Australia: The Land Where Time Began

A biography of the Australian continent 

Riversleigh Monotremes

The only mammal known from the Cretaceous of Australia is a monotreme, Steropodon galmani, that was similar to a platypus, but it has been suggested it should be assigned to its own family.

A complete skull of Obdurodon dicksoni form the Early Miocene has been found at Riversleigh. It is the only ornithorynchid skull so far found. It was first seen as a braincast that had preserved an impression of the blood vessel. After releasing the skull from the limestone with acetic acid the complete skull emerged from the limestone block.

This complete Obdurodon skull and a complete dentition, apart from the missing last lower molar, has provided the material necessary to not only see the complete structure of the head, but to work out its relationships and the biogeography of platypuses. Various aquatic sites at Riversleigh have produced material of Obdurodon. At Riversleigh, they were among a rich fauna that included turtles, lungfish, other fish and crocodiles, and n some sites, especially the site called Neville's Garden, there were also many terrestrial mammals. It has been suggested that this site may have formed from a deep water body with a wide shallow margin, and a deeper main body where the aquatic animals lived.

63-61 million-year-old teeth of anObdurodon-lke animal have been found in Patagonia, in the south of Argentina. Obviously they must have originated in Gondwana, becoming extinct everywhere except Australia.

An anomaly of the Riversleigh deposits with regard to monotremes is that no echidnas have been found. At present they exist all over Australia in all types of environment, so why not Riversleigh?

Sources & Further reading

Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland