By Jonathan Amos
BBC News Online science staff
Australia's ancient lands had their fair share of big, fierce carnivores, a Sydney team of researchers argues.
Dr Wroe: Examined "a great natural experiment"
It has been a popular theory of the last 10 years that poor soils had acted to restrict the size and diversity of the country's warm-blooded predators.
But Stephen Wroe and colleagues have compared Australia and South America over 25 million years to show this stunted view of history is a myth.
They cite examples such as killer kangaroos and marsupial lions.
Other predators from Australia's past include 13 kinds of Tasmanian "tiger" - and new fossil discoveries are steadily extending the list of known species.
"Actually, given its size and isolation, Australia did pretty well for large, mammalian carnivores," Dr Wroe, from the University of Sydney's School of Biological Sciences, told BBC News Online.
It was a 1994 book, The Future Eaters, that popularised the idea that the apparently low numbers and small sizes of Australia's mammals could be attributed to a lack of food.
The continent's nutrient-poor soils, it was claimed, restricted plant growth, which led to smaller herbivores and impeded the growth of the carnivores that fed on them.
The theory was appealing and intuitive, argued Dr Wroe, but it was never really backed up by hard quantitative science.
Wroe's team tested the idea by examining a "great natural experiment" - the period of isolation South America experienced before it crashed into North America three million years ago.
For tens of millions of years, the southern landmass had conditions not dissimilar to those in Australia.
And both were dominated by a spectacular and bizarre range of predatory marsupials. South America, for example, had a marsupial sabre-tooth, an animal the size of a jaguar, but with fangs up to 15cm long.
The Sydney team have told the journal Proceedings B of the Royal Society that the diversity and size of the meat-eaters on the two landmasses were broadly the same for 22 million years.
The picture only changes when the American continents are joined and new predators move south from North America.
"And these were some of the meanest animals that ever stalked the Earth," Dr Wroe said. "You had at least three species of cats with body weights over 300kg; there was even a bear that was 6-700kg - absolutely gigantic. So, size and diversity in South America naturally then goes through the roof."
However, it is the Sydney team's assessment of the available data that Australia has had the sort of diversity of large warm-blooded carnivores one would expect for a continent of its relatively small size and isolation.
South America had its unique creatures, such as giant "terror birds"
Indeed, when compared with many of the other continents, it may even have done slightly better than expected.
"The smaller and more isolated a continent, the fewer the number of species there will be. That basically comes down to immigration and extinction rates, and also origination rates - the rate at which new species evolve in that area," said Dr Wroe.
"Low productivity almost certainly did play a role in shaping diversity but it wasn't the critical factor."
He continued: "Big fierce mammalian carnivores have always been rare on all continents.
"Over the last 65,000 years, only around 45 exceeding even 2.5kg have existed on the entire planet.
"Despite only amounting to around 6% of the world's land surface area, more than 10% of these were big, furry, sharp-toothed Australians that packed a helluva pouch."
The Future Eaters was written by Dr Tim Flannery, director of the South Australian Museum in Adelaide.
The text has become a popular and influential description of the country's past - and was turned into a major ABC TV documentary.
Dr Flannery told BBC News Online that the Wroe team's analysis contained interesting data but failed, in his view, to unseat impoverished soils as the "fundamental driver behind change in evolutionary trajectories".
"I say that because the database I drew on to make that hypothesis was a very broad one.
"It included the responses of Australian plants; it drew on the behaviour of birds; it drew on the lifecycles and physiology of some of the marsupials.
"And it noted that in the carnivore guilds of Australia, there was only one species in each of the major guilds - the major guilds being dog-like, cat-like and scavenger - whereas, overseas it was usual for there to be more than one.
"It looked to me, also, as though, on average, the Australian carnivores were rather small for the size of the continent they came from."
Dr Flannery said one of his major reservations about the Wroe team analysis was that it had not, as far as he could see, analysed soil values in South America.