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emerged.
    By the time of the latest Cretaceous, the period preserved in the Hell Creek rocks, Antarctica, Australia, and South America had separated from the unified landmass and were not connected to other continents. But Africa was still attached to Europe and northern land bridges connected North America, Europe, and Asia.
    The sauropods were gone, and among the dominant land animals was Tyrannosaurus rex . Duck-billed dinosaurs abounded, as did Triceratops and other dinosaurs like the Pachycephalosaurus . The late Cretaceous was a time of mountain building in North America. The rising mountains that we now call the Rockies were being eroded as fast as they grew and were drained by rivers and streams that dumped sediment on the plains during floods, burying and preserving dinosaur bones.
    The rivers and streams in what is now Montana flowed into the inland sea, which expanded and contracted over millions of years. By the time of the latest Cretaceous, when the Hell Creek sediments were deposited, the sea had retreated and the section of the formation in Garfield County, from which paleontologists have drawn so many fossils, was a river delta, with winding channels, and both land and water habitats, near the coast of the inland sea.
    The vegetation was thick and varied, including ferns, conifers, and flowering trees. These are known both from fossils and the microscopic analysis of pollen grains in the rock. Herds of dinosaurs fed on the lush plant life and were preyed on by packs of smaller, hunting dinosaurs like Troodon. Lizardlike predatory mosasaurs and long-necked plesiosaurs swam in the inland sea. Mollusks were present in the seas, as well as ponds and rivers, which played host to fish, amphibians, crocodilians, and turtles; all of these survived and continued to thrive when the dinosaurs disappeared. There were land, shore, and diving birds and numerous mammals, primitive relatives of today’s egg-laying platypus and echidna, marsupials, and placental mammals. Some of these would have run up and down the trees like squirrels. Others would have lived on the ground, perhaps in burrows.
    One day, while life hummed, chewed, killed, and died, as usual in this tropical environment, a meteor ten to fifteen kilometers in diameter—six to nine miles—entered the earth’s atmosphere, headed for the sea near the Yucatán Peninsula at about seventy thousand miles per hour (thirty-two thousand meters per second). It was traveling at a thirty-degree angle when it struck. The meteorite vaporized, as did its target. One estimate is that twelve thousand cubic miles of debris were sent into the atmosphere. And the energy released has been estimated at one hundred million megatons. The blast at Hiroshima is estimated at fifteen kilotons. In other words, the explosive energy released when the meteorite hit the earth was the equivalent of 6.6 billion atomic bombs like the one dropped on Hiroshima exploding simultaneously.
    The result was a worldwide disaster, although it is hard to pin down the exact effects. The impact coincided with a mass extinction that wiped out 35 percent of the species on earth, including all of the nonavian dinosaurs. For a brief time the foundation of the world ocean ecology, the community of microorganisms that harvest the energy in sunlight, was devastated.
    The evidence for this event was first published in 1980. Walter Alvarez, a geologist, had been in Italy studying the rate of accumulation of cosmic dust in geological strata as a way of dating them independent of fossils or other methods, when he found that right at the K/T (Cretaceous-Tertiary) boundary, there was much more iridium than in any of the other strata. He was near the town of Gubbio, and in the rocks he was studying, the K/T boundary was marked by a layer of clay. Below it were fossils of microorganisms of the Cretaceous. And above it were fossils of different microorganisms, from the Tertiary.
    This boundary is visible in other