Mass Extinctions Face Downsizing

Risen. This Cenomanian sea urchin disappeared from the fossil record for 30 million years.

A bunch of sea urchins have turned up in the Cretaceous, millions of years after they were believed to have gone extinct. Their reappearance, reported in the current issue of Paleobiology, casts doubt on the existence of a mass extinction and by implication that of several others. "This is going to shake up the paleo world for a while," says paleontologist Lisa Park of the University of Akron in Ohio.

Scattered among the five major crises in the history of life--such as the Cretaceous-Tertiary (K-T) extinction 65 million years ago that marked the end of the dinosaurs--are a half dozen lesser extinction events, including the Cenomanian-Turonian (C-T) mass extinction 94 million years ago. By a simple count, the C-T extinction event claimed 71% of existing echinoderm genera, which include the sea urchins.

A group of paleontologists led by Andrew Smith of The Natural History Museum in London wondered just how reliable the fossil record of the C-T is and thus how real the C-T extinctions were. Smith and his colleagues made a detailed study of sediments and echinoderm fossils in samples from England, France, and Germany.

Almost half of the apparently extinct genera reappeared in the record as much as 30 million years after the Turonian, they found. These "Lazarus taxa" obviously had not disappeared from the world, only from the rock preserved in Western Europe around the C-T boundary. And other more recent genera appear to be descendants of supposedly extinct genera. By Smith's count, the C-T extinction of echinoderms shrinks to an unremarkable 17%. The problem, Smith says, is that falling sea levels and erosion after the C-T decreased the number of shallow-water taxa preserved in the fossil record, creating the appearance of an extinction event.

The study is "a cautionary note for the lesser extinction events," says paleontologist Anthony Hallam of the University of Birmingham, United Kingdom. And because most of the Big Five mass extinctions also occurred at times of rising sea level, the magnitude--though not the existence--of even the largest mass extinctions will now come under scrutiny.