A previously unknown mass extinction of plants occurred
- The Ice Age, or Meteor Bombardments wiped out massive breeds of earth's plants?In Australia seven distinct craters (above), ranging in size from about 25 m to around 1 km in diameter, have been recognised. Five of these are associated with meteorites. In addition, there are another 30 very much larger but deeply eroded and enigmatic circular scars that present some evidence of an origin by impact.(Among the largest are Woodleigh in Western Australia (60-70 km across), Lake Acraman in South Australia (greater than 35 km) and Tookoonooka in Queensland (50 km in diameter)).A previously unknown mass extinction of plants occurred...in the southeast corner of Australia, an analysis of fossilised leaves shows.Fossil leaves uncover mass plant extinctionAustralia's south-west region has a spectacular diversity of sclerophyll plantsThe findings, by Australian researchers, helps explain a spectacular but mysterious diversity of sclerophyll plants elsewhere - in Australia's southwest region.Australian Research Council Fellow Dr Kale Sniderman, of the University of Melbourne, says the diversity of sclerophyll plants in Australia's southwest, and also in South Africa's western Cape, is a global "anomaly"."They are unusually rich in species diversity [and] have the most diverse floras [in the world] outside the tropics," he says.
Sniderman says it has long been assumed this diversity was the result of the arrival of Mediterranean-type climates to these regions.
However their study suggests otherwise. It shows that about 1.5 million years ago southeastern Australia was awash with sclerophyll flora, every bit as spectacularly diverse as the southwest corner.
Importantly, says co-author Dr Greg Jordan, at the University of Tasmania, this sclerophyll flora diversity in southeastern Australia occurred at a time when the region was much wetter and warmer than it is today.
"Think northern New South Wales, Coffs Harbour and growing bananas," says Jordan.
This finding, published in today's PNAS journal , is based on an analysis of "exceptionally preserved" sclerophyll fossilised leaves, flowers and fruits from Stony Creek Basin, an infield lake near Daylesford, about 100 kilometres from Melbourne.
Sclerophyll are tough-leaved, woody plants common to Australia such as banksias, grevilleas, hakeas, acacias and eucalyptas.
The researchers found leaf and stem fossils of 69 sclerophyll trees and shrubs at the site. They then compared this find with the diversity of leaves deposited in a modern-day equivalent lake system - Lake Dobson in Tasmania.
Sniderman says by knowing how much leaf deposit in this modern-day equivalent reflected local diversity, they were able to extrapolate the diversity at Stony Creek Basin.'Something special'
Jordan says it was traditionally believed there was "something special" about the climate in southwestern Australia and the western Cape that allowed lots of species to flourish...
"These results undermine the notion that summer-dry Mediterranean-type climates are necessary for the hyperdiverse sclerophyll diversity," the authors write.
Instead the study suggests the diversity can then be explained by the fact southwestern Australia and the western Cape did not suffer a severe loss of flora during the Ice Age.
"It looks like the size of the climate cycles between warm and wet and cold and dry [in the west] weren't as great as in eastern Australia," says Jordan. "So these were places where lots of species didn't disappear."
There had been an assumption that sclerophyll flora had "chased" rainforests, into small enclaves in a process known as ecological substitution, he says.
However the study shows the loss of sclerophyll diversity in the east coincided with the decline of rainforests.
"What our results show is that sclerophylls have also undergone quite large-scale loss of species [and] are a skeleton of their former diversity," says Sniderman.
http://www.abc.net.au/science/articles/2013/02/12/3686410.htm(Source: istockphoto/annadunning)Fossil leaves uncover mass plant extinction