Abstract

Contents Updated: Wednesday, December 15, 1999

"Learn to see in another’s calamity the ills which you should avoid."

How are the Mighty Fallen?

The extinction of the dinosaurs is the “greatest of all titillating puzzles” according to writer, John Noble Wilford.

Something unusual certainly happened at the end of the Cretaceous Period. Many genera of all kinds died off. Any terrestrial creature weighing more than 50 pounds as an adult became extinct; sea organisms of all sizes were devastated, including many minute sea creatures like the foraminifera; many of the sea bottom filter feeders such as bivalves disappeared; only about 30 per cent of sponges remained.

But not all living things were equally affected:

• marine genera were badly affected but not freshwater ones
• tropical plants suffered more than those in more northerly climes
• pterosaurs died but not birds
• most mammals survived but few marsupials
• squid survived but their relatives the ammonites died
• dinosaurs died but not crocodiles
• mammals, which were all small, and small lizards mostly survived but even the smallest dinosaurs died out.

Though the Cretaceous mass extinction was bad, the earlier Permo-Triassic mass extinction, when 70 per cent of all known types of animal became extinct, was worse.

The Permian extinction is explained by the theory of plate tectonics. Several continents that had been slowly drifting separately around the globe crunched together creating the supercontinent of Pangaea. Most marine organisms live on the continental shelves but, where the continent buckled together, these were thrust up into mountain ranges leaving their inhabitants stranded. The mass of the new supercontinent also suppressed the welling up of magma at the mid-oceanic plate boundaries. Lacking the buoyancy of the underlying magma, the mid-ocean ridges then settled down under their own weight, increasing the depth of the oceans. Sea level fell as much as 200 feet, draining much of the remaining continental shelf and disposing of many more marine species. On land, species diversity was reduced because previously isolated species now met and only the fittest creatures were to survive the furious competition between them.

There is no such explanation for the extinctions at the end of the Cretaceous. By then the supercontinent of Pangaea had began to break up again and the continents were drifting apart slowly moving to their present positions. Alternative theories put forward to explain the Cretaceous extinctions often ignore the variety of species extinguished and cannot be credible. In our search for the truth behind the tragedy we shall examine some of these hypotheses, credible or not.

Table: Genera surviving the Late Cretaceous extinctions.

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.

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.

Another group of theories blame the extinctions on an increase in radiation from space, whether from a weakening of the earth’s magnetic shielding, by depletion of ozone in the upper air, from a nearby supernova or from an anomalous flair up on the sun.

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!

S.Gartner and co-workers in 1978 and 1979 sought the explanation of the extinctions in a sudden cooling. Their mechanism was that continental movements cut off the Arctic Ocean which effectively became a big inland sea or estuary with brackish or even fresh waters. Eventually the land barrier broke and the cold trapped water flowed into the more southerly oceans. The merely brackish water, though cold, would float on the warmer but saltier denser water of the oceans killing off lots of surface dwellers. Regrettably there is no sign that the Arctic Ocean was ever a freshwater lake. A sudden cooling could be the answer but a more convincing mechanism is needed.

Could volcanoes have triggered the cooling? Could they even have triggered an ice age? Louis Agassiz, the Swiss-American naturalist who studied under Cuvier, first discovered evidence of ice ages, or more explicitly The Ice Age (the most recent was originally thought to have been the only one) from his studies of the Jura mountains. Needless to say, orthodox geologists rejected Agassiz’s idea for 25 years, preferring the Biblical flood as the explanation of the scratched rocks, erratics, and glacial tills he noted in the mountains. “The earth covered in ice… Absurd!” Now we know it was not absurd—volcanic eruptions can indeed cool the earth.

The year 1826 was long remembered as “the year without a summer”. In the previous year an enormous volcanic eruption had occurred in Java. The volcano, Tambora, blew up in a cataclysm bigger even than the more often quoted case of Krakatoa in 1883. According to H. and E. Strommel writing in the Scientific American, the eruption reduced the height of the volcano by 4200 feet, and blew out 25 cubic miles of matter, much of it into the atmosphere. In the vicinity of the eruption, the miasma blocked out the sun so thoroughly that the blackness was “palpable”. The explosion of Mount St—Helena in the North West USA bears witness to such a palpable blackness. An extended period of continuous vulcanism on this scale could severely change the climate, triggering global cooling.

But could it have destroyed the viability of the most successful animals the world had ever known? It is unlikely to have caused an ice age at the end of the Cretaceous. The earth’s climate was not unstable then as it is now—it was not teetering on the edge of an ice age as it has been for the last few million years.

Plate tectonics cause ice ages. Through plate tectonics lighter rocks forming the continents drift on the heavier basaltic rocks below. From time to time a piece of a continent drifts over one of the poles and snow falls on its cool highlands. As time passes ice builds up until a vast ice sheet has been created. At the continental margins the glaciers calf into the sea as icebergs which drift into warmer waters, warming and melting as they progress. Melting abstracts heat from the surface water which is therefore cooled ultimately to 0 degrees Celsius. Salt water at 0 degrees Celsius is heavier than slightly warmer water, unlike pure water which has its maximum density at 4 degrees Celsius. The icy salt water therefore sinks to the depths, warmer water rises and the temperature of the whole ocean falls. On the shores of the polar continent the ocean freezes. When salt solutions freeze the ice which appears, apart from occluded salt (that trapped in tiny bubbles in the ice crystals), is pure, leaving a solution which is even saltier than before. The water gets denser, is obviously at 0 degrees Celsius and again sinks to the ocean’s depths. Once the icy water is at the bottom of the ocean the sun cannot heat it—it only heats the surface. Thus the oceans continue to fall in temperature as long as a continental mass remains near one of the poles.

Emiliani has shown by isotopic methods that the temperature of the deep oceans has fallen monotonically from about 12 degrees Celsius to its present value of 2 degrees Celsius over the last 40 million years. The reason?—Antarctica is passing over the South Pole.

At the end of the Cretaceous period 65 million years ago, the surface temperature of the oceans was 25 degrees Celsius almost as far as the British Isles. By ten million years ago surface water at this temperature did not get beyond the tropics.

At the time of the dinosaur extinctions no continent was at a pole, though Antarctica and Australia were near the South Pole. There were no vast ice sheets locking up water and calving off bergs. The earth was warmer and climatically more stable than it is now.

64 per cent of the sun’s incident radiation at present gets through to the earth’s surface. An increase in the reflectivity of the earth’s atmosphere would cut the proportion below 64 per cent and the whole earth would become colder. Minute ice crystals in the upper atmosphere—the ones that cause halos round the sun and moon, mock suns and moons and so on—are highly reflective. By returning incident radiation to space, only a small increase in them could seriously reduce the sun’s radiation getting to the surface.

Fred Hoyle shows that ice crystals of this sort form at temperatures below -40 degrees Celsius. But heat from the lower atmosphere normally prevents water vapor high in the air from reaching such low temperatures. The water vapor releases the heat as latent heat of condensation when it forms rain droplets. Rainfall of 25 inches (63 cm) supplies enough latent heat to prevent ice crystals from forming in the upper atmosphere but less rain would not provide enough heat to suppress crystal formation. Today the only extended regions where there is less than 25 inches of rain and this condition is met are the polar regions—exactly where the earth is cold enough for glaciation to occur. The average rainfall on the earth is about 30 inches (75cm), only 5 inches (12 cm) above the critical level. If it were to fall below 25 inches then ice crystals would form almost everywhere and a worldwide ice age would have arrived.

The cooling of the oceans as Antarctica passes over the South Pole has brought the risk of a freeze up within the bounds of possibility. Before about seven million years ago it was impossible. The earth’s heat store is the ocean and the heat stored in the warm surface waters until then easily caused sufficient evaporation to put plenty of latent heat into the upper atmosphere. With warm oceans there is no chance that ice crystal formation could occur in the skies and therefore heat from the sun gets to the surface of the oceans, keeping it warm. It is a positive feedback system. But once ocean temperatures drop so much that the threshold of ice formation is crossed a new feedback system starts to operate. Ice crystals in the upper atmosphere spread to lower latitudes cutting down the sunlight and cooling the oceans even more. Evaporation reduces further still and the ice crystals become more permanent. The land cools quickly and snow gradually builds up to form an icecap. When it is sufficiently large, glaciers start to calf into the sea, again cooling it and reducing the earth’s store of heat. There is less evaporation, less latent heat transfer, more ice crystals form, less sunlight penetrates—a new feedback system locked into ice age has arrived.

Today the oceans of the world have a heat store of about ten years of sunlight. If the light of the sun were cut down for ten years it could switch on an ice age. Fine dust in the upper atmosphere could do it. It would mimic the action of the ice crystals, reflecting solar heat away from the surface of the earth. Fine dust of diameter less than a thousandth of a millimeter will stay in the stratosphere for long periods, certainly for a year but possibly for a decade or more depending upon the amount, its height and its size, and may be carried for hundreds or thousands of miles. The explosion of Thera in the Aegean Sea in 1500 BC carried dust to Egypt causing some of the plagues of Pharaoh at the time of Joseph. Very fine particles could stay aloft for a very long time indeed and, though they are not in themselves as effective as reflectors of heat as coarser ash or ice crystals, they can act as nuclei for ice crystals to condense upon.

The greatest volcanic explosion in the last two million years was the eruption of Toba in Sumatra 73,000 years ago. It was a hundred times bigger than the eruption of Krakatoa, throwing 500 cubic miles (2000 km^3) of dust into the air and creating a crater 25 miles (40 km) in diameter. Changes in the pollen in European sediments from this time show a marked cooling followed by a period of erratic weather and, after a delay of a thousand years, an ice age. This enormous eruption eventually triggered an ice age—but it failed to cause a mass extinction!

What then of Late Cretaceous times? Although a world wide drop in temperature did occur, there is no evidence of even a short ice age when the dinosaurs became extinct. As far as we know there were no icecaps, no calving glaciers and the oceans were warm. A Cretaceous freeze up did not occur because no highlands or landlocked seas were sufficiently near to the poles. Furthermore the heat capacity of the oceans was greater than it is now because little water was locked up as ice and sea levels were higher. The higher temperature and greater heat store in the oceans provided a greater safety margin over the possibility of an ice age starting. Instead of only ten years, 50 years or more of darkness would have been needed to trigger an ice age. Only in the last seven million years has the threshold of climatic instability been crossed which permitted the recent ice ages.

One explanation, it is claimed, can take in all the feasible theories so far reviewed—an asteroid of exceptional size hit the earth. Edmund Halley, of Halley’s comet fame, suggested two and a half centuries ago that a comet had collided with the Earth gouging out the Caspian Sea and causing the Biblical flood.

Billy P Glass and Bruce C Heezen revived the possibility of the earth having catastrophic collisions with cosmic bodies in 1967. They linked the fall of large meteorites with geomagnetic reversals, the extinction of species and the distribution of tektites (curious glassy droplets varying in size from several inches to microscopic, formed from molten rock projected through the air, and widely distributed over the earth).

Lately the idea of a collision with an interplanetary body has been strengthened.

The Cretaceous-Tertiary boundary is marked by the virtual disappearance of the foraminifera, tiny creatures which live in the sea and whose shells of calcium carbonate, sinking to the bottom of the sea over millennia, form limestone. In the Gubbio district in Italy only a single species survived. Vast beds of Cretaceous limestone composed of foraminifera gave way to half an inch of reddish-grey clay which contained no fossils. Then another layer of limestone began. There was no sign of a reversal of the earth’s magnetic field.

This anomaly in the limestone rocks was found by Walter Alverez. His father, the late Luis Alvarez, a Nobel Prize winner, determined to estimate the age of the boundary layer. Micrometeorites shower the earth daily at a constant rate. Knowing that rate and analyzing the sediments for extraterrestrial material would show how quickly the sediments had deposited. Slowly deposited sediments should contain more interplanetary matter because it had been falling on them longer. He chose to look at iridium, a dense metal similar to platinum, rare on the surface of the earth (being dense, it had settled into the earth’s interior when the planet was still molten, as did most other heavy elements) but more common in meteors.

Luis Alvarez was a specialist in nuclear activation analysis. A sample is bombarded with neutrons in a nuclear reactor until some of the elements present become radioactive. They can then be identified by their differing modes of radioactive decay. Alvarez’s analysis of the clay gave odd results. The amount of iridium was 30 times higher than in the deposits above and below. The quantities were only a few parts per million but that is unusually large for rare elements like iridium, osmium and platinum. Tests on sites in different parts of the world gave similarly high concentrations. Other elements found in meteorites were also detected in comparatively high concentrations and tektites were present.

The Alvarezes’ believed tektites and meteoric materials confirmed that a massive meteorite, the size of an asteroid, had shaken the earth.

Scars on the surfaces of the Moon, Mercury, Mars, Venus and the moons of Saturn and Jupiter show they have been bombarded by meteorites throughout time. The earth also has traces of large meteor impacts in the form of craters, crater lakes or, in older weathered rocks, crater impressions. Only in 1908 some object from space (probably a fragment of the comet Encke) hit Tungusku in Siberia devastating a large area of forest.

R Grieve of the Canadian Department of Energy, Mines and Resources says that 5000 asteroids with diameters of more than 3000 feet (1 km) have struck the earth in the past 600 million years. Meteors 1000 feet in diameter have hit the earth every 10,000 years on average (corresponding roughly with the cycle of ice ages). G—W Weatherill writing in Icarus in 1979 and E—W Shomaker at the Snowbird Conference on large body impacts in 1981 have given the frequency of cometary impacts as—one km wide every 250,000 years, five km wide every 20 million years and 15 km wide every 100 million years.

A 1000 feet wide asteroid would throw up one or two cubic miles of debris depending upon its entry speed. A 3000 feet wide meteorite would crash with a force equal to that of 10,000 ten megaton hydrogen bombs. It might be expected to leave a crater 12 miles across and displace 25 to 50 cubic miles of debris, more than sufficient to disrupt weather patterns and possibly enough to trigger an ice age in today’s conditions.

The Alvarezes postulated an asteroid six miles across hurtling into the earth at 45,000 miles per hour, gouging out a crater over 100 miles across and shooting debris amounting to 60 times the asteroid’s volume (8000 cubic miles) into the atmosphere. Sunlight would be blotted out for a long period, there would be prolonged cooling, photosynthesis would stop, the base of the food chain would die, animals higher up the food chain would starve. The asteroid must have approached more or less vertically. If it had approached obliquely spending more time in the atmosphere prior to impact, Allaby and Lovelock maintain it would have destroyed all life. Friction would have heated the air to such a temperature that nitrogen and oxygen would have reacted forming nitric acid, sterilizing the earth with its corrosive and oxidizing action.

Astronomers have observed suitable objects. The Apollo class of asteroids are prime candidates. They include planetoids of the right size and they cross the earth’s orbit making it likely that they would collide with the earth from time to time. But where is the crater?

Only one large impact crater has an age of 65 million years, Manson Crater, recently found underneath sediments in Idaho, but it is too small to have caused destruction on the scale envisaged. It is only 20 miles across not the 100 miles that would be needed. Conceivably the asteroid broke into fragments on entering the atmosphere and the Manson Crater is the scar of just one of the fragments. Some scientists argued that the asteroid was most likely to have fallen into the sea, vaporizing huge quantities of water, causing torrential rain for months on end, and a temporary greenhouse effect. No crater would then be obvious. If the asteroid disintegrated before impact some fragments might have landed in the sea and some on land, adding to the complexity of the climatic effects.

Others said that the meteor would have made a crater even though it fell into the ocean, but this has now been subducted under continents by the action of plate tectonics. About half of the ocean floors have disappeared under the edges of continents and reformed at the mid-ocean ridges since the end of the Cretaceous period.

“But there is a crater”, said others “or a scar of one at any rate”. Fred Whipple, the originator of the dirty snowball theory of comets, largely confirmed by Giotto, claimed that the impact was so energetic that the earth’s crust shattered allowing vast amounts of magma to well up filling and destroying the crater but leaving a massive scar still volcanically active—it is Iceland! Iceland has no rocks more than 65 million years old.

The volcanic “hot spot” under the Hawaiian chain of islands in the Pacific was suggested as an alternative point of impact but that is unlikely. It seems to have been active for too long, at least 80 million years.

A crater has been found in the Yucatan peninsula and is now considered the prime candidate but, like the Manson crater, it seems too small. But it may have been one of several impacting one after the other.

The Alverez team used the idea of a cosmic collision to encompass four previous explanations:

1. suppression of photosynthesis—the global darkness of three to six months curtailed photosynthesis and led particularly to extinctions in the oceans
2. the greenhouse effect, especially of an impact into the ocean—the temperature rise caused by a blanket of water vapor would have killed many land animals
3. an ice age—exclusion of the sun’s radiation for many months would lead to a global cooling which would kill off many species
4. pollution and poisoning—the impact heated the air to such high temperatures that large amounts of nitrogen oxides were created by chemical reaction between the normally inert nitrogen and the oxygen in the air, and the acid rain, which subsequently fell, devastated life for a long period afterwards.

Some scientists were not convinced that the meteorite impact could trigger mass extinctions any more than a large volcanic eruption could. Would the debris thrown up stay aloft long enough to cause any lasting damage? After all many genera and individual species did survive showing that, despite darkness, dust and poison gases, conditions could not have been bad for too long. A fairly short period of darkness could explain the excessive extinction of water dwelling species relative to land types because the food reserves of the plankton in the sea is only sufficient to last for between ten and a hundred days, but land plants can suffer the absence of light for longer.

A period of darkness of three years, as the Alverezes supposed, would have destroyed most genera, perhaps all higher ones. Many species were not seriously affected—and many others, we know, had been suffering decline before the hypothetical collision. According to E.G.Kauffman of Colorado University, 75 per cent of marine organisms were in decline at the end of the Cretaceous period. They had been on the wane for two to five million years and few species seem to have died off at exactly the same time.

Ammonites had fluctuated in population previously. Some ammonite genera declined and expanded periodically while others seemed relatively steady. When extinctions had occurred before, the steady species had tended to survive while the fluctuating species had died off. In the upturn the steady species had radiated into available niches including those suiting the fluctuating species and the cycle continued. At the end of the Cretaceous the ammonites were in such decline that there were no reliable steady species to bring them through. What was different? What had killed off the steady species of ammonites?

Dinosaurs were similarly on the wane and by the end of the Cretaceous survived in numbers only in the West of North America, having died out in South America and possibly Europe. Even in North America the decline was severe. Half of the 36 genera of dinosaurs alive about ten million years before the end of the Cretaceous had died out by the time the final million years was entered.

Robert Bakker is among those who do not support the catastrophe theories. He claims there is no doubt that dinosaurs did not die out in a geological instant but petered out over thousands if not millions of years. Plainly life was under stress. What was its cause?

Leigh Van Valen and Robert Sloane attributed the extinctions to climatic changes over the last five to ten million years of the Cretaceous. At the beginning of the period vegetation was prolific and typically tropical or sub-tropical. Towards the end of the period the climate had become typically temperate with cool woodlands. Dinosaurs thrived in the warmer climate but in the cooler one mammals had the advantage. The proposed reason for the change in climate was that the ocean floor had lifted with renewed mid-ocean spreading and sea levels had risen. Shallow seas divided North America and also divided Europe from Asia. Ocean currents and wind patterns may therefore have altered.

On the other hand, Bakker argues a fall in sea level draining the shallow continental seas could have triggered the mass extinctions. This accounts for the loss of a lot of marine species, those preferring the continental margins and intra-continental seas obviously, but the loss of light warm water draining from them on to the ocean surfaces would also lead to the demise of many open sea species that could not adapt to the colder surface conditions. What though of the land vertebrates? Surely they would have had more lebensraum and should have multiplied. No. The linking of previously isolated continents by land bridges created conditions similar to those in the great Permian extinction—hypercompetition between species and the unchecked spread of disease and parasites in populations not adapted to be immune from them. Large active animals like the dinosaurs could migrate faster, therefore they experienced more competition and disease, and suffered most. Smaller creatures like the mammals could migrate, but more slowly, having more time to adapt and freshwater species, which seemed least affected, could not migrate, though they were affected to a lesser extent by pests or diseases carried in by the migrants.

Late in the Cretaceous some Asian genera of dinosaurs appeared in North America having crossed the Bering Straits (or whatever the dinosaurs might have called them) showing the two continents had linked. A more recent example was the land bridge which formed between North and South America, about 30 million years later than the dinosaurs, when many South American species failed to survive competition from invaders from the North.

Bakker puts particular emphasis on the spread of diseases unrestricted by hereditary immunity. Warm blood is at the ideal temperature for bacteria and viruses to multiply. The warm blooded dinosaurs would therefore be susceptible to the new pathogens being introduced to all the continents. He quotes examples like the spread of the Black Death and the carrying of rinderpest from India to Africa with devastating effects on the antelope herds. V.D. and smallpox devastated the Amerindians. Virulent strains of myxomitosis were deliberately developed by CSIRO in Australia and introduced into the wild to control rabbits. All have depended upon the intelligent mammal with its capacity for travel and his insensitivity towards other species.

Bakker contradicts his advocacy of the evolutionary resilience of the dinosaurs. The dinosaurs, he persuades us, are great competitors and the forming of land bridges, while leading to mass extinctions, also provides lots of empty niches for enterprising species to adapt to. If the theory is true at all, and it might be partly true, we need to know why the genetic variability of the dinosaurs had been reduced to such an extent that they could not cope with the new challenge as they had always done before.

Those doubtful of the Alverez’s theory thought the iridium anomaly could be explained by differential sedimentation rates or volcanic activity. Further careful testing of the iridium layer showed that the iridium concentration seemed to build up slowly during the last few thousand years prior to the supposed cataclysm. Volcanic activity occurring over an extended period, they argue, would be more likely to match such a pattern. India had broken from Africa and raced (in geological terms) across the Indian Ocean to hit Asia. The collision pushed up the Himalayas and created such friction that lava spewed forth for centuries to form the Deccan Peninsula. This was the extended vulcanism they sought.

Calder noted that 65 million years ago in Western North America plants were “dusted with exotic elements”. Besides iridium, volcanoes emit other metals present in the K-T sediments not commonly present in meteors, like arsenic and antimony. For Charles Officer and Charles Drake of Dartmouth College this proved the boundary layer and the extinctions were related to the break up of the old continents. The level of the seas fell to their lowest for 200 million years. The warmth which gave subtropical conditions to northerly climes subsided. Widespread volcanic activity over a long period (but short on a geological timescale) led to pollution, climatic changes and ecological damage which destroyed species.

You will, by now, have noticed that some experts have postulated high sea levels and others low sea levels as reasons for the Cretaceous extinctions. You may well ask: “Don’t they know where the sea level was? Wasn’t it where it usually is?” Sea level provides a riddle of its own. Geologists, notably those working for oil companies have built up a detailed knowledge of changes in sea level over the ages. In the last 200 million years it reached its highest consistent level from about 85 to 67 million years ago when about twice the area of continental shelf presently inundated was flooded. In the preceding 100 million years sea levels had steadily risen due to the activity of the mid-oceanic ridges and the sea floor spreading associated with continental drift. The welling up of magma under the mid-oceanic ridges displaces the water of the oceans causing higher sea levels.

Sudden unexplained falls in sea level occur periodically but are rarely linked with meteorite falls. The sudden onset of an ice age freezing large amounts of water in extensive ice sheets might explain some recent sea level changes but not those in the Cretaceous. They also do not normally coincide with mass exterminations, throwing doubt on an idea like Bakker’s. The sea level did fall rapidly about 67 million years ago, the event identifiable with Bakker’s theory, but there was a greater fall in sea level 95 million years ago which is associated with only a minor turnover of species compared with the extinctions terminating the Cretaceous. Why did this earlier event not have the impact of the later one if Bakker’s idea is correct?

Any convincing explanation of the extinctions has to account for all the genera that became extinct not just those that are representative of the dinosaurs. It also has to be sudden, at least on a geological timescale. The dinosaurs had shown that they were well able to adapt over 140 million years and were still evolving in the Cretaceous. A gradual change of conditions was unlikely therefore to overwhelm them—they would have adapted into the new conditions.

Though doubt is being cast on the asteroid impact, many of its anticipated effects like adverse climatic disturbance and pollution of the environment remain persuasive—but were these the shadow of an asteroid or did they come from closer to home? Today we see similar effects created by the intelligent mammal. Volcanoes and asteroid impacts do not have to be invoked to explain the environmental problems we are experiencing, or the mass extinction of species currently taking place. Similar things are happening today to events at the end of the Cretaceous.

Skeptical Resources—Internet infidels | Jesus Never Existed | Steven Carr’s Website | Christianism | Early Christian Writings | God is Imaginary | “Religion Detoxification” | Our Judaio-Christian Heritage | Archaeoastronomy | No Deity | No Beliefs | Evil Bible | Bible God | ex-Christians | Jesus Police | Islamic Faith Freedom | Atheist Experience | American Atheists | Jovial Atheist | Askwhy! books
Other Resources—Early Christian Docs | Resources for Study | Traditional Bible-History | Traditional Bible World History | Traditional Bible History | about.com biblical history | Apologetics web sites | Advent Ch Fathers | Orion center links | Wikipedia | Traditional Jewish History