The survival game
Of the many great dinosaurian lineages, only the birds made it through the mass extinction at the end of the Cretaceous – but nobody is quite sure why.
Sixty-six million years ago, in the last few moments of quiet before the Cretaceous era comes crashing to a close, a herd of duck-billed hadrosaurs is gathered around a small stream at the edge of a forest on a cool and clear night. The reflection of the moon in the water ripples into a thousand pieces as several stoop to drink. Illuminated by the moonlight, small nocturnal dinosaurs festooned in feathers move between the nearby branches; some scramble, others glide or flap, on the hunt for tasty morsels. Sharing the same trees are flocks of birds, most silently perched and sleeping in the darkness. The peace is broken when one of the duck-bills hoots in alarm – a trumpeting sound made with the bony crest on its head. Slowly chewing on ferns and other foliage, the herd peers upward, its attention caught by something searing bright and soundless in the sky.
For many centuries a great celestial ballet has played out across the stage of the solar system. A comet, shaken loose from its original orbit in the distant parts of the sun’s dominion beyond Pluto, has been dancing with the planet earth. At first infrequently – but with increasing regularity, during its many circuits of the sun – it has provided a spectacular show in the night sky as the orbits of two satellites wend ever closer. This comet is 10 kilometres in diameter, taller than Mt Everest or larger than the Martian moon Deimos. This harbinger of destruction is travelling at a speed of more than 100 000 kilometres per hour and its energy of motion has the destructive force of 100 million hydrogen bombs.
Tonight the light show will be like nothing dinosaurs have ever witnessed before. As the comet makes its final approach, the earth’s gravity draws it into a tight embrace, speeding its passage even further. If this were a film, the movement of this rolling ball of rock and ice might be accompanied by a roaring or rumbling, but space is a vacuum that doesn’t transmit sound, so in reality it would have been silent until it hit the planet’s atmosphere. As the earth slowly spins, southern North America comes into the comet’s sights, specifically the area around what we know today as the Gulf of Mexico and the Yucatán Peninsula. Although the continents have assumed much of their modern configuration by the Late Cretaceous, the climate is warmer and sea levels are higher, so the coastlines look quite different.
It has been a very long time since the earth has endured the impact of an object this size, and the dinosaurs have lived in a relatively stable world for 135 million years. During this time they have adapted, diversified and spread to all its landmasses. All of that is now over, and few animals bigger than 25 kilograms will make it through the next few months alive.
Before the herd of duck-bills even has time to process what it’s looking at, there’s a blinding flash as the comet punches through the atmosphere and ocean and then plunges beneath the earth’s crust, throwing out molten rock from deep below the surface.
The initial crater is 30 kilometres deep and 100 kilometres across, although this rapidly widens and shallows as the rock beneath rebounds. The comet itself is completely vaporised upon impact, and a vast fireball consisting of this vapour, molten rock, dust and other debris is blasted out of the earth’s atmosphere and into space. Massive geological activity is triggered across the planet, with earthquakes the size of which we have no concept today. Around the Atlantic itself, gigantic tsunamis, perhaps a kilometre high, race in every direction; fossil evidence of these has been found as far away as Illinois, more than 2000 kilometres to the north.
Smouldering netherworld of the Cretaceous
Everything living within a few hundred kilometres of the impact site – such as the herd of duck-bills – would have been killed almost instantly by the initial fireball, the shockwave of superheated air, or the rain of molten rock falling back to the planet’s surface. But the impacts were felt much more widely than just this central zone of devastation; some theories suggest the atmosphere itself was briefly heated to hundreds of degrees, roasting any animals that couldn’t take cover and igniting cataclysmic wildfires that raged across all the continents. According to geologist Walter Alvarez in his book T. rex and the Crater of Doom:
Within hours of the impact, most of Mexico and the United States must have been reduced to a desolate wasteland of the most appalling, agonising destruction. Where only the day before there had been fertile landscapes, full of animals and plants of all kinds, now there was a vast, smouldering netherworld, mercifully hidden from view by black clouds of roiling smoke.
The most distant landmasses from the impact, such as Australia and Antarctica, might have escaped the worst of the initial firestorm, but tragedy would befall them in the weeks and months ahead. Within days, temperatures across the planet began to plummet as a thick blanket of smoke, dust and soot blocked out the sun’s warming rays. Then acid rain, formed from the nitrous oxide and sulfates clogging the atmosphere, began to hammer down on the surface, killing plants and animals and even dissolving rocks. This rain would have been as corrosive as battery acid and its most devastating effect would have been to destroy the shells of small marine organisms. Combined with the atmospheric pollution that blocked out the sun and stopped planktonic algae from photosynthesising, this would have been catastrophic for marine food chains globally, and halted primary productivity for decades. Once the atmosphere cleared and the earth began to warm again, vast quantities of greenhouse gases led to a global warming event that sent temperatures soaring once more for perhaps 1000 years.
‘Regionally, there is little doubt that the North American continent would have been absolutely devastated’, writes Richard Cowen of the University of California, Davis. ‘Globally, even a short-lived catastrophe among land plants and surface plankton at sea would drastically affect normal food chains. Pterosaurs, dinosaurs, and large marine reptiles would have been vulnerable to food shortage.’
Some estimates suggest that as many as 80 per cent of all species went extinct at this time. The dinosaurs, and other large reptiles such as the flying pterosaurs and swimming mosasaurs, weren’t the only kinds of animals hit hard. Around 80 per cent of mammals, 83 per cent of snakes and lizards, and even the majority of birds are thought to have gone extinct, although the numbers are unclear because birds preserved as fossils from the late Cretaceous are rare. Birds that disappeared include the enantiornithines – the ‘birds with teeth’ that were used to make a mockery of OC Marsh in the 1890s (see chapter 3) – and the hesperornithiforms – swimming birds with huge feet that shared the seas with ichthyosaurs and plesiosaurs. Up to 50 per cent of land plants were lost in some areas as well as many plankton species and molluscs, such as the coiled ammonites that make popular fossils today. Whatever combination of events came together at the end of the Cretaceous, we can tell it was a major planet-wide cataclysm, the effects of which were felt in every environment.
The end-Cretaceous mass extinction was devastating, and is the best known because it killed off the dinosaurs, but it’s not the biggest extinction event we know about. One earlier catastrophe killed off an even larger proportion of the tree of life. The Permian mass extinction, 252 million years ago, is estimated to have killed 93–97 per cent of all species. The forebears of all living things were in the 3 per cent that survived. The fact we are here at all today is a small miracle considering our ancestors also made it through three other mass extinctions – at the end of the Ordovician (444 million years ago), Devonian (375 million years ago) and Triassic (201 million years ago) periods. You may be wondering why mass extinctions always neatly occur at the end of geological periods. It’s because these events are so devastating in the history of life and mark such a change in the species found in fossil deposits that they themselves define how geological time is named and divided. Theories for the causes of these mass extinctions vary from climate change and sea level rise to massive volcanism and extraterrestrial impacts.
One of the more exotic ideas is that there is a 42-million-year cycle in the path of our solar system across the plane of the galaxy. This periodically sees it move above or below of the main disc of the Milky Way making it more likely to be bombarded with comets and cosmic radiation that has a sterilising effect on life. This may have been what killed the dinosaurs at the end of the Cretaceous, the experts claim, but they have little idea how regular the trend is and even whether it really exists or not. In case you’re wondering, right now we’re hurtling around the galaxy at around 200 kilometres per second and will complete a full circuit in 225 million years – so the last time we were in this spot was during the Triassic, when the first dinosaurs were starting to diversify. When you consider that, on top of that, we’re spinning around the axis of the earth at close to 1700 kilometres per hour, and the earth itself is travelling around the sun at 107 000 kilometres per hour, it’s enough to make you feel pretty dizzy.
Dino-death whodunnit
A rare element was the clue that led Walter Alvarez and his Nobel Prize–winning physicist father, Luis Alvarez, to the truth about the demise of the dinosaurs. In 1977 Walter was in the central Italian village of Gubbio, 160 kilometres north of Rome, where was he studying magnetism in late Cretaceous rocks. He found something very puzzling – a fine layer of red clay between the limestones of the Cretaceous period and the Paleogene (or Tertiary) period that followed it.
Walter took samples back to the University of California, Berkeley, where he and his father were both based. Luis had the samples chemically analysed for trace elements, which only deepened the mystery. The results showed the red layer had plenty of soot and a level of a metallic element called iridium 30 times that in the surrounding rocks. Silvery-white, brittle and similar to platinum, iridium is very rare on earth but much more common in asteroids and meteorites. Working with the team that had helped them analyse the samples, Walter and Luis went on to find the same layer of iridium in Denmark, New Zealand and numerous other parts of the world. There was only one startling conclusion – something enormous from outer space had smashed into the planet at the end of the Cretaceous period, leaving a fine layer of extraterrestrial iridium all over the globe.
The Alvarezes and their co-workers published their findings in an influential 1980 paper in the journal Science, but the idea was greeted with derision from some in the scientific community, who argued that an object large enough to have created a global iridium layer would have left a crater, and none was known from around 66 million years ago. More evidence came in the form rock particles in the same layer (shocked quartz and tektites) that could only have been formed in an exceptionally hot and violent event, such as a nuclear explosion or a meteorite impact.
But the clincher came in 1991, when studies of magnetic and gravitational fields revealed a staggeringly huge crater largely hidden beneath the seafloor at Chicxulub, off the coast of Mexico’s Yucatán Peninsula. At 180 kilometres across, this vast feature had been hidden in plain sight. Today, more than 30 years after the first puzzling discovery in Gubbio, the iridium layer has been found at 350 different sites around the world.
While the great majority of experts now agree that an asteroid or comet (the jury is still out on precisely which) did strike the earth around 66 million years ago, not everyone agrees that this was the only reason for the mass extinction, nor on what the effects of the impact would have been.
‘There’s general agreement that a meteorite hit’, says Paul Barrett from the Natural History Museum in London. ‘Everyone’s happy with that. There’s much less agreement on exactly when it hit and what its effects were globally. Some argue it hit a few hundred thousand years before the extinction happened, which might have meant it was a contributing factor but not the factor. A number argue that the impact would have had primarily local effects – so a devastating effect on North and South America, but not necessarily a global effect. Also, at this time we know there are massive volcanic eruptions going on in central India, which have been going on for 2 million years sporadically. This would have had significant effects on global climate, throwing sulfur dioxide and carbon dioxide into the atmosphere in vast quantities.’
Gradual changes had been taking place at the end of the Cretaceous anyway. Plate tectonics had caused the continents to drift apart, leading to new ocean circulation and air currents that cooled the planet (and that would soon cause the formerly lush southern landmass of Antarctica to freeze over). There were also major vegetation changes as the new flowering plants started to take over from the conifers.
An Australian comet strike
You may not be able to see the Yucatán crater from the surface of the earth, but if you visit the Australian outback you can clearly see the remains of another large comet that struck the planet during the reign of the dinosaurs. Gosse Bluff, a site of stark desert beauty 175 kilometres west of Alice Springs in the Northern Territory, is truly a marvel to behold. A comet hit with such force there it threw up a small mountain range in a circle around the impact site, part of which is still there today. Known as Tnorala to the local Western Arrernte Aboriginal people, the 5-kilometre-wide site is all that remains of a much bigger 20-kilometre-wide crater created 142.5 million years ago when a 600-metre object plummeted to the earth’s surface.
‘You have this perfect storm, if you like’, Barrett says. ‘Everything is changing in the last few million years of the Cretaceous. Some of it is very rapid, as with the meteorite impact, and some of it is occurring over millions of years, as with the vegetation changes. So all these things are going on … The extinction itself is rapid, but it looks like the global environment is stressed for a long time leading up to this point. We’ve kind of solved it, because we know all this stuff was going on, but we’re never going to know what actually killed the T. rex … It will only be answered if we find more fossil sites right at that moment in time. Most of what we know about it currently comes from western North America, as that’s where the best record for that time is.’
The unanswered questions are many. A few studies have even hinted that some large dinosaurs might have persisted for hundreds of thousands of years after the main extinction event.
Research from the University of Alberta in Canada suggests that the femur of a hadrosaur from New Mexico is 64.8 million years old. Only time will tell if some dinosaurs really clung onto survival for 700 000 to 1.2 million years after the asteroid hit, or if the difference is just due to discrepancies between dating methods.
One of the biggest unanswered questions, however, is why some animals made it through when others perished. Most particularly, why did some small feathered dinosaurs survive when their much more robust relatives vanished, leaving just a few scattered clues marked in stone for us to attempt to decipher tens of millions of years later?
An idiot’s guide to surviving extinction
There appear to be a few golden rules to surviving a mass extinction. Animals that are small, mobile, have lots of offspring, are generalists in habits, range over a wide area and are good at coping with stress are better represented among the survivors of these events than other species. It’s clear that birds could tick off many more of these traits than some of their much more massive and specialised dinosaur relatives. Giant sauropods, for example, are likely to have reproduced slowly and taken many years to grow to adult size. They also required vast volumes of specific types of plants, and were much less mobile than small flying animals.
Small animals typically have larger populations and wider genetic diversity. They also reproduce more quickly and can therefore evolve and adapt at a faster rate. Having many offspring creates a wider variety of individuals and makes it more likely that some of them will survive in trying conditions.
Size in particular seems to be an important factor in the survival of birds – no species of any kind larger than 25 kilograms made it past the end of the Cretaceous. ‘The obvious explanation for that is that if you are a big animal you need a lot of food’, says Barrett. ‘If there’s any kind of stress in ecosystems and you have food chains collapsing, then you are not going to do very well. Dinosaurs on average weighed a tonne, so they are all going to be hit very hard … All birds are small at this time. None would have been bigger than a seagull.’
Being wide-ranging and mobile is something else birds do particularly well today. The Arctic tern, for example, flies an annual round trip of approximately 70 000 kilometres between its Arctic breeding grounds and Antarctic wintering grounds. The oldest known Arctic tern, a bird of 34 ringed in the United States, is therefore likely to have travelled 2.4 million kilometres in its life, further than flying to the moon and back three times. In all, around half of all living bird species migrate in some way each year. And if a species has a distribution across the planet, at least some of those individuals are likely to be somewhere less impacted by the crisis, or to be able to move quickly to such an area. ‘Nearly all of the effort of flying is getting off the ground’, Barrett says. ‘This means that if they live in an area where the resources are really bad, they can just go somewhere else. So these are two big reasons birds might have done well – because they are small and very mobile.’
Being a generalist in habits makes a species more likely to survive trying conditions. Pigeons, which eat whatever they can get their beaks on in cities across the planet, are far more successful, for example, than pandas, which eat only specific types of bamboo in small montane patches of China. ‘If you have only one source of food, or need a specific plant to shelter in, then you’re doomed if that is affected or taken out’, writes Dave Hone. If, however, you can eat pretty much anything, your chance of finding enough to get you through tough times is much greater.
Another factor birds and mammals share is a fluffy covering of insulation in the form of fur and feathers – perhaps this helped shield them from the initial catastrophe or protected them from the severe cold snap that gripped the planet in the weeks and months of darkness after the impact. Some birds and mammals may have been shielded from the heat in burrows or in termite mounds, or sought sanctuary in marine environments and wetlands. Many mammals also hibernate, so may have slept through the cold snap and could have found nuts, seeds and insects in the ground to tide them over until the sun returned.
But this doesn’t totally answer the question of how birds, crocodiles and turtles survived the extinction at the end of the Cretaceous while the smallest non-avian dinosaurs (some of which were likely generalists that were common and similar to birds in many ways) didn’t. ‘The survival of birds is the strangest of all the K–T [Cretaceous–Tertiary] boundary events, if we are to accept the catastrophic scenarios’, writes palaeontologist Richard Cowen in his book History of Life:
Smaller dinosaurs overlapped with larger birds in size and in ecological roles as terrestrial bipeds. How did birds survive while dinosaurs did not? Birds seek food in the open, by sight; they are small and warm-blooded, with high metabolic rates and small energy stores. Even a sudden storm or a slightly severe winter can cause high mortality among bird populations.
The surprise is not so much that some birds made it, but that some of the small dinosaurs didn’t, argues Hone on his blog. ‘Things like dromaeosaurs and troodontids in particular had nearly all of the same characteristics as birds, as far as we can tell, and did similar things, in similar places, and in similar ways. It could of course have just been the luck of the draw; these things do happen. But if you knew in advance some birds would make it, it would have been a decent bet that dromaeosaurs would, too.’
Barrett doesn’t have a good explanation for this either. ‘Lots of other things the size of small dinosaurs all make it through relatively unscathed’, he says. ‘There were a number of small-bodied meat eaters and plant eaters that you might have imagined would be immune if body size was the key factor, but for some reason they all go.’
The diets of some birds today are quite malleable, so if they face a period of tough conditions they can switch from eating insects and seeds, or whatever they normally eat, to eating just about anything. The answer might be that small dinosaurs were too specialised in their diet, but we may never have a simple solution.
The benefits of being bird-brained
Other research teams have looked to features of early bird brains, deduced from the shape of the inside of their skulls, to understand why they fared better than other species. These studies have shown that the ancestors of modern birds were not only quicker witted than dinosaurs but also may have had a better sense of smell – both of which would have been useful skills for searching out food in the darkness and chaos that followed the impact.
In a study published in 2009, scientists at the Natural History Museum in London, led by Stig Walsh, put the case that the evolution of a larger and more complex brain may have given the ancestors of modern birds an edge over dinosaurs, pterosaurs and other groups of early birds. ‘Birds today are the direct descendants of the Cretaceous extinction survivors, and they went on to become one of the most successful and diverse groups on the planet’, Walsh, now at the Natural Museums of Scotland, told journalists. ‘There were other flying animals around, such as ptero- saurs and older groups of birds, but we’ve not really known why the ancestors of the birds we see today survived the extinction event and the others did not. It has been a great puzzle for us.’
Among living birds, the most intelligent species – such as New Caledonian crows and many parrots – have more flexible and complex behaviours, such as making use of tools. If the ancestors of modern birds were able to employ their cunning and guile to find food it would have given them a major advantage over other species. And research does indeed suggest that bigger brained birds display behavioural flexibility and are better able to survive in new environments than those with smaller brains – a skill that would have been very useful after the extinction event. ‘In the aftermath of the extinction event, life must have been especially challenging’, said Walsh. ‘Birds that were not able to adapt to rapidly changing environments and food availability did not survive, whereas the flexible behaviour of the large-brained individuals would have allowed them to think their way around the problem.’
Working with palaeontologist Angela Milner, Walsh used CT scans to look at the 55-million-year-old fossil brain cases (as a proxy for the shape of the brain itself) of two early birds that lived in the then warm, tropical conditions of England. They were surprised to find that the brains were likely to have been similar to those of modern birds in the regions that controlled sight, flight and memory, showing that – in organisation and appearance – the brains were essentially modern.
Phoenix rising
Whatever the reason for their survival, the various lineages of modern birds rose phoenix-like from the ashes of the extinction event to diversify, spread and succeed across the planet. They eventually assumed a much wider range of forms and sizes than during the Cretaceous, evolving into creatures as diverse as ducks, emus, penguins, falcons, toucans, kingfishers, flamingos, loons and wrens. They would fly in great flocks that darkened the skies of the Australian outback and populate the icy seas of the newly frozen Antarctica. A million or more of them would stand, at any one time, in the salt lakes of East Africa, while others would seek out and nest in every nook and cranny along the cliffs of the British coastline. Others would develop plumages of brilliant, vibrant colours and swoop screeching between the vines and great hardwoods of the rainforests that grew to dominate the tropics. Though the world was sadly empty of the incredible giants that had dominated it for hundreds of millions of years, the feathered dinosaurs had made it, and their journey into the modern day was well underway.
