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updated 7/7/2011 4:50:42 PM ET 2011-07-07T20:50:42

The rise of modern mammals, which came with a dramatic warming of the Earth, may be traced to hot magma, new research suggests.

The Eocene epoch, which saw the emergence of the first ancestors of modern mammals such as hoofed animals, began almost 56 million years ago with global temperatures rising 9 degrees Fahrenheit (5 degrees Celsius) on average over a few thousand years.

The oldest known members of these species were relatively small, weighing less than 22 pounds (10 kilograms) — hinting the heat favored smaller creatures. Larger animals, with their greater body mass, would not be able to handle the excess heat so well. [ Gallery: The World's Biggest Beasts ]

Earth's wobbles

But what caused this ancient warming event, known as the Paleocene-Eocene Thermal Maximum?

Some scientists have linked the thermal maximum to a peak of an approximately 400,000-year cycle of variations in Earth's orbit that would have maximized the solar energy the planet received.

"This could have resulted in the warming of deep ocean waters, which would have destabilized large quantities of frozen methane known as methane hydrates stored on the marine continental shelves and slopes," researcher Adam Charles, a paleooceanographer at the University of Southampton in England. "Once released, the methane would diffuse through the seawater and subsequently be transformed to carbon dioxide when reaching the atmosphere."

However, other studies hinted the warming event was caused by activity within the Earth, with magma baking carbon-rich rocks. This would similarly release carbon-rich greenhouse gases into the atmosphere, via volcanic vents.

The ash test

To resolve the debate, Charles and other scientists focused on pinning down when the Paleocene-Eocene Thermal Maximum started.

So a group of scientists ventured into the Arctic and tested ancient volcanic ash.

"The group went into the field to analyze the Paleocene-Eocene Thermal Maximum sediments armed with rifles in case of polar bear attacks," Charles told LiveScience.

They employed one of the most accurate dating techniques available, known as uranium-lead radioactive isotope dating. This method relies on finding zircon encased in ancient volcanic ash, which they discovered in exposed mountainside rock and a tube of sediment 1,800 feet long (550 meters) drilled from Spitsbergen, the largest and westernmost island of the Arctic Svalbard archipelago.

Their analysis put the date of this major warming event at between 55,728,000 and 55,964,000 years ago. When the scientists compared the age of this episode with models of the Earth's orbital cycles, "we found the onset of the event occurred on a downward swing in the 400,000-year cycle," Charles said. "This suggests that this event was not triggered by changes in the Earth's orbital cycles, but by geological processes."

So the researchers conclude that hot magma was more likely to have led to the spike in global temperatures. This, in turn, may have led to the rise of today's mammals.

Records of how the Arctic has changed over time "are especially important as the region is changing rapidly today," Charles added. By studying how the Arctic behaved in the past, we can "examine the implications for the future of the region as it warms."

The scientists detailed their findings June 7 in the journal Geochemistry Geophysics Geosystems.

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