What really happened to the dinosaurs

The debate on the death of dinosaurs has two sides. On one side are the 'gradualists' who point out that the fossil record shows a steady decline in species diversity starting several hundred thousand years before the end of the Cretaceous. This decline happened because of several environmental changes. The gradualists do not deny an asteroid impact, but they say it only 'killed off a few stragglers'.

'Catastrophists', on the other side, believe that the dinosaurs and 75 per cent of other species were wiped out in the space of few months or a few years. But the catastrophists do not agree on a single cause: some say it was an asteroid; others say it was a volcano; some even believe in 'double whammy' scenarios.

Changes in climate and sea level have occurred throughout Earth's history. These changes take much longer to occur than the extinctions at the end of the Cretaceous. It is possible that these changes played a part in changing the environment, which affected the populations of dinosaurs. Many dinosaur species had been declining before they all finally disappeared: the number of dinosaur types dropped 70 per cent between 73 million and 65 million years ago. It suggests a slow extinction. Extinction is a natural phenomenon, and all species eventually become extinct. Dinosaurs had a good innings. They just ran out of steam.

The supporters of the impact theory would shout: 'No, they were clean bowled by a fireball from the sky.' The American palaeontologist David Jablonski claims that there is widespread agreement in his field that an asteroid or comet did indeed strike Earth 65 million years ago, and generated the huge Chicxulub crater in Mexico. The British palaeontologist Norman MacLeod disagrees: 'Whatever wiped out the dinosaurs was a lot more complicated than a single hammer blow from an asteroid.'

Other exotic theories fail to account for the 75 per cent of other species that also vanished from the face of the planet with the dinosaurs. They all fail the test of a good theory - that it explains as many events as possible. Walter Alvarez says that all the suspects listed under natural causes (from cataracts to mammals eating dinosaurs' eggs) have an airtight alibi: they could not have killed all the different organisms that died with the dinosaurs.

The idea of Nemesis, the Sun's so-called companion star, or Planet X causing mass extinctions every 26 million years is an interesting one. The American palaeontologist Dewey McLean's verdict on Nemesis and Planet X theories is a bit harsh: 'It's science gone absolutely bonkers.' Anyway, most scientists have now rejected the theory.

Walter Alvarez says that murder suspects must typically have means, motive and opportunity. An impact certainly had the means, and the evidence that the impact occurred at exactly the right time points to the opportunity. The asteroid impact theory provides, if not motive, then at least a mechanism behind the crime, he says.

The proof of the pudding is in the eating: a theory can be judged good or bad only when other scientists can test it. The impact theory predicted that all K-T boundary rocks should have whopping amounts of iridium. This prediction was testable, and scientists have found the predicted amounts of iridium in many K-T boundary rocks around the world.

But the cornerstone of the asteroid theory - the large amounts of iridium in certain rocks - has also been challenged. Now it can be proved without any reasonable doubt that the source of iridium can be on Earth (volcanoes) or in outer space (asteroids, meteorites or comets). This leads to 'double whammy' scenarios: an asteroid as well as a volcano. There are some who even go for a 'multiple whammy' scenario: an asteroid, a volcano and a change in climate.

Gerta Keller, a palaeontologist at Princeton University, favours such a scenario. She arrived at this conclusion after studying microfossils at the Chicxulub crater and other sites for more than a decade. Her studies showed that the asteroid struck about 300,000 years before the dinosaurs became extinct, and that the crater was smaller than originally believed. By the time of the impact, there were already many signs of stress in organisms: species were already endangered, their populations having declined and become dwarfed. Instead of an instant 'wipeout', Keller says, this and other mass extinctions can be tied to an intensive period of volcanic activity and resulting greenhouse effects, and probably a series of many asteroid hits. However, she agrees that her theory may not be as riveting as a massive space object hitting Earth. 'Dinosaurs are very popular, and the asteroid theory is sexy, it's a perfect story, and in the past few years it's all you've read in the popular press', she adds.

Everyone agrees that there was a nasty end. Was it sudden or slow? There is no simple answer. Scientists are using the same fossil record, but why are they coming up with different conclusions? The problem is that the fossil record is patchy. Especially when it comes to dinosaurs, the number of known fossils is very small. The American palaeontologist Douglas Erwin has the right advice for his fellow scientists: 'They have to spend more time studying the corpses.'

The debate on the question of the death of the dinosaurs shows no sign of ending soon. Now to another fireball and another debate.

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