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 referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. "[[Ambondro mahabo|Ambondro]]" is more closely related to monotremes than to therian mammals while "[[Amphilestes]]" and "[[Amphitherium]]" are more closely related to the therians; as fossils of all three genera are dated about in the [[Middle Jurassic]], this is a reasonable estimate for the appearance of the crown group. T. S. Kemp has provided a more traditional definition: "[[synapsid]]s that possess a