February 18, 2010 | 4
The gargantuan plankton-eating whales of today—such as the humpback whale and the blue whale–have long been thought to be the evolutionary masters of their filter-feeding technique. New research, published online February 18 in two papers in Science, however, shows that millions of years before these massive mammals evolved, huge fish fruitfully employed the same approach—and that it wasn’t necessarily these cetaceans’ skill, but rather changing climate and new food source diversity, that allowed them to eventually dominate their domain.
One group of prehistoric big fish known as the pachycormids was known to have used filter feeding (in which water is sucked into the mouth and small organisms such as plankton are trapped inside when the water is filtered back out through baleen). But fossils had been scarce, leading researchers to figure that this "poorly understood clade" was a brief adaptational assay.
But by reexamining fossils housed—and largely forgotten—in several museum collections, a team of researchers found that the pachycormids actually roamed the prehistoric seas for more than 100 million years. This long history of this clade, which includes the largest known bony fish, the nine-meter Leedsichthys, "alters the picture of the evolution of this ecological guild in the Mesozoic and afterward," the researchers, led by Matt Friedman of the Department of Earth Sciences at the University of Oxford in the U.K., wrote. And the new findings imply that rather than being an unsuccessful, empty niche, the filter-feeding function was dominated by these large fish from about 170 million to 65 million years ago, "suggesting that familiar modern groups of planktivores diversified into the ecospace vacated by giant pachycormids."
The other group of researchers, led by Felix Marx, of the Department of Geology at the University of Otago in New Zealand, studied the changes in food sources to better understand just how and when the oceans could begin to support the ancestors of the huge filter feeders we know today. "Our study offers an entirely biological explanation of cetacean diversity," the researchers noted in their paper. They concluded that early whale diversity was likely moderated by climate-related ocean oxygen levels and, in turn, the diversity of diatoms—a major staple in suspension-feeding diets.
By gathering the latest evidence about the environments just before modern leviathans emerged, scientists aim to gain a clearer picture about the evolutionary forces at work. "The two papers change our view of the natural history of these evolutionary distant organisms, which share similar trophic resources," Lionel Calvin, of the Department of Geology and Paleontology at the Natural History Museum in Geneva, Switzerland, wrote in a commentary that accompanied the two papers. The two different study topics also demonstrate that "phylogenic reconstructions can be the starting point for investigating major events in the history of life," he asserted.
Image of filter-feeding pachycormid Bonnerichthys, which lived some 70 million years ago, courtesy of Robert Nicholls