 Opossums are frequently considered to be "living fossils", and as a result are often used to approximate the ancestral therian condition in comparative studies.However, this is inaccurate, as the oldest opossum fossils are from the early Miocene (roughly 20 million years old).The last common ancestor of all living opossums approximately dates to the Oligocene-Miocene boundary (only 23 million years ago) and is at most no older than Oligocene in age.