Theria is a subclass of mammals amongst the Theriiformes. Theria includes the metatherians. Therian mammals give birth to live young without a shelled egg, it is possible thanks to key proteins called syncytins, which allow exchanges between the mother and its offspring through a placenta. Genetic studies have enlighted the viral origin of syncytins through the endogenization process; the marsupials and the placental mammals evolved from a common therian ancestor that gave live-birth by suppressing the mother's immune system. While the marsupials continued to give birth to an underdeveloped fetus after a short pregnancy, the ancestors of placental mammals evolved a prolonged pregnancy. Therian mammals no longer have the coracoid bone, contrary to their cousins, monotremes. Pinnae are a distinctive trait, a therian exclusivity, though some therians, such as the earless seals, have lost them secondarily; the earliest known therian mammal fossil is Juramaia, from the Late Jurassic of China. However, molecular data suggests that therians may have originated earlier, during the Early Jurassic.
The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. gives the following: In the above system Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell it is ranked as a supercohort under the subclass Theriiformes: Another classification proposed by Luo et al. does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead. Marsupials Monotremes Placental mammals Theria — supercohort — Tree of Life
Synapomorphy and apomorphy
In phylogenetics and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history. An apomorphy is a character, different from the form found in an ancestor, i.e. an innovation, that sets the clade apart from other clades. A synapomorphy is a shared apomorphy. In other words, it is an apomorphy shared by members of a monophyletic group, thus assumed to be present in their most recent common ancestor. An apomorphy is a character, different from the form found in an ancestor, i.e. an innovation, that sets the clade apart from other clades. A synapomorphy is a shared apomorphy. In other words, it is an apomorphy shared by members of a monophyletic group, thus assumed to be present in their most recent common ancestor. In most groups of mammals, the vertebral column is conserved, with the same number of vertebrae found in the neck of a giraffe, for example, as in mammals with shorter necks. However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae.
This is a synapomorphy of the clade: a shared feature considered to be derived from a common ancestor. The word synapomorphy—coined by German entomologist Willi Hennig—is derived from the Greek words σύν, syn = shared; these phylogenetic terms are used to describe different patterns of ancestral and derived character or trait states as stated in the above diagram in association with synapomorphies. Symplesiomorphy – an ancestral trait shared by two or more taxa. Plesiomorphy – a symplesiomorphy discussed in reference to a more derived state. Pseudoplesiomorphy – is a trait that cannot be identified as neither a plesiomorphy nor an apomorphy, a reversal. Reversal – is a loss of derived trait present in ancestor and the reestablishment of a plesiomorphic trait. Convergence – independent evolution of a similar trait in two or more taxa. Apomorphy – a derived trait. Apomorphy shared by two or more taxa and inherited from a common ancestor is synapomorphy. Apomorphy unique to a given taxon is autapomorphy.
Synapomorphy/Homology – a derived trait, found in some or all terminal groups of a clade, inherited from a common ancestor, for which it was an autapomorphy. Underlying synapomorphy – a synapomorphy, lost again in many members of the clade. If lost in all but one, it can be hard to distinguish from an autapomorphy. Autapomorphy – a distinctive derived trait, unique to a given taxon or group. Homoplasy in biological systematics is when a trait has been gained or lost independently in separate lineages during evolution; this convergent evolution leads to species independently sharing a trait, different from the trait inferred to have been present in their common ancestor. Parallel Homoplasy – derived trait present in two groups or species without a common ancestor due to convergent evolution. Reverse Homoplasy – trait present in an ancestor but not in direct descendants that reappears in descendants. Hemiplasy A new method of measuring phylogenetic characteristics is the use of Relative Apparent Synapomorphy Analysis.
The objective of analysis is to determine if a given characteristic is common between taxa as a result of either shared ancestors or the process of convergence. This method allows for several advantages such as computational efficiency and it administers an unbiased and reliable measure of phylogenetic signal; the concept of synapomorphy is relative to a given clade in the tree of life. What counts as a synapomorphy for one clade may well be a primitive character or plesiomorphy at a less inclusive or nested clade. For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example. So the concept can be understood as well in terms of "a character newer than" and "a character older than" the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered vertebral column is a plesiomorphy.
Cladograms are diagrams. These illustrations are accurate predictive device in modern genetics, they are depicted in either tree or ladder form. Synapomorphies create evidence for historical relationships and their associated hierarchical structure. Evolutionarily, a synapomorphy is the marker for the most recent common ancestor of the monophyletic group consisting of a set of taxa in a cladogram. Cladistics, Berkeley
Yinotheria is a proposed basal mammalian subclass clade that contains a few fossils of the Mesozoic and the extant monotremes. Today, there are only five surviving species, which live in Australia and New Guinea, but fossils have been found in England, China and Argentina; the surviving species consist of four species of echidna. According to genetic studies, Yinotheria diverged from other mammals around 220 to 210 million years ago, at some point in the Triassic or Early Jurassic; the oldest-known fossils are a bit younger, dating around 168 to 163 million years in the Middle Jurassic. These fossils are the genera Pseudotribos of China, Shuotherium of both China and England, Itatodon of Siberia and Paritatodon of Kyrgyzstan and England. These, which belong to the family Shuotheriidae, are the only known northern hemisphere group of yinotherians; the infraclass Australosphenida appeared around the same time as Shuotheriidae. The family Henosferidae, comprising the genera Henosferus and Asfaltomylos, has been found in the southern hemisphere at locations in Argentina and Madagascar.
This suggests that this family could have been more widespread and diverse in Gondwana during that time. The family Ausktribosphenidae and the first monotremes appeared in the Early Cretaceous, in the region, now known as Australasia. Despite being found in the same region of the world and in the same time period, recent work has found that the older Henosferidae is the sister taxon to Monotremata, with Ausktribosphenidae being the next sister taxa in Australosphenida. Ausktribosphenidae includes the genera Ausktribosphenos; some 110-million-year-old monotreme fossil jaw fragments were found at Lightning Ridge, New South Wales. These fragments, from the species Steropodon galmani, are the oldest known monotreme fossils. Fossils from the genera Kollikodon and Obdurodon have been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina. Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19 to 48 million years ago, but another putting it at 17 to 89 million years ago.
All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. Monotremes were classified as a subclass of mammals known as Prototheria; the names Prototheria and Eutheria refer to the three mammalian groupings that have living representatives. Each of the three may be defined as a total clade containing a living crown-group plus any fossil species that are more related to that crown-group than to any other living animals; the threefold division of living mammals into monotremes and placentals was well established when Thomas Huxley proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Treated as subclasses and Eutheria are by convention now grouped as infraclasses of the subclass Theria, in more recent proposals have been demoted further, as cladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank.
Prototheria, on the other hand, was recognised as a subclass until quite on the basis of a hypothesis that defined the group by two supposed synapomorphies: formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians. These characters appeared to unite monotremes with a range of Mesozoic fossil orders in a broader clade for which the name Prototheria was retained, of which monotremes were thought to be only the last surviving branch; the evidence, held to support Prototheria is now universally discounted. In the first place, the examination of embryos has revealed that the development of the braincase wall is identical in therians and in'prototherians': the anterior lamina fuses with the alisphenoid in therians, therefore the'prototherian' condition of the braincase wall is primitive for all mammals, while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is primitive for all mammals.
Therefore, neither of these states can supply a uniquely shared derived character that would support a'prototherian' grouping of orders in contradistinction to Theria. In a further reappraisal, the molars of embryonic and fossil monotremes appear to demonstrate an ancestral pattern of cusps, similar to the triangular arrangement observed in therians; some peculiarities of this dentition support an alternative grouping of monotremes with certain discovered fossil forms into a proposed new clade known as the Australosphenida, suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians. Australosphenida is characterized by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a low trigonid, a triangula
Echidnas, sometimes known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. The four extant species, together with the platypus, are the only surviving members of the order Monotremata, are the only living mammals that lay eggs; the diet of some species consists of ants and termites, but they are not related to the true anteaters of the Americas. Echidnas live in New Guinea. Echidnas evolved between 50 million years ago, descending from a platypus-like monotreme; this ancestor was aquatic. The echidnas are named after Echidna, a creature from Greek mythology, half-woman, half-snake, as the animal was perceived to have qualities of both mammals and reptiles. Echidnas are medium-sized, solitary mammals spines. Superficially, they resemble the anteaters of South America and other spiny mammals such as hedgehogs and porcupines, they are black or brown in colour. There have been several reports of albino echidnas, their eyes pink and their spines white.
They have slender snouts that function as both mouth and nose. Like the platypus, they are equipped with electrosensors, but while the platypus has 40,000 electroreceptors on its bill, the long-beaked echidna has only 2,000 electroreceptors, the short-beaked echidna, which lives in a drier environment, has no more than 400 located at the tip of its snout, they have short, strong limbs with large claws, are powerful diggers. Their claws on their hind limbs are curved backwards to help aid in digging. Echidnas have tiny toothless jaws; the echidna feeds by tearing open soft logs and the like, using its long, sticky tongue, which protrudes from its snout, to collect prey. The ears are slits on the sides of their heads that are unseen, as they are blanketed by their spines; the external ear is created by a large cartilaginous funnel, deep in the muscle. At 33 °C, the echidna possess the second lowest active body temperature of all mammals, behind the platypus; the short-beaked echidna's diet consists of ants and termites, while the Zaglossus species eat worms and insect larvae.
The tongues of long-beaked echidnas have tiny spines that help them capture their prey. They have no teeth, break down their food by grinding it between the bottoms of their mouths and their tongues. Echidnas' faeces are cylindrical in shape. Echidnas do not tolerate extreme temperatures. Echidnas are found in woodlands, hiding under vegetation, roots or piles of debris, they sometimes use the burrows of animals such as wombats. Individual echidnas have mutually overlapping territories. Despite their appearance, echidnas are capable swimmers; when swimming, they expose their snout and some of their spines, are known to journey to water in order to groom and bathe themselves. Echidnas and the platypus are the only egg-laying mammals, known as monotremes; the average lifespan of an echidna in the wild is estimated around 14–16 years. When grown, a female can weigh up to 4.5 kilograms and a male can weigh up to 6 kilograms. The echidnas' sex can be inferred from their size; the reproductive organs differ, but both sexes have a single opening called a cloaca, which they use to urinate, release their faeces and to mate.
Male echidnas have non-venomous spurs on the hind feet. The neocortex makes up half compared to 80 % of a human brain. Due to their low metabolism and accompanying stress resistance, echidnas are long-lived for their size. Contrary to previous research, the echidna does enter REM sleep, but only when the ambient temperature is around 25 °C. At temperatures of 15 °C and 28 °C, REM sleep is suppressed; the female lays a single soft-shelled, leathery egg 22 days after mating, deposits it directly into her pouch. An egg is about 1.4 centimetres long. While hatching, the baby echidna opens the leather shell with a reptile-like egg tooth. Hatching takes place after 10 days of gestation; the mother digs a nursery burrow and deposits the young, returning every five days to suckle it until it is weaned at seven months. Puggles will stay within their mother's den for up to a year before leaving. Male echidnas have a four-headed penis. During mating, the heads on one side "shut down" and do not grow in size.
Each time it copulates, it alternates heads in sets of two. When not in use, the penis is retracted inside a preputial sac in the cloaca; the male echidna's penis is 7 centimetres long when erect, its shaft is covered with penile spines. These may be used to induce ovulation in the female, it is a challenge to study the echidna in its natural habitat and they show no interest in mating while in captivity. Therefore, no one has seen an echidna ejaculate. There have been previous attempts, trying to force the echidna to ejaculate through the use of electrically stimulated ejaculation in order to obtain semen samples but has on
Eutheria is one of two mammalian clades with extant members that diverged in the Early Cretaceous or the Late Jurassic. Except for the Virginia opossum, from North America, a metatherian, all post-Miocene mammals indigenous to Europe, Africa and North America north of Mexico are eutherians. Extant eutherians, their last common ancestor, all extinct descendants of that ancestor are members of Placentalia. Eutherians are distinguished from noneutherians by various phenotypic traits of the feet, ankles and teeth. All extant eutherians lack epipubic bones; this allows for expansion of the abdomen during pregnancy. The oldest-known eutherian species is Juramaia sinensis, dated at 161 million years ago from the Jurassic in China. Eutheria was named in 1872 by Theodore Gill. Distinguishing features are: an enlarged malleolus at the bottom of the tibia, the larger of the two shin bones the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset farther back than the joint between the second metatarsal and middle cuneiform bones—in metatherians these joints are level with each other various features of jaws and teeth Eutheria contains several extinct genera as well as larger groups, many with complicated taxonomic histories still not understood.
Members of the Adapisoriculidae and Leptictida have been placed within the out-dated placental group Insectivora, while Zhelestids have been considered primitive ungulates. However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia; the weakly favoured cladogram favours Boreoeuthearia as a basal Eutherian clade as sister to the Atlantogenata
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
Thomas Henry Huxley
Thomas Henry Huxley was an English biologist and anthropologist specialising in comparative anatomy. He is known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution; the stories regarding Huxley's famous debate in 1860 with Samuel Wilberforce were a key moment in the wider acceptance of evolution and in his own career, although historians think that the surviving story of the debate is a fabrication. Huxley had been planning to leave Oxford on the previous day, after an encounter with Robert Chambers, the author of Vestiges, he changed his mind and decided to join the debate. Wilberforce was coached by Richard Owen, against whom Huxley debated about whether humans were related to apes. Huxley was slow to accept some of Darwin's ideas, such as gradualism, was undecided about natural selection, but despite this he was wholehearted in his public support of Darwin. Instrumental in developing scientific education in Britain, he fought against the more extreme versions of religious tradition.
Coining the term in 1869, Huxley elaborated on "agnosticism" in 1889 to frame the nature of claims in terms of what is knowable and what is not. Huxley statesAgnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle... the fundamental axiom of modern science... In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration... In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable. Use of that term has continued to the present day. Much of Huxley's agnosticism is influenced by Kantian views on human perception and the ability to rely on rational evidence rather than belief systems. Huxley had little formal schooling and was self-taught, he became the finest comparative anatomist of the 19th century. He worked on invertebrates, clarifying relationships between groups little understood, he worked on vertebrates on the relationship between apes and humans.
After comparing Archaeopteryx with Compsognathus, he concluded that birds evolved from small carnivorous dinosaurs, a theory accepted today. The tendency has been for this fine anatomical work to be overshadowed by his energetic and controversial activity in favour of evolution, by his extensive public work on scientific education, both of which had significant effects on society in Britain and elsewhere. Huxley’s 1893 Romanes Lecture, “Evolution and Ethics” is exceedingly influential in China. Thomas Henry Huxley was born in Ealing, a village in Middlesex, he was the second youngest of eight children of Rachel Withers. Like some other British scientists of the nineteenth century such as Alfred Russel Wallace, Huxley was brought up in a literate middle-class family which had fallen on hard times, his father was a mathematics teacher at Ealing School until it closed, putting the family into financial difficulties. As a result, Thomas left school after only two years of formal schooling. Huxley's parents were Anglicans, although it was against organized religion Huxley sympathized with the town's Nonconformist.
Despite this unenviable start, Huxley was determined to educate himself. He became one of the great autodidacts of the nineteenth century. At first he read Thomas Carlyle, James Hutton's Geology, Hamilton's Logic. In his teens he taught himself German becoming fluent and used by Charles Darwin as a translator of scientific material in German, he learned Latin, enough Greek to read Aristotle in the original. On, as a young adult, he made himself an expert, first on invertebrates, on vertebrates, all self-taught, he was skilled in drawing and did many of the illustrations for his publications on marine invertebrates. In his debates and writing on science and religion his grasp of theology was better than most of his clerical opponents. Huxley, a boy who left school at ten, became one of the most knowledgeable men in Britain, he was apprenticed for short periods to several medical practitioners: at 13 to his brother-in-law John Cooke in Coventry, who passed him on to Thomas Chandler, notable for his experiments using mesmerism for medical purposes.
Chandler's practice was in London's Rotherhithe amidst the squalor endured by the Dickensian poor. Here Thomas would have seen poverty and rampant disease at its worst. Next, another brother-in-law took him on: his eldest sister's husband. Now 16, Huxley entered Sydenham College, a cut-price anatomy school whose founder, Marshall Hall, discovered the reflex arc. All this time Huxley continued his programme of reading, which more than made up for his lack of formal schooling. A year buoyed by excellent results and a silver medal prize in the Apothecaries' yearly competition, Huxley was admitted to study at Charing Cross Hospital, where he obtained a small scholarship. At Charing Cross, he was taught by Thomas Wharton Jones, Professor of Ophthalmic Medicine and Surgery at University College London. Jones had been Robert Knox's assistant when Knox bought cadavers from Hare; the young Wharton Jones, who acted as go-between, was exonerated of crime, but thought it best to leave Scotland. He was a fine teacher, up-to-date in physiology and an ophthalmic surgeon.
In 1845, under Wharton Jones' guidance, Huxley published his first scientific paper demonstrating the existence of a hitherto unrecognised layer in the inner sheath of hairs, a layer, known sin