and the Marsupials - A Historical and Modern Perspective
Marsupials are essentially similar to the placentals in many respects,
though they do possess a number of structural differences, primarily in
respect to the reproductive organs, skull and skeleton (which includes
the presence of two additional bones, the epipubics, located in
the pelvic girdle). Unlike placentals, marsupials are born in a semi-embryonic
state, and in most
species they are then protected within their mother's abdominal pouch where
they are milk-fed until until their early development is complete.
Two thirds of the
young Virginia opossum (Didelphis virginiana). In the Americas,
the Virginia opossum is the only marsupial species found north of Mexico.
living marsupial species are found in Australia and its neighboring islands
including New Guinea. Most of the remaining species live in
South America. Currently, the only marsupial that naturally occurs
in North America is the common and widespread Virginia opossum (Didelphis
The third and most ancient group of living mammals are the monotremes.
They feed their young milk as do other mammals, but they also possess a
number of reptilian characteristics, particularly in
fossil skull of the Late Oligocene platypus Obdurodon dicksoni,
as compared to that of the modern platypus (Ornithorhynchus
anatinus). Apart from being physically larger, unlike
the living species, Obdurodon possessed teeth.
to their skeletal structure. Also, like reptiles, they lay eggs.
The monotremes are represented today by only three genera:
(Long-nosed echidna), Tachyglossus (Short-nosed echidna), and Ornithorhynchus
(platypus). The echidnas have shallow pouches in which they hold
their single egg and resultant young, but the platypus does not.
Instead, the platypus keeps its eggs (most often two) in a leaf and stick
nest built within a chamber which it excavates in the bank of a river or
creek. The young are raised there until old enough to venture outside,
usually by 17 weeks of age.
The precise ancestral
history of the three living mammal groups is still not completely understood,
and numerous theories exist about the evolutionary relationships between
them. As compared to the monotremes, placentals and marsupials share
a closer common ancestry with each other. Extremely early marsupials,
or mammals very similar to them in form (proto-marsupials), are considered
by some to be ancestral to the placentals, having split into two groups
perhaps 125+ million years ago. How the monotremes relate to the
marsupials and placentals is less certain, and has been the subject of
| Whatever the actual
details of these evolutionary relationships may be, the three extant groups
of mammals have been distinct from one another for a considerable length
of time. However, time itself is the only thing that separates
them, since all mammal groups are descended from a common Mesozoic ancestor
which probably appeared about 195 million years ago.
| When zoologists
began to classify the mammals of Australia, representatives of all three
living groups were found among them, although marsupials were by far the
The great variety of
Australian marsupials include such families as the kangaroos (Macropodidae),
koala (Phascolarctidae), the possums (Phalangeridae, Burramyidae, Petaluridae),
wombats (Vombatidae), quolls, Tasmanian devil,
and marsupial "mice" (Dasyuridae), bandicoots (Peramelidae), the marsupial
"mole" (Notoryctidae), the Numbat (Myrmecobiidae), and the thylacine
(Thylacinidae). Many Australian marsupials are analogues to the placentals
of other continents in that they occupy the same types of ecological niches.
Through evolutionary convergence, a number of them have even come to resemble
certain placentals in general physical form.
koala (Phascolarctos cinereus), a phascolarctid marsupial.