Fotografías de mamíferos, animales de granja. Photography about mammals and farm animals.
Most mammals also possess sweat glands and specialized teeth, and the largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals.
With the old order Insectivora shown to be polyphylectic and more properly subdivided (as Afrosoricida, Erinaceomorpha, and Soricomorpha), the following classification for placental mammals contains 21 orders: For more details on this topic, see Evolution of mammals. It is not certain when true placental mammals evolved - the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes the last common ancestor of modern marsupial and placentals - and therefore that all earlier mammals had sprawling limbs. Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands.
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