page_title: Mammal
section_title: Earliest appearances of features
hierarchical_section_title: Mammal / Evolution / Earliest appearances of features
caption_attribution_description: 230 - 225 million years ago Early Triassic
context_page_description: Mammals are vertebrate animals constituting the class Mammalia, and characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Carboniferous, approximately 300 million years ago. Around 6,400 extant species of mammals have been described. The largest orders are the rodents, bats and Eulipotyphla. The next three are the Primates, the Cetartiodactyla, and the Carnivora.
  In terms of cladistics, which reflects evolutionary history, mammals are the only living members of the Synapsida; this clade, together with Sauropsida, constitutes the larger Amniota clade. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that included the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds.
context_section_description: Hadrocodium, whose fossils date from approximately 195 million years ago, in the early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.
  The earliest clear evidence of hair or fur is in fossils of Castorocauda and Megaconus, from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur; it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina that are almost identical to those found in the nonmammalian cynodont Thrinaxodon. Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon. Studies on Permian coprolites suggest that non-mammalian synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts.
  When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids. Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals, but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians. Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.
  The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago. Epipubic bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae, suggesting that it is a synapomorphy between them and mammaliformes. They are omnipresent in non-placental mammaliformes, though Megazostrodon and Erythrotherium appear to have lacked them.
  It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.
caption: Fossil of Thrinaxodon at the National Museum of Natural History
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