Monotremes are egg-laying mammals of the sub-class Prototheria, the most primitive of living mammals. There are only two families, the duck-billed platypus of Australia and the spiny anteater (Echidna) of New Guinea and Australia.
Monotremes have certain features similar to reptiles, birds, marsupials and placental mammals and their classification was the center of controversy between anatomists in the 19th century.
Monotremes comprise the order Monotremata of the class Mammalia and are represented by the Platypus and the Spiny Anteater or Echidna. They are found only in Australia and New Guinea and no fossilized evidence of their existence has been found in any other part of the world.
Monotremes share a number of features with reptiles: both lay eggs with shells and large yolks; monotremes have the combined urino-genital and anal openings common in all reptiles; and the structure of some parts of a monotreme's skeleton, such as the shoulder girdle, is reptile-like.
However, apart from the fact that they lay eggs instead of giving birth to live young, monotremes possess all the features of mammals, being warm-blooded, furred vertebrates which breathe air and suckle their young. The young are protected in a temporary pouch (marsupium). Male platypuses are unique in that they posses a venom gland and a spine on the hind-foot; the use of these features is disputed.
Unfortunately, the only fossils of monotremes found so far resemble the living species. Because monotremes are very specialised animals, one living on land and feeding on insects, the other leading an aquatic existence and feeding on aquatic invertebrates, the fossils give no clues as to the relationships and ancestry of these animals. Furthermore, the adults do not have teeth which are an important guide to mammal relationships as they are often the only fossil remains.