Large impacts would seem to be bad for dinosaurs. After all, a huge asteroid or comet ended the giant reptiles' reign when it hit Earth 65 million years ago. But now a group of researchers suggests that an impact also triggered the final rise of dinosaurs to dominance 200 million years ago.
In the early 1990s, palynologist Sarah Fowell at the University of Alaska, Fairbanks, and geologist Paul Olsen of the Lamont-Doherty Earth Observatory in Palisades, New York, found a rock layer rich in the spores of ferns--plants that rush in when the landscape is devastated--in southeastern Pennsylvania. The fern fossils appear in rocks formed at the Triassic-Jurassic (T-J) boundary of 200 million years ago, and Fowell and Olsen argued they could be evidence of an impact. But their colleagues were skeptical.
In a paper in the 17 May issue of Science, Olsen, Fowell, and colleagues show that at three sites in Pennsylvania, the same rock layer that contains the fern spores has high levels of iridium--an element rare on Earth but abundant in asteroids. "There was something interesting going on 200 million years ago," says Olsen. What was happening to the dinosaurs during the iridium-dusted fern spike? To find out, Olsen and his colleagues--especially amateur paleontologists Michael Szajna and Brian Hartline of Reading, Pennsylvania--collected footprints left in the mud of the string of lakes that ran through the middle of what was then the supercontinent Pangea. Lumping together more than 10,000 tracks found in former lake basins from Virginia to Nova Scotia, they found that "the nondinosaurs were getting wiped out" across the boundary, says Olsen; dinosaurs jumped from 20% to more than 50% of taxa in less than 20,000 years. At the same time, meat-eating dinosaurs ballooned to twice their previous mass, to judge by the size of their tracks.
Paleontologists like what Olsen and his colleagues did with their huge footprint database. "They've definitely pinned [the evolutionary transition] right on the boundary," says paleontologist Michael Benton of the University of Bristol, U.K. Impact specialists are less impressed. The iridium levels aren't high enough above background levels to be convincing, says cosmochemist David Kring of the University of Arizona in Tucson. However, he allows that the discovery of other, iridiumlike elements could prove that the iridium is extraterrestrial. Then the dinosaurs could feel ambivalent about visitors from outer space.