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12. 34: Mammal Classification

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    How would you classify this mammal?

    Obviously it is a camel, but is there more to it than that? There are 17 orders of placental mammals. But then these mammals need to be further classified into families, genera, and finally species.

    Classification of Placental Mammals

    Traditional classifications of mammals are based on similarities in structure and function. Increasingly, mammals are being classified on the basis of molecular similarities. DNAanalyses has recently shown that the traditional orders include mammals that may not be closely related.

    Traditional Classification

    The most widely accepted traditional classification of mammals divides living placental mammals into 17 orders. These orders are shown in Table below. This classification of mammals was widely accepted for more than 50 years. Placental mammals are still commonly placed in these orders. However, this classification is not very useful for studies of mammalian evolution. That’s because it groups together some mammals that do not seem to be closely related by descent from a recent common ancestor.

    Order Example Sample Trait
    Insectivora

    mole

    small sharp teeth
    Edentata

    anteater

    few or no teeth
    Pholidota

    pangolin

    large plate-like scales
    Chiroptera

    bat

    digits support membranous wings
    Carnivora

    coyote

    long pointed canine teeth
    Rodentia

    mouse

    incisor teeth grow continuously
    Lagomorpha

    rabbit

    chisel-like incisor teeth
    Perissodactyla

    horse

    odd-toed hooves
    Artiodactyla

    deer

    even-toed hooves
    Cetacea

    whale

    paddle-like forelimbs
    Primates

    monkey

    five digits on hands and feet
    Proboscidea

    elephant

    tusks
    Hyracoidea

    hyrax

    rubbery pads on feet
    Dermoptera

    colugo

    membrane of skin between legs for gliding
    Pinnipedia

    seal

    feet with fins
    Sirenia

    manatee

    paddle-like tail
    Tubulidentata

    aardvark

    teeth without enamel

    Phylogenetic Classification

    The mammalian supertree classifies placental mammals phylogenetically. It uses the analysis of DNA sequences to group together mammals that are evolutionarily closely related, sharing a recent common ancestor. These groups are not necessarily the same as the traditional groups based on structure and function.

    The supertree classification places placental mammals in four superorders. The four superorders and some of the mammals in them are:

    • Afrotheria—aardvarks, elephants, manatees.
    • Xenarthra—anteaters, sloths, armadillos.
    • Laurasiatheria—bats, whales, hoofed mammals, carnivores.
    • Supraprimates—primates, rabbits, rodents.

    All four superorders appear to have become distinct from one another between 85 and 105 million years ago. The exact relationships among the superorders are still not clear. Revisions in this classification of mammals may occur as new data become available.

    Summary

    • Mammals used to be classified on the basis of similarities in structure and function into 17 different orders.
    • Recently, DNA analyses have shown that the traditional orders include mammals that are not closely related.
    • Phylogenetic classification, based on DNA data, groups placental mammals in four superorders. The superorders appear to have become distinct from each other 85–105 million years ago.

    Review

    1. Compare traditional and phylogenetic classifications of placental mammals. Explain which type of classification is more useful for understanding how mammals evolved.
    2. Assume that a new species of placental mammal has been discovered. Scientists have examined it closely and studied its DNA. It has wings similar to a bat that it uses for gliding. Its DNA is most similar to the DNA of rodents such as mice. How would you classify the new mammal? Explain your answer.