The guinea pig or domestic guinea pig known as cavy or domestic cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, guinea pigs are not native to Guinea, nor are they biologically related to pigs, the origin of the name is still unclear, they originated in the Andes of South America, studies based on biochemistry and hybridization suggest they are domesticated descendants of a related species of cavy such as C. tschudii, therefore do not exist in the wild. In Western society, the domestic guinea pig has enjoyed widespread popularity as a household pet, a type of pocket pet, since its introduction by European traders in the 16th century, their docile nature, friendly responsiveness to handling and feeding, the relative ease of caring for them have made and continue to make guinea pigs a popular choice of pet. Organizations devoted to the competitive breeding of guinea pigs have been formed worldwide, many specialized breeds with varying coat colors and textures are selected by breeders.
The domestic guinea pig plays an important role in folk culture for many indigenous Andean groups as a food source, but in folk medicine and in community religious ceremonies. The animals are used for meat and are a culinary staple in the Andes Mountains, where they are known as cuy. A modern breeding program was started in the 1960s in Peru that resulted in large breeds known as cuy mejorados and prompted efforts to increase consumption of the animal outside South America. Biological experimentation on domestic guinea pigs has been carried out since the 17th century; the animals were so used as model organisms in the 19th and 20th centuries that the epithet guinea pig came into use to describe a human test subject. Since that time, they have been replaced by other rodents such as mice and rats. However, they are still used in research as models for human medical conditions such as juvenile diabetes, tuberculosis and pregnancy complications; the scientific name of the common species is Cavia porcellus, with porcellus being Latin for "little pig".
Cavia is New Latin. Cabiai may be an adaptation of the Portuguese çavia, itself derived from the Tupi word saujá, meaning rat. Guinea pigs are called quwi or jaca in Quechua and cuy or cuyo in the Spanish of Ecuador and Bolivia. Breeders tend to use the more formal "cavy" to describe the animal, while in scientific and laboratory contexts, it is far more referred to by the more colloquial "guinea pig". How the animals came to be called "pigs" is not clear, they are built somewhat like pigs, with large heads relative to their bodies, stout necks, rounded rumps with no tail of any consequence. They can survive for long periods in small quarters, like a'pig pen', were thus transported on ships to Europe; the animal's name alludes to pigs in many European languages. The German word for them is Meerschweinchen "little sea pig", translated into Polish as świnka morska, into Hungarian as tengerimalac, into Russian as морская свинка; this derives from the Middle High German name merswin. This meant "dolphin" and was used because of the animals' grunting sounds.
Many other less scientifically based explanations of the German name exist. For example, sailing ships stopping to reprovision in the New World would pick up stores of guinea pigs, which provided an transportable source of fresh meat; the French term is cochon cobaye. This is not universal; the Chinese refer to them as 豚鼠, sometimes as Netherlands pig or Indian mouse. The Japanese word for guinea pig is "モルモット", which derives from the name of another mountain-dwelling rodent, the marmot; the other Japanese word for guinea pig, using kanji, is tenjiku-nezumi, which translates as India rat. The origin of "guinea" in "guinea pig" is harder to explain. One proposed explanation is that the animals were brought to Europe by way of Guinea, leading people to think they had originated there. "Guinea" was frequently used in English to refer to any far-off, unknown country, so the name may be a colorful reference to the animal's exotic appeal. Another hypothesis suggests the "guinea" in the name is a corruption of "Guiana", an area in South America.
A common misconception is that they were so named because they were sold for the price of a guinea coin. Others believe; the guinea pig was first domesticated as early as 5000 BC for food by tribes in the Andean region of South America (the present-day sout
Bushmeat, wildmeat, or game meat is meat from non-domesticated mammals, reptiles and birds hunted for food in tropical forests. Commercial harvesting and the trade of wildlife is considered a threat to biodiversity. Bushmeat provides a route for a number of serious tropical diseases to spread to humans from their animal hosts, such as HIV/AIDS and Ebola. Bushmeat is used for sustenance in remote areas, while in major towns and cities in bushmeat eating societies it is treated as a delicacy. Today the term bushmeat is used for meat of terrestrial wild or feral mammals, killed for sustenance or commercial purposes throughout the humid tropics of the Americas and Africa. In West Africa, Achatina achatina, a giant African snail, is gathered, sold and monitored as part of the bushmeat trade. To reflect the global nature of hunting of wild animals, Resolution 2.64 of the IUCN General Assembly in Amman in October 2000 referred to wild meat rather than bushmeat. A more worldwide term for terrestrial wild animals is game.
The term bushmeat crisis is sometimes used to describe unsustainable hunting of endangered wild mammals in West and Central Africa and the humid tropics, depending on interpretation. African hunting predates recorded history; the volume of the bushmeat trade in West and Central Africa was estimated at 1-5 million tonnes per year at the turn of the century. According to the Center for International Forestry Research in 2014 5 million tonnes were still being consumed per year in the Congo Basin. For the people of this region, bushmeat represents a primary source of animal protein in the diet, making it a significant commercial industry. According to a 1994 study in Gabon, annual sales were estimated at US$50 million; the study found that bushmeat accounted for more than half of meat sold in local markets, with primates representing 20% of the total bushmeat. Logging concessions operated by companies in African forests have been linked to the bushmeat trade; because they provide roads and other access to remote forests, they are the primary means for the transportation of hunters and meat between forests and urban centres.
Some, including the Congolaise Industrielle du Bois in the Republic of Congo, have partnered with governments and international conservation organizations to regulate the bushmeat trade within the concessions where they operate. Numerous solutions are needed. In the case of Ghana, international over-exploitation of African fishing grounds has increased demand for bushmeat. Both EU-subsidized fleets and local commercial fleets have depleted fish stocks, leaving local people to supplement their diets with animals hunted from nature reserves. Over 30 years of data link sharp declines in both mammal populations and the biomass of 41 wildlife species with a decreased supply of fish. In the case of Liberia in West Africa, bushmeat is eaten and is considered a delicacy. A 2004 public opinion survey found that bushmeat ranked second behind fish among residents of the capital Monrovia as a preferred source of protein. Of households where bushmeat was served, 80% of residents said they cooked it "once in a while", while 13% cooked it once a week and 7% cooked bushmeat daily.
The survey was conducted during the last civil war, bushmeat consumption is now believed to be far higher. There has been extensive literature on the topic of bushmeat, related to field anthropology, the process of ethnology has been repeated in many different regions to ensure accuracy and precision; the transmission of variable retrovirus chains causes zoonotic diseases. Outbreaks of the Ebola virus in the Congo Basin and in Gabon in the 1990s have been associated with the butchering of apes and consumption of their meat. Bushmeat hunters in Central Africa infected with the human T-lymphotropic virus were exposed to wild primates. Results of research on wild chimpanzees in Cameroon indicate that they are infected with the simian foamy virus and constitute a reservoir of HIV-1, a precursor of the acquired immunodeficiency syndrome in humans. There are several distinct strains of HIV, indicating that this cross-species transfer has occurred several times. Simian Immunodeficiency Virus, present in modern day chimpanzees, is derived from older strains of the virus present in the Red Capped Mangabey and Greater Spot-nosed Monkey.
It is that HIV was transferred to humans after having come into contact with infected bushmeat. The Ebola virus, for which the primary host is suspected to be fruit bats, has been linked to bushmeat. Between the first recorded outbreak in 1976 and the largest in 2014, the virus has transferred from animals to humans only 30 times, despite large numbers of bats being killed and sold each year. Bats drop eaten fruits and pulp land mammals such as gorillas and duikers feed on these fallen fruits; this chain of events forms a possible indirect means of transmission from the natural host to animal populations. Although primates and other species may be intermediates, evidence suggests people contract the virus from bats. Since most people buy smoked bushmeat and people preparing the food have the highest risk of infection. Hunters shoot, scavenge or catapult their prey, butcher the bats without gloves, getting bites or scratches and coming in contact with their blood. In 2014, the suspected index case for the Ebola outbreak in West Africa is a two-year-old child in Guéckédou in south-eastern Guinea, the child of a family tha
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
Rainforests are forests characterized by high rainfall, with annual rainfall in the case of tropical rainforests between 250 and 450 centimetres, definitions varying by region for temperate rainforests. The monsoon trough, alternatively known as the intertropical convergence zone, plays a significant role in creating the climatic conditions necessary for the Earth's tropical rainforests. Around 40% to 75% of all biotic species are indigenous to the rainforests. There may be many millions of species of plants and microorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there. Rainforests are responsible for 28% of the world's oxygen turnover, sometimes misnamed oxygen production, processing it through photosynthesis from carbon dioxide and consuming it through respiration; the undergrowth in some areas of a rainforest can be restricted by poor penetration of sunlight to ground level.
If the leaf canopy is destroyed or thinned, the ground beneath is soon colonized by a dense, tangled growth of vines and small trees, called a jungle. The term jungle is sometimes applied to tropical rainforests generally. Rainforests as well as endemic rainforest species are disappearing due to deforestation, the resulting habitat loss and pollution of the atmosphere. Tropical rainforests are characterized by a warm and wet climate with no substantial dry season: found within 10 degrees north and south of the equator. Mean monthly temperatures exceed 18 °C during all months of the year. Average annual rainfall is no less than 168 cm and can exceed 1,000 cm although it lies between 175 cm and 200 cm. Many of the world's tropical forests are associated with the location of the monsoon trough known as the intertropical convergence zone; the broader category of tropical moist forests are located in the equatorial zone between the Tropic of Cancer and Tropic of Capricorn. Tropical rainforests exist in Southeast Asia to the Philippines, Indonesia, Papua New Guinea and Sri Lanka.
Tropical forests have been called the "Earth's lungs", although it is now known that rainforests contribute little net oxygen addition to the atmosphere through photosynthesis. Tropical forests cover a large part of the globe, but temperate rainforests only occur in few regions around the world. Temperate rainforests are rainforests in temperate regions, they occur in North America, in Europe, in East Asia, in South America and in Australia and New Zealand. A tropical rainforest has a number of layers, each with different plants and animals adapted for life in that particular area. Examples include the emergent, canopy and forest floor layers; the emergent layer contains a small number of large trees called emergents, which grow above the general canopy, reaching heights of 45–55 m, although on occasion a few species will grow to 70–80 m tall. They need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas. Eagles, butterflies and certain monkeys inhabit this layer.
The canopy layer contains the majority of the largest trees 30 metres to 45 metres tall. The densest areas of biodiversity are found in the forest canopy, a more or less continuous cover of foliage formed by adjacent treetops; the canopy, by some estimates, is home to 50 percent of all plant species. Epiphytic plants attach to trunks and branches, obtain water and minerals from rain and debris that collects on the supporting plants; the fauna is similar to that found in the emergent layer, but more diverse. A quarter of all insect species are believed to exist in the rainforest canopy. Scientists have long suspected the richness of the canopy as a habitat, but have only developed practical methods of exploring it; as long ago as 1917, naturalist William Beebe declared that "another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles." True exploration of this habitat only began in the 1980s, when scientists developed methods to reach the canopy, such as firing ropes into the trees using crossbows.
Exploration of the canopy is still in its infancy, but other methods include the use of balloons and airships to float above the highest branches and the building of cranes and walkways planted on the forest floor. The science of accessing tropical forest canopy using airships or similar aerial platforms is called dendronautics; the understory or understorey layer lies between the forest floor. It is home to a number of birds and lizards, as well as predators such as jaguars, boa constrictors and leopards; the leaves are much larger at this level and insect life is abundant. Many seedlings that will grow to the canopy level are present in the understory. Only about 5% of the sunlight s
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
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