Earth's wobbly orbit blamed for mammal extinctions
IAN SAMPLE
SCIENTISTS HAVE named a prime suspect in the mysterious case of the missing mammals.
The reason history is littered with suspiciously regular extinctions is all down to the Earth's wobbly orbit, according to research published recently.
Records reveal that mammal species die out, usually 2.5 million years after they first emerge.
The answer
Archaeologists have struggled to explain it, with some blaming competition with neighbouring creatures and others suspecting dramatic swings in climate.
After examining the remains of 80,000 fossilised teeth from 132 different rodents that span a 22 million-year period, Jan van Dam at Utrecht University in the Netherlands believes he has the answer.
He conducted detailed analyses of the fossil fragmentsto work out when each species emerged and when they became extinct.
Focus on rodents
The study, which focused on rodents because they are easily identified from their dental remains, included fossils of squirrels, mice, beavers, voles and hamster-like rodents, dating to between 24.5 million and 2.5 million years ago and only a few of which survive today.
Dr. van Dam correlated the results with natural cycles in the Earth's orbit and found that every 2.4 million years there was a flurry of both mammal extinctions and new species.
The extinctions coincided with a cycle that sees the Earth's orbit vary from almost perfectly circular to elliptical.
Another cycle of extinctions and emerging species overlapped with changes in the tilt of the Earth's axis, which occur every one million years.
Writing in the journal Nature, Dr. van Dam says that when the Earth is in a very circular orbit the climate is less changeable, and summer heat will be less extreme.
The more mild weather encourages glaciers to grow down from the poles, and causes a drop in ocean levels and changes to rainfall.
Ice expansion
"The ice expansion affects the global climate via atmospheric and oceanic currents," Dr. van Dam said.
The climatic upheaval put pressure on mammals by wiping out food sources and fragmenting their habitats.
For some species, the climate change is enough to wipe them out, while others, by being forced to live in smaller, isolated communities, are likely to become new species.
- © Guardian Newspapers Limited 2006
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