How are the Mighty Fallen? 2

Contents Updated: Monday, September 13, 1999

• Excess Oxygen
• High Temperature
• Radiation from Space

Excess Oxygen

Richard Owen was the first to attempt a convincing explanation of the extinctions. He reasoned that dinosaurs (then considered to be cold-blooded) had thrived at a time when the earth’s atmosphere was less rich in oxygen than it is now. The lack of oxygen disadvantaged reptiles less than it did mammals, therefore the reptiles dominated. When the air became richer in oxygen, mammals took advantage of their warm-bloodedness and higher mobility and saw off the dinosaurs.

The theory could be persuasive but for the evidence. The oxygen content of the atmosphere was not lower in Cretaceous times than today. Indeed analysis of air bubbles trapped in amber, the resin exuded by coniferous trees, indicates rather a higher oxygen level. These tiny air bubbles, which can give amber a cloudy appearance, are trapped when the resin hardens effectively becoming fossilized atmosphere. Air bubbles in Cretaceous amber apparently contain 30 per cent oxygen, nine per cent more than today, an incredibly high figure—the biosphere would have been unstable in respect of atmospheric oxidation.

In our lower levels of atmospheric oxygen the lodgepole pine uses the instability of organic matter in the air as a weapon against competing species. The wood of this pine is so dense that it does not burn easily. The tree also has seeds which are able to survive fierce conflagrations. Yet the seasonal litter it produces, its pine needles and broken branches, are readily inflammable. Once enough litter has accumulated, it is liable to ignite and start a forest fire which wipes out competitors, leaving more space for the lodgepole pines and their seeds which survive the blaze.

Our atmosphere is rather delicately balanced. An increase in atmospheric oxygen of only four per cent, let alone nine, could be disastrous for us. At 21 per cent of oxygen a moisture content of only one sixth in organic waste prevents it from burning. But the probability of a spontaneous bush fire doubles for each 1.3 per cent rise in oxygen level above 21 per cent. Four per cent more oxygen increases the chance of a bush fire eight times: nine per cent more increases the chance of a bush fire 128 times. At 25 per cent of oxygen, plant tissue containing as much as a third water would burn easily. Even damp forests like the rain forests could ignite.

Methane from bacteria prevents such a horror by removing each year 2000 million tons of oxygen from the air. Without this reaction the oxygen level in the air would rise by one per cent every 12,000 years and the earth would be incinerated after only about 50,000 years. It is unlikely that creatures in the Cretaceous could have coped with such conditions. The oxygen level in the amber must have been enhanced by some chemical or diffusion process not yet known.

Some theories are comical. Did you hear the one about the clever little mammals which surreptitiously robbed the nests of the dinosaurs who were too stupid to notice their disappearing eggs? Eventually no dinosaur reached maturity and they died out. This theory has several problems. First, we cannot be sure all dinosaurs laid eggs. We know that icthyosaurs did not. Nor can we believe the idea that the dinosaurs were stupid. Nor does it make ecological sense for a species to cause the extinction of another which is its source of food: if the hunted decline then so do the hunters—a balance is maintained. Finally and conclusively, this idea takes no account of the simultaneous deaths of other quite distinct genera including all those marine species.

Another joke is that flowering plants, the angiosperms, murdered the dinosaurs. The newly evolved flowering plants devised chemical weapons—like the alkaloids—that the stupid dinosaurs could not cope with. The dinosaurs could not understand such clever plants and continued to eat them causing their own extinction. Evidently the poisons evolved by the plants were very slow acting—dinosaurs proliferated for fifty million years after the emergence of the flowering plants, actually benefiting from them as a nutritious new source of food. Only then did they quickly decline. Nor does this idea explain the simultaneous extinctions in many unrelated species.

High Temperature

R.B.Cowles and others believe that the dinosaurs became sterile because a rise in temperature affected their testes. Sperm is sensitive to temperature, this being the reason why a mammal’s scrotum sac hangs outside his body. Foklore has it that a man who wears underpants is less virile than one who does not. Possibly sterility rather than virility is meant, and so it might have been for the dinosaurs. A marked increase in temperature could have caused their extinction through infertility.

Alternatively a higher temperature could have altered the sex ratios of their offspring. Incubation temperature affects the sex of present day reptiles and conceivably the dinosaurs’ hatchlings could have emerged all the same sex. Similarly a loss of fertility caused by the rise in temperature at the end of the last ice age might have eclipsed the mammoths, mastodons and sabre toothed tigers.

But is there any evidence for a sustained rise in temperature at the end of the Cretaceous? Scientists on the oceanographic exploration vessel, The Glomar Challenger, examined rock cores taken from the ocean floor and indeed did discover a global warming 65 million years ago. They believed the reason for the warming was volcanoes spewing forth the greenhouse gas, carbon dioxide.

The greenhouse effect is the warming of the surface of the earth by gases trapping radiation from the sun. Carbon dioxide is transparent to the incident radiation from the sun but not to the longer wavelength heat radiation re-emitted by the earth. Carbon dioxide in the atmosphere traps the sun’s energy and the earth’s surface gets warmer. An increase of the gases in the atmosphere like carbon dioxide that are good trappers of heat radiation heats up the biosphere.

D.McLean of the Virginia Polytechnic Institute has proposed that heating by the greenhouse effect is a likely cause of the extinction of the dinosaurs and would also have affected many other species. Measurements of the proportions of carbon-13 and oxygen-18 in rocks at the Cretaceous-Tertiary boundary indicate a brief sharp cooling followed by a longer warm period perhaps caused by a greenhouse effect.

Radiation from Space

Robert Uffen, Chairman of the Defence Research Board of Canada, reasoned that a weakening of the earth’s magnetism would lead to extinctions. The earth’s magnetic field deflects charged cosmic particles that otherwise would penetrate to the surface with seriously harmful effects. Periodically the earth’s magnetic field undergoes a reversal which is quite sudden in geological terms but might actually take many, probably thousands, of years. The old field falls to zero then grows from zero in the reversed direction. On completion what was the magnetic north has become the magnetic south and vice versa. Note that only the magnetic field, not the earth itself, has reversed direction. A compass would point south if the present field direction reversed. The danger is not in the reversal itself, the direction of the field is unimportant, it is in the changeover period when the field strength is too low to prevent cosmic particles from hitting earth’s lifeforms. The weakness in the theory is that the intensity of the cosmic bombardment can only double and though that might affect life on land it is unlikely to affect many of the species protected by water.

M.L.Keith, a professor at Pennsylvania State University, believes a depletion of the ozone layer in the upper atmosphere caused the K-T extinctions. His mechanism was the emission of excessive amounts of hydrogen chloride by volcanoes. The importance of the ozone layer is that it absorbs much of the incoming ultraviolet light. This high frequency light is healthy in small doses (being responsible for your holiday tan) but when more intense is very harmful, killing cells and causing skin cancers. Bare-skinned dinosaurs, like pigs and humans today, would have been vulnerable but furry and feathered creatures would have had protection, as would animals and plants dwelling in water below the surface layer. But we have seen that some dinosaurs had feathers and pterosaurs had fur. Other dinosaurs had scales. Feathers and scales would shield the soft skin beneath from the harmful rays.

A nearby exploding star, a supernova, would bathe the solar system in huge levels of radiation that would quickly put paid to many of the earth’s inhabitants. Jacques Bergier, who with Louis Pauwels had most of Von Daniken’s ideas about visiting gods a decade earlier, has even gone so far as to suggest that superbeings in space deliberately exploded a star to kill off the mammals’ competitors, the dinosaurs. They thus broke an evolutionary bottleneck clearing the way for a surge in evolution leading to intelligent beings like themselves. Unfortunately the pattern of extinctions does not match this idea. Photosynthetic algae like the coccoliths are very resistant to direct radiation, largely because they are unsophisticated organisms more difficult to damage than the higher organisms. Yet they were more severely affected than land based reptiles and dinosaurs. Then again marine animals would have been expected to survive the influx of cosmic rays better than land animals because water is an effective shield against radiation. Yet the extinction of marine organisms was more severe than that of land organisms.

Although cosmic radiation theories cannot match the pattern of extinctions, some authorities argue that the exclusion of visible radiation (ordinary light upon which all life depends through the action of photosynthesis by plants) for a year or more could match it. Absence of light would cool the earth and interrupt photosynthesis for long enough to destroy the larger animals at the top of the food chain. The most serious effects would occur in the oceans where the whole ecosystem depends upon photosynthetic plankton. Stopping photosynthesis in plankton for a year would undoubtedly have dire consequences with a clear likelihood of extinctions. Terrestrial animals that depend upon foraging for fruit or nuts or decaying matter could survive the darkness. Similarly plants which leave seeds around that are viable for long periods would be able to re-establish themselves when conditions improved again. The same is true of some of the plankton which can enter a dormant phase to wait for better times. Whatever caused the global darkening would have to have been severe enough to cut down the light considerably because plankton can function efficiently, indeed more efficiently, at light levels of only a few per cent of normal daylight. Even if an ice age were triggered the plankton would be quite happy—maybe more so!

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