They first appear in the late Oligocene, and were even more diverse in the Oligocene–Miocene period than they are now. Another example of adaptive radiation is the development of different Australian Marsupials from a single ancestral stock in the Australian subcontinent. Adaptive Radiation. Diprotodontia are by far the most abundant and diverse Australian marsupials. In vertebrates, imprinting is believed to have originated approximately 150 million years ago, in the common ancestor of the two Therian infra-classes: eutherians (placental mammals like humans and mice), and metatherians (marsupials like the opossum and tammar wallaby which have a less invasive placenta)[5]. We do know that early marsupials (or metatherians, as they're sometimes called by paleontologists) spread from Asia to North and South America, and then from South America to Australia… For tens of millions of years after Sinodelphys, the marsupial fossil record is frustratingly scattered and incomplete. The distance data support the view that the echidna and platypus lineages diverged from their last common ancestor at least 50 to 57 Ma (million years ago) and that monotremes diverged from marsupials and eutherian mammals about 163 to 186 Ma. A biography of the Australian continent : Marsupial Origins. This explains divergent evolution for the ancestral stock from which a number of species arise. Diprotodontidae, an extinct family, includes the largest known marsupials: the very largest, Diprotodon, was a fully quadrupedal form estimated to weigh more than one tonne. In the book Australia's Mammal Extinctions, Chris Johnson has a section on what is known of the origin of marsupials, and their spread from South America to Australia via Antarctica, and discusses the possible reasons for the dominance of marsupials … Yet today no monotremes exist outside of Australia (and New Guinea), and no placental mammals that didn't fly or swim there—for example, bats or dugongs—exist in Australia except for rodents (which arrived only about five million years ago) and mammals that were introduced by people (who arrived by 60,000 years ago).