Australia: The Land Where Time Began

A biography of the Australian continent 

Based on the fossil record, monotremes are believed to have evolved a bit over 100 million years ago, and though they continued to evolve, are a relic group that are mostly not much changed from their early form. It has been assumed that echidnas evolved as early as platypuses, the only other monotremes. Some studies suggested that they may have split from the platypuses prior to the separation of monotremes from the therian line.

An anomaly in the fossil record has been that although platypuses, and platypus-like animals, have been found going back more than 100 million years, echidnas have made their appearance in the fossil record only about 15 million years ago.

A recent molecular study has now indicated that the platypus and echidna diverged only about 30 million years ago. This would fit more closely with the timing indicated by the known fossil record. If this is indeed the case it would suggest that monotremes, far from being a relic group, have actually been evolving and diversifying in competition with marsupials, and at a relatively rapid rate over the past 30 million years.

The new results have been disputed on the grounds that a 106 million years old bone found in the Dinosaur Cove deposit in Victoria appears to have come from an echidna-like animal.

The Australian fossil record also includes some specimens of Giant Echidnas from the Pleistocene era. These were as large as sheep, and have been found in Western Australia (in Mammoth Cave), and constitute the largest monotremes ever discovered. (Zaglossus hacketti).

The fossil beds of Naracoorte Caves in South Australia, have produced an animal similar to the Papua New Guinea long beaked echidna, but much larger, Megalibgwilia ramsayi.

Links

• The oldest platypus and its bearing on divergence timing of the platypus and echidna clades
• Molecules, morphology and ecology indicate a recent, amphibious ancestry for echidnas
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Sources & Further reading

• Echidnas
• Marsupial vs Placental
• Monotremes
• Murgon Fossil Site
• Naracoorte Fossils Site
• Perameloids
• Riversleigh Fossil Site