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 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 ammonites4 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 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 innumerable animal species. In support of this idea, it should be said that another geological deposit, the Siberian Traps, contains ten times as much lava as the Deccan system, and it so happens that the Siberian Traps were laid down at the time of another mass extinction, the great Permian extinction, which we mentioned earlier. To pile on further evidence: some geologists believe they have found another meteorite impact site in modern Australia, which in Permian times was opposite to Siberia.
Is there any evidence for this suggestion? In 2000, Dallas Abbott and Ann Isley analysed impact events to see whether they coincided with volcanic ‘superplumes’, where large amounts of liquid rock well up from the Earth’s mantle. A superplume is thought to be responsible for the Hawaiian island chain, for instance, whose relics stretch across half the Pacific Ocean. The likely explanation is that as the continents drifted, the Pacific Plate moved, so that the superplume welled up in what now seem to be different places. However, very recent observations suggest that superplumes can move, too. The Deccan and Siberian Traps were also probably caused by superplumes.
The result of the analysis is that impacts and superplumes occur together far more often than they would if their association is due to chance. That suggests that impacts can make superplumes bigger, or more likely. (It’s hard to see how superplumes could make impacts more likely, except by attracting invading aliens who are heavily into vulcanism.) However, thanks to continental drift, it is not certain that the Deccan Traps were in fact on the exact far side of the Earth from Chicxulub at the time when the K/T meteorite hit. So we can’t yet be sure that the meteorite really did trigger massive volcanic activity.
The moral of this tale is that we should not look for ‘the’ cause of the dinosaur extinction. It is very rare for there to be just one cause of a natural event, unlike scientific experiments which are specially set up to reveal unique explanations.
On Discworld, not only does Deat
h come for humans, scythe in hand, but diminutive sub-Deaths come for other animals – for example the Death of Rats in Soul Music, from whom a single, typical quote will suffice: ‘SQUEAK.’
The Death of Dinosaurs would have been something to see, with volcanoes in one hand and an asteroid in the other, trailing a cloak of ice …
They were wonderfully cinematic reptiles, weren’t they? Trust the wizards to get it wrong.
There is another lesson to be learned from our emphasis on the demise of the dinosaurs. Many other large and/or dramatic reptiles died out at the end of the Cretaceous, notably the plesiosaurs (famous as a possible ‘explanation’ of the mythical Loch Ness monster), the ichthyosaurs (enormous fish-shaped predators, reptilian whales and dolphins), the pterosaurs (strange flying forms, of which the pterodactyls appear in all the dinosaur films and are labelled, wrongly, dinosaurs), and especially the mosasaurs …
Mosasaurs?
What were they? They were as dramatic as the dinosaurs, but they weren’t dinosaurs. They didn’t have as good a PR firm, though, because few non-specialists have heard of them. They are popularly known as fish-lizards – not as good a name as ‘terrible lizard’ – and it describes them well. Some were nearly as fish-like as ichthyosaurs or dolphins, some were rather crocodile-like, some were fifty-foot predators like the great white shark, some were just a couple of feet long and fed on baby ammonites and other common molluscs. They lasted a good twenty million years, and for much of that time they seem to have been the dominant marine predators. Yet most people meet the word in stories about dinosaurs, assume that the mosasaur was a not-very-interesting kind of dinosaur, and promptly forget them.
The other really strange thing about the K/T extinction – probably not a ‘thing’ in any meaningful sense, because in this context a thing would be an equation of unknowns, whereas what we have is a diversity of related puzzles – is which creatures survived it. In the sea, the ammonites all died out, as did the other shelled forms like belemnites – unrolled ammonites – but the nautilus came through, as did the cuttlefish, squids, and octopuses. Amazingly the crocodiles, which to our eyes are about as dinosaur-like as you can get without actually being one, survived the K/T event with little loss of diversity. And those little dinosaurs called ‘birds’ came through pretty well unscathed. (There’s a story here that we need to tell, quickly. Not so long ago, the idea that birds are the living remnants of the dinosaurs was new, controversial, and therefore a hot topic. Then it rapidly turned into the prevailing wisdom. New fossil discoveries, however, have shown conclusively that the major families of modern birds diverged, in an evolutionary sense, long before the K/T event. So they aren’t remnants of the dinosaurs that otherwise died – they got out early by ceasing to be dinosaurs at all.)
Myths, not least Jurassic Park itself, have suggested that dinosaurs are not ‘really’ extinct at all. They survive, or so semi-fact semi-fiction accounts lead us to believe, in Lost World South American valleys, on uninhabited islands, in the depths of Loch Ness, on other planets, or more mystically as DNA preserved inside bloodsucking insects trapped and encased in amber. Alas, almost certainly not. In particular, ‘ancient DNA’ reportedly extracted from insects fossilized in amber comes from modern contaminants, not prehistoric organisms – at least if the amber is more than a hundred thousand years old.
Significantly, no one has made a film bringing back dodos, moas, pygmy elephants, or mosasaurs – only dinosaurs and Hitler are popular for the reawakening myth. Both at the same time would be a good trick.
Dinosaurs are the ultimate icon for an evolutionary fact which we generally ignore, and definitely find uncomfortable to think about: nearly all species that have ever existed are extinct. As soon as we realize that, we are forced to look at conservation of animal species in new ways. Does it really matter that the lesser spotted pogo-bird is down to its last hundred specimens, or that a hundred species of tree-snail on a Pacific island have been eaten out of existence by predators introduced by human activity? Some issues, like the importation of Nile Perch into Lake Victoria in order to improve the game fishing – which has resulted in the loss of many hundreds of fascinating ‘cichlid’ fish species – are regretted even by the people responsible, if only because the new lake ecosystem seems to be much less productive. Everyone (except purveyors of bizarre ancient ‘medicines’, their even more foolish customers, and some unreconstructed barbarians) seems to agree that the loss of magnificent creatures like the great whales, elephants, rhinos and of course plants like ginkgoes and sequoias would be a tragedy. Nevertheless, we persist in reducing the diversity of species in ecosystems all around the planet, losing many species of beetles and bacteria with hardly any regrets.
From the point of view of the majority of humans, there are ‘good’ species, unimportant species, and ‘bad’ species like smallpox and mosquitoes, which we would clearly be better off without. Unless you take an extreme view on the ‘rights’ of all living creatures to a continued existence, you find yourself having to pass judgment about which species should be conserved. And if you do take such an extreme view, you’ve got a real problem trying to preserve the rights of cheetahs and those of their prey, such as gazelles. On the other hand, if you take the task of passing judgment seriously, you can’t just assume that, say, mosquitoes are bad and should be eliminated. Ecosystems are dynamic, and the loss of a species in one place may cause unexpected trouble elsewhere. You have to examine the unintended consequences of your methods as well as the intended ones. When worldwide efforts were made to eradicate mosquitoes, with the aim of getting rid of malaria, the preferred route was mass sprayings of the insecticide DDT. For a time this appeared to be working, but the result in the medium term was to destroy all manner of beneficial insects and other creatures, and to produce resistant strains of mosquitoes which if anything were worse than their predecessors. DDT is now banned worldwide – which unfortunately doesn’t stop some people continuing to use it.
In the past, the environment was a context for us – we evolved to suit it. Now we’ve become a context for the environment – we change it to suit us. We need to learn how to do that, but going back to some imaginary golden age in which primitive humans allegedly lived in harmony with nature isn’t the answer. It may not be politically correct to say so, but most primitive humans did as much environmental damage as their puny technology would allow. When humans came to the Americas from Siberia, by way of Alaska, they slaughtered their way right down to the tip of South America in a few thousand years, wiping out dozens of species – giant tree sloths and mastodons (ancient elephants, like mammoths but different), for example.
Some scientists have disagreed with this interpretation, claiming that humans could never have made that great an impact. In 2001, to test that claim, John Alroy simulated the effect of hunting in computer models of 41 North American species. It turned out that extinctions were virtually unavoidable, especially for the larger animals. Even highly incompetent hunters would have wiped them out. The simulations correctly ‘postdicted’ the fate of 32 of the 41 species, which adds credibility to their conclusions. ‘Whodunnit?’ New Scientist asked, offering its own suggestion: ‘Mr Sapiens in the Americas with a large axe.’
There have been many similar examples of ecological devastation wrought by ‘primitive’ humans. The Anasazi Indians in the southern part of today’s USA cut down forests to build their cliff dwellings, creating some of the most arid areas of the United States. The Maoris killed off the moas. Modern humans may be even more destructive, but there are more of us and technology can amplify our actions. Nevertheless, by the time humans were able to articulate the term ‘natural environment’, there wasn’t one. We had changed the face of continents, in ways big and small.
To live in harmony with nature, we must know how to sing the same song as nature. To do that, we must understand nature. Good intentions aren’t enough. Science might be – if we use it wisely.
1 Readers of the Discworld b
ook The Last Continent will recall that, by an amazing coincidence, beetles were something of a passion for the God of Evolution.
2 Rincewind would add some more:
‘Is it safe?’
‘Are you sure?’
‘Are you absolutely sure?’
3 A worse case is what used to be called Eohippus, the Dawn Horse – a beautiful, poetic name for the animal that formed the main stem of the horse’s family tree. It is now called Hyracotherium, because somewhat earlier somebody had given that name to a creature that they thought was a relative of the hyrax, represented by a single fossil shoulder-blade. Then it turned out that the bone was actually part of an Eohippus. Unfortunately, whoever officially names a species first must get priority, so now the Dawn Horse has a silly, unpoetic name that commemorates a mistake.
We lost ‘Brontosaurus’ – thunder-lizard – for a similar reason. Thunder Lizard … what a marvellous name. ‘Apatosaurus’? It probably means ‘Gravitationally challenged Lizard’.
The moral of this tale is that when learned committees of elderly scientists meet to discuss an exceptional issue they can always be trusted to make a completely ridiculous decision. Quite unlike the wizards of Unseen University, naturally.
4 Lots of ammonite species died out 5–10 million years before the K/T boundary, so it looks as if their extinction genuinely was gradual. But whatever it was that happened at the K/T boundary finished them off.
THIRTY-NINE
BACKSLIDERS
GLOOM HAD SETTLED over the wizards. Some of them had even refused a third helping at dinner.
The Science of Discworld Revised Edition Page 33