Megafauna Extinctions in Ancient Australia

In an effort to discover the connections in Australia’s past between climate change, the vanishing of the large Ice Age animals (megafauna) there, and the arrival of humans, Gifford Miller turned to fossil eggshells he found in dune deposits along the ancient shoreline of a now vanished lake. The eggshells had been left by emus, ostrich- like flightless birds that still walk Australia’s savannas and woodlands, and by Genyornis newtoni , an extinct bird species whose massive bones suggest each individual would have weighed about 550 pounds. Australia’s acid soils and severe climate quickly leach all organic matter out of bone. But because eggshells have a different mineral structure than bone, the ancient shells retained traces of protein. This made it possible to date them using a technique called amino acid racemization. Results from a large set of samples- -1,200 dates collected from three different sites- -showed that the emu and Genyornis had coexisted for millennia. Then, about 45,000 50,000 years ago, Genyornis vanished.

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The eggshells also offered clues to the big birds’ diets. Grasses that thrive in hot habitats use a unique chemical pathway to capture the Sun’s energy, a process that distorts the amount of the stable isotope carbon-13 they contain relative to most other plants. Miller and his colleagues compared the carbon isotope signatures of fossil emu and Genyornis eggshells. The results show that the two bird species relied on different food sources. Grass must have been abundant right before the extinction event, because the emu of that era ate little else. The doomed Genyornis ate both shrubs and grass.

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Two large, flightless birds living in the same habitat would have to develop different survival strategies, and different food sources, or constantly clash with each other. The emu is now, as it was during the Pleistocene (1.8 million years ago-11,700 years ago), a flexible eater. If necessary, it ate only grass, but it could also make do with shrubs and herbs. Genyornis included shrubs in its menu but seems to have been unable to get by without grass. About 45,000 years ago, the vegetation changed throughout the Lake Eyre basin. Emus shifted their tactics and began eating lots of shrubs. Genyornis could not adapt, and the species died out. More recently, Miller has analyzed the carbon isotope signatures of teeth fossils of the wombat (a pouched mammal). Like the emu, the wombat survived the ecological shift that came 45,000 years ago because it fed on shrubs, making do without grass.

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According to Miller, the arrival of people, carrying fire sticks in their hands, changed everything. The harsh landscape of interior Australia does not hold well-preserved deposits of ancient pollen that might precisely track the changing vegetation. Based on the shifting diets of the herbivores he has studied, however, Miller can envision how and why the balance tipped.“Before people came, there must have been a savanna mixed with open woodland, the kind of habitat where grasses are abundant in years of good rainfall and the trees and shrubs sustain the animals when water is scarce,” he explains. Frequent burning tends to favor grasses over shrubs and trees, yet at the moment of human arrival, emus and wombats stopped eating grass, and Genyornis simply vanished.

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Miller argues that burning did promote one kind of grass growth, but that grass was spinifex, an unpleasant tasting species that now dominates the region. Spinifex leaves are heavily loaded with silica and are nutrient-poor. No native mammal eats it, and introduced cattle will take only the freshest, youngest shoots that sprout up after a fire. And spinifex loves fire: it grows in hummocks (low mounds) where live and dead blades intermix, laden with flammable resin, and it resprouts quickly after a burn. Australian plants had evolved with fire for millions of years. Before people came, the bush burned regularly at the end of the dry season, the time of many lightning strikes. But humans could light fires at any time of the year. In Miller’s scenario, the increased frequency of fire wiped out many plants. Many ecosystems in the interior, he says, are adapted to burn every 20 to 50 years, the kind of frequency that occurs if lightning is the only ignition source. Torched frequently by Aboriginal people, such habitats would not have enough time to recover between burns.

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?In an effort to discover the connections in Australia’s past between climate change, the vanishing of the large Ice Age animals (megafauna) there, and the arrival of humans, Gifford Miller turned to fossil eggshells he found in dune deposits along the ancient shoreline of a now vanished lake.?The eggshells had been left by emus, ostrich- like flightless birds that still walk Australia’s savannas and woodlands, and by Genyornis newtoni , an extinct bird species whose massive bones suggest each individual would have weighed about 550 pounds.?Australia’s acid soils and severe climate quickly leach all organic matter out of bone. But because eggshells have a different mineral structure than bone, the ancient shells retained traces of protein. This made it possible to date them using a technique called amino acid racemization. Results from a large set of samples- -1,200 dates collected from three different sites- -showed that the emu and Genyornis had coexisted for millennia. Then, about 45,000 50,000 years ago, Genyornis vanished.

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