when the pups are nursing less; the pups begin to dive from the age of 6–10 months, yet weaning is known to occur between the ages of 8 and 11 months, so the young pups do not have much time to learn to forage. The pups need to progressively develop nocturnal diving skills while they still have their mothers' milk to fall back on if dives are unsuccessful. Age, physiological development, experience are important factors for success in hunting and contribute to the development of the pups' diving ability and behaviour; this transitional period when young pups are becoming nutritionally independent while their foraging efficiency is rather low, is a time of high risk, mortality can be high. Based on scat samples, it has been found that the pups start by eating cephalopods and making their way to fish, but this may just be a result of prey availability during different times of the year.
Males vocalise through a bark or whimper, either a guttural threat, a low-intensity threat, a full threat, or a submissive call. Females growl and have a high-pitched pup attraction wail call. Pup-attraction calls allow communication from longer distances. Once together, females use olfactory recognition to confirm the pup as their own. In males, the full neck display is a non-combative posture that functions as a threat to surrounding males by which they are able to assess each other's dominance status. Females mature between 4 and 6 years old, males mature between 8 and 10 years old; these seals are polygynous. Males guard territory in late October before females arrive. Females mate only once a year, this occurs eight days postpartum for about 13 minutes on average. Females have a delayed implantation of the fertilised egg, so that implantation on the uterine wall does not occur for 3 months. Gestation occurs for 9 months Females are more aggressive near the time of birth, do not like to be approached right after birth.
Females will continue to reproduce until their death, on average between 14 and 17 years of age. Females first arrive on the shore between November and January, just a few days before giving birth, stay
The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east. At 165,250,000 square kilometers in area, this largest division of the World Ocean—and, in turn, the hydrosphere—covers about 46% of Earth's water surface and about one-third of its total surface area, making it larger than all of Earth's land area combined; the centers of both the Water Hemisphere and the Western Hemisphere are in the Pacific Ocean. The equator subdivides it into the North Pacific Ocean and South Pacific Ocean, with two exceptions: the Galápagos and Gilbert Islands, while straddling the equator, are deemed wholly within the South Pacific, its mean depth is 4,000 meters. The Mariana Trench in the western North Pacific is the deepest point in the world, reaching a depth of 10,911 meters; the western Pacific has many peripheral seas. Though the peoples of Asia and Oceania have traveled the Pacific Ocean since prehistoric times, the eastern Pacific was first sighted by Europeans in the early 16th century when Spanish explorer Vasco Núñez de Balboa crossed the Isthmus of Panama in 1513 and discovered the great "southern sea" which he named Mar del Sur.
The ocean's current name was coined by Portuguese explorer Ferdinand Magellan during the Spanish circumnavigation of the world in 1521, as he encountered favorable winds on reaching the ocean. He called it Mar Pacífico, which in both Portuguese and Spanish means "peaceful sea". Important human migrations occurred in the Pacific in prehistoric times. About 3000 BC, the Austronesian peoples on the island of Taiwan mastered the art of long-distance canoe travel and spread themselves and their languages south to the Philippines and maritime Southeast Asia. Long-distance trade developed all along the coast from Mozambique to Japan. Trade, therefore knowledge, extended to the Indonesian islands but not Australia. By at least 878 when there was a significant Islamic settlement in Canton much of this trade was controlled by Arabs or Muslims. In 219 BC Xu Fu sailed out into the Pacific searching for the elixir of immortality. From 1404 to 1433 Zheng He led expeditions into the Indian Ocean; the first contact of European navigators with the western edge of the Pacific Ocean was made by the Portuguese expeditions of António de Abreu and Francisco Serrão, via the Lesser Sunda Islands, to the Maluku Islands, in 1512, with Jorge Álvares's expedition to southern China in 1513, both ordered by Afonso de Albuquerque from Malacca.
The east side of the ocean was discovered by Spanish explorer Vasco Núñez de Balboa in 1513 after his expedition crossed the Isthmus of Panama and reached a new ocean. He named it Mar del Sur because the ocean was to the south of the coast of the isthmus where he first observed the Pacific. In 1519, Portuguese explorer Ferdinand Magellan sailed the Pacific East to West on a Spanish expedition to the Spice Islands that would result in the first world circumnavigation. Magellan called the ocean Pacífico because, after sailing through the stormy seas off Cape Horn, the expedition found calm waters; the ocean was called the Sea of Magellan in his honor until the eighteenth century. Although Magellan himself died in the Philippines in 1521, Spanish Basque navigator Juan Sebastián Elcano led the remains of the expedition back to Spain across the Indian Ocean and round the Cape of Good Hope, completing the first world circumnavigation in a single expedition in 1522. Sailing around and east of the Moluccas, between 1525 and 1527, Portuguese expeditions discovered the Caroline Islands, the Aru Islands, Papua New Guinea.
In 1542–43 the Portuguese reached Japan. In 1564, five Spanish ships carrying 379 explorers crossed the ocean from Mexico led by Miguel López de Legazpi, sailed to the Philippines and Mariana Islands. For the remainder of the 16th century, Spanish influence was paramount, with ships sailing from Mexico and Peru across the Pacific Ocean to the Philippines via Guam, establishing the Spanish East Indies; the Manila galleons operated for two and a half centuries, linking Manila and Acapulco, in one of the longest trade routes in history. Spanish expeditions discovered Tuvalu, the Marquesas, the Cook Islands, the Solomon Islands, the Admiralty Islands in the South Pacific. In the quest for Terra Australis, Spanish explorations in the 17th century, such as the expedition led by the Portuguese navigator Pedro Fernandes de Queirós, discovered the Pitcairn and Vanuatu archipelagos, sailed the Torres Strait between Australia and New Guinea, named after navigator Luís Vaz de Torres. Dutch explorers, sailing around southern Africa engaged in discovery and trade.
In the 16th and 17th centuries Spain considered the Pacific Ocean a mare clausum—a sea closed to other naval powers. As the only known entrance from the Atlantic, the Strait of Magellan was at times patrolled by fleets sent to prevent entrance of non-Spanish ships. On the western side of the Pacific Ocean the Dutch threatened the Spanish Philippines; the 18th cen
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
Zalophus is a genus of the family Otariidae of order Carnivora. It includes these species, of which one became extinct: Z. californianus: California sea lion Z. japonicus: Japanese sea lion † Z. wollebaeki: Galápagos sea lion
Northern fur seal
The northern fur seal is an eared seal found along the north Pacific Ocean, the Bering Sea, the Sea of Okhotsk. It is the largest member of the fur seal subfamily and the only living species in the genus Callorhinus. A single fossil species, Callorhinus gilmorei, is known from the Pliocene of Japan and western North America. Northern fur seals have extreme sexual dimorphism, with males being 30–40% longer and more than 4.5 times heavier than adult females. The head is foreshortened in both sexes because of the short, down-curved muzzle, small nose, which extends beyond the mouth in females and moderately in males; the pelage is luxuriant, with a dense underfur in a creamy color. The underfur is obscured by the longer guard hairs, although it is visible when the animals are wet. Features of both fore and hind flippers are diagnostic of the species. Fur is absent on the top of the fore flippers and an abrupt "clean line" is seen across the wrist where the fur ends; the hind flippers are proportionately the longest in any otariid because of long, cartilaginous extensions on all of the toes.
Small claws are on digits 2–4, well back from the flap-like end of each digit. The ear pinnae are long and conspicuous, naked of dark fur at the tips in older animals; the mystacial vibrissae can be long, extend beyond the ears. Adults have all white vibrissae and subadults have a mixture of white and black vibrissae, including some that have dark bases and white ends, pups and yearlings have all black vibrissae; the eyes are proportionately large and conspicuous on females and juveniles. Adult males are stocky in build, have enlarged necks. A mane of coarse, longer guard hairs extends from the lower neck to the shoulders, and covers the nape, neck and upper back. While the skulls of adult males are large and robust for their overall size, their heads appear short because of the combination of a short muzzle, the backs of the head behind the ear pinnae being obscured by the enlarged necks. Adult males have abrupt foreheads formed by the elevation of the crown from development of the sagittal crests, thicker fur of the mane on the top of their heads.
Canine teeth are much longer and have a greater diameter in adult males than those found on adult females, this relationship holds to a lesser extent at all ages. Adult females and juveniles are moderate in build. Distinguishing the sexes is difficult until about age five; the body is modest in size and the neck and shoulders are sized in proportion with the torso. Adult females and subadults have more variable coloration than adult males, they are dark silver-gray to charcoal above. The flanks, chest and underside of the neck forming a chevron pattern in this area, are cream to tan with rusty tones. Variable cream to rust-colored areas are on the sides and top of the muzzle, as a "brush stroke" running backwards under the eye. In contrast, adult males reddish to dark brown all over, their manes can have variable amounts of yellowish tinting on the guard hairs. Pups are blackish at birth, with variable oval areas of buff on the sides, in the axillary area, on the chin and sides of the muzzle. After three to four months, pups molt to the color of adult subadults.
Males can be as large as 2.1 270 kg. Females can weigh 50 kg or more. Newborns weigh 5.4–6 kg, are 60–65 cm long. The teeth are haplodont, i.e. sharp and single-rooted, as is common with carnivorous marine mammals adapted to tearing fish flesh. As with most caniforms, the upper canines are prominent; the dental formula of the adult is 188.8.131.52.1.4.1 Like other otariids, northern fur seals are built for efficient terrestrial locomotion. Their hind limbs are in a plantigrade stance and are able to rotate under the body for quadrupedal locomotion and support; when swimming, there are two different types of movement: diving. These seals swim with forelimb propulsion due to their physiology, they have flexible joints between vertebrae for better maneuverability in the water as well as “greater muscular leverage” for pectoral strokes. Stroke patterns are different for different dive types and locomotion, stroke rates vary for individuals since there’s a relationship between maximum stroke rate and body size.
The northern fur seal is found in the north Pacific – its southernmost reach is a line that runs from the southern tip of Japan to the southern tip of the Baja California Peninsula, the Sea of Okhotsk, the Bering Sea. An estimated 1.1 million northern fur seals occur across the range, of which half breed on the Pribilof Islands in the east Bering Sea. Another 200–250 thousand breed on the Commander Islands in the west Bering Sea, some 100,000 breed on Tyuleniy Island off the coast of Sakhalin in the southwest Sea of Okhotsk, another 60–70 thousand in the central Kuril Islands in Russia. Smaller rookeries are found on Bogoslof Island in the Aleutian Chain, San Miguel Island in the Channel Island group and South Farallon Island off the coast of California. Recent evidence from stable isotope analysis of Holocene fur seal bone collagen indicates that before the maritime fur trade, it was more common for these animals to breed at local rookeries in British Columbia and along much of the northwest coast of North America.
During the winter, northern fur seals display a net movement southward, with animals from Russian rookeries entering Japanese and Korean waters in the Sea of Japan and Alaskan animals moving along the central a
California sea lion
The California sea lion is a coastal eared seal native to western North America. It is one of six species of sea lion, its natural habitat ranges from southeast Alaska to central Mexico, including the Gulf of California. Sea lions are sexually dimorphic, they haul-out on sandy or rocky beaches, but they frequent manmade environments such as marinas and wharves. Sea lions feed on a number of species of fish and squid, are preyed on by orcas and white sharks. California sea lions have a polygynous breeding pattern. From May to August, males try to attract females with which to mate. Females are free to move in between territories, are not coerced by males. Mothers nurse their pups in between foraging trips. Sea lions communicate with numerous vocalizations, notably with mother-pup contact calls. Outside their breeding season, sea lions spend much of their time at sea, but they come to shore to molt. Sea lions are intelligent, can be trained to perform various tasks and display limited fear of humans if accustomed to them.
Because of this, California sea lions are a popular choice for public display in zoos and oceanariums, are trained by the United States Navy for certain military operations. The International Union for Conservation of Nature lists the species as Least Concern due to its abundance. To protect fish, the US states of Oregon and Washington engage in annual kill quotas of sea lions; the California sea lion was described by René Primevère Lesson, a French naturalist, in 1828. It is grouped with other sea lions and fur seals in the family Otariidae. Otariids known as eared seals, differ from true seals in having external ear flaps, proportionately larger foreflippers and pectoral muscles. Along with the Galapagos sea lion and the extinct Japanese sea lion, the California sea lion belongs to the genus Zalophus, which derives from the Greek words za, meaning "intensive," and lophus, meaning "crest." This refers to the protruding sagittal crest of the males. Traditionally, the Galapagos sea lion and Japanese sea lion were classified as subspecies of the California sea lion.
However, a genetic study in 2007 found. The lineages of the California and Japanese sea lion appear to have split off 2.2 million years ago during the Pliocene. The California sea lion differs from the Galapagos sea lion in its greater sexual dimorphism; the Steller sea lion is the closest extant relative of the Zalophus sea lions. Being sexually dimorphic, California sea lions differ in size and coloration between the sexes. Males are around 2.4 m long and weigh up to 350 kg, while females are around 1.8 m and weigh up to 100 kg. Females and juveniles have a tawny brown pelage, although they may be temporarily light gray or silver after molting; the pelage of adult males can be anywhere from light brown to black, but is dark brown. The face of adult males may be light tan in some areas. Pups have a dark brown pelage at birth. Although the species has a slender build, adult males have robust necks and shoulders. Adult males have a protruding crest which gives them a "high, domed forehead", they have manes, which are less developed than those of adult male South American and Steller sea lions.
Both sexes have narrow muzzles. As an otariid, the California sea lion relies on its foreflippers to propel itself; this form of aquatic locomotion, along with its streamlined body reduces drag underwater. Its foreflipper movement is not continuous; the flexibility of its spine allows the sea lion to bend its neck backwards far enough to reach its hindflippers. This allows the animal to maintain a streamlined posture; when moving on land, the sea lion is able to walk on all fours. It moves the foreflippers in a transverse, rather than fashion. In addition, it relies on movements of its head and neck more than its hindflippers for terrestrial locomotion. Sea lions may travel at speeds of around 10.8 km/h, can dive at depths of 274 m and for up to 9.9 minutes, though most dives are 80 m and last less than 3 minutes. Sea lions have color vision; this is an adaptation for living in marine coastal habitats. Sea lions have acute underwater hearing, with a hearing range of 0.4–32 kHz. Sea lions rely on their whiskers or vibrissae for detection of vibrations underwater.
Compared to the harbor seal, the California sea lion's vibrissae are smoother and less specialized and thus perform less when following hydrodynamic trails, although they still perform well. The California sea lion ranges along the western coast and islands of North America, from southeast Alaska to central Mexico. Mitochondrial DNA sequences in 2009 have identified five distinct California sea lion populations: the U. S. or Pacific Temperate stock, the Western Baja California or Pacific Tropical stock, the Southern and Northern Gulf of California stocks. The U. S. stock breeds in the Channel Islands, although some breeding sites may be established in northern California, females are now found there. The Western Baja California stock breeds near Punta Eugenia and at Isla Santa Margarita; the above-mentioned stocks are separated by the Ensenada Front. The stocks of the Gulf of California live in the shallow wa
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