Mammals of Australia introductions of placental mammals into Australian fauna were about (rats), several thousand years ago (dingo), and 200 years ago (many species); the last two were made by humans. Some claims were made about placental mammal fossils from the Eocene of Australia, called "Tingamarra". These claims are based on only one found tooth having some characteristic features of condylarth, and were discussed widely. But both the age and placental nature of these fossils were challenged by other researchers. Monotremes are mammals with a unique method of reproduction: they lay eggs instead of giving birth to live young. Two of the five
Mammal contain their [[fetus]] during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore [[Pregnancy (mammals)|pregnancy]] was short, as in modern marsupials. This suggests that the placenta was a later development. One of the earliest known monotremes was "[[Teinolophos]]", which lived about 120 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce ([[cloaca]]) – as lizards and birds also do – and they lay [[Egg (biology)|eggs]] which are leathery and uncalcified. "Hadrocodium", whose fossils date