Location: Queensland, Australia
Age: 25 million years to present
The geological history of Riversleigh started with the deposition of the Thorntonia Limestone Formation, 500 million years ago, in a shallow sea. Over geological time, sea level drop and the wandering of Australia and Antarctica by global tectonics uplifted the limestone. Fifty-five million years ago, Australia split from Antarctica and moved northwards. This allowed ocean currents to circulate between the two continents and so climate changed.
The result was the warming of Australia's climate and the production of vast rain forests by 25 million years ago. This is where the fossils began to be deposited, by the late Oligocene (25ma). The uplifted limestone plateau was deeply eroded by the climate and environment change. Water carved out cave systems and deep channels into the rock. The caves became living spaces for many animals and so when they died, they fell to the cave floors. The fossils of the rainforest era can be seen in the Riversleigh sites dating between the Late Oligocene (25ma) and the Middle Miocene (15ma).
Late Miocene (15ma) to Pliocene (5ma) environments were still forested but they were becoming more open as the climate dried. Fossils of this time can be found on the Alcoota Station in the Northern Territory.
As Australia moved northwards, the climate became even more arid and so rainforests began to recede. This climate change can be seen in the faunas from the Pliocene (5ma) sites. The area gradually became mostly grassland and so a large change in fauna occurred. Continued movement of Australia into the Asia plate caused the uplift of the Indonesian Islands.
This created opportunities for animals to migrate from Asia down into Australia by island hopping. This occurred sometime after 5 million years ago as new, alien fauna can be seen in sites from the Pliocene (40,000 years ago).
Collection of plants and animals in the Riversleigh Lagerstätte has not stopped as animals still live in the area and so are deposited. This means the fossil area is over 25 million years and still on-going. Since deposition took a long time, it is possible to get a good insight into the evolution of Australia's fauna. The fossil sites are from times since Australia was isolated from the rest of the world and so its unique evolution is important. Later invasion can help show how fauna changed due to competition and new opportunities.
Fauna and flora
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