What Theories Are There To Explain The Origin Of Monotremes?

Monotremes have very many unique features and there has been a lot of speculation about their origins. So far, fossils of early monotremes have only been found in Australia. The earliest platypus like fossil dates from about 10 million years ago.

One theory of their origin suggests that monotremes developed in America. South America and Australia both have their own species of marsupial, showing that these relatively primitive animals shared the land mass before the area that is now Australia broke away. However, no fossils have yet been found in South America to support this theory.

It is a more accepted theory that monotremes broke away from mainstream evolution a very long time ago and never developed the advanced mammalian features such as giving birth to live young or feeding the unborn young via a placenta. It is likely that monotremes developed from a completely different ancestor much further back in evolution and there are no related fossils to be found in South America.