ASK anyone you meet how the dinosaurs met their end 65 million years ago, and they鈥檙e likely to blame it all on an asteroid. And why not? It鈥檚 a dramatic answer and the image of one huge blow obliterating those 鈥渢errible lizards鈥� at a stroke is almost irresistible. But could it have been that simple? Was that really all it took to wipe out 65 per cent of Earth鈥檚 species?
The impact theory has had a firm grip on the headlines ever since Luis Alvarez of the University of California, Berkeley, proposed it back in 1980. And 11 years later, the discovery that the giant Chicxulub crater off Mexico鈥檚 Yucat谩n peninsula dated back to just the right time gave the 鈥渃atastrophists鈥� a further boost.
But despite the success story, the catastrophists鈥� viewpoint is far from universally accepted. A group of scientists feel that it is flawed. 鈥淲hatever wiped out the non-avian dinosaurs was a lot more complicated than a single hammer blow from an asteroid,鈥� says Norman MacLeod of London鈥檚 Natural History Museum, who is one of the catastrophists鈥� leading opponents.
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For one thing, some scientists maintain that it is by no means established that the impact coincides with the Cretaceous-Tertiary (K/T) boundary, which is when the dinosaurs became extinct.
What鈥檚 more, when Alvarez first proposed the asteroid theory, his principal piece of evidence was a global layer of iridium found exactly at the K/T boundary. Iridium is rare on the Earth鈥檚 surface, but more abundant in space, making the impact of an asteroid a likely cause. Yet there are alternative explanations, both cosmic and terrestrial, which MacLeod and others in the 鈥済radualist鈥� school believe account equally well for the iridium.
Their own view of the extinctions presents a picture of lengthy decline before the dinosaurs鈥� final coup de gr芒ce at the K/T boundary. So the two camps are quite distinct: one adhering to a vision of a quick and nasty end, the other to a slow and nasty end. But both, in the end, have to rely on the main evidence in the dinosaur debate-the fossil record. According to Peter Sheehan of Wisconsin鈥檚 Milwaukee Museum, himself firmly in the catastrophists鈥� camp: 鈥淭he community is split right down the middle. We all have the same data but we come to very different conclusions.鈥�
It is not surprising. The fossil record is sparse, difficult to interpret, and reveals an extraordinarily complex extinction scenario at the end of the Cretaceous. Many groups of living things besides the dinosaurs received their marching orders, while others sailed through entirely unaffected. Bizarrely, the latter group included amphibians such as frogs and salamanders, commonly considered to be vulnerable to the slightest change in the environment.
But according to MacLeod, many dinosaur families had been declining for millions of years before they all finally vanished. Other creatures-for instance, two prominent groups of mollusc-actually took a nose-dive more than a million years before the boundary, he says.
The catastrophists beg to differ. Sheehan admits that the molluscs died early, but argues that it had nothing to do with the eventual extinction of the dinosaurs. 鈥淚 see very little in the record that doesn鈥檛 agree with a catastrophic extinction,鈥� he argues. Dale Russell of North Carolina State University agrees. Though he accepts that some dinosaur groups seem to disappear in the last 2 million years of the Cretaceous, he believes this was a local effect confined to continental North America, where a vast inland sea was withdrawing at the same time.
Gerta Keller, a palaeontologist from Princeton University, is a convinced gradualist, but she bases her theories not on dinosaur bones, but on microfossils. Dinosaur remains are astonishingly sparse-a mere 3000 fossils exist from a timespan of 150 million years. But Keller鈥檚 microfossils, the remains of tiny creatures that also suffered from severe extinctions, are much more abundant. She has found that the microfossils that suffered most from the extinctions began to die out more than 100 000 years before the K/T boundary.
The overall picture, then, is a muddy one. But what does emerge is that the gradualist alternative to the impact theory-in all its flavours-has not received nearly the same attention. No one doubts that the impact of a huge asteroid would have been bad news for life on Earth. But for many such an impact is not the cause of the majority of the extinctions, but a very secondary event.
Choking dust
So, what of the primary event? There are both extraterrestrial and terrestrial possibilities. Anthony Allen of Queen Mary and Westfield College, University of London, and Shin Yabushita of Kyoto University in Japan back an extraterrestrial candidate. They argue that the Cretaceous extinctions came about when the Solar System passed through the dense core of a giant molecular cloud (GMC).
Stars like the Sun are born out of these massive clouds of cold gas. Typically, a GMC will contain the mass of a million suns in dust and molecular gas, mostly hydrogen. Yabushita and Allen say it is inevitable that the Sun occasionally passes through the fringes of such a cloud as it sails through the Galaxy. Such an encounter could take a million years because GMCs can be several light years across.
While it passed through, dust from the cloud would accumulate in the Earth鈥檚 atmosphere, where it would reflect sunlight back into space and severely cool the planet. Simultaneously, the GMC鈥檚 gravity would shake up the Sun鈥檚 reservoir of comets, known as the Oort cloud, which orbits far beyond Pluto. This would in turn send more comets than usual towards the Sun, where they would pose a threat to life on Earth. Such events may account for several of the mass extinctions that have happened over time, believes Allen.
But choking dust and comet impacts make up only part of the jigsaw. Allen and Yabushita claim the Cretaceous extinction event was unique among all such encounters in that the Solar System passed not through the outskirts of a GMC, but through its dense core, exposing life to the threat of molecular hydrogen.
Usually, sunlight can ionise all the hydrogen within the Solar System. But if the solar neighbourhood were filled with the gas to a density of 100 000 molecules per cubic centimetre, the Sun could not ionise hydrogen as far as the limits of the Earth鈥檚 orbit. Crucially, say Allen and Yabushita, this is the hydrogen density found in a typical GMC core.
So if the Sun passed through such a core, the Earth would spend hundreds of thousands of years embedded in a cloud of neutral hydrogen. This would threaten life on Earth, say the researchers, because the hydrogen would mop up about a third of the oxygen in the atmosphere. The hydrogen would penetrate the atmosphere because the planet鈥檚 magnetic field, which shields us from ionised gas, is no defence against neutral gas. And once the hydrogen was in, lightning would cause it to mix with oxygen and so make water (Monthly Notices of the Royal Astronomical Society, vol 238, p 146).
According to Allen and Yabushita, the amount of atmospheric oxygen theoretically mopped up by this hydrogen matches the reduction in oxygen found in air bubbles trapped in late-Cretaceous amber. Although such measurements are controversial, with some researchers adamant they are not pristine, Allen and Yabushita say the drop in oxygen is consistent across all amber samples.
Deadly cloud
So, the chain of events set off by an encounter between the Solar System and a GMC would explain why the extinctions took place gradually rather than all at once, and also account for the Chicxulub crater-presumably the result of a comet destabilised from the Oort cloud. It could even help to explain why the Cretaceous extinctions were so selective. 鈥淲e believe that the creatures that were disadvantaged were the largest ones-the largest insects, largest ammonites and largest dinosaurs,鈥� says Allen. 鈥淭he smaller species with faster inspiration rates were less affected.鈥�
Allen admits that such a differential effect of reduced oxygen on organisms is unproven. Nevertheless, he says that anecdotal evidence appears to support it. Other gradualists do not believe that a killing mechanism as elaborate as Allen and Yabushita鈥檚 is needed. In the mid-1980s, Victor Clube of the University of Oxford and Bill Napier of Armagh Observatory in Northern Ireland pointed out that the lion鈥檚 share of cometary mass is to be found in relatively few giant comets, measuring as much as 100 kilometres across.
Meanwhile, working independently of Clube and Napier, Fred Hoyle and Chandra Wickramasinghe of the University of Wales College, Cardiff, explored the effect that such a 鈥渟upercomet鈥� might have on the Earth. Now the four have teamed up to present a comprehensive picture.
They believe that 鈥渢idal鈥� forces exerted by the Galaxy as a whole loosen giant comets from the Oort cloud. Occasionally, such a body becomes trapped in the inner Solar System, where the Sun sublimates it over many millennia, causing it to dump large amounts of dust into interplanetary space. The dust is swept up by the Earth during its orbit.
Eventually, the terrestrial atmosphere becomes clogged with enough dust to drastically cool the planet. The process takes thousands of years-a factor crucial to how organisms die off because the oceans store only about a 10-year supply of sunlight. 鈥淥nce this supply is gone, there is no warmth left to stop the ice crystals forming in the upper atmosphere,鈥� says Napier. 鈥淭he presence of the ice crystals mean that even when the dust is gone the climate remains cool.鈥� MacLeod points out that there is no evidence of glaciation at the end of the Cretaceous, but Napier counters that a climate change as severe as an ice age isn鈥檛 necessary to cause a mass extinction. 鈥淧rolonged cooling, even without glaciation, will be perfectly sufficient.鈥�
In Napier鈥檚 scenario, major impacts occur in the dying days of the supercomet when most of its ice has gone, at which point the rest of it disintegrates into 鈥渃ometary asteroids鈥�. Even so, these major impacts are not the main killer: that privilege, Napier insists, belongs to the dust. 鈥淏y focusing on impacts, everyone has completely overlooked a much more serious threat to Earth,鈥� he says.
Napier鈥檚 gradualist scenario could even help to explain an enduring mystery: why extinctions and impact craters seem to occur every 28 million years or so. Though many palaeontologists are sceptical about whether this periodicity really exists, it is true that every 30 million years the Oort cloud is subjected to maximum tidal stresses for a period of a few million years because the Sun passes through the plane of the Galaxy at those intervals.
Hellish acid rain
Extraterrestrial models are compelling solutions to the dinosaur extinction dilemma. But MacLeod believes that an equally dramatic, but home-grown, event precipitated the mass die-offs of the Cretaceous: supervolcanism.
The Deccan Traps, a vast plateau in southern India, were created over a few million years at the end of the Cretaceous when about a million cubic kilometres of lava gushed onto the Earth鈥檚 surface. No eruption in recorded history was even a thousandth as big. 鈥淣ear Bombay, the flood basalt is still 2.4 kilometres thick, after millions of years of erosion,鈥� says Dewey McLean of the Virginia Polytechnic Institute and State University in Blacksburg, who was the first to argue that supervolcanism caused the extinctions back in 1979.
The eruption was undoubtedly severe. 鈥淭he stress it imposed on the environment would have been phenomenal,鈥� says Andrew Kerr of the University of Leicester. 鈥淚t beggars belief that the asteroid impact people can ignore it.鈥�
A very similar outburst of supervolcanism, responsible for the Siberian Traps, coincided with the biggest of all mass extinctions, the Permian-Triassic extinction, which happened about 250 million years ago. 鈥淭hat involved perhaps as many as 95 per cent of all species compared with 65 per dent at the K/T,鈥� says Kerr. 鈥淎nd there is little evidence of an asteroid impact at the time.鈥�
Sulphur dioxide unleashed by supervolcanism would have created hellish acid rain, while an increase in carbon dioxide would have caused dramatic greenhouse warming. It was this warming that McLean sees as the principal culling mechanism. 鈥淚t killed off embryos,鈥� he says.
McLean鈥檚 mechanism, refined over the past few years, is not an obvious one. When the air temperature rises, he says, adult animals cool themselves by diverting blood to the surface of their skin. This reduces the flow of blood to the oviducts and uterus-the sites of fertilisation and of embryonic development.
This is important because uterine blood flow carries damaging heat away from the region of the uterus and carries nutrients to the developing embryo. 鈥淚n cows, the embryo is killed if the oviduct temperature rises by a mere 1 to 1.5 掳C above the optimum during the first cleavage of the fertilised egg-the day after insemination,鈥� says McLean. 鈥淩educed blood flow within the uterus while an embryo is developing can also cause dwarfing and abnormalities of the skeleton.鈥�
Large reptiles also control their body temperature by varying the blood flow between their core and skin. McLean says that, during the rapid greenhouse warming caused by supervolcanism at the end of the Cretaceous, the process of shunting blood away from the oviducts and towards the skin in female dinosaurs would have caused their oviducts to heat above the optimum, killing or damaging any embryos during the critical first cleavage. 鈥淭imes of rapid greenhouse warming are especially hard on large animals,鈥� says McLean. 鈥淭heir small surface-to-volume ratios mean that they retain excess body heat, increasing the mortality rate of embryos.鈥� So the supervolcanic hypothesis could perhaps explain the selectivity of the extinctions as well as their gradual nature.
Geologists believe that a superhot blob of mantle rising from the boundary between the Earth鈥檚 core and mantle caused the Deccan supervolcanism. Once this mantle plume reached the crust and spread out, its tail provided a conduit allowing the tremendous heat of the Earth鈥檚 core to melt vast amounts of rock.
Such a mantle plume can explain the presence of a global layer of iridium at the end of the Cretaceous. Iridium has an affinity for iron and would have been concentrated in the Earth鈥檚 iron core when it formed. 鈥淎 mantle plume would transport it back to the surface,鈥� says McLean.
If the supervolcanism scenario is correct, the impact of an asteroid at about the same time was simply a coincidence. In MacLeod鈥檚 view, the Earth鈥檚 biosphere is subjected to a variety of different stresses such as volcanism and impacts, all of which happen randomly in time. 鈥淥ccasionally, however, such events come together,鈥� he says. 鈥淭his happened at the end of the Cretaceous, a period of profound environmental change when even the sea level dropped by 100 metres.鈥�
Cretaceous cocktail
Napier has no objection to such a 鈥渕ultiple-whammy鈥� scenario. 鈥淚 wouldn鈥檛 want to ignore the effect of the Deccan Traps,鈥� he says. 鈥淚鈥檇 be perfectly happy if the Cretaceous extinctions turned out to have a cocktail of causes.鈥�
With so many possibilities, why has the asteroid theory become so dominant? 鈥淚t鈥檚 simple and sensational and it was brilliantly promoted by Alvarez, a Nobel laureate whose ideas demanded to be taken seriously,鈥� says MacLeod. Proof of how effectively the idea has been publicised was provided last November by an article about Chicxulub in the Sunday Times. It ran under the headline 鈥淒inosaur death: mystery solved鈥�.
鈥淲hen I read it, I couldn鈥檛 believe it,鈥� says Kerr. 鈥淭he story simply wasn鈥檛 true.鈥� And MacLeod adds: 鈥淚鈥檝e heard it said that the only thing the impact idea doesn鈥檛 have is sex and the Royal family.鈥�
Competing with such an attractive idea leaves the gradualists with an uphill struggle. And the fossil record is messy enough for the issue to remain unresolved for some time. 鈥淚鈥檓 not going to change my mind and Norm [MacLeod] is not going to change his,鈥� says Sheehan. Whatever it was that left the dinosaurs in the lurch, we can only hope that it鈥檚 not likely to recur for a good while yet.
