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 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 kid 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.
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 Death 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 tens of thousands of years, wiping out dozens of species, giant tree sloths and mastodons (ancient elephants, like mammoths but different), for example. 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.
35. BACKSLIDERS
GLOOM HAD SETTLED OVER THE WIZARDS. Some of them had even refused a third helping at dinner. 'It's not as if they were very advanced,' said the Dean, in an attempt to cheer everyone up. 'They weren't even using metal. And their writing was frankly nothing but pictograms.'
'Why doesn't that sort of this thing happen here?' said the Senior Wrangler, merely toying with his trifle.
'Well, there have been historical examples of mass extinction,' said Ponder.
'Yes, but only as a result of argumentative wizardry. That's quite different. You don't expect rocks to drop out of the sky.'
'You don't expect them to stay up?' said Ridcully. 'In a proper universe, the turtle snaps up most of them and the elephants get the rest. Protects the world. Y'know, it seems to me that the most sensible thing any intelligent lifeform could do on that little world would be to get off it.'
'Nowhere to go,' said Ponder.
'Nonsense! There's a big moon. And there's other balls floating around this star.'
'All too hot, too cold, or completely without atmosphere,' said Ponder.
'People would just have to make their own entertainment. Anyway ... there's plenty of other suns, isn't there?'
'All far too far away. It would take ... well, lifetimes to get there.'
'Yes, but being extinct takes forever.'
Ponder sighed. 'You'd set out not even knowing if there's a world you could live on, sir,' he said.
'Yes, but you'd be leavin' one that you'd know you couldn't,' said Ridcully calmly. 'Not for any length of time, anyway.'
'There are new lifeforms turning up, sir. I went and checked before dinner.'
'Tell that to the lizards,' sighed the Senior Wrangler. 'Any of the new ones any good?' said Ridcully. 'They're ... more fluffy, sir.' 'Doin' anything interesting?'
'Eating leaves, mainly,' said Ponder. 'There are some much more realistic trees now.'
'Billions of years of history and we've got a better tree,' sighed the Senior Wrangler.
'No, no, that's got to be a step in the right direction,' said Ridcully, thoughtfully.
'Oh? How so?'
'You can make paper out of trees.'
The wizards stared into the omniscope.
'Oh, how nice,' said the Lecturer in Recent Runes. 'Ice again. It's a long time since we've had a really big freeze.'
'Well, look at the universe,' said the Dean. 'It's mainly freezing cold with small patches of boiling hot. The planet's only doing what it knows.'
'You know, we're certainly learning a lot from this project,' said Ridcully. 'But it's mainly that we should be grateful we're living on a proper world.'
A few million years passed, as they do.
The Dean was on the beach and almost in tears. The other wizards appeared nearby and wandered over to see what the fuss was about.
Rincewind was waist deep in water, apparently struggling with a medium-sized dog.
'That's right,' the Dean shouted. 'Turn it round! Use a stick if you have to!'
'What the thunder is going on here?' said Ricully.
'Look at them!' said the Dean, beside himself with rage. 'Backsliders! Caught them trying to return to the ocean!'
Ridcully glanced at one of the creatures, which was lying in the shallows and chewing on a crab.
'Didn't catch them soon enough, did you,' he said. 'They've got webbed paws.'
'There's been too much of this sort of thing lately!' snapped the Dean. He waved his finger at one of the creatures, who watched it carefully in case it turned out to be a fish.
'What would your ancestors say, my friend, if they saw you rushing into the water just because times are a bit tough on land?' he said.
'Er ... '"Welcome back"?' suggested Rincewind, trying to avoid the snapping jaws.
"Long time no sea'?' said the Senior Wrangler, cheerfully.
The creature begged, uncertainly.
'Oh, go on, if you must,' said the Dean. 'Fish, fish, fish ... you'll turn into a fish one of these days!'
'Y'know, going back to the sea might not be a bad idea,' said Ridcully, as they strolled away along the beach. 'Beaches are edges. You always get interestin' stuff on the edge. Look at those lizards we saw on the islands. Their world was all edges.'
'Yes, but giving up the land to just go swimming around in the water? I don't call that evolution.'
'But if you go on land where you have to grow a decent brain and some cunning and a bit of muscle in order to get anything done, and then you go back to see the sea where the fish have never had to think about anything very much, you could really, er, kick butt.'
'Do fish have...?'
'All right, all right. I meant, in a manner of speaking. It was just a thought, anyway.' Uncharacteristically, the Archchancellor frowned.
'Back to the sea,' he said. 'Well, you can't blame them.'
36. MAMMALS ON THE MAKE
AFTER THE DINOSAURS CAME THE MAMM
ALS -Not exactly.
Mammals constitute the most obvious class of animal alive on Earth today. When we say 'animal' in ordinary conversation, we're mostly referring to mammals, cats, dogs, elephants, cows, mice, rabbits, whatever. There are about 4,000 species of mammals, and they are astonishingly diverse in shape, size, and behaviour. The largest mammal, the blue whale, lives in the ocean and looks like a fish but isn't; it can weigh 150 tons (136,000 kg). The smallest mammals, various species of shrew, live in holes in the ground and weigh about an ounce (30 g). Roughly in the middle come humans which, paradoxically, have specialized in being generalists. We are the most intelligent of the mammals, sometimes.
The main distinguishing feature of mammals is that when they are young their mother feeds them on milk, produced by special glands. Other features that (nearly) all mammals have in common include their ears, specifically the three tiny bones in the middle ear known as the anvil, stirrup, and hammer, which send sound to the eardrum; hair (except on adult whales); and the diaphragm, which separates the heart and lungs from the rest of the internal organs. Virtually all mammals bear live young: the exceptions are the duckbilled platypus and the echidna, which lay eggs. Another curious feature is that mammalian red blood cells lack a nucleus, whereas the red cells of all other vertebrates possess a nucleus. All is this is evidence for a lengthy common evolutionary history, subject to a few unusual events of which the most significant was the early separation of Australia from the rest of Gondwanaland. Modern studies of mammalian DNA confirm that basically we are all one big happy family.
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