are also many placental
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
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Origins of Australian of Placental Mammals
Australian placental animals (aka eutherian animals) are not as famous Australian
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 Gondwanaland
had broken up.
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.
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Placental Mammals are More Successful than Marsupial
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.
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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
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Evolution of Placentals
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
The SECOND branch started with medium-size
herbivores that eventually gave rise to Lagomorpha
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
such as dugong), Proboscidea (elephants), and Cetacea
(whales and porpoise dolphins).
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A THIRD branch includes the order Carnivora,
which includes our
carnivores like dogs and cats, raccoons, skunks, and the pinnipedes
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.
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