for National Geographic News
Small changes in Earth's orbit and tilt may have regulated the cyclical rise and fall of many prehistoric mammal species, new research suggests.
Earth's orbital patterns are believed to drive long-term climate change.
Over millions of years these climatic shifts may have regularly spawned events that give rise to new mammal species.
They may have also caused the periodic extinctions that doomed other mammal lineages to oblivion, says a team of researchers led by paleontologist Jan van Dam of Utrecht University in the Netherlands.
"The question of climate's role in causing both evolution and extinction has been a big area of contention," said Tony Barnosky, a paleobiologist at the University of California, Berkeley.
Barnosky is not affiliated with van Dam's new research, which will appear in tomorrow's edition of the journal Nature.
The work is the first to document direct correlations between changes in Earth's orbit and patterns of speciation—the rise of new species—and extinction.
"I think they've nicely demonstrated a correlation between periodic climate events and what we'd recognize as normal turnover in fauna that would happen over and over again," Barnosky said.
Life and Death
Van Dam and colleagues combed through a fossil record from central Spain spanning some 22 million years—from 24.5 to 2.5 million years ago (map of Spain).
They studied about 80,000 fossils, predominately molars and premolars, to reveal when 132 different species of small rodents first arose—and when the animals disappeared.
The fossils, found scattered across more than 200 sites, appear to reveal a striking symmetry between the rise and fall of small mammal species and two cyclical "wobbles" in Earth's orbit. (Related: "Did Million-Year-Long Eruptions Cause Mass Extinction?" [May 2, 2006].)
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