Wednesday, January 24, 2007

 

Australia: Nullarbor caves reveal ancient megafauna thrived in dry climate

Paleontology: In 2002, a team of cavers exploring the vast, treeless Nullarbor Plain of southern Australia made what some palaeontologists described as the "find of the century" - a dozen skeletons of the extinct marsupial 'lion', Thylacoleo carnifex [images], as well as the bones of giant wombats, short-faced kangaroos and thylacines.

Until then, no complete skeleton of Thylacoleo was known to science, so the Western Australian Museum launched "Operation Leo" to recover these fossils, supported by the Rio Tinto WA Future Fund.

Now in early 2007, the first analysis of these subterranean treasure troves - led by Dr Gavin Prideaux*, Rio Tinto Research Fellow at the Western Australian Museum - has appeared in the latest issue of the prestigious international journal Nature.

Fossils from the caverns, dubbed the Thylacoleo Caves, show that the Nullarbor ('no trees' in poor Latin) was once home to at least 69 species of mammals, birds and reptiles, including 23 different types of kangaroo, ranging from pint-sized bettongs to 3-metre tall giants. Eight of the kangaroo species are entirely new to science, and include an unusual wallaby with large 'brow-ridges', and, ironically (in the now treeless Nullarbor Plain), two tree-kangaroos - modern relatives of which inhabit rainforests in tropical Australia and New Guinea.

Expedition crews led by Western Australian Museum palaeontologist, Dr John Long (now Head of Science at Museum Victoria in Melbourne), spent a month camped in the middle of the Nullarbor during 2002, 2003 and 2004.

"We collected hundreds of specimens during each expedition and were stunned by the amazing preservation of the fossils. Many of the skeletons are complete. As palaeontologists, we spend most of our time trying to identify and reconstruct extinct animals from fragments. All of a sudden, it was information overload," said Dr Long.

Fossils were dated using changes in the Earth's magnetic field, uranium-series dating and optically stimulated luminescence by a team of geochronologists including Professor Richard 'Bert' Roberts (University of Wollongong). The researchers found that animals fell into the caves between about 400,000 and 800,000 years ago, through the same narrow entrances that cavers abseil through today. Since then, the caves have been sealed for much of the time, which accounts for the pristine condition of the bones. "The dates place the animals in a period when Australia was locked into a long-term trend of increasing aridity in one of the driest regions of the continent", Professor Roberts said.

The researchers reconstructed the ancient Nullarbor environment from oxygen and carbon isotopes contained in the enamel of kangaroo and wombat teeth, and from the modern geographic ranges of species in the fossil fauna that are still living today.

Surprisingly, the climate half-a-million years ago was very similar to that of today, although the region must have had substantially more tree cover to support such a high diversity of herbivores.

"Some time during the last 400,000 years, the Nullarbor vegetation changed from fire-sensitive woodland to the shrub/grass mosaic we see today. We think that an increase in wildfires best explains the shift, given that climate change was not a significant factor," Dr Prideaux said.

The study also has implications for the debate over what finally drove the megafauna to extinction 40,000-50,000 years ago. The authors argue that if the Nullarbor animals were well adapted to dry conditions for at least 400,000 years before they disappeared, then it is unlikely they succumbed to Ice Age aridity.

"Our work removes another pillar of support from the idea that the megafauna were driven to extinction by climate change, especially given that most of the large species were not Nullarbor specialists - they were widespread across much of Australia. But the jury's still out on whether or not the change in the Nullarbor vegetation is correlated with the arrival of humans, as seems to have been the case in other parts of Australia," said Dr Prideaux, who earlier this month published a separate study in the journal Geology [2] revealing that megafauna from the Naracoorte Caves in southeastern Australia were resilient to climate change for half-a-million years before humans arrived.

The palaeontologists now have their eye on a deep sediment pile in one of the Thylacoleo Caves, which may preserve a record extending back millions of years. Study of bones from the various layers will help build up a detailed view of the evolution of the Nullarbor animals, and their responses to an increasingly dry climate.

"The Thylacoleo Caves are undoubtedly one of Australia's most important fossil localities. It's been a privilege to work on these sites, and we can't wait to get back out there to start the next phase of the project,” Dr Prideaux said. [Source: University of Wollongong]

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[1] An arid-adapted middle Pleistocene vertebrate fauna from south-central Australia

Gavin J. Prideaux, John A. Long, Linda K. Ayliffe, John C. Hellstrom, Brad Pillans, Walter E. Boles, Mark N. Hutchinson, Richard G. Roberts, Matthew L. Cupper, Lee J. Arnold, Paul D. Devine and Natalie M. Warburton

doi:10.1038/nature05471

How well the ecology, zoogeography and evolution of modern biotas is understood depends substantially on knowledge of the Pleistocene. Australia has one of the most distinctive, but least understood, Pleistocene faunas. Records from the western half of the continent are especially rare.

Here we report on a diverse and exceptionally well preserved middle Pleistocene vertebrate assemblage from caves beneath the arid, treeless Nullarbor plain of south-central Australia. Many taxa are represented by whole skeletons, which together serve as a template for identifying fragmentary, hitherto indeterminate, remains collected previously from Pleistocene sites across southern Australia. A remarkable eight of the 23 Nullarbor kangaroos are new, including two tree-kangaroos.

The diverse herbivore assemblage implies substantially greater floristic diversity than that of the modern shrub steppe, but all other faunal and stable-isotope data indicate that the climate was very similar to today. Because the 21 Nullarbor species that did not survive the Pleistocene were well adapted to dry conditions, climate change (specifically, increased aridity) is unlikely to have been significant in their extinction.

[2] Mammalian responses to Pleistocene climate change in southeastern Australia (open access)

Gavin J. Prideaux, Richard G. Roberts, Dirk Megirian, Kira E. Westaway4 John C. Hellstrom, Jon M. Olley

doi: 10.1130/G23070A.1

Resolving faunal responses to Pleistocene climate change is vital for differentiating human impacts from other drivers of ecological change. While 90% of Australia's large mammals were extinct by ca. 45 ka, their responses to glacial-interglacial cycling have remained unknown, due to a lack of rigorous biostratigraphic studies and the rarity of terrestrial climatic records that can be related directly to faunal records.

We present an analysis of faunal data from the Naracoorte Caves in southeastern Australia, which are unique not only because of the species richness and time-depth of the assemblages that they contain, but also because this faunal record is directly comparable with a 500 k.y. speleothem-based record of local effective moisture. Our data reveal that, despite significant population fluctuations driven by glacial-interglacial cycling, the species composition of the mammal fauna was essentially stable for 500 k.y. before the late Pleistocene extinctions.

Larger species declined during a drier interval between 270 and 220 ka, likely reflecting range contractions away from Naracoorte, but they then recovered locally, persisting well into the late Pleistocene. Because the speleothem record and prior faunal response imply that local conditions should have been favorable for megafauna until at least 30 ka, climate change is unlikely to have been the principal cause of the extinctions.

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Also of interest, a 2005 paper from the Proceedings of the National Academy of Sciences (open access):

Prolonged coexistence of humans and megafauna in Pleistocene Australia

Clive N. G. Trueman, Judith H. Field, Joe Dortch, Bethan Charles, and Stephen Wroe

PNAS | June 7, 2005 | vol. 102 | no. 23 | 8381-8385

Recent claims for continent wide disappearance of megafauna at 46.5 thousand calendar years ago (ka) in Australia have been used to support a "blitzkrieg" model, which explains extinctions as the result of rapid overkill by human colonizers. A number of key sites with megafauna remains that significantly postdate 46.5 ka have been excluded from consideration because of questions regarding their stratigraphic integrity. Of these sites, Cuddie Springs is the only locality in Australia where megafauna and cultural remains are found together in sequential stratigraphic horizons, dated from 36-30 ka. Verifying the stratigraphic associations found here would effectively refute the rapid-overkill model and necessitate reconsideration of the regional impacts of global climatic change on megafauna and humans in the lead up to the last glacial maximum. Here, we present geochemical evidence that demonstrates the coexistence of humans and now-extinct megafaunal species on the Australian continent for a minimum of 15 ka.

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Gavin Prideaux is author of:

Systematics and Evolution of the Sthenurine Kangaroos

This work represents an exhaustive review of one of the most important late Cenozoic radiations of Australian marsupials: the short-faced, or sthenurine kangaroos. Sthenurines originated in the Miocene, diversified in the Pliocene, and radiated in the Quaternary to become one of Australiaâs most conspicuous mammal groups, the only lineage of browsing marsupials comparable in diversity to the browsing artiodactyl guilds of other continents.

The culmination of 12 years research, the monograph details the taxonomy of the sthenurines, redescribing each of the six genera (two new) and 26 species (four new), and is amply illustrated with line drawings and more than 100 pages of plates. It presents the first cladistic analysis of sthenurines, and by synthesizing systematic, functional morphological, biochronologic and zoogeographic data, considers the major directions of adaptive change within the group, and the major environmental factors that drove their evolution.

It is one of the most comprehensive studies of an extinct marsupial lineage ever made, and should be an essential reference for students of Australian late Cenozoic vertebrates, marsupial evolution, environmental change and Pleistocene extinctions.

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Recent posts include:

"Humans Responsible for Australian Extinctions"

"Wollemi Pine: University acquires rare plant from dinosaur age"

"Fossil discovery turns scientific theory on its head"

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Comments:
Hi Guys,

Just to let you know, I have some Australian Megafauna fossils I'd be happy to trade or sell to those with interest in the area.

I have 16 pieces of Procoptodon (giant kangaroo) bones of varying sizes, that I was allowed to keep after an expedition by the state museum.

I also have an enormous tooth, about a foot long and still showing its original enamel, and a couple of skull fragments from a Diprotodon (giant wombat) that I found myself on a geology field trip.

I have documentation authorising the export of these fossils outside Australia.

Not meaning to spam, its just that I'm aware this kind of material is very rarely available and I'd like to see the specimens go to an appreciative home ;]

I can be contacted at leary.joe@gmail.com for photos and further information

Cheers

Joe
 
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