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
Steropodon Steropodon Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived about 105 million years ago (mya) in the Lower Cretaceous period. It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries. The dentition of "Steropodon" is somewhat similar to that of therians–the placentals and the marsupials–specifically the presence of the tribosphenic molar tooth which was thought to be exclusive to therians since the Cretaceous. This, along with the tribosphenic molar discoveries of monotreme-relatives "Ausktribosphenos" and "Ambondro" of which the latter evolved in the Jurassic, led to the conclusion that