Fossil by proxy

Kelps dominate the reefs of cool seas. They represent most of the biomass in these richly productive ecosystems, yet relatively little is known about their origins. One theory holds that kelps became widespread in the Northern Hemisphere sometime between 5 and 10 million years ago, when northern oceans cooled off and became rich enough in nutrients for kelps to flourish. But evidence supporting the theory is hard to come by, because the fossil record is bereft of kelps--their soft tissue simply does not mineralize well.

So one must approach the question sideways. James A. Estes, a marine biologist at the University of California, Santa Cruz, and two colleagues noted a pattern among species of abalone that grow to more than six inches long. Throughout the world, those larger species live only in cold seas that have plenty of kelp, their preferred food. Unlike kelps, though, abalones leave fossils behind. Examining the fossil record, Estes and his coworkers found that small species of abalone have been around for more than 60 million years, but large ones only S million years. That sharpens the time estimate for the kelps' population surge in northern seas.

The work also offers insight into another question: How could large abalones evolve in kelp forests that also harbor otters? Otters dine on abalones and prefer the big ones. Estes and his colleagues think abalones survive in protective crevices on the rocky seafloor; meanwhile, otters prey on other kelp eaters, such as sea urchins. The reduction in predators enables the kelp to grow so lush that the dead bits raining from their rubbery fronds provide the sheltered abalones with food aplenty. (Paleobiology 31:591-606, 2005)

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