A huge asteroid smashing into Earth may have let dinosaurs take over the Earth 135 million years before another one wiped them out, a controversial new study suggests. Dinosaurs flourished in the wake of a mass extinction 200 million years ago. But no one is sure what made so many plants and animals disappear. "The simplest scenario is that a very large asteroid struck our planet," says Paul Olsen of Columbia University in New York. Olsen's international research team have new evidence that an earlier impact could be to blame. Levels of the metal iridium - which rarely occurs naturally on the Earth's surface and mostly arrives on extraterrestrial objects - shoot up in rocks from the time when many species died out. Simultaneously, spores of ferns, the first plants to colonize devastated areas, also rise dramatically1. Similar evidence from sites worldwide supports the final extinction of dinosaurs by an impact 65 million years ago. Olsen has investigated only one site, the Newark rift basin in eastern North America. Until more evidence is accumulated, some palaeontologists remain cautious. "I tend not to believe it yet," says palaeontologist Mike Benton of the University of Bristol. If people find iridium and spores in other places around the world, "it will become more convincing", he says. Before and after Around 200 million years ago, large plant-eating dinosaurs grazed the Earth alongside primitive meat-eaters as large as ostriches. After the mysterious event that marked the Triassic-Jurassic boundary, herbivores dwindled and large carnivores flourished. The precursors to Tyrannosaurus rex were born. Any explanation for the boundary has to explain why some animals lived when so many others perished. Olsen thinks that only the hardiest creatures would have survived the extreme conditions following an asteroid strike. Dust clouds masking the Sun would have plunged the Earth into cold gloom, followed by intense warming as clouds of greenhouse gases built up. Warm-blooded dinosaurs that could withstand the cold or those that scavenged many food types would have fared best, Olsen suggests. Big feet An alternative explanation is that massive eruptions of volcanic lava, whose ancient remnants have been found, could have caused the mass extinction. For example, gases pumped out with the lava could have cooled the Earth and gradual climate change killed off certain species. Olsen admits he cannot rule this out. But his team does offer additional evidence that the mass extinction at the Triassic-Jurassic boundary was sudden and extreme. This is more consistent with the impact hypothesis. Olsen and his colleagues sifted through sedimentary rocks in several sites in the Newark basin for dinosaur and reptile footprints to find out which animals wandered around when. Previous timing for the extinction was based on sparse fossil finds from around the world. In lakes that were rapidly filling with sediment, a thick layer of rock was laid down in a short period of time. This means that more precise times can be pinned on the disappearance and reappearance of animals. Rocks are dated on the basis of the types of pollen they contain. Half of the animal footprints vanish within as little as 50,000 years, the researchers found, while dinosaur footprints suddenly appear that are up to a fifth larger than before. This time period "is instantaneous on the geological timescale", says geologist Walter Alvarez of the University of California at Berkeley. Alvarez agrees that the weight of evidence now points to an impact as the likely cause of this extinction - and possibly others too. The rise and demise of dinosaurs are only two of five massive extinctions in world history. "It becomes more likely that they could be impacts too," says Alvarez. References Olsen, P. E. et al. Ascent of dinosaurs linked to iridium anomaly at the Triassic-Jurassic boundary. Science, 296, 1305 - 1307, (2002). © Nature News Service / Macmillan Magazines Ltd 2002
