February 6, 2009

Professor traces dinosaur extinction through clam fossil analysis

The mass extinction that ended the reign of the dinosaurs millions of years ago can be traced through the relative ages of modern species, according to the latest research at the University of Chicago and the University of California–Berkeley.

Geophysical sciences postdoc Andrew Krug was hired by the University’s David Jablonski to study why the variety of species in a tropical climate is so much higher than that in a colder area. UC–Berkeley’s Jim Valentine, who studies evolutionary records as well, was visiting the lab when Krug decided to plot the relative ages of various species of marine bivalves, such as clams and oysters. The three scientists immediately saw an important correlation on the graph.

“It was one of those great ‘Aha!’ moments,” Jablonski said.

The graph showed a kink, or spot, where the number of species at a certain age dropped dramatically, roughly 65 million years ago, when the Cretaceous period ended. Scientists hypothesize that at that time a large asteroid hit the earth, throwing dust clouds into the air that blocked light from the sun. Plants began to die, starting a chain reaction that ended much of the life on Earth. Ultimately, the fossil record shows that roughly 60 percent of the living species that were around during the Cretaceous period died out.

The species that lived, however, went on to evolve at an enhanced rate. Jablonski compared it to the post–World War II baby boom that occurred in the U.S.

“What we did was sort of as if you had gone into your town and took a census of all the people living there,” he said. If that town were like the species on Earth, there would be lots of young people, fewer middle-aged people, and surprisingly, more older people than would be expected. That spike in numbers represents the baby boomer generation, although for the bivalves, the “evolutionary rate never went back to the pre-extinction rate,” Jablonski said. Essentially, the baby boom continued expanding, causing the overall population rate to rise.

The data they collected also revealed that the age of the local species increases farther from the equator, suggesting that “small, specialized species can get a start in tropical climates.”

Krug, along with Jablonski and Valentine, published a paper in Science that will appear February 6 detailing their methods and the mathematics they used to analyze their data. They worked with bivalves, which according to Krug are an exceptional group due to their extensive fossil record, but hope that further research will apply their method to other plants and animals.

“I bet you would find similar patterns,” Jablonski said.

The researchers plan to continue this line of study, looking for new answers to questions such as why the boom never stopped expanding.