Science of Discworld

No. Until Jurassic Park, there were precisely three, and the times when those particular animals lived have a spread of five million years. Three more fossilized T. rexes have been found since, because Jurassic Park gave dinosaurs a lot of favourable publicity, making it possible to drum up enough money to go out and find some more. With that rate of success, the chance of a future race finding any fossil humanoids, over the whole period of our and our ancestors' existence, would be negligible. So if some species had survived on Earth for a five million year period, it is entirely likely that no fos­sils of it will have been found, especially if it lived on dry land, where fossils seldom form. This may suggest that the fossil record isn't much use, but quite the contrary applies. Every fossil that we find is proof positive that the corresponding species did actually exist; moreover, we can get a pretty accurate impression of the grand flow of Life from an incomplete sample. One lizard fossil is enough to establish the presence of lizards, even if we've found only one species out of the ten thousand that were around.

Bearing 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 ail. 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 undrarnatic species instead. When they do, they find that ichthyosaurs also died out about then, as did the last of the ammonites 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-wi

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 die-backs 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 series of climatic changes at the end of the Cretaceous, with very drastic changes of sea level as ice caps grew or melted. There is also good evidence that some seas, perhaps all, lost their oxygen-based ecology to become vast, stinking, black, anaerobic sinks. The fossil evidence for this consists of black iron-and sulphur-rich lines in sediments. The most dramatic terrestrial events were undoubtedly associated with the vulcanism which resulted in the so-called Deccan Traps, huge geological deposits of lava. The whole of Asia seems to have been covered with volcanoes, and they produced enough lava that it would have formed a layer 50 yards (45 m) thick if it had been spread over the whole continent. Such extensive vulcanism would have had enormous effects on the atmosphere; carbon dioxide emissions that warmed the atmosphere by the greenhouse effect, sulphur compounds resulting in terrible acid rain and freshwater pollution over the entire planet, and tiny rock particles blocking sunlight and causing 'nuclear winters' for decades at a time. Could the volcanoes that formed the Deccan Traps have killed the dinosaurs, instead of a meteorite? Much depends on the timing.

Our preferred theory, not because there is good independent evidence for it but because it would explain so much, and because it has a moral, is that the two causes are linked. The Chicxulub crater is very nearly opposite the Deccan Traps, on the other side of the planet. Perhaps volcanic activity in Asia began some millions of years before the K/T boundary, causing occasional ecological crises for the larger animals but nothing really final Then the meteorite hit, causing shockwaves which passed right through the Earth and converged, focused as if by a lens on just that fragile region of the planet's crust. (A similar effect happened on Mercury, where a gigantic impact crater called the Caloris Basin is directly opposite 'weird terrain' caused by focused shockwaves.)

There would then have been a gigantic, synchronized burst of vulcanism, on top of all the events of the collision, which would have been pretty bad on their own. The combination could have polished off i