Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change in Australia - Vertebrates of Quaternary Rainforests Response

The first Quaternary record of an Australian tropical rainforest fauna, that has been dated by U disequilibrium series, has been found of a vertebrate fossil fauna from the Middle Pleistocene. This fauna, that was exceptionally rich, underwent extinction following a long period spanning several glacial cycles during which there was relative faunal stability, and appear to have persisted until about 280,000 years ago. A xeric-adapted fauna replaced the rainforest fauna at some point between 280,000-205,000 years ago, following which a mesic-adapted fauna replaced the xeric-adapted fauna, becoming established by the Holocene. According to the authors¹ this is the first evidence of a vertebrate fauna in Australia from the mid-Brunhes Climatic Event (MBE) of the Middle Pleistocene, a major reorganisation of climate leading to an increase of aridity in northern Australia that began about 300,000 years ago. The climatic shift to aridity is suggested by several independent palaeoclimate proxies to be the result of increasing variability of the climate and weaker northern monsoons, that have been suggested to have possibly been manifested in the extinction of the aseasonal rainforest fauna and replacement by an arid-adapted fauna. The authors¹ say they have extended the temporal ranges of several taxa from the Pliocene to the Middle Pleistocene, and also reveal a longer palaeogeographic connection of rainforest taxa and linages of Australia, that were shared with New Guinea than was previously believed, showing that they went extinct on the Australian mainland some time after 280,000 years ago.

Conclusions of the authors¹

At Mt Etna, central Queensland, cave deposits have been found that contain fossils representing the only known rainforest vertebrates from the Quaternary of Australia. The fossils had been believed previously to be from the Pliocene, based on biocorrelation, but it has been revealed by dating using direct chronometric dating that most of the sites are actually from the Middle Pleistocene, from earlier than 500 ka to about 280 ka. The temporal range of several taxa from the Pliocene and Early Pleistocene have thus been extended into the Middle Pleistocene.

After remaining stable for a long period during which the fauna remained relatively stable there was a major faunal turnover that occurred between about 280 ka and 205 ka resulting in the diverse rainforest fauna being replaced by a xeric-adapted fauna, which is the first vertebrate fossil evidence that large-scale climatic shifts occurred at this time, supporting other evidence that indicates aridity in Northern Australia, which has been documented by many other palaeoclimatic methods. The Holocene and modern fauna that is mesic-adapted resulted from at least 1 more faunal change, suggested by the authors¹ to be a continued response of habitats of northern Australia to a sustained shift to a climate that is more variable, though for the latest faunal changes they do not rule out additional anthropogenic changes.

Unlike these results the records from southern Australia indicate a remarkable level of faunal stability in the Quaternary for the last 500 ka for fauna other than the megafauna, though in southern Australia the stability may be a reflection of long-term adaptation to increasing aridity that occurred earlier in the southern parts of the continent. The authors¹ suggest their results indicate there was a suite of rainforest taxa that have no representatives in Australia at the present and therefore indicates a longer connectivity between the faunas of northern Australia and those of New Guinea than was previously believed. The athors¹ suggest that when considering the vulnerabilities of rainforest taxa in far north Queensland, a bioregion that is currently under several climatic and human-induced threats, it is fundamental to have an understanding of the causes for the extinction of these taxa on mainland Australia.

In this paper the authors¹ demonstrate that climate change in the Quaternary influenced different regions of the Australian continent in different ways over a similar period of time and to a degree that has not previously been recorded. Though this is in itself an intuitive statement it illustrates the inherent complexity involved in determining the possible future responses of flora and fauna climate change.

Sources & Further reading

1. Hocknull, Scott A., Jian-xin Zhao, Yue-xing Feng, and Gregory E. Webb. "Responses of Quaternary Rainforest Vertebrates to Climate Change in Australia." Earth and Planetary Science Letters 264, no. 1–2 (12/15/ 2007): 317-31.