Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus Rattus. Other rat genera include Neotoma and Dipodomys. Rats are distinguished from mice by their size; when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonomically specific. In other words, rat is not a scientific term; the best-known rat species are the brown rat. This group known as the Old World rats or true rats, originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but weigh over 500 grams in the wild; the term rat is used in the names of other small mammals that are not true rats. Examples include the North American pack rats, a number of species loosely called kangaroo rats, others. Rats such as the bandicoot rat are murine rodents related to true rats but are not members of the genus Rattus.
Male rats are called bucks. A group of rats is referred to as a mischief; the common species are opportunistic survivors and live with and near humans. They may cause substantial food losses in developing countries. However, the distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat. Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, Campylobacter; the Black Death is traditionally believed to have been caused by the microorganism Yersinia pestis, carried by the tropical rat flea, which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages. Another zoonotic disease linked to the rat is foot-and-mouth disease. Rats become sexually reach social maturity at about 5 to 6 months of age; the average lifespan of rats varies by species.
The black and brown rats diverged from other Old World rats in the forests of Asia during the beginning of the Pleistocene. The characteristic long tail of most rodents is a feature, extensively studied in various rat species models, which suggest three primary functions of this structure: thermoregulation, minor proprioception, a nocifensive-mediated degloving response. Rodent tails—particularly in rat models—have been implicated with a thermoregulation function that follows from its anatomical construction; this particular tail morphology is evident across the family Muridae, in contrast to the bushier tails of Sciuridae, the squirrel family. The tail is hairless and thin skinned but vascularized, thus allowing for efficient countercurrent heat exchange with the environment; the high muscular and connective tissue densities of the tail, along with ample muscle attachment sites along its plentiful caudal vertebrae, facilitate specific proprioceptive senses to help orient the rodent in a three-dimensional environment.
Lastly, murids have evolved a unique defense mechanism termed degloving that allows for escape from predation through the loss of the outermost integumentary layer on the tail. However, this mechanism is associated with multiple pathologies that have been the subject of investigation. Multiple studies have explored the thermoregulatory capacity of rodent tails by subjecting test organisms to varying levels of physical activity and quantifying heat conduction via the animals' tails. One study demonstrated a significant disparity in heat dissipation from a rat's tail relative to its abdomen; this observation was attributed to the higher proportion of vascularity in the tail, as well as its higher surface-area-to-volume ratio, which directly relates to heat's ability to dissipate via the skin. These findings were confirmed in a separate study analyzing the relationships of heat storage and mechanical efficiency in rodents that exercise in warm environments. In this study, the tail was a focal point in measuring heat modulation.
On the other hand, the tail's ability to function as a proprioceptive sensor and modulator has been investigated. As aforementioned, the tail demonstrates a high degree of muscularization and subsequent innervation that ostensibly collaborate in orienting the organism; this is accomplished by coordinated flexion and extension of tail muscles to produce slight shifts in the organism's center of mass, etc. which assists it with achieving a state of proprioceptive balance in its environment. Further mechanobiological investigations of the constituent tendons in the tail of the rat have identified multiple factors that influence how the organism navigates its environment with this structure. A particular example is that of a study in which the morphology of these tendons is explicated in detail. Namely, cell viability tests of tendons of the rat's tail demonstrate a higher proportion of living fibroblasts that produce the collagen for these fibers; as in humans, these tendons contain a high density of golgi tendon organs that help the animal assess stretching of muscle in situ and adjust accordingly by relaying the information to higher cortical areas associated with balance and movement.
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 term Hystricomorpha has had many definitions throughout its history. In the broadest sense, it refers to any rodent with a hystricomorphous zygomasseteric system; this includes the Hystricognathi, Ctenodactylidae and Pedetidae. Molecular and morphological results suggest the inclusion of the Anomaluridae and Pedetidae in Hystricomorpha may be suspect. Based on Carleton & Musser 2005, these two families are treated here as representing a distinct suborder Anomaluromorpha; the modern definition of Hystricomorpha known as Entodacrya or Ctenohystrica, is a taxonomic hypothesis uniting the gundis with the hystricognath rodents. Considerable morphological and strong molecular support exists for this relationship. If true, this hypothesis renders the traditional view of Sciurognathi invalid, as it becomes a paraphyletic group; the hystricomorph rodents, or at least members of Caviomorpha, are sometimes not regarded as rodents. Most molecular and genetic research, confirms the monophyly of rodents.
Support for rodent polyphyly appears to be a product of long branch attraction. Hystricomorph rodents appeared in South America in the Eocene, a continent which had metatherians and meridiungulates as the only resident nonflying mammals, they arrived by rafting across the Atlantic from Africa. The same type of migration may have occurred with primates, which appeared in South America in the Eocene when it was an isolated continent, long before the Great American Interchange. All of this is still controversial, new scientific discoveries on this subject are published regularly; the following list of families is based on the taxonomy of Marivaux et al. 2002 and Marivaux, Vianey-Liaud & Jaeger 2004, who subjected a number of early fossil rodents to parsimony analysis and recovered support for the Hystricomorpha or Entodacrya hypothesis. Their results rendered the suborder Sciuravida as defined by McKenna & Bell 1997 to be polyphyletic and invalid; the symbol "†" is used to indicate extinct groups.
Suborder Hystricomorpha Superfamily Ctenodactyloidea Ctenodactylidae - gundis †Tammquammyidae Diatomyidae - Laotian rock rat †Yuomyidae †Chapattimyidae Hystricognathiformes †Tsaganomyidae Hystricognathi - true hystricognaths †Baluchimyinae Hystricidae - Old World porcupines Phiomorpha †Myophiomyidae †Diamantomyidae †Phiomyidae †Kenyamyidae Petromuridae - dassie rat Thryonomyidae - cane rats Bathyergidae - blesmols †Bathyergoididae Caviomorpha - New World hystricognaths Superfamily Erethizontoidea Erethizontidae - New World porcupines Superfamily Cavioidea †Cephalomyidae Dasyproctidae - agoutis and acouchis Cuniculidae - pacas †Eocardiidae Dinomyidae - pacarana Caviidae - cavies and guinea pigs Superfamily Octodontoidea Octodontidae - degus and relatives Ctenomyidae - tuco-tucos Echimyidae - spiny rats Myocastoridae - nutria Capromyidae - hutias †Heptaxodontidae - giant hutias Superfamily Chinchilloidea Chinchillidae - chinchillas and viscachas †Neoepiblemidae Abrocomidae - chinchilla rats
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals, they are used to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola and dens, tooth. Molars show a great deal of diversity in shape across mammal groups. In humans, the molar teeth have either five cusps. Adult humans have 12 molars, in four groups of three at the back of the mouth; the third, rearmost molar in each group is called a wisdom tooth. It is the last tooth to appear, breaking through the front of the gum at about the age of 20, although this varies from individual to individual. Race can affect the age at which this occurs, with statistical variations between groups. In some cases, it may not erupt at all; the human mouth contains lower molars. They are: maxillary first molar, maxillary second molar, maxillary third molar, mandibular first molar, mandibular second molar, mandibular third molar. In mammals, the crown of the molars and premolars is folded into a wide range of complex shapes.
The basic elements of the crown are the more or less conical projections called cusps and the valleys that separate them. The cusps contain both dentine and enamel, whereas minor projections on the crown, called crenullations, are the result of different enamel thickness. Cusps are joined to form ridges and expanded to form crests. Cingula are incomplete ridges that pass around the base of the crown; these mammalian, multicusped cheek teeth evolved from single-cusped teeth in reptilians, although the diversity of therapsid molar patterns and the complexity in the molars of the earliest mammals make determining how this happened impossible. According to the accepted "differentiation theory", additional cusps have arisen by budding or outgrowth from the crown, while the rivalling "concrescence theory" instead proposes that complex teeth evolved by the clustering of separate conical teeth. Therian mammals are agreed to have evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in a triangle.
Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, ento-. Suffixes are added to these names: -id is added to cusps on a lower molar. A shelf-like ridge on the lower part of the crown is called a cingulum; the design, considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This molar design has two important features: the trigonid, or shearing end, the talonid, or crushing heel. In modern tribosphenic molars, the trigonid is towards the front of the jaw and the talonid is towards the rear; the tribosphenic tooth is found in young platypuses. Upper molars look like three-pointed mountain ranges; the tribosphenic design appears primitively in all groups of mammals. Some paleontologists believe that it developed independently in monotremes, rather than being inherited from an ancestor that they share with marsupials and placentals. For example, the dentition of the Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals, but, on the other hand, the status of neither of these two groups is well-established.
Some Jurassic mammals, such as Shuotherium and Pseudotribos, have "reversed tribosphenic" molars, in which the talonid is towards the front. This variant is regarded as an example of convergent evolution. From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the hypocone, subsequently evolved. Quadrate molars have an additional fourth cusp on the lingual side called the hypocone, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including hedgehogs and many primates, including humans. There may be a fifth cusp. In many mammals, additional smaller cusps called, they are named after their locations, e.g. a paraconule is located between a paracone and a metacone, a hypoconulid is located between a hypoconid and an entoconid. In bunodont molars, the cusps rounded hills rather than sharp peaks, they are most common among omnivores such as pigs and humans. Bunodont molars are effective crushing devices and basically quadrate in shape.
Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cattle and horses, all animals that feed on gritty, fibrous material. Hypsodont molars can continue to grow for example in some species of Arvicolinae. Hypsodont molars lack both a neck; the occlusal surface is rough and flat, adapted for crushing and grinding plant material. The body is covered with cementum both above and below the gingival line, below, a layer of enamel covering the entire length of the body; the cementum and the enamel invaginate into the thick
The Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era known as the Age of Reptiles; the start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, the Tithonian event at the end; the Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations; the Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, Gondwana to the south; this created more coastlines and shifted the continental climate from dry to humid, many of the arid deserts of the Triassic were replaced by lush rainforests.
On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life; the oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, he named it "Jura-Kalkstein" in 1799.
The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", borrowed into Latin as a place name, evolved into Juria and Jura. The Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations known as Lias and Malm in Europe; the separation of the term Jurassic into three sections originated with Leopold von Buch. The faunal stages from youngest to oldest are: During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Jurassic North Atlantic Ocean was narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, the Neotethys basin appeared. Climates were warm, with no evidence of a glacier having appeared; as in the Triassic, there was no land over either pole, no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation; the Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus common, along with calcitic ooids, calcitic cements, invertebrate faunas with dominantly calcitic skeletons; the first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are found in Russia, South America, Japan and the United Kingdom.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well studied in Africa. Late Jurassic strata are poorly represented apart from the spectacular Tendaguru fauna in Tanzania; the Late Jurassic life of Tendaguru is similar to that found in western North America's Morrison Formation. During the Jurassic period, the primary vertebrates living in the sea were marine reptiles; the latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in rivers. In the invertebrate world, several new groups appeared, including rudists (a reef-formi
A mouse, plural mice, is a small rodent characteristically having a pointed snout, small rounded ears, a body-length scaly tail and a high breeding rate. The best known mouse species is the common house mouse, it is a popular pet. In some places, certain kinds of field mice are locally common, they are known to invade homes for shelter. Species of mice are found in Rodentia, are present throughout the order. Typical mice are found in the genus Mus. Mice are distinguished from rats by their size; when someone discovers a smaller muroid rodent, its common name includes the term mouse, while if it is larger, the name includes the term rat. Common terms rat and mouse are not taxonomically specific. Scientifically, the term mouse is not confined to members of Mus for the deer mouse. Domestic mice sold as pets differ in size from the common house mouse; this is attributable both to different conditions in the wild. The best-known strain, the white lab mouse, has more uniform traits that are appropriate to its use in research.
Cats, wild dogs, birds of prey and certain kinds of arthropods have been known to prey upon mice. Because of its remarkable adaptability to any environment, the mouse is one of the most successful mammalian genera living on Earth today. Mice, in certain contexts, can be considered vermin which are a major source of crop damage, causing structural damage and spreading diseases through their parasites and feces. In North America, breathing dust that has come in contact with mouse excrement has been linked to hantavirus, which may lead to hantavirus pulmonary syndrome. Nocturnal animals, mice compensate for their poor eyesight with a keen sense of hearing, rely on their sense of smell to locate food and avoid predators. Mice build long intricate burrows in the wild; these have long entrances and are equipped with escape tunnels or routes. In at least one species, the architectural design of a burrow is a genetic trait. Order Dasyuromorphia marsupial mice, smaller species of Dasyuridae order Rodentia suborder Castorimorpha family Heteromyidae Kangaroo mouse, genus Microdipodops Pocket mouse, tribe Perognathinae Spiny pocket mouse, genus Heteromys suborder Anomaluromorpha family Anomaluridae flying mouse suborder Myomorpha family Cricetidae Brush mouse, Peromyscus boylii Florida mouse Golden mouse American Harvest mouse, genus Reithrodontomys family Muridae typical mice, the genus Mus Field mice, genus Apodemus Wood mouse, Apodemus sylvaticus Yellow-necked mouse, Apodemus flavicollis Large Mindoro forest mouse Big-eared hopping mouse Luzon montane forest mouse Forrest's mouse Pebble-mound mouse Bolam's mouse Eurasian Harvest mouse, genus Micromys Mice are common experimental animals in laboratory research of biology and psychology fields because they are mammals, because they share a high degree of homology with humans.
They are the most used mammalian model organism, more common than rats. The mouse genome has been sequenced, all mouse genes have human homologs; the mouse has 2.7 billion base pairs and 20 pairs of chromosomes. They can be manipulated in ways that are illegal with humans, although animal rights activists object. A knockout mouse is a genetically modified mouse that has had one or more of its genes made inoperable through a gene knockout. Reasons for common selection of mice are small size, inexpensive varied diet maintained, can reproduce quickly. Several generations of mice can be observed in a short time. Mice are very docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental. Mice and rats have the same organs in the same places, with the difference of size. Many people buy mice as companion pets, they can be playful and can grow used to being handled. Like pet rats, pet mice should not be left unsupervised outside as they have many natural predators, including birds, lizards and dogs.
Male mice tend to have a stronger odor than the females. However, mice are as pets they never need bathing. Well looked-after mice can make ideal pets; some common mouse care products are: Cage – Usually a hamster or gerbil cage, but a variety of special mouse cages are now available. Most should have a secure door. Food – Special pelleted and seed-based food is available. Mice can eat most rodent food Bedding – Usually made of hardwood pulp, such as aspen, sometimes from shredded, uninked paper or recycled virgin wood pulp. Using corn husk bedding is avoided because it promotes Aspergillus fungus, can grow mold once it gets wet, rough on their feet. In nature, mice are herbivores, consuming any kind of fruit or grain from plants. However, mice adapt well to urban areas and are known for eating all types of food scraps. In captivity, mice are fed commercial pelleted mouse diet; these diets are nutritionally complete. Mice are a staple in the diet of many small carnivores. Humans have eaten mice since prehistoric times and still eat them as a delicacy throughout eastern Zambia and northern Malawi, where they are a seasonal source of protein.
Mice are no longer consumed by humans elsewhere. However, in Victorian Britain, fried mice were still given to children as a folk remedy for bed-wetting. Prescribed cures in Ancient Egypt included mice as medicine. In Ancient Egypt, when infant
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