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Shell fossils tell life came out of the tropics

This outcrop shows major shell concentrations of Miocene age (approximately 20 million years old), exposed in the Calvert Cliffs along the western shore of Chesapeake Bay in Maryland. (Image courtesy of Susan M. Kidwell, University of Chicago)

Researchers studied 11 million years worth of shell fossils and learned that the tropics are where new types of life called species begin and old species continue to live.

"If you came from outer space and you started randomly observing life on Earth, at least before people were here, the first thing you would see was this incredible amount of life at the tropics," said Dr. David Jablonski, one of the three researchers on the project.

The tropics are the area just north and south of the equator and are home to many exotic types of life. Why are the tropics so rich in different types of life? Biologists have asked this question for many years.

For the past 30 years, biologists debated whether the tropics are a cradle of diversity where species begin, or a museum of diversity where old species remain.

To find the answer, three researchers spent about a 10 years organizing 11 million years worth of fossils of bivalves. Bivalves are marine life including clams, mussels and oysters.

"For example, we took a whole group of clams and organized them," Jablonski said. "Then we made maps of the clams to show where they first occurred and then what happened to them. We tracked if they stayed in the same place, if they moved or if they died."

Researchers found that the clams and other bivalves started growing in the tropics and stayed there. But they also spread north and south over time without leaving the tropics. This is like a family that has lived in Florida for a long time. Some may move to New York and others stay in Florida but they are still the same family.

"Our study shows that the tropics are both a cradle and a museum of diversity," Jablonski said.

This collection of marine mollusks from Panama provide a glimpse into the very rich communities found on tropical sea floors. (Image courtesy of Susan M. Kidwell, University of Chicago)

The scientists published this research in the October 6 issue of Science magazine. David Jablonski teaches at the University of Chicago. Kaustuv Roy works at the University of California, Santa Diego. James W. Valentine is at the University of California, Berkeley.

Spotlight on the Scientist

Dr. David Jablonski is a paleontologist teaching and performing research at the University of Chicago. He grew up in New York City near the Natural History Museum and visited it often to see the fossils, including the dinosaurs. "I knew by the time I was five that I wanted to be a paleontologist," he said.

When he finished college he decided he wanted to study clams and snails instead of dinosaurs. "Dinosaurs are rare and expensive to collect and repair," he said. "I can collect hundreds of clams and snails with a shovel. Each one is a puzzle or a mystery."

Dr. Jablonski remembers that he was always "interested in how the variety of life originates and becomes extinct. I wanted to understand how evolution happens over time and distance," he said.

This paleontologist travels the world studying fossils, but he spends most of his time teaching college and graduate students.