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 mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal [[insectivore]] niche from the mid-Jurassic onwards; The Jurassic "[[Castorocauda]]", for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish. Most, if not all, are thought to have remained nocturnal (the [[Nocturnal bottleneck]]), accounting for much of the typical mammalian traits. The majority of the mammal species that existed in the [[Mesozoic|Mesozoic Era]] were multituberculates, eutriconodonts and [[spalacotheriid]]s. The earliest known [[metatherian]] is "[[Sinodelphys]]", found