(KT), 65 myr. Lesser extinctions include the late Cambrian, 510 myr to 520 myr; the Frasnian-Famennian at 365 myr; the Triassic-Jurassic, 210 myr; the late Cenomanian at 91 myr; and the late Eocene, 34 myr.

It is not clear how many have been due to asteroid or comet impacts. The Permian-Triassic event, for example, was the most severe. It wiped out 90 percent or more of all species, including half the families of marine invertebrates. However, very little is known about this mass extinction.

At the Cretaceous-Tertiary boundary (KTB), by contrast, extinctions were somewhat less severe. Yet there is abundant evidence for a bolide impact: iridium, shocked quartz, spherules, and tektites. In addition, the extinctions evidently were quite sudden. Planktonic organisms, notably coccoliths and Foraminifera, flourished right up to the boundary before being cut off. The paleontologist Peter Sheehan also finds no falloff in the abundance and diversity of dinosaurs during the last 3 million years of the Cretaceous. At the KTB, though, the dinosaurs also go extinct.

Jan Smit of the Free University of Amsterdam, along with Walter Alvarez and his colleagues Alessandro Montanari and Nicola Swinburne of the University of California at Berkeley, have explored the candidate impact site: a 300-kilometer-diameter crater centered near the town of Chicxulub in northern Yucatan. Alvarez and co-workers propose that the impact took place on land but produced a kilometer-high tsunami in the adjacent Gulf of Mexico, as ejecta fell into the water. They find evidence for a backwash from this immense wave, in a thick deposit at the KTB boundary that contains abundant plant remains. Smit describes these as driftwood from coastal swamps, swept into the sea. They also find ripple marks indicative of surface waves—at depths below 400 meters. This suggests that the great wave sloshed back and forth within the Gulf. Alvarez describes these findings as consistent with the impact of an object with a diameter of 10 kilometers, traveling at tens of kilometers per second. The bolide would have struck with an energy of 108 megatons, some 10,000 times greater than that in the world's nuclear arsenal.

The astronomer Piet Hut, of the Institute for Advanced Study, proposes that the impactor was probably a comet, not an asteroid. He notes that there indeed are two craters that appear to have the right age, 65 myr: Chicxulub in Yucatan and Manson in Iowa. He describes such an impact sequence as resulting from the breakup of the comet as it rounded the sun, producing closely spaced fragments resembling a swarm of buckshot.

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