Mammals are divided into three groups: placental, marsupial, and monotreme. Monotremes include only the platypus and four species of echidna. Placental mammals are dominant with over 5,500 species, while marsupials have only 334 species and are mostly found in Australia. Marsupials differ from placental mammals in their mode of reproduction, genitalia, and slightly lower blood temperature. They leave the mother’s body at an early stage and ascend to a special nipple sac for milk. Well-known marsupials include the koala, kangaroo, and possums.
There are three groups of mammals: placental, marsupial, and monotreme mammals. Monotremes include only the platypus and four species of echidna. The remaining mammals are all placentals or marsupials. Placental mammals are clearly dominant, as they have been for over 65 million years, with over 5,500 species on every continent except Antarctica. Marsupials, numbering just 334 species, have carved out a niche for themselves in Australia, where they have largely displaced placental mammals. Marsupials are also found in small numbers in South America and Central America, with only one species (Virginia Opossum) in North America.
Marsupials and placental mammals are both mammals, i.e. warm-blooded tetrapods with sweat glands that provide milk for the young and are covered with a layer of hair or fur. What distinguishes the two from each other are 1) their mode of reproduction, 2) their genitalia, and 3) their blood temperature, which is slightly lower than that of placental mammals. For many centuries, marsupials were considered “primitive” mammals, but there is little real evidence for this outside of cultural biases. Because marsupials primarily occupy Australia and remote areas of South America, they were poorly characterized by scientists until the mid-1800s.
Instead of remaining in a uterus like placental mammals, marsupials leave the mother’s body at a very early stage, ascending from the genital opening to a special nipple sac for milk. This evolutionary strategy avoids the need for a complex placenta to protect a growing fetus from the mother’s immune system. Marsupials and placental mammals both evolved around the same time, about 125 million years ago, and although they have evolved separately since that time, they have shown significant parallel evolution as they radiated from shrew-like ancestors. For the casual observer with no prior knowledge of the species, the only way to distinguish a marsupial from a placental mammal is to examine the genitalia.
Some well-known marsupials include the koala, kangaroo, possums, opossums, wombats, Tasmanian devil, marsupial rats, bandicoot, and lesser-known species such as the marsupial mole, numbat, bile, and musk kangaroo. Placental mammals include rodents, whales, elephants, otters, horses, cats, dogs, humans, and many more.
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