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
Aquatic mammal to these ancient forms and that at least "Teinolophos" was a rather different mammal lacking several speciations seen in platypi. However, the last common ancestor between platypi and echidnas probably was aquatic, and echidnas thus secondarily became terrestrial. "Monotrematum sudamericanum" is currently the oldest aquatic monotreme known. It has been found in Argentina, indicating monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica, or that monotremes existed along the shorelines of Antarctica in the early Cenozoic. Marine mammals are aquatic mammals that rely on the ocean for their existence.