families, 3 genera and 3 species currently represent
the monotreme order. The fossil history of monotremes
goes back from early Cretaceous to Recent in New Guinea
and Australia, and early Pliocene in South America.
In Australia, two diverse families are represented from
the early Cretaceous, which indicates that the order
may actually predate this period. Because modern monotremes
are only found in Australia and New Guinea, the presence
of fossil evidence in South America may be indicative
of a dispersal of monotremes via Antarctica to South
America when the two landmasses were joined.
has been much discussion about the taxonomical placement
of these unusual species. Some taxonomists have categorized
them in a subclass with extinct lineages separate from
all other extant mammals. Others have placed them as
living therapsid reptiles. The source of the confusion
is the fact that monotremes possess characteristics
of reptiles, mammals and marsupials.
is the reptilian characteristics that set monotremes
furthest apart from the other mammals. Among the most
noteworthy is that female monotremes lay eggs. The embryos
develop inside leathery shells that are incubated and
hatched externally from the mother. Monotremes also
possess a single external opening for the reproductive,
excretory and digestive systems called the cloaca. Another
reptilian resemblance is the lack of a corpus callosum
bridging the two hemispheres of the monotreme brain.
This connection of nervous tissue is present in all
other mammals but not in reptiles or marsupials.
possess a potentially significant, yet puzzling marsupial
characteristic in the presence of marsupiam bones in
the pelvis. These bones are usually associated with
support of the marsupial pouch, however the purpose
in the monotreme anatomy is more obscure because the
bones are present in both sexes. Scientists have proposed
that the bones may have functioned at one time as support
to a pouch structure or, perhaps be remnants of skeletal
and muscular structures supporting the large hindquarters
of a reptilian ancestry.
with all of these reptilian characteristics, the current
consensus is that monotremes are in more ways mammalian.
They have fur, they are warm-blooded, they nurse their
young and their heart is four-chambered (however with
some differences in the atrioventricular valve and left
aortic arch). The skeletal system also features significant
mammalian characteristics, such as the existence of
one bone on each side of the lower jaw and three middle
ear bones. As mentioned, aspects of the monotreme brain
are reptilian, however a notable mammalian characteristic
is a more developed pallial region than striatal region.
The opposite is true for reptiles and birds.