are primitive mammals. They have many characteristics of the
other mammals, but they also have features that are reptilian
in nature. Monotremes are the only mammals that lay eggs, and
they are the only representatives of the subclass Prototheria.
It is widely believed that mammals, including the monotremes,
are monophyletic, having evolved only once from the therapsid
reptiles in the late Triassic period.
Platypus is the representative of the family Ornithorynchidae,
and the Echidnas are of the family Tachyglossidae.
do not have teeth as adults, have several unusual distinguishing
anatomical skeletal characters, including the presence of cervical
ribs, absence of lacrimal bones and of auditory bullae. The
jaws are covered with rubbery, hairless skin, Vibrissae are
lacking, and a cloaca is present. The uteri are completely unfused,
mammary glands lack nipples. In males the penis is bifurcate
at the tip, and is attached to the ventral wall of the cloaca.
The word "monotreme" refers to the single opening
(or cloaca) into which the digestive, excretory and reproductory
Platypus is both semiaquatic and semifossorial, spends most
of its time in the water and feeds on aquatic invertebrates.
The Echidnas are terrestrial and also semifossorial, feeding
upon termites and other insects and larvae, which they excavate
with their powerful claws.
are known only from the Australian region (including Australia,
Tasmania, and New Guinea).
history of the Montremata is poorly known but is thought to
represent a line distantly related to the other living mammals.
A few fossil forms of each of the two living families have been
found from the Pleistocene of Australia. They may have evolved
from a group of therapsids that were distinct from those that
were the ancestors of marsupials and the placental mammals.
A Manual of Mammalogy, with keys to Families of the world, by
Anthony DeBlase and Robert E. Martin, second edition, 1981;
Wm. C. Brown Company Publishers, Dubuque, Iowa).