The Science of Discworld Revised Edition

this in mind, though, we can easily see that even if the death of the dinosaurs was extremely sudden, then the fossil record might easily give a different impression. Suppose that fossils of a given species turn up randomly about every five million years. Sometimes they’re like buses, and three come along at once – that is, within a million years of each other. Other times, they’re also like buses: you wait all day (ten million years) and don’t see any at all. During the ten million year run-up to the K/T boundary, you find random fossils. For some species, the last one you find is from 75 million years ago; for others it’s from 70 million years ago. For a few, by chance, it’s from 65 million years ago. So you
seem
to see a gradual fade-out.
    Unfortunately, you’d see much the same if there really
had
been a gradual fade-out. How can you tell the difference? You should look at species whose fossils are far more common. If the demise was a sudden one, those ought to show a sharper cut-off. Species that live wholly or partially in water get fossilized more often, so the best way to time the K/T mass extinction is to look at fossils of marine species. Wise scientists therefore mostly ignore the dinosaur drama and fiddle around with tiny snails and other undramatic species instead. When they do, they find that ichthyosaurs also died out about then, as did the last of the ammonites 4 and many other marine groups. So something sudden and dramatic really did happen at the actual boundary, but there may well have been a succession of other events just before it too.
    What kind of drama? An important clue comes from deposits of iridium, a rare metal in the Earth’s crust. Iridium is distinctly more common in some meteorites, particularly those from the asteroid belt between Mars and Jupiter. So if you find an unusually rich deposit of iridium on Earth, then it may well have come from an impacting meteorite.
    In 1979 the Nobel-winning physicist Luis Alvarez was musing along such lines, and he and his geologist son Walter Alvarez discovered a layer of clay that contains a hundred times as much iridium as normal. It was laid down right at the K/T boundary, and it can be found over the whole of the Earth’s land mass. The Alvarezes interpreted this discovery as a strong hint that a meteorite impact caused the K/T extinction. The total amount of iridium in the layer is estimated to be around 200,000 tons (tonnes), which is about the amount you’d expect to find in a meteorite 6 miles (10 km) across. If a meteorite that size were to hit the Earth, travelling at a typical 10 miles per second (16 kps), it would leave an impact crater 40 miles (65 km) in diameter. The blast would have been equivalent to thousands of hydrogen bombs, it would have thrown enormous quantities of dust into the atmosphere, blanking out sunlight for years, and if it happened to hit the ocean – a better than 50/50 chance – it would cause huge tidal waves and a short-lived burst of superheated steam. Plants would die, large plant-eating dinosaurs would run out of food and die too, carnivorous dinosaurs would quickly follow. Insects would on the whole fare a little better, as would insect-eaters.
    Much evidence has accumulated that the Chicxulub crater, a buried rock formation in the Yucatan region of southern Mexico, is the remnant of this impact. Crystals of ‘shocked’ quartz were spread far and wide from the impact site: the biggest ones are found near the crater, and smaller ones are found half way round the world. In 1998 a piece of the actual meteorite, a tenth of an inch (2.5 mm) across, was found in the north Pacific Ocean by Frank Kyte. The fragment looks like part of an asteroid – ruling out a possible alternative, a comet, which might also create a similar crater. According to A. Shukolyukov and G.W. Lugmair, the proportions of chromium isotopes in K/T sediment confirm that view. And Andrew Smith and Charlotte Jeffery have found that mass diebacks of sea urchins which occurred at the K/T boundary were worst in the regions around central America, where we think the meteorite came down.
    Although the evidence for an impact is strong, and has grown considerably over the twenty years since the Alvarezes first advanced their meteorite-strike theory, a strongly dissenting group of palaeontologists has looked to terrestrial events, not dramatic astronomical interference, to explain the K/T extinction. There was certainly a rapid