Before the Dinosaurs' Demise, a Clambake Extinction?

摘要

Most researchers think the dinosaurs, many plants on land, and much of the life in the sea succumbed to a huge cosmic impact 65.5 million years ago. But new evidence from the sea floor just off Antarctica points to a major extinction there a geologic moment before the impact. The culprit in this earlier cataclysm may well have been humongous volcanic eruptions in India-the same eruptions that some researchers have credited with wiping out the dinosaurs. Nailing down what killed the dinosaurs and the 65% of other species that disappeared with them has been a matter of timing. The impact left a layer of distinctive debris around the world, a perfect marker for paleontologists trying to connect the impact to extinctions recorded in the same sediments. In the fossil record of ocean sediments, microscopic plants and animals disappeared in the same geologic instant that the impact debris was laid down, clinching the case that the impact caused the ocean extinction. On land, abundant plant pollen and leaves built a similarly strong case. As to the land-dwelling dinosaurs and mammals, their less-abundant fossil record made their case less airtight, but most researchers still consider it proven beyond a reasonable doubt.