Mammals (from latin mamma, meaning "breast") are amniotes characterized by the presence of mammary glands to which the females produce milk for feeding the young, a neocortex in their brain, fur (very likely analogous to the avemetatarsalian filaments) and three middle ear bones. These characteristics make them different from reptiles, whose common ancestor with the mammals existed in the Late Carboniferous. Currently there are almost 5500 extant species of mammals. Nearly all mammals are endothermic. They form the Amniota clade alongside the reptiles. The first mammals evolved in the Late Triassic, with most modern mammalian orders arriving only in the Cenozoic era, after the K-Pg extinction event that ended the dinosaurs's domain on land, and allowed the mammals to successfully conquer the globe. The basic body plan of a mammal is a quadruped one, although they can adapt it for water, arboreal, fossorial and even bipedal locomotion, among others. Mammals range in size from the size of a grasshopper to the length of a sauropod dinosaur, making them the largest animals ever to have lived on the planet, although only with the arrival of the Cenozoic mammals got so large, remaining diminute in the Mesozoic, not getting much larger than a cat. They can also have varying lifespans, depending on the species and their size. Most mammals give birth to live young (viviparity). Mammals are considered intelligent animals, with some having large brains, self-awareness and even tool use. Mammals communicate and vocalize in different ways, such as with ultrasound, scent-marking, singing, etc. They can organize themselves in groups and hierarchies, or be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous. We humans belong to this class of amniotes, one that had some quite humble begginings.
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