To judge by the haul of fossil clams, snails, and sea urchins that paleontologists have retrieved from ancient sediments around the world, marine life has gotten more and more varied during the past quarter-billion years. But some worried that their less-than-systematic search may have been misleading. Now a group of paleontologists largely lays those worries to rest.
The potential problem with the fossil record has been called the "Pull of the Recent." Species from the Recent--the last 10,000 years--can inflate the trend in diversity by improving the completeness of the younger fossil record far more than that of the older record (Science, 25 May 2001, p. 1481).
Simply cleaning up the existing fossil record, reports a team led by paleontologist David Jablonski of the University of Chicago, eliminates reservations about the reality of rising diversity, at least since the death of the dinosaurs. The researchers first corrected some mismatches between the way fossils have been classified over the decades and the way living animals are classified today. Taking class Bivalvia with its oysters, clams, and mussels, they found that many genera present today that had seemed to be missing from the fossil record of the past 5 million years were actually there all along. Paleontologists and biologists had simply put many species in two different genera. Other species had been lumped into broadly defined genera by paleontologists; Jablonski and colleagues moved them into more modern, narrowly defined genera.
When the researchers calculated the diversity trend from the updated record, they found that the Pull of the Recent had been affecting just 5% of taxa. Because the Pull of the Recent operates on so few taxa, they say, it can't be biasing diversity estimates much at all.
“I'm convinced this demonstrates there has been a major diversity increase in the” past 65 million years, says paleontologist Richard Bambach of Harvard University. Now paleontologists can focus on what might have increased diversity lately and on fixing the more numerous problems of the older fossil record.
Neogene Marine Biota of Tropical America project