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 buffalo]] is most threatened with extinction by genetic pollution from the [[Water buffalo|domestic water buffalo]]. Such extinctions are not always apparent from a [[morphology (biology)|morphological]] standpoint. Some degree of [[gene flow]] is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species. [[File:Extinctions Africa Austrailia NAmerica Madagascar.gif|thumb|upright=1.4|Biodiversity of large mammal species per continent before and after humans arrived there]] The loss of species from ecological communities, [[defaunation]], is primarily driven by human activity. This has resulted in [[empty forest]]s, ecological communities depleted of large vertebrates. In the [[Quaternary extinction event]], the mass die-off of [[megafauna]]l variety coincided with