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In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria.

The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria.

The largest orders are the rodents, bats and Soricomorpha (shrews and allies).

There are around 5450 species of mammal, depending on which authority is cited.The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon.At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds.The largest group of mammals, the placentals, have a placenta, which enables the feeding of the fetus during gestation.Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30-meter (98 ft) blue whale.

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