Mammals are classified into marsupials, monotremes and placental, or eutherian animals. While marsupials and monotremes are mainly only found in Australia, eutherian animals rule the rest of the world. Australia has, however, got some eutherian animals that have either arrived air- or waterways, or have been introduced by humans. Here are some facts about placental mammals, and origin, evolution and characteristics of placental animals.
Origins of Australian of Placental Mammals Australian placental animals (aka eutherian animals) are not as famous Australian animals as are marsupials, and fair enough - Gondwana continent was the home of marsupials and no eutherian mammals developed here. All Australian placentals have arrived in Australia after Gondwana had broken up. Seals, whales and dugongs arrived waterways, bats airways and rodents crossed the narrowing gap between Asia and Australia about five million years ago. Rising sea levels stopped other land mammals from getting to Australia, until humans arrived with dingos, rabbits and hares, horses and camels, dogs and cats that don’t do any good for Australian ecosystems today.
Placental Mammals are More Successful than Marsupial Animals All Gondwana continents except Australia became connected to northern continents of Laurasia where there were eutherian animals, and marsupials got extinct (there are a few marsupial species left in South America). The reason why eutherians are more successful and outcompete marsupials is not their reproductive system, but their larger brain. Bats communities for example have a very complex social structure and whales are so smart they can form a phrase when communicating with each other.
Characteristics of Placental Mammals But the reason for classifying marsupials and eutherians separately is their reproductive system. Marsupials give birth only weeks after fertilisation to a tiny undeveloped young that makes its way to the pouch where it stays sucking milk, growing and completing its development. Placentals, or eutherians, do all that development in the uterus, joined to the mother by placenta, while she is pregnant. The period of pregnancy is longer, but there is no pouch life and once the young is born, it is ready to live in the outside world. Fossil evidence indicates that placentals and marsupials are more closely related to each other than either is to monotremes. Eutherians and marsupials are believed to have diverged from a common ancestor about 80 to 100 million years ago.
Evolution of Placental Mammals Mammalogists nowadays recognise four evolutionary lines in eutherians. The FIRST branch consists of the orders Insectivore (shrews) and Chiroptera (bats). Bats are thought to have evolved from insectivores that fed on flying insects. The SECOND branch started with medium-size herbivores that eventually gave rise to Lagomorpha (rabbits and their relatives), Perissodactyla (odd-toed ungulates such as horses and rhinoceroses (ungulates walk on toe tips), Artiodactyla (even-toed ungulates like deer and swine), Sirenia (sea-cows, such as dugong), Proboscidea (elephants), and Cetacea (whales and porpoises).
A THIRD branch includes the order Carnivora, which includes our carnivores like dogs and cats, raccoons, skunks, and the pinnipedes (walruses, seals and sea lions). Pinnipedes evolved from Cenozoic carnivores that became adapted to swimming and so returned to water. A FOURTH adaptive radiation of placental mammals produced the extensive primate-rodent group which includes order Rodentia (rats, mice, beavers and squirrels - by far the largest group of eutherian mammals with about 1770 species where “Rodentia” – the Latin word for “gnawing” refers to a pair of large front teeth that resist heavy wear by growing continuously); and Primates - monkeys, apes and humans ourselves.
NOTE: This website is written in British English, which is the English we use in Australia. You will find words like "traveller", "harbour" and "realise", and they are all correct in the language used in Australia.
