The monotremes are made up of two families, three genera and two species of Australia and New Guinea. They are the most distinctive of the 21 orders of living mammals currently recognized by scientists. Classification of the monotremes caused no end of trouble for those seeking to attribute the affinities of mammals above the ordinal level.
According to Griffiths (1978), the following four points should be considered when looking at this issue:
(1) monotremes are actually living therapsid reptiles, being nearly as far removed from mammals as alligators are from birds;
(2) monotremes are closely related to marsupials, and the two groups are best placed together in one mammalian subclass;
(3) monotremes, along with certain other extinct groups, compose one great subclass of mammals, while marsupials and placentals are included in another subclass;
(4) monotremes are closely related to the multituberculates, a group of herbivorous mammals with some superficial resemblance to rodents, that flourished in North America from the late Cretaceous period through the Eocene.
While points 3 and 4, above, seem to be popular among zoologists. There is also wide acceptance of the view that mammals, including the monotremes, are monophyletic, having evolved only once from the therapsid reptiles in the late Triassic period.
Monotremes resemble reptiles and differ from other mammals in that they lay shell-covered eggs that are incubated and hatched outside the body of the mother. They also resemble reptiles in the structure of the eye and certain bone groups. Monotreme digestive, reproductive and excretory systems are also quite similar to those of reptiles. While they resemble reptiles in these ways, in others they are decidedly mammalian. They are furred, have a four-chambered heart, nurse their young from milk secreted by specialized glands and are warm-blooded. They also have certain skeletal similarities to mammals - those that do not conform to the reptilian type.
Similar to marsupials, the monotremes possess epipubic bones associated with the pelvis, which are said to aid in the supporting of a pouch. This feature is common in both males and females, and as such, is likely a holdover from reptilian ancestry. Male monotremes have horny spus on their ankles - some of which are grooved to permit the secretion of a glandular poison.
Monotremes are known to have existed as far back as the early Cretaceous in Australia and New Guinea.
Echidnas, or Spiny Anteaters MONOTREMATA; TACHYGLOSSIDAE
Short-nosed Echidna MONOTREMATA; TACHYGLOSSIDAE; TACHYGLOSSUS
Long-nosed Echidna MONOTREMATA; TACHYGLOSSIDAE; ZAGLOSSUS
Duck-billed Platypus MONOTREMATA; ORNITHORTHYNSHIDAE; ORNITHORHYNCHUS
Griffiths M 1978. The Biology of the Monotremes. Academic Press, New York.
Griffiths M 1989. Tachyglossidae. In: "Fauna of Australia". Eds.
Griffiths, M., 1988. The Platypus. Scientific American. 258(5), pp.84-91.
Grant, T.R., & Griffiths, M., & Leckie, R.M.C., 1983. Aspects of lactation in the platypus, Ornithorhynchus anatinus (Monotremata), in waters of eastern New South Wales. Australian Journal of Zoology. 31, pp.881-889.