Among the
mammals, there are three major variations in reproductive systems. This
is the basis for dividing them into subclasses and infraclasses.
| class: | Mammalia | ||
|---|---|---|---|
| subclass: | Prototheria | Theria | |
| infraclass: | Metatheria | Eutheria | |
Members of
the subclass Prototheria
lay eggs
like non-mammalian vertebrates. However, they feed their newborn with mammary gland secretions like all other mammals. The
Prototheria are also referred to
as monotremes
, which literally means that they
have one opening for excretion and reproduction. This is similar to
birds. There are only three surviving rare
species groups of Prototheria. These are the Australian platypus and 2
echidna (spiny anteater) species of Australia and New Guinea.
 |
 |
|
|
Platypus (subclass Prototheria) |
Echidna (subclass Prototheria) |
All other
living mammalian species, including humans, are in the subclass
Theria
.
They have in common the fact that they give birth to live young. Therian
mammals apparently did not evolve from the Prototheria. The relatively primitive
prototherian reproductive system evidently evolved after their evolutionary line separated
from the other early mammals.
The oldest infraclass of
therian mammals is the Metatheria
, or the
marsupials
.
Their young are born very immature and cannot live without further development
in the mother's pouch. The word marsupial comes from
marsupium, the Latin word for purse. Marsupials include kangaroos, koalas,
opossums, and many other similar animals. Most of them are native only to Australia
and New Guinea.
 |
 |
|
| Kangaroo and koala (infraclass Metatheria) | ||
 |
|
| Human fetus in utero |
Most mammal
species, including humans, are in the infraclass Eutheria
. They are
also referred to as placental mammals. Eutherian mothers carry their unborn children within the uterus where they are
nourished and protected until an advanced stage is reached. This is made possible by the umbilical
cord and placenta which connects the fetus to the uterus wall and enables
nutrients and oxygen to get to the offspring as well as provides a means of eliminating waste.
Placental mammals have been extremely successful in out-competing monotremes and marsupials for ecological niches. Placental mammals are found on all continents, in the air, and in the seas. Primates, cats, dogs, bears, hoofed animals, rodents, bats, seals, dolphins, and whales are among the dominant placental mammal groups today. Nearly 94% of all mammal species now are placental mammals.
 |
| Whale, dolphin, monkey, and zebra (infraclass Eutheria) |
The next tutorial in this series, The Primates, investigates all of the Linnaean classification categories below the infraclass level for humans, apes, monkeys, and some other closely related animals. This will take us from the "order" level down to "species."