Domain Eucarya
Kingdom Animalia
Phylum Chordata
Subphylum Vertebrata
Class Mammalia

Mammals evolved on land during the late Triassic period, but were not nearly as successful as the dinosaurs which evolved at the same time.

During the Cenozoic Era, mammals radiated into so many ecological niches that the Era is often called the "Age of Mammals".

Defining mammalian characteristics include:

  • Mammary glands from which females feed their newborn offspring
  • Three bones in the middle ear
  • Molar and premolar "cheek teeth" (where they have not evolved away)
  • Bodies covered in hair

  • Paleoryctoidae
  • Subclass Prototheria:

    • Infraclass Eotheria:
  • Infraclass Ornithodelphia:
  • Infraclass Allotheria:

  • Subclass Theria:
    • Infraclass Pantotheria:

  • Infraclass Metatheria (marsupials):

  • Infraclass Eutheria:

  • Mammals are a class of bony fishes that have evolved to live on land, and whose scales have attenuated into long, thin projections called "hair". A cladistic lineage of the mammals looks something like this1:

    Eukaryota - cells with nuclei
    Metazoa - animals
    Bilateria - triploblast embryos, bodies with bilateral symmetry
    Chordata - vertebrates
    Craniata - vertebrates with skulls
    Vertebrata - vertebrates with backbones (i.e. instead of a cartaliginous notochord
    Gnathostomata - vertebrates with jaws
    Teleostomi - mouth at end, three ear bones
    Osteichthyes - bony fishes
    Sarcopterygii - lobed fins
    Tetrapoda - four limbs
    Stegocephalia - digits
    Amniota - young develop in amniotic egg
    Synapsida - ear hole in skull

    University of Michigan Animal Diversity Web - Mammalia

    Smithsonian Institution, National Museum of Natural History, MSW Scientific Names

    The Ultimate Ungulate Page, your guide to the world's Hoofed Mammals]


    1Tree of Life Web Project

    MAMMALS: A World Listing of Living and Extinct Species, Edited by John H. Burkitt, Tennessee Department of Agriculture, Nashville, Tennessee, Second Edition, 1995

    The DNA revolution of the early 2000s produced comparatively few major changes in the taxonomy of mammals. Most of the orders survived intact. The biggest surprise was that the Cetacea (whales and dolphins) form a clade with the hippopotamuses: that is, these two groups are more closely related to each other than they are to other artiodactyls such as pigs and cattle. In consequence Cetacea is now subsumed under Artiodactyla, which is renamed Cetartiodactyla.

    The big news was that the dozen or so traditional orders of Eutheria (placental mammals) could be grouped into four clades, three of them new. The Xenarthra, the mainly South American edentates, remained, and group alongside the other three as follows:

    It was no surprise that the tree shrews and flying lemurs grouped with primates, nor that rodents and lagomorphs went together. At first it seemed the extended primate group would include bats as well, and that was called Archonta; then this turned out to be wrong, and the remainder was renamed Euarchonta (see eu-: "the good ones that are left when we take some out"). It had long been doubted whether the Microchiroptera (echo-locating bats) and Macrochiroptera (fruit bats) really do go together, or independently invented flight, but it seems they are indeed a clade together.

    The big loser is Insectivora. It was disbanded after all sorts of 'shrews' and 'moles' and their protean Madagascan equivalents the tenrecs were removed from it, mostly into the impressively varied new clade Afrotheria. Under its new name Eulipotyphla it still contains the European shrews and moles. Basically, a 'shrew' isn't a kind of animal, it's what a mammal has always looked like if it hasn't changed into something sexy like an okapi or an ocelot. And lo and behold, the little and whimsically-named elephant shrew turns out to actually be closer to elephants than to shrews.

    Unfortunately the otherwise very good Tree of Life website hasn't updated its Eutheria page in many years. They know they have to, but they seem to have just dried up. Richard Dawkins's book The Ancestor's Tale is a good popularization that is up-to-date enough to use the new phylogeny.

    Mam*ma"li*a (?), n. pl. [NL., from L. mammalis. See Mammal.] Zool.

    The highest class of Vertebrata. The young are nourished for a time by milk, or an analogous fluid, secreted by the mammary glands of the mother.

    Mammalia are divided into threes subclasses; --

    I. Placentalia. This subclass embraces all the higher orders, including man. In these the fetus is attached to the uterus by a placenta.

    II. Marsupialia. In these no placenta is formed, and the young, which are born at an early state of development, are carried for a time attached to the teats, and usually protected by a marsupial pouch. The opossum, kangaroo, wombat, and koala are examples.

    III. Monotremata. In this group, which includes the genera Echidna and Ornithorhynchus, the female lays large eggs resembling those of a bird or lizard, and the young, which are hatched like those of birds, are nourished by a watery secretion from the imperfectly developed mammae.


    © Webster 1913.

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