Armadillos as big as Volkswagens and other grass-eating mammals became more diverse in South America about 6 million years ago, report researchers.
Why? Because shifts in atmospheric circulation drove changes in climate and vegetation.
Geoscientists already knew the Earth was cooling 7 to 5.5 million years ago, a period of time known as the late Miocene.
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 mammal species that were good at eating grasses increased.
Lead author Barbara Carrapa, professor and head of the University of Arizona department of geosciences, and colleagues 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.
“We found a strong correlation between this big change in late Miocene climate and circulation that affected the ecology—the plants and animals,” she says. “It has implications for ecosystem evolution.”
Carrapa says the new research offers a new understanding of the late Miocene, a time when near-modern ecosystems became established.
Ancient soils
Geoscientists use the geochemistry of ancient soils, specifically forms of the elements oxygen and carbon, to infer past precipitation and vegetation. Researchers had thought the precipitation at the time the soil formed was mostly a function of the site’s topography and elevation.
Carrapa wanted to test that idea by looking at the geochemistry of ancient soils on a continental scale. She teamed up with paleontologist Mark Clementz of the University of Wyoming.
They compiled the published data of the oxygen-18/oxygen-16 ratio and carbon-13/carbon-12 ratio from ancient soils covering a wide swath of South America—from 15 degrees South latitude to 35 degrees South latitude, or about the change from La Paz, Bolivia, to Buenos Aires, Argentina. Changes in the oxygen ratio provide information on past precipitation, while changes in carbon ratio indicate what plants were growing at the time.
Clementz scoured the published studies. The results were surprising, Carrapa says. The changes in soil geochemistry during the late Miocene changed in latitudinal bands from north to south, indicating an underlying cause spanning much of South America, not just local changes in elevation or topography.
Hadley circulation
The two researchers thought the systematic shifts in soil geochemistry were related to changes in climate, so they asked Ran Feng of the University of Connecticut to help them by applying the global climate model she used for research.
“Looking at geological pasts is like looking at different planets.”
Feng loaded known information about the Miocene-to-late-Miocene climate, including atmospheric carbon dioxide concentrations and the ocean temperatures, into the computer model and then asked it to simulate three different versions of late Miocene climate—not much cooler, cooler, and much cooler than before. In each case, the simulation indicated what soil geochemistry would have occurred under that climate regime.
The team found the geochemistry of South American ancient soils predicted by the model matches the geochemistry of the actual soil samples.
Feng figured out that the Earth’s Hadley circulation intensified from 7 to 6 million years ago. “The records compiled by Barbara and Mark could be explained by a significant change in the strength of the Hadley circulation,” she says.
Feng’s work with the global climate model shows how the past climate could have created the patterns the team was seeing in the soil geochemistry, Clementz says.
Ever-growing teeth
The carbon ratio from the ancient soils reflects the vegetation of the time and indicates that in the late Miocene, grasslands were expanding as the climate was changing.
“During the late Miocene, things are starting to dry out, particularly in the 25-30 degree South zone,” he says. “There’s also an increase in the numbers of animals with high-crowned or ever-growing teeth.”
Grasses contain silica, an abrasive substance, which is why grass-eaters have either high-crowned teeth or teeth that continue to grow. The mammals that became more prevalent in the late Miocene included giant armadillos and rhinoceros-like animals and also smaller mammals, he says.
Carrapa says, “Looking at geological pasts is like looking at different planets. The state of the Earth we see today is very different from the Earth of 10 million years ago, 6 million years ago—it’s a different planet. You have the possibility of looking at a different planet through the lens of time, and with the geological record we can do that.”
The paper appears in the Proceedings of the National Academy of Sciences. The National Science Foundation funded the research.
Source: University of Arizona
