Robert Broom FRS FRSE was a Scottish South African doctor and paleontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow. From 1903 to 1910 he was professor of zoology and geology at Victoria College, South Africa, subsequently he became keeper of vertebrate paleontology at the South African Museum, Cape Town. Broom was born at 66 Back Sneddon Street in Paisley, the son of John Broom, a designer of calico prints and Paisley shawls, Agnes Hunter Shearer. In 1893 he married Mary Baird Baillie. In his medical studies at the University of Glasgow Broom specialised in midwifery. After graduating in 1895 he travelled to Australia, he settled in South Africa in 1897, just prior to the South African War. From 1903 to 1910 he was professor of Zoology and Geology at Victoria College, but was forced out of this position for promoting belief in evolution, he established a medical practice in the Karoo region of South Africa, an area rich in Therapsid fossils.
Based on his continuing studies of these fossils and mammalian anatomy he was made a Fellow of the Royal Society in 1920. Following the discovery of the Taung child he became interested in the search for human ancestors and commenced work on much more recent fossils from the dolomite caves north-west of Johannesburg Sterkfontein Cave; as well as describing many mammalian fossils from these caves he identified several hominin fossils, the most complete of, an Australopithecine skull, nicknamed Mrs Ples, a partial skeleton that indicated that Australopithecines walked upright. Broom died in Pretoria, South Africa in 1951. Broom was first known for his study of mammal-like reptiles. After Raymond Dart's discovery of the Taung Child, an infant australopithecine, Broom's interest in paleoanthropology was heightened. Broom's career seemed over and he was sinking into poverty, when Dart wrote to Jan Smuts about the situation. Smuts, exerting pressure on the South African government, managed to obtain a position for Broom in 1934 with the staff of the Transvaal Museum in Pretoria as an Assistant in Palaeontology.
In the following years, he and John T. Robinson made a series of spectacular finds, including fragments from six hominins in Sterkfontein, which they named Plesianthropus transvaalensis, popularly called Mrs. Ples, but, classified as an adult Australopithecus africanus, as well as more discoveries at sites in Kromdraai and Swartkrans. In 1937, Broom made his most famous discovery, by defining the robust hominin genus Paranthropus with his discovery of Paranthropus robustus; these discoveries helped support Dart's claims for the Taung species. The remainder of Broom's career was devoted to the exploration of these sites and the interpretation of the many early hominin remains discovered there. For his volume, The South Africa Fossil Ape-Men, The Australopithecinae, in which he proposed the Australopithecinae subfamily, Broom was awarded the Daniel Giraud Elliot Medal from the National Academy of Sciences in 1946, he continued to write to the last. Shortly before his death he finished a monograph on the Australopithecines and remarked to his nephew: "Now that's finished... and so am I."
Broom was a nonconformist and was interested in the paranormal and spiritualism. Broom was a believer in spiritual evolution. In his book The Coming of Man: Was it Accident or Design? he claimed that "spiritual agencies" had guided evolution as animals and plants were too complex to have arisen by chance. According to Broom, there were at least two different kinds of spiritual forces, psychics are capable of seeing them. Broom claimed there was a plan and purpose in evolution and that the origin of Homo sapiens is the ultimate purpose behind evolution. According to Broom "Much of evolution looks as if it had been planned to result in man, in other animals and plants to make the world a suitable place for him to dwell in."After discovering the skull of Mrs. Ples, Broom was asked if he excavated at random, Broom replied that spirits had told him where to find his discoveries. Among hundreds of articles contributed by him to scientific journals, the most important include: "Fossil Reptiles of South Africa" in Science in South Africa "Reptiles of Karroo Formation" in Geology of Cape Colony "Development and Morphology of the Marsupial Shoulder Girdle" in Transactions of the Royal Society of Edinburgh "Comparison of Permian Reptiles of North America with Those of South Africa" in Bulletin of the American Museum of Natural History "Structure of Skull in Cynodont Reptiles" in Proceedings of the Zoölogical Society.
The South Africa Fossil Ape-Men, The Australopithecinae. Books The origin of the human skeleton: an introduction to human osteology The mammal-like reptiles of South Africa and the origin of mammals The coming of man: was it accident or design? The South African fossil ape-man: the Australopithecinae Sterkfontein ape-man Plesianthropus Finding the missing link List of fossil sites List of hominina fossils Bernard Price Institute for Palaeontological Research Biographies: Robert Broom TalkOrigins Archive Robert Broom: A Short Bibliography of his Evolutionary Works, MPRInstitute.org site. British metaphysics as reflected in Robert Broom's evolutionary theory, translation of an article by Václav Petr published in Bulletin of the Czech Geological Survey, 75: 73-85. Praha 2000. Text and photos displayed entire at the MPRInstitute.org site
A coprolite is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology; the name is derived from the Greek words κόπρος and λίθος. They were first described by William Buckland in 1829. Prior to this they were known as "fossil fir cones" and "bezoar stones", they serve a valuable purpose in paleontology because they provide direct evidence of the predation and diet of extinct organisms. Coprolites may range in size from a few millimetres to over 60 centimetres. Coprolites, distinct from paleofaeces, are fossilized animal dung. Like other fossils, coprolites have had much of their original composition replaced by mineral deposits such as silicates and calcium carbonates. Paleofaeces, on the other hand, retain much of their original organic composition and can be reconstituted to determine their original chemical properties, though in practice the term coprolite is used for ancient human faecal material in archaeological contexts.
In the same context, there are the urolites, erosions caused by evacuation of liquid wastes and nonliquid urinary secretions. The fossil hunter Mary Anning noticed as early as 1824 that "bezoar stones" were found in the abdominal region of ichthyosaur skeletons found in the Lias formation at Lyme Regis, she noted that if such stones were broken open they contained fossilized fish bones and scales as well as sometimes bones from smaller ichthyosaurs. It was these observations by Anning that led the geologist William Buckland to propose in 1829 that the stones were fossilized feces and name them coprolites. Buckland suspected that the spiral markings on the fossils indicated that ichthyosaurs had spiral ridges in their intestines similar to those of modern sharks, that some of these coprolites were black with ink from swallowed belemnites. By examining coprolites, paleontologists are able to find information about the diet of the animal, such as whether it was a herbivorous or carnivorous, the taphonomy of the coprolites, although the producer is identified unambiguously with more ancient examples.
In some instances, knowledge about the anatomy of animal digestive tracts can be helpful in assigning a coprolite to the animal that produced it, one example being the finding that the Triassic dinosauriform Silesaurus may have been an insectivore, a suggestion, based on the beak-like jaws of the animal and the high density of beetle remains found in associated coprolites. Further, coprolites can be analyzed for certain minerals that are known to exist in trace amounts in certain species of plant that can still be detected millions of years later. In another example, the existence of human proteins in coprolites can be used to pinpoint the existence of cannibalistic behavior in an ancient culture. Parasite remains found in human and animal coprolites have shed new light on questions of human migratory patterns, the diseases which plagued ancient civilizations, animal domestication practices in the past. Organic molecules found in fossil faecal matter can be very informative about the producer of the coprolite, its diet, or the paleoenvironment where it was deposited.
The application of the faecal biomarker approach in archaeological sites has provided groundbreaking evidence in key questions such as the peopling of the Americas, the Neanderthal diet, the origin of the domestication of animals. The recognition of coprolites is aided by their structural patterns, such as spiral or annular markings, by their content, such as undigested food fragments, by associated fossil remains; the smallest coprolites are difficult to distinguish from inorganic pellets or from eggs. Most coprolites are composed chiefly of calcium phosphate, along with minor quantities of organic matter. By analyzing coprolites, it is possible to infer the diet of the animal. Coprolites have been recorded in deposits ranging in age from the Cambrian period to recent times and are found worldwide; some of them are useful as index fossils, such as Favreina from the Jurassic period of Haute-Savoie in France. Some marine deposits contain a high proportion of fecal remains. However, animal excrement is fragmented and destroyed, so has little chance of becoming fossilized.
In 1842 the Rev John Stevens Henslow, a professor of Botany at St John's College, discovered coprolites just outside Felixstowe in Suffolk in the villages of Trimley St Martin and Kirton and investigated their composition. Realising their potential as a source of available phosphate once they had been treated with sulphuric acid, he patented an extraction process and set about finding new sources. Soon, coprolites were being mined on an industrial scale for use as fertiliser due to their high phosphate content; the major area of extraction occurred over the east of England, centred on Cambridgeshire and the Isle of Ely with its refining being carried out in Ipswich by the Fison Company. There is a Coprolite Street near Ipswich docks; the industry declined in the 1880s but was revived during the First World War to provide phosphates for munitions. A renewed interest in coprolite mining in the First World War extended the area of interest into parts of Buckinghamshire as far west as Woburn Sands.
Bromalite Fecalith Fossil Fossils and the geological timescale Gastrolith Lloyds Bank coprolite Regurgitalith Spencer, P. K.. "The "coprolites" that aren't: the straight poop on specimens from the Miocene
Dinocephalia is a clade of large-bodied early therapsids that flourished for a brief time in the Middle Permian between 270 and 260 million years ago, but became extinct, leaving no descendants. Dinocephalians included both herbivorous and omnivorous forms. Many species had thickened skulls with bony projections. Dinocephalian fossils are known from Russia, Brazil, South Africa and Tanzania. Apart from the Biarmosuchians, the dinocephalians are the least advanced therapsids, although still uniquely specialised in their own way, they retain a number of primitive characteristics shared with their pelycosaur ancestors, although they are more advanced in possessing therapsid adaptations like the expansion of the ilium and more erect limbs. They include carnivorous and omnivorous forms; some were semiaquatic, others were terrestrial. They were among the largest animals of the Permian period. Dinocephalians were large; the biggest herbivores and omnivores may have massed up to 2 tonnes, were some 4.5 metres long, while the largest carnivores were at least as long, with heavy skulls 80 centimetres long, overall masses of around a half tonne.
All dinocephalians are distinguished by the interlocking incisor teeth. Correlated features are the distinctly downturned facial region, a deep temporal region, forwardly rotated suspensorium. Shearing contact between the upper and lower teeth is achieved through keeping a fixed quadrate and a hinge-like movement at the jaw articulation; the lower teeth are inclined forward, occlusion is achieved by the interlocking of the incisors. The dinocephalians improved on this system by developing heels on the lingual sides of the incisor teeth that met against one another to form a crushing surface when the jaws were shut. Most dinocephalians developed pachyostosis of the bones in the skull, which seems to have been an adaptation for intra-specific behaviour for territory or a mate. In some types, such as Estemmenosuchus and Styracocephalus, there are horn-like structures, which evolved independently in each case; the dinocephalians are an ancient group and their ancestry is not clear. It is assumed that they must have evolved during the earlier part of the Roadian, or even the Kungurian epoch, but no trace has been found.
These animals radiated at the expense of the dying pelycosaurs, who dominated during the early part of the Permian. The earliest members, the estemmenosuchids and early brithopodids of the Russian Ocher fauna, were a diverse group of herbivores and carnivores. During the Wordian and early Capitanian, advanced dinocephalians radiated into a large number of herbivorous forms, representing a diverse megafauna; this is well known from the Tapinocephalus Assemblage Zone of the Southern African Karoo. At the height of their diversity, all the dinocephalians died out; the reason for their extinction is not clear. They were replaced by much smaller therapsids. CLASS SYNAPSIDA Order THERAPSIDA Suborder DINOCEPHALIA? Eccasaurus? Pelosuchus? Tappenosaurus Family Estemmenosuchidae Estemmenosuchus Molybdopygus? Parabradysaurus? Family Phreatosuchidae Phreatosaurus Phreatosuchus? Family Phthinosuchidae Phthinosuchus? Family Rhopalodontidae? Phthinosaurus Rhopalodon Clade Anteosauria Family Anteosauridae Family Brithopodidae Family Deuterosauridae Clade Tapinocephalia?
Dimacrodon Family? Driveriidae Family? Mastersoniidae Family Styracocephalidae Family Tapinocephalidae Family Titanosuchidae Evolution of mammals Permian tetrapods Carroll, R. L. Vertebrate Paleontology and Evolution, WH Freeman & Co. Dinocephalia at Palaeos Dinocephalia at Palaeocritti
Fur is a thick growth of hair that covers the skin of many animals. It is a defining characteristic of mammals, it consists of a combination of oily guard hair on thick underfur beneath. The guard hair keeps moisture and the underfur acts as an insulating blanket that keeps the animal warm; the fur of mammals has many uses: protection, sensory purposes and camouflage, with the primary usage being thermoregulation. The types of hair include definitive. Hair length is negligible in thermoregulation, as some tropical mammals, such as sloths, have the same fur length as some arctic mammals but with less insulation; the denseness of fur can increase an animal's insulation value, arctic mammals have dense fur. Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool. Aquatic mammals, trap air in their fur to conserve heat by keeping the skin dry. Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection and physiological processes such as temperature regulation.
Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey. Aposematism, warning off possible predators, is the most explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger. In arctic and subarctic mammals such as the arctic fox, collared lemming and snowshoe hare, seasonal color change between brown in summer and white in winter is driven by camouflage. Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection; some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their arboreal habitat due to convergent evolution. The green coloration of sloths, however, is the result of a symbiotic relationship with algae. Coat color is sometimes sexually dimorphic, as in many primate species.
Coat color may influence the ability to retain heat, depending on. Mammals with a darker colored coat can absorb more heat from solar radiation, stay warmer, some smaller mammals, such as voles, have darker fur in the winter; the white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin. The term pelage – first known use in English c. 1828 – is sometimes used to refer to an animal's complete coat. The term fur is used to refer to animal pelts which have been processed into leather with their hair still attached; the words fur or furry are used, more casually, to refer to hair-like growths or formations when the subject being referred to exhibits a dense coat of fine, soft "hairs". If layered, rather than grown as a single coat, it may consist of short down hairs, long guard hairs, in some cases, medium awn hairs. Mammals with reduced amounts of fur are called "naked", as with the naked mole-rat, or "hairless", as with hairless dogs.
An animal with commercially valuable fur is known within the fur industry as a furbearer. The use of fur as clothing or decoration is controversial; the modern mammalian fur arrangement is known to have occurred as far back as docodonts and eutriconodonts, with specimens of Castorocauda and Spinolestes preserving compound follicles with both guard hair and underfur. Fur may consist of each with a different type of hair. Down hair is the bottom—or inner—layer, composed of wavy or curly hairs with no straight portions or sharp points. Down hairs, which are flat, tend to be the shortest and most numerous in the coat. Thermoregulation is the principal function of the down hair, which insulates a layer of dry air next to the skin; the awn hair can be thought of as a hybrid, bridging the gap between the distinctly different characteristics of down and guard hairs. Awn hairs begin their growth much like guard hairs, but less than half way to their full length, awn hairs start to grow thin and wavy like down hair.
The proximal part of the awn hair assists in thermoregulation, whereas the distal part can shed water. The awn hair's thin basal portion does not allow the amount of piloerection that the stiffer guard hairs are capable of. Mammals with well developed down and guard hairs usually have large numbers of awn hairs, which may sometimes b
Therapsida is a group of synapsids that includes mammals and their ancestors. Many of the traits today seen as unique to mammals had their origin within early therapsids, including having their four limbs extend vertically beneath the body, as opposed to the sprawling posture of reptiles; the earliest fossil attributed to Therapsida is Tetraceratops insignis from the Lower Permian. Therapsids evolved from "pelycosaurs" within the Sphenacodontia, more than 275 million years ago, they replaced the "pelycosaurs" as the dominant large land animals in the Middle Permian and were replaced, in turn, by the archosauromorphs in the Triassic, although one group of therapsids, the kannemeyeriiforms, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts survived the Triassic–Jurassic extinction event; the last of the non-mammalian therapsids, the tritylodontid cynodonts, became extinct in the Early Cretaceous 100 million years ago.
Compared to their pelycosaurian ancestors, early therapsids had similar skulls but different post-cranial morphology. Therapsid legs were positioned more vertically beneath their bodies than were the sprawling legs of reptiles and pelycosaurs. Compared to these groups, the feet were more symmetrical, with the first and last toes short and the middle toes long, an indication that the foot's axis was placed parallel to that of the animal, not sprawling out sideways; this orientation would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs. Therapsids' temporal fenestrae were larger than those of the pelycosaurs; the jaws of some therapsids were more complex and powerful, the teeth were differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, molars for shearing and chopping food. Several characteristics in therapsids have been noted as being consistent with the development of endothermy: the presence of turbinates, erect limbs vascularized bones and tail proportions conducive to the preservation of body heat, the absence of growth rings in bones.
Therefore, like modern mammals, non-mammalian therapsids were most warm-blooded. Recent studies on Permian coprolites showcase. Hair is by any means present in the docodont Castorocauda and several contemporary haramiyidans, whiskers are inferred from therocephalians and cynodonts. Therapsids evolved from a group of pelycosaurs called sphenacodonts. Therapsids became the dominant land animals in the Middle Permian. Therapsida consists of four major clades: the dinocephalians, the herbivorous anomodonts, the carnivorous biarmosuchians, the carnivorous theriodonts. After a brief burst of evolutionary diversity, the dinocephalians died out in the Middle Permian but the anomodont dicynodonts as well as the theriodont gorgonopsians and therocephalians flourished, being joined at the end of the Permian by the first of the cynodonts. Like all land animals, the therapsids were affected by the Permian–Triassic extinction event; the dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, the medium-sized cynodonts, flourished worldwide throughout the Early and Middle Triassic.
They disappear from the fossil record across much of Pangea at the end of the Carnian, although they continued for some time longer in the wet equatorial band and the south. Some exceptions were the still further derived eucynodonts. At least three groups of them survived, they all appeared in the Late Triassic period. The mammal-like family, survived into the Early Cretaceous. Another mammal-like family, are unknown than the Early Jurassic. Mammaliaformes was the third group, including similar animals. Many taxonomists refer to these animals as "mammals", though some limit the term to the mammalian crown group; the non-eucynodont cynodonts survived the Permian-Triassic extinction. By the Middle Triassic, only the eucynodonts remained; the therocephalians, relatives of the cynodonts, managed to survive the Permian-Triassic extinction and continued to diversify through the Early Triassic period. Approaching the end of the period, the therocephalians were in decline to eventual extinction outcompeted by the diversifying Saurian lineage of diapsids, equipped with sophisticated respiratory systems better suited to the hot and oxygen-poor world of the End-Triassic.
Dicynodonts were long thought to have become extinct near the end of the Triassic, but there is evidence that they survived into the Cretaceous. Their fossils have been found in Gondwana; this is an example of Lazarus taxon. Other animals that were common in the Triassic took refuge here, such as the temnospondyls. Mammals are the only living therapsids; the mammalian crown group, which evolved in the Early Jurassic period, radiated from a group of mammaliaforms that included the docodonts. The mammaliaforms themselves evolved from a lineage of the eucynodont suborder. Class Synapsida ORDER THERAPSIDA *? Family † Tetraceratopsidae Suborder † Biarmosuchia * Family † Biarmosuchidae Family † Eotitanosuchidae Eutherapsida Suborder † Dinocephalia Family † Estemmenosu
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
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