The Earth's predicted near-miss with asteroid XF11 in the year 2028 has once again focussed attention on the fear that a large asteroid or comet hitting our planet could trigger a global catastrophe.

To back this up, every article and television program about XF11 boldly asserted that the dinosaur extinction was caused by a giant asteroid impacting into the Earth 65 million years ago. This has typically been accompanied by a picture of frightened dinosaurs looking skyward at a huge flaming meteorite streaking across the horizon. This scenario is so widely accepted that few commentators bother to question it any more. There is, however, much evidence to suggest that an asteroid may not have hit the Earth 65 million years ago and that, even if it did, it did not cause the mass extinction of life attributed to it. There is also the possibility that dinosaurs may not have been around to witness it!

By the second world war it was known that, after a reign of 180 million years, the dinosaurs had died out 65 million years ago at the end of the Mesozoic era. During the 1960s and 70s, palaeontologists discovered that, at the same time as the dinosaurs had died out, a large number of other fossil groups became extinct or suffered huge drops in species numbers. Amongst those that became extinct were the ammonites, belemnites, marine reptiles, flying reptiles and the reef-buildingbivalves whilst those suffering partial extinction included many types of marine plankton, sea urchins and some corals. All of this occurred around the transition from the Cretaceous period to the Tertiary period and became known as the 'K-T boundary' mass extinction event (K stands for keta, the Greek for chalk, T for Tertiary).

In rock outcrops, the K-T boundary itself is represented by a thin layer of red clay, only a couple centimetres in thickness. Below this clay are Mesozoic fossils, such as the dinosaurs, whilst above it are Tertiary ones. This was the situation, as science saw it when, in 1976, Dr. Walter Alvarez came across an exposure of K-T boundary red clay in Italy and collected some samples for later analysis. Walter Alvarez mentioned the mass extinction and his find to his physicist father, Dr. Louis Alvarez, who suggested chemically analysing the clay.

The results showed that the sample unexpectedly contained huge quantities of iridium, an element that is so rare on Earth that it was hard to explain its association with the K-T boundary clay. Other K-T boundary clays from around the world showed similarly high levels suggesting that the iridium spike (as it was known) was a global phenomenon and not just a local geological anomaly in Italy.

The Alvarez' knew that such high levels of iridium could only be found during volcanic eruptions or in asteroids from outer space. At that time there was no evidence for widespread volcanic eruptions 65 million years ago and so an extraterrestrial solution was sought.
At the time of this research, a group of anti-nuclear scientists had begun promoting their ideas about the global effects of a nuclear holocaust that, in the depths of the cold war, seemed a realistic possibility. Their chief proposal was that a global nuclear war would produce a `nuclear winter' in which dust, water and other material sucked into the upper atmosphere would form a blanket around the Earth effectively preventing the heat and light of the sun from reaching the planet's surface. Beneath this atmospheric blanket the Earth would freeze, destroying tropical and temperate animal species, whilst the lack of sunlight would remove the base of the food chain by killing off all the plant life and plankton. In addition to this, large amounts of radioactive fall-out would accumulate inside the world's sediments poisoning the soils and seabed. The result would be the sudden mass extinction of almost all the plant and animal life on Earth with only the hardiest of creatures, such as rats and cockroaches, able to survive. It is this that caught the attention of Walter and Luis Alvarez. They adapted the nuclear winter scenario to suggest that an asteroid impact 65 million years ago would have had a similar effect by throwing a huge quantity of debris into the upper atmosphere, blocking out the sun. This would have shattered the food chain and led to the demise of many groups and species, including the dinosaurs. Instead of radiation, the Alvarez envisaged that the iridium found inside the asteroid would also be thrown into the upper atmosphere where it would have rained down into the world's sediments to form the elevated levels seen in the K-T boundary clay. To do this, the asteroid was calculated to have been at least 10 kilometres across and travelling at a speed of 80,000 kilometres an hour. In this form, the Alvarez published their theory in 1980. Under normal circumstances, the scientific community reacts badly to new and radical ideas and it can take a generation for them to become widely accepted. This was not the case with the asteroid impact theory; it was quickly grasped and heralded as a masterpiece of scientific deduction. The K-T boundary suddenly became a hot research topic. It is here that, in my opinion, the first problem occurred. Having agreed with the impact theory, now a large generation of scientists were setting out with the sole aim of finding the evidence to reinforce it. As most forteans will know, people looking for evidence to prove an existing theory will by and large find it, whether it is there or not. A more scientific way to approach a research topic is to first gather together the evidence and to build up conclusions from that database. This did not occur and scientists began to find circumstantial evidence for widespread bush fires, tidal waves5, freezing temperatures and acid rain as well as the iridium spikes and mass extinction of life at every K-T boundary site around the world. Each new observation reinforced the asteroid-impact theory with very few objections being tolerated by the scientific press. The only variation was the discovery of huge levels of volcanic activity in India at the time of the K-T boundary which led to a small band of people advocating this as the cause of the extinction and accompanying iridium spike. By the mid-1980s it was common knowledge that it was an asteroid that had wiped out the dinosaurs and today this is cited as a fact in most popular scientific journals and television programs. There is, however, a small minority of palaeontologists whose research was at odds with the asteroid impact theory. Chief amongst these are the micropalaeontologists who study the fossil remains of plants and animals which are less than one millimetre in size. Unlike dinosaurs, ammonites, corals and trees, which are large and rare in the fossil record, a small chip of rock can contain millions of microfossils (my record is 24 million per gram) and can therefore give a better clue as to what the local environment was like when they died. They are particularly common in deep sea sediments and, throughout the 1980s, a world-wide program of deep sea coring produced a whole series of new and well-preserved K-T boundary sediments in which the micropalaeontologists began to find inconsistencies with the asteroid theory. Some of the common microfossil groups did seem to suffer a mass extinction of species across the K-T boundary. For example, the calcareous nannofossils lost 92 percent of their species and planktonic foraminifera between 50 and 85 percent.7 However, when a detailed examination of these groups was done, it was discovered that many of the species did not die out all at once but in a series of stages that began at least 300,000 years before the K-T boundary and carried on for 200,000 years afterwards. Other microfossil groups proved to be unaffected by the K-T boundary at all (eg. the radiolaria, diatoms, dinoflagellates and bottom-dwelling foraminifera) something that was completely at odds with the theory of nuclear winter followed by plankton collapse in the oceans. Together these and other discoveries cast doubt on the notion of a sudden mass extinction of species within a few years following a large asteroid impact. Many of the fossils that had started the debate about the K-T boundary in the first place were found not have died out suddenly at the K-T boundary, but to have been in a serious decline for millions of years beforehand. Groups like the reef-buildingbivalves, marine and flying reptiles were proved to have become extinct before the K-T boundary whilst the dinosaurs, ammonites, belemnites, scleractinian corals and sea urchins had been in decline long before it. The evidence was now beginning to point towards a long term global decline in many fossil groups in the final stages of the Cretaceous rather than a sudden extinction event at its end. For example, the dinosaur diversity had dropped from 45 genera, 85 million years ago, to just 12 at the K-T boundary. No dinosaur remains at all are known from the K-T boundary and the youngest known dinosaur fossil was found some 3 metres below the iridium clay, suggesting that they may have become extinct before the proposed asteroid impact. New geochemical techniques seemed to back this up by producing evidence of serious climatic fluctuations in the three million years prior to the K-T boundary event. The Mesozoic world of the dinosaurs had a stable greenhouse climate with a much higher sea level and warmer temperatures than present. There were inland seas across central of America, Europe, Africa, Russia and South America and tropical conditions occurred as far north as Britain and New York. There were no polar ice caps. This began to change 70 million years ago when the sea level began to drop causing the climate to de-stabilise and the globe to cool rapidly. The gradual decline in many fossil species including the dinosaurs, ammonites, belemnites, reef-buildingbivalves, etc., appears to be closely linked to the dropping sea level and climate change in this last part of the Cretaceous period. Recent geochemical and fossil evidence now suggests that, 300,000 years before the K-T boundary, there was a large and sudden drop in sea level that triggered the final collapse in the world's biosphere which resulted in the extinction of some fossil groups and a dramatic loss of species in others. This change possibly due to a change-over from the `greenhouse' climate of the Cretaceous, where ocean circulation and weather were driven by heat in the tropics, to our Tertiary `icehouse' climate where they are driven by the polar ice caps. The evidence for this comes from the observation that Polar fossil species were largely unaffected by the mass extinction. Further research on K-T boundary exposures world-wide seemed to suggest that they were not as complete as was first thought. Many of the sections were shown to have long periods of time where sediment had not been deposited causing small, but significant, gaps in the fossil record. Thus, when data from a number of locations was assembled together and compared, it was realised that many more species had survived into the Tertiary than was previously thought and that the `sudden' extinction within some fossil groups was in fact staggered over a period of 500,000 years. Alvarez' iridium spike also came under suspicion after widespread geochemical analysis found that iridium clays are actually quite common in the geological record, can be caused by natural processes and did not need to coincide with a mass extinction. By the beginning of the 1990s it was beginning to look like the small band of opponents to the asteroid impact theory were winning ground. One particular problem for the pro-asteroid camp was the visible lack of an impact crater. A solid object 10 kilometres in diameter could be expected to have done a massive amount of crustal damage on its impact, the signs of which ought still to be detectable after 65 million years. As there are no known large craters of the right age on land, the ocean floors were next logical place to look. Here, too, there were no obvious candidates and for a long time it was supposed that the crater must have been recycled into the Earth's interior by the action of plate tectonics. However, exploration around the Gulf of Mexico, in 1990, revealed a large circular structure, buried beneath 1.5 kilometres of sediment, that appeared to be the right size, rock type and age for it to be associated with the K-T boundary asteroid impact. In addition to this, K-T boundary sediments on local Caribbean islands were shown to contain crystals which had been `shocked' (ie. cracked) by the force of a near-by explosion. The pro-asteroid camp declared that it had found its impact crater, situated at Chicxulub, to the south of USA, off the Yucatán peninsular.
The evidence for this being an impact crater is quite strong although, being buried under 1.5 kilometres of sediment and only accessible using samples from six deep boreholes, there is still some controversy about the structure's exact age, size and origin. The discovery of the Chicxulub crater has added considerable weight to the theory that an asteroid impacted into the Earth 65 million years ago and many scientists believe that the matter of the `death of the dinosaurs' is now fully resolved. Objections by palaeontologists are largely ignored and are certainly not reflected in the mainstream media. Beyond this, the debate has reached a stalemate and there are currently three main areas of belief on the matter ... The first belief is that an asteroid impact at the K-T boundary was responsible for a sudden mass extinction of approximately 50 to 85 percent of life on Earth. This still has by far the greatest scientific following. The second belief is that most of the extinction had occurred prior to the K-T boundary and that the effect of an asteroid impact, if it occurred, was minimal. This is the view taken here. The third group believes in a combination of the two with an asteroid, sometimes in conjunction with volcanic activity, delivering the coup de grace to already declining fossil groups. This is possible but, in my view, not supported by the evidence.

The answer is clearly not straight forward and this leads onto the final point I would like to make. The asteroid impact theory as proposed by Walter and Luis Alvarez is too neat and convenient to be applicable to the complex nature of the evidence that has been found since 1980. The impact theory has all the features of a Hollywood film script (ie. dinosaurs, explosions, death, widespread disaster and, with the rise of us mammals, a happy ending) which is probably why it has been so widely publicised and accepted by the media. Although there is evidence for a bolide impact at, or near, the K-T boundary, it would have come too late to seriously affect many already extinct or declining fossil groups, especially the dinosaurs who were well and truly on their way out by then, whilst it appears not to have affected others at all. Add to this the gaps in the fossil record, problems with dating rocks, the changes in global environment and other iridium spikes in the geological record, and the Hollywood script starts to have too many subplots to become coherent.
This proves that we currently do not know the cause(s) of the mass extinction at the end of the Cretaceous and anybody that claims to have a definite answer is wrong. Even the assertion that the dinosaurs became extinct is wrong. As was covered in , one important branch of the dinosaurs not only survived into the Tertiary but continues to thrive in our mammal dominated world. In fact, I can see half a dozen of these dinosaur ancestors from where I sit and I ate part of one in a curry last night. I refer, of course, to the birds who, like some scientists, are prone to flights of fantasy.

SURVIVAL OF THE FITTEST? When, in 1980, Walter and Luis Alvarez proposed their asteroid impact theory, very little detailed or integrated work had been done on fossil groups in relation to the K-T boundary. It was estimated that 50 percent of life on Earth had died at the K-T boundary a figure that is meaningless when it is considered that we have no information at all on many fossil groups such as the beetles (currently the most diverse group of animals on Earth) and all soft-bodied organisms. Amongst the animals deemed to have become suddenly extinct were the dinosaurs, ammonites, belemnites, reef-buildingbivalves, marine and flying reptiles whilst those badly affected include all the plankton, all shallow water marine life and land plants. It was also known that some fossil groups, such as the mammals and crocodiles, survived rather well, although this was little commented on. Now, eighteen years later, we know much more about the global distribution of species across the K-T boundary and what happened in the few million years preceding it. This has thrown up some inconsistencies. The `nuclear winter' was envisaged to have wiped out all life on Earth evenly with only a few hardy species surviving. Yet on land, we see the dinosaurs become extinct whilst their close and cold-blooded relatives the crocodiles, amphibians and lizards survive with no unusual effects. The mammals and birds also survived well, leaving the dinosaurs as the only major on land animal group to become extinct.
A similar story is seen in the oceans where, amongst the plankton, the diatoms, radiolaria and dinoflagellates all come across the K-T boundary unscathed whilst the nannofossils and planktonic foraminifera were badly affected. All the plankton groups lived in the same environment and have the same requirements, so why did some die and some live? An asteroid should have affected them all to the same degree. Similarly, in the shallow seas, the ammonites were dying whilst their very close relatives, the nautiloids, survived intact. Other fossil groups such as the reef-buildingbivalves, marine and flying reptiles and possibly the belemnites, were distinct before the K-T boundary. Another problem is the apparent difference between the extinction rates in the polar regions and those in the tropics. On land there is a noticeable decline in temperate and tropical plant species as the polar trees spread south with the changing climate whilst in the seas warmer water species, including the ammonites, belemnites and plankton foraminifera, also show a decline.In general, polar species survive the K-T boundary event much better than their tropical relatives and in one Antarctic rock outcrop there is so little evidence of a mass extinction that the boundary itself has been very hard to pin-point at all.

CHICXULUB