Some scientists believe that dinosaurs were not 'killed'; their end was natural. The traditional favourite for the death of the dinosaurs has been the slow change in climate. The list of suspects for a slow climate change includes ice ages, collision of continents and the greenhouse effect. Dinosaurs and many other species were unable to adapt to the changes and therefore died. Mammals and some other species adapted and survived.
Another traditional favourite is the drop in sea level. Large sea level changes are caused either by the movements of the Earth's crust or by changers in the ice caps. Scientists agree that the sea level dropped by 100 metres at the end of the Cretaceous, which caused severe environmental changes. The species died out because of these changes, and the asteroid impact in Chicxulub killed off a few stragglers.
Besides these traditional favourites, there is a plethora of exotic theories. Some samples:
The dinosaurs died because their bodies continued to grow bigger while their brains remained small. As the dinosaurs became progressively less intelligent, they lost the ability to adapt and survive in a changing environment. This theory was very popular for many decades because those who proposed it could point to the dinosaurs' tiny heads relative to their body size. Yes, we know dinosaurs' brains were not big enough to solve an algebra equation, but the tyrant lizards were smart enough to rule the planet for more than 140 million years. It certainly beats the humans.
Biologist David Archibald believes that the dinosaurs died because of bad genes. They were probably born losers. 'Survival is a game of luck and skill - some species make it, others don't', he says. 'Extinction may always have been on the cards for dinosaurs.' He argues that there is no record of dinosaur extinction throughout the world. No one can say whether dinosaurs died out overnight around the world, or whether they lived for several million years in some places after disappearing elsewhere.
A novel but serious attempt to explain the demise of the dinosaurs comes from the American astrophysicist Juan Collar. He claims that dinosaurs were wiped out by epidemics of cancer. No, it wasn't caused by smoking. The cancer was triggered by massive bursts of neutrinos released by dying stars.
In the final stages of their death, massive stars radiate most of their energy in the form of neutrinos. These dying stars are not as bright as supernovas, and therefore difficult to find. Collar calls them 'silent' dying stars. He predicts that a silent star death occurs within 20 light years of Earth about once every 100 million years. He suggests that a collapsing star would produce twelve malignant cells per kilogram of tissue, each of which could trigger a tumour. The effect would be more severe in dinosaurs because they had more tissue to become cancerous. He advises cataloguing possible 'neutrino bombs' - sources of neutrinos - in the galaxy to save us from the same fate as that of the dinosaurs.
A new theory is that a gamma-ray burster may have busted the big beasts. When a neutron star is sucked into a black hole it produces massive bursts of energy that are detected millions of light years away as gamma rays. Some of these bursts also reach our skies, but they are very faint and last only for a few moments.
Thousands of gamma-ray bursts have been detected so far. They are all immensely far away - half-way across the universe. If a burst happened 3,000 light years away in our galaxy, the gamma rays striking Earth's upper atmosphere would create a blue patch in the sky glowing about as bright as the Moon. Much worse events would follow, warn the American astronomers Peter Leonard and Jerry Bonnell. The blast of gamma rays would trigger a chemical reaction in Earth's atmosphere which would wipe out the entire ozone layer.
A few days after the gamma-ray burst, Earth would be immersed in a cosmic-ray bath which would last perhaps for a month. 'At this stage', say Leonard and Bonnell, 'Earth turning on its axis could be portrayed as a chicken roasting on a spit'. The catastrophe would kill 'all but the most well-protected or radiation-resistant species'.
The good news is that a dying neutron star in our vicinity can be predicted many million years in advance. To save Earth, Leonard and Bonnell have a plan: use an asteroid as a shield to block the gamma- and cosmic-ray bath.
Cosmic rays continually bombard Earth from all directions. They are particles such as protons and electrons that travel at very high speeds within our galaxy and elsewhere in the universe. Some of the particles have as much energy as a tennis ball moving at 300 kilometres per hour - they travel nearly as fast as light. Scientists divide cosmic rays into two groups: low-energy and high-energy rays. Low-energy rays are produced in supernovas, the giant exploding stars. Scientists are not yet sure of the source of high-energy rays; they think some of them come from neutron stars. Wherever they come from, how could they kill the dinosaurs?
Two theoretical physicists, John Ellis and David Schramm, seem to have the answer. They say that if a supernova occurred within 33 light years of Earth, it would bombard the upper atmosphere with about 100 times the normal amount of cosmic rays. Such a high radiation would totally destroy the ozone layer, and then the ultraviolet radiation would destroy anything on the surface or close to the surface of the sea. The researchers estimate that one supernova occurs within killing range of Earth once every 240 million years. When a supernova explodes, it ejects enormous amounts of matter which contains unusual isotopes of common elements rarely found on our planet. Earth could have swept up some of this material. The researchers say that these tell-tale signatures of a supernova can be found in the rocks formed at the time.
Dinosaurs were blinded by cataracts caused by excessive ultraviolet light. This was the conclusion drawn by biochemist R. Croft in a little book published in the 1980s. He presents a convincing case, but his theory has been challenged on the ground that he had a poor understanding of dinosaur anatomy. But considering the increased ultraviolet light we receive these days because of the thinning ozone layer, Croft's theory does make sense to us.
Chaos, the study of disorder, is an exciting area of science. The weather is the most familiar example of a chaotic - disorderly - system. In a chaotic system, the finest change can bring about a major upheaval. This rule has a flashy title, the 'butterfly effect'. It is possible that an effect as small as a butterfly flapping its wings, say, in Hong Kong can bring about a snowstorm in London.
The American biologist Stuart Kauffman, who has applied chaos theory to mass extinctions, says: 'Mass extinctions, chaos theory suggests, do not require comets or volcanoes to trigger them.' Dinosaurs were part of a chaotic living system, and in such a system superior fitness does not provide a safety net. Their extinction probably occurred for no obvious reason. Explosions and extinctions of various life forms is a pattern that can be found in any chaotic system. 'We are all part of the same pageant', says Kauffman. Homo sapiens is not exempt from a fate like the dinosaurs'. Watch out for that butterfly in the garden. Its flapping wings may start the next ice age.
A magnet wiped out the dinosaurs: the biggest magnet on Earth - Earth itself. Earth has a very strong magnetic field which extends some 60,000 kilometres out in space. This tear-shaped magnetic field shields Earth from deadly radiation such as cosmic rays. You can imagine this magnetic field as a big bar magnet inside Earth. It has north and south poles, and is slowly moving. At present, Earth's geographical north and south poles are not pointing in the same direction as its magnetic poles. There is a difference of about 11 degrees.
When certain rocks are formed, small grains of iron act like tiny compasses and line up in the direction of Earth's magnetic field. When the rocks solidify, these little 'compasses' are locked in. The magnetic field is 'fossilised' in the rocks. The study of these rocks shows that the magnetic field has reversed itself many times in the past. Magnetic reversals do not have a regular cycle. Over the past 170 million years, it has reversed nearly 30 times. The last reversal was about 700,000 years ago, when compass needles would have pointed south. No reversal took place for 35 million years during the Cretaceous.
Everything under the Sun has been accused of killing the tyrant lizards. No wonder 'magnetic reversals', as geologists prefer to call the changes in Earth's magnetic field, also appear on this list. The reason? Experts blame these 'reversals' for the ice ages. Earth's magnetic field is slowly weakening. If it continues to weaken at the same rate, the field will completely vanish in a mere 1,500 years - about when the next ice age is expected.
Hot Weather = Stress = Infertility
In 1978, palaeontologist Dewey McLean suggested that the dinosaurs died because of a slight but critical increase in the global temperature. The effect of the heat was not actually to kill the dinosaurs but effectively to castrate them. Because large animals do not shed excess heat as efficiently as small animals do, a temperature increase of just 2 degrees could have baked the considerable reproductive apparatus of a 10-tonne male dinosaur enough to kill its sperm.
The argument that McLean presented to support his theory went something like this. A large number of completely unhatched dinosaur eggs have been found in rocks, possibly suggesting failure of fertilisation. At the same time, eggs show thinning of the shells. Modern birds under stress also lay thin eggs. Put two and two together and you have a theory: hot weather stressed dinosaurs; stress made them infertile.
Don't blame the heat - it was drugs. Flowering plants, the angiosperms, evolved around the same time as the dinosaurs died. Many of these plants contain poisonous substances. Modern animals avoid them today because of their bitter taste. Ronald Siegel, an American psycho-pharmacologist, has suggested that dinosaurs had neither the taste for the bitterness nor livers effective enough to detoxify the substance. They died of massive overdoses.
The British palaeontologist Anthony Hallam has looked at the emerging angiosperms from a different angle: the dinosaurs died because of constipation caused by eating the flowering plants that replaced ferns, a dinosaur dietary staple containing laxative oils.
These lateral thinking exercises - poison and constipation - have been knocked on the head by those who say that the angiosperms appeared 40 million years before the dinosaurs' death.
More ingenious theories
Self-destruction. The dinosaurs 'self-destructed' themselves: vicious meat-eating dinosaurs ate all the plant-eating dinosaurs.
Overpopulation. Some blame overpopulation for the dinosaurs' demise. Overcrowding made the females stressed, and stress can cause an imbalance in the hormones. Therefore, they laid dangerously thin eggs. Those who propose this theory are certainly walking on eggshells.
Senility. They became senile and forgot to breed and find food. Abnormal and useless features such as the wild neck frills of horned dinosaurs and the bizarre crests of duck-billed dinosaurs prove that the dinosaurs were becoming senile. These features, in fact, enabled dinosaurs to adapt to the changing environment.
Laziness. They starved to death. As their bodies continued to grow bigger, they were no longer able to support their big size with enough food. Fossil evidence contradicts this idea. Dinosaurs were not lazybones. They roamed in herds for hundreds of kilometres for food.
Thieving mammals. Clever little mammals developed a taste for dinosaurs' eggs, raiding their nests to steal them. The dinosaurs could not fight back because the little thieves were too fast for them. Ben Sloan, an American palaeontologist, has a different theory on mammals. He says that some 65 million years ago a receding sea level created a land bridge between North America and the long-isolated Asian continent. Asian mammals invaded North America and began eating the same plants that most dinosaurs ate. 'The mammals ate much less food per animal. But there were so many of them. They ate the last of the dinosaurs out of house and home', he says.
Slipped discs. The dinosaurs suffered from slipped discs, which left them unable to forage for food. A theory without rhyme or reason.
Fussy eaters. They were fussy eaters. 'If they ate mainly one plant, just as the koala bear lives on eucalyptus, they would be in trouble if that plant were no longer available', says James Hopson, an American dinosaur expert.
Itching eyes. 'Itching eyes and dinosaur demise' is the title of a paper written by a distinguished geologist, R.H. Dott Jr. His theory: it was pollen in the air that killed the dinosaurs. A theory not to be sneezed at.
Too much gas! The dinosaurs were wiped out by a serious flatulence problem: the methane expelled by dinosaurs was enough to blast a hole in the ozone layer. The ozone hole in turn damaged vegetation and caused a food shortage which ended the dinosaurs' long reign.
Radiation. In 1984, Moscow News reported that dinosaur fossils showed an unusually high uranium content. The report suggests that they may have been killed by radiation in the lagoons in which they lived.
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