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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 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.

Living mammals are divided into the Yinotheria (platypus and echidnas) and Theriiformes (all other mammals).

There are around 5450 species of mammal, depending on which authority is cited.

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

The next three biggest orders, depending on the biological classification scheme used, are the Primates (apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).

Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics.

Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial.

Most mammals are polygynous, but some can be monogamous or polyandrous.

The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemurs; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap").

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