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| Marsupials
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Marsupials are mammals in which the female typically has a pouch (called the marsupium, from which the name 'Marsupial' derives) in which it rears its young through early infancy. They differ from placental mammals ( Placentalia) in their reproductive traits. The female has two vaginas, each of which leads to a different compartment in the uterusThe uterus or womb is the major female reproductive organ of most mammals, including humans. One end called the cervix, opens into the vagina, and the wider end, called the body of uterus, is connected on both sides with the Fallopian tubes. It comes in a. Males usually have a two-pronged penis which corresponds to the females' two vaginas. The pregnant female develops a kind of yolk sack in her placentaThe placenta is an ephemeral (temporary) organ present only in female placental mammals during gestation ( pregnancy). The placenta is composed of two parts, one of which, the chorion, is genetically and biologically part of the fetus, the other part of t which delivers nutrients to the embryotadpole) of the wrinkled frog Rana rugosa . An embryo is an animal or a plant in its earliest stage of development. Plants In botany, a plant embryo is part of a seed, consisting of precursor tissues for the leaves, stem (see hypocotyl , and root (see rad. The embryo is born at a very early stage of development (at about 4-5 weeks), upon which it crawls up its mother's belly and attaches itself to a nipple. It remains attached to the nipple for a number of weeks. The offspring later passes through a stage where it temporarily leaves the pouch, returning for warmth and nourishment.
Fossil evidence does not support the once-common belief that marsupials were a primitive forerunner of the placental mammals: both main branches of the mammal tree appear to have evolved at around the same time, toward the end of the MesozoicThe Mesozoic is one of four (sometimes more) geologic eras. The division of time into Eras dates back to the 19th century. The Mesozoic includes three Geologic Periods; from oldest to youngest, the Triassic, Jurassic and Cretaceous. It extended from rough era, and have been competitors since that time. In most continents, placentals were much more successful and no marsupials survived; in South America the opossums retained a strong presence; in Australia's harsh climate the placentals died out and only marsupials and monotremeKollikodontidae extinct Ornithorhynchidae Platypus Tachyglossidae Echidnas Steropodontidae extinct Monotremes are mammals that are best known for laying eggs, instead of giving birth to live young like marsupials and placental mammals ( Eutheria). The subs survived.
The early birth of marsupials removes the developing young much sooner than in placental mammals, and marsupials have not needed to develop a complex placenta to protect the young from its mother's immune systemThe immune system is any system present in an organism to prevent predation by biological agents. All living organisms have these protective measures, although they vary radically in scope and mechanism. In humans and domesticated animals, the immune syst. Early birth places the tiny new-born marsupial at greater risk, but significantly reduces the risk of pregnancy, as there is no need to carry a large foetus to full-term in bad seasons.
Because a newborn marsupial must climb up to its mother's nipples, the otherwise minimally developed newborn has front limbs that are much better developed than the rest of its body. This requirement is responsible for the more limited range of locomotory adaptations in marsupials than placentals; marsupials must retain a grasping forepaw and cannot develop it into a hoof, wing, or flipper as some groups of placental mammals have done.
There are between 260 and 280 species of marsupials, almost 200 of them native to Australia and nearby islands to the north. There are also many extant species in South America and one species, the Virginia Opossum, native to North America.
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