Armadillos as big as Volkswagens and other grass-eating mammals became more diverse in South America about 6 million years ago because shifts in atmospheric circulation drove changes in climate and vegetation, scientists report. Geoscientists already knew the Earth was cooling 7 to 5.5 million years ago, a period of time known as the Late Miocene. However, the changes in ocean climate during that time have been better understood than changes in the continental climate. The new research shows that about 7 to 6 million years ago, the global tropical atmospheric circulation known as the Hadley circulation intensified. As a result, the climate of South America became drier, subtropical grasslands expanded and the numbers of mammalian species that were good at eating grasses increased. The researchers used a computer model to figure out that the Hadley circulation had strengthened in the Late Miocene, altering the climate. They then compared the model's predictions of the past climate with the natural archives of rainfall and vegetation stored in ancient soils. The model's predictions agreed with the natural archives. The new research -- an unusual blend of mammalian paleontology, the geochemistry of ancient soils and global climate computer models -- provides a new understanding of the Late Miocene, a time when near-modern ecosystems became established.
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