We often think that most animals start off small and get bigger. It's a survival method for the species. In evolutionary terms, smaller animals have a better chance at long-term survival than larger animals. That is before humans interfered. Humans hunt anything now, big or small. And most of the extinctions going on now are due to over hunting and global warming. But anyway, in this article, it is discovered that modern koalas are not descended from the larger koalas that died off about 30,000 years ago. In fact they were simply 2 separate, and closely related species. This article also explains how if we could find out how this giant koala died off, we might be able to solve the puzzle as to how all other Australian megafauna died off.
http://dsc.discovery.com/news/2008/12/16/koala-dwarf-prehistoric.html
Koala Not a Dwarf of Prehistoric Versions
Dani Cooper, ABC Science Online
Dec. 16, 2008 -- Australia's iconic koala is not a dwarf and the finding has major implications for theories on what happened to the continent's prehistoric megafauna, a Queensland palaeontologist said.
Gilbert Price, of the University of Queensland's Center for Microscopy and Microanalysis, said there has been a long-held view that modern koalas were a dwarf version of the giant prehistoric koala that lived between 30,000 and 700,000 years ago.
However, in a paper published in Quaternary Science Reviews, Price said modern koalas are a separate species that at times coexisted with the Late Pleistocene giant Phascolarctos stirtoni, which weighed between 20 and 30 kilograms.
Gilbert used improved dating techniques to analyze fossils and found that between 300,000 and 500,000 years ago the two koalas were both living in Australian trees.
"It's fascinating that up until fairly recently in geological terms we did have two types of koalas kicking around," said Gilbert. "The fossil records do suggest that they lived in the same place at the same time [perhaps] there is something about their sizes that allowed them to fill a slightly different ecological niche.
"The big question is why one koala species survived past 50,000 years ago and the other didn't make it."
Gilbert said understanding this may help prevent the modern-day koala from becoming extinct. However he said a lack of well-dated fossil records makes it difficult to determine the true ancestor of the modern koala.
He said his finding also suggests that the debate about why Australia's megafauna became extinct need to be revisited.
"My work shows we've got to sort out this dwarfing hypothesis first," he said.
The "dwarfing hypothesis" was originally developed to explain the body-size relationship between extinct Pleistocene mammalian megafauna and smaller-sized, similar-looking, modern-day animals, said Gilbert.
It has been applied to other present-day Australian mammals including the grey kangaroo, Tasmanian devil and the koala. Gilbert said the dwarfing phenomena has been used to support opposing megafauna extinction theories.
He said on one side dwarfing is viewed as the result of a physiological response to climate-induced changes in habitat and food supplies, while the counter view holds that it was human induced through targeting of larger animals in hunting.
However Gilbert said his study shows that dwarfing itself "has not been fully tested".
"A combination of more intensive physical dating, better stratigraphic control in regard to collecting methodologies, and up-to-date taxonomic information is critical for testing such hypotheses," he wrote. "Until such information becomes available an understanding of the fate of many Late Pleistocene forms, and the origins of many extant taxa, will remain elusive."
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