A lion's feast

There was a time, yesterday, geologically speaking,  when kangaroos 3 m tall hopped about Australia, elephants roamed North America and 2-m high birds strutted around in New Zealand. Since the end of the nineteenth century, many different writers have noticed that the world does not have abundant creatures over 40 kg. Collectively, these large beasts are known as megafauna. Alfred Wallace, who wrote the first paper on evolution by natural selection with Charles Darwin, noted that we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared.

We now know that the extinction of many of these creatures was global and that they died out quite recently. Their bones, when explorers and researchers found them, were not fossilized, suggesting their death was a matter of thousands of years ago. But the extinction seems to have happened at different times in different places. Some regions even kept their megafauna.  In Australia, the best estimate is that around 94% of megafauna became extinct around 46,000 years ago.  What happened to them all and are we to blame?

As with any good mystery, there are two main suspects: in this case, climate and humans.  The idea that our animal-skin clad ancestors may have hunted the huge beasts to extinction was first suggested as long ago as the mid-nineteenth century. The alternative is that a rapidly changing climate caused the habitat of the megafauna to shrink or disappear.

An important contribution to this debate has just been reported in Nature (DOI: 10.1038/nature05471).  A research team led by Gavin Prideaux at the Western Australian Museum have identified a cornucopia of fossils in a cave complex on the vast treeless Nullarbor Plain of southern Australia. 

The Nullarbor Plain from space
The discovery provides a unique glimpse of life in the region between 780,000 and 200,000 years ago. It seems likely that hapless animals became trapped in the caves once they’d fallen through holes in the roofs.  Most probably survived the fall of 20 m but died trying to try to find a way out. 

The result is an Aladdin's Cave of fossils.  Before this work there was only a few skeletal fragments of the extinct marsupial lion, Thylacoleo.  Now there are a dozen complete skeletons.  The chambers are so rich in lion remains they have been dubbed the Thylacoleo caves.  But the lions are just part of the story; a whole host of bird and animal remains have been discovered.   Excitingly, 23 species of kangaroo were found; 8 of these are entirely new to science and in spite of the absence of trees today, 2 species are tree kangaroos.

Climate reconstructions show that conditions in the past weren’t much different from today. In spite of the aridity, the vast region was capable of supporting a huge diversity of fauna.  Nullarbor mean no trees and yet the presence of tree kangaroos shows the vegetation in the past must have been completely different to today. The woodland has gone, replaced by shrubs and grasses. 

Gavin sums it up nicely: Three independent lines of evidence, including stable isotope signatures retrieved from herbivore tooth enamel, tell us that the climate on the Nullarbor Plain 400,000 years ago was as dry as it is now. This demonstrates, for the first time, that a good proportion of the Australian megafauna were adapted to arid conditions well before they disappeared, which undermines the idea that it was aridity that drove them to extinction.

What a tragedy they’re not with us today.

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