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Theria
Theria (; Greek language, Greek: , wild beast) is a Scientific classification, subclass of Mammal, mammals amongst the Theriiformes (the sister taxon to Yinotheria). Theria includes the Eutheria, eutherians (including the placental mammals) and the Metatheria, metatherians (including the marsupials). Therian mammals are the dominant group of terrestrial amniotes on Earth since the Cretaceous–Paleogene extinction event, K-Pg extinction event extinguished the non-avian dinosaurs. Characteristics Therian mammals give birth to live young without a shelled egg (biology), egg. This is possible thanks to key proteins called Syncytin-1, syncytins which allow exchanges between the mother and its offspring through a placenta, even Marsupial#Reproductive system, rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the Endogenous retrovirus, endogenization process. The marsupials and the placental mammals evolved from a common theria ...
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Mammal
Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three ossicles, middle ear bones. These characteristics distinguish them from reptiles and birds, from which they Genetic divergence, diverged in the Carboniferous, over 300 million years ago. Around 6,400 extant taxon, extant species of mammals have been described. The largest Order (biology), orders are the rodents, bats and Eulipotyphla (hedgehogs, Mole (animal), moles, shrews, and others). The next three are the Primates (including humans, apes, monkeys, and others), the Artiodactyla (cetaceans and even-toed ungulates), and the Carnivora (cats, dogs, pinniped, seals, and others). In terms of cladistics, which reflects evolutionary history, ma ...
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Yinotheria
Yinotheria is a proposed basal subclass clade of crown mammals 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, Russia, Madagascar and Argentina. The surviving species consist of the platypus and four species of echidna. Contrary to other known crown mammals, they retained postdentary bones as shown by the presence of a postdentary trough. The extant members (monotremes) developed the mammalian middle ear independently. Evolutionary history 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.http://timetree.org/pdf/Madsen2009Chap68.pdf 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 E ...
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Eutheria
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineage (evolution), lineal descen ... consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic characteristic of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...s of the feet, ankles, jaws and teeth. All extant eutherians lack epipubic bones, which are present in all other living mammals (marsupials and monotreme Monotremes are one of the three main groups of living mammal Mammals (fro ...
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Monotreme
Monotremes are one of the three main groups of living mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...s, along with placentals ( Eutheria) and marsupials ( Metatheria). The monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts compared to the more common mammalian types. In addition, they lay eggs rather than bearing live young, but, like all mammals, the female monotremes nurse their young with milk Milk is a nutrient-rich liquid food produced by the mammary gland A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast .... Monotremes are traditionally referred to as ...
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Theriiformes
Theriiformes is a subclass of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...s. The term was coined in 1997 by McKenna & Bell in their classification of mammals. In the strict sense, it is defined as all mammals more closely related to theria Theria (; Greek language, Greek: , wild beast) is a Scientific classification, subclass of Mammal, mammals amongst the Theriiformes (the sister taxon to Yinotheria). Theria includes the Eutheria, eutherians (including the placental mammals) and ...ns than to monotreme Monotremes are one of the three main groups of living mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mamm ...s. Referen ...
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Amniotes
Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a clade of tetrapod vertebrates comprising reptiles (class Reptilia), birds (class Aves) and mammals (class Mammalia). Amniotes lay their Egg (biology), eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the Great chain of being#Scala naturae in evolution, evolutionary great chain of being. Amniotes are tetrapods (descendants of four-limbed and backboned animals) that are characterised by having an Egg (biology), egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anam ...
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Syncytin-1
Syncytin-1 also known as enverin is a protein found in humans and other primates that is encoded by the ERVW-1 gene (Human Endogenous Retrovirus-W, endogenous retrovirus group W envelope member 1). Syncytin-1 is a membrane fusion protein, cell-cell fusion protein whose function is best characterized in placental development. The placenta in turn aids in embryo attachment to the uterus and establishment of a nutrient supply. The gene encoding this protein is an endogenous viral element, endogenous retroviral element that is the remnant of an ancient retroviral infection integrated into the primate germ line. In the case of syncytin-1 (which is found in humans, apes, and Old World monkey, Old World but not New World monkeys), this integration likely occurred more than 25 million years ago. Syncytin-1 is one of two known syncytin proteins expressed in catarrhini primates (the other being syncytin-2) and one of many viral genomes incorporated on multiple occasions over evolutionary tim ...
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