Monotremes 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.

The Platypus is the representative of the family Ornithorynchidae, and the Echidnas are of the family Tachyglossidae.

Monotremes 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 tracts open.

The 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.

Monotremes are known only from the Australian region (including Australia, Tasmania, and New Guinea).

The 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.

(From: 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).

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Family Tachyglossidae

• Short-beaked Echidna (Tachyglossus aculeatus)

Family Ornithorhynchidae

• Platypus (Ornithorhynchus anatinus)