by Tim Flannery
The cause of the extinction of the dinosaurs has been long debated. Some palaeontologists argued that climatic or geological changes interrupted the dinosaurs’ food supply, but no one could convincingly explain what had happened until, in 1980, a team of researchers—led by the physicist Louis Alvarez and his geologist son Walter—suggested that an asteroid had struck the Earth, causing a nuclear winter severe enough to trigger a mass extinction event. The team announced that they had evidence of this—in the form of sedimentary layers rich in iridium derived from the asteroid—in rocks from around the globe. Building on this pioneering work, in 2013, a team led by Professor Paul Renne of the Berkeley Geochronology Center used argon dating to pinpoint the moment of the strike: 66,038,000 years ago, give or take 11,000 years.2
Some palaeontologists seemed outraged by the bolide theory of extinction or—more accurately, perhaps—that a person from outside their discipline dared dabble in their business. They argued that the dinosaurs existed for millennia after the impact, or that at the time of the disaster they were in gentle decline anyway. Others denied that an asteroid impact could have such a catastrophic effect.3 Despite counterarguments, it is now accepted that some sort of heavenly body (a bolide) struck Earth and caused the extinctions. Increasingly, scientists are convinced that the offending object was a meteorite or comet of about the size of Manhattan Island.
So how bad could an asteroid strike be? One answer comes from the fact that it takes a great deal of force to shock quartz. Indeed, until recently, it wasn’t believed that quartz was shockable. Then scientists examined some sand grains from the vicinity of an underground nuclear test. The power of the blast had been sufficient to deform the quartz’s crystal structure, which showed up as microscopic lines in the grains. It takes more than two gigapascals (two billion pascals) of pressure to shock quartz in this manner (for comparison, the atmosphere at sea level exerts a little over 100,000 pascals of pressure). Volcanoes, incidentally, don’t shock quartz. Even though they can generate the required pressure, shock requires temperatures to remain relatively low, and volcanoes are too hot. The bolide that exterminated the dinosaurs released two million times more energy than the largest nuclear test ever conducted, creating the greatest volume of shocked quartz in the history of our planet: the stuff is ubiquitous in rocks formed at that time.
The bolide struck near the Equator, on what is now the Yucatan Peninsula in Mexico. The impact displaced about 200,000 cubic kilometres of sediment, and the shockwaves would have rung Earth like a bell, triggering volcanic eruptions and earthquakes globally.4 The mega-tsunami it caused is estimated to have been several kilometres high—one of the largest in the planet’s history, and must have still been substantial by the time it reached the European Archipelago. In the aftermath, flaming debris rained from the sky, triggering firestorms that consumed forests wholesale, leaving behind great layers of charcoal. Because oxygen levels were higher then, even wet vegetation would have burned.*
When the fires died down, a nuclear winter, initiated by particles blasted into the atmosphere that obscured the sun, commenced. To add to the destructive effect, the bolide had landed in a bed of gypsum, creating enormous volumes of sulphur trioxide that mixed with water to produce sulphuric acid, reducing the amount of sunlight reaching Earth by as much as 20 per cent, exacerbating the nuclear winter so that it caused freezing temperatures and prevented photosynthesis for about a decade. Paradoxically, the nuclear winter was followed by global warming caused by CO2 released by fires and volcanic activity. Ocean circulation would have ceased abruptly and may have remained severely impaired for thousands of years. Marine life was devastated. Never again would the world see the likes of the glorious ammonite, or the ungainly plesiosaur. Nor would it have the rudist clams or the artillery-shell-like actaeonellids.
The impact site was relatively close to the European Archipelago, and we can anticipate that the consequences of tsunami and wildfire there were severe. Nowhere on Earth could have escaped the nuclear winter that followed. Almost everything weighing more than a few kilograms—including Europe’s stunted dinosaurs and Bajazid’s turtle—became extinct. Even many smaller creatures, including Europe’s whiptail lizards, its matsoiid snakes and some primitive mammals, also vanished. Sic transit gloria mundi!
Europe’s freshwaters, however, provided important refuges. Its amphibians came through largely unscathed, as did some of its aquatic turtles. Deep water buffers extremes of heat and cold, and freshwater ecosystems can survive for a time without photosynthesis, because bacteria and fungi feeding off detritus washing in from the devastated land provide a base for the food chain. Eventually, at the top of the food chain, frogs and turtles can scavenge. So it was that the ancestors of the delicate salamanders and the midwife toads survived global catastrophe.
Frustratingly, we have almost no European fossils from the moment of the bolide strike to tell us what was happening on land. We are more fortunate when it comes to the seas. In Italy and the Netherlands, among other places, the precise moment of the strike can be seen—and touched—in stone. Indeed, it was at Gubbio in Italy’s Apennines that the iridium layer was first identified and studied, after being splendidly exposed in a roadside cutting. The layer proved to be rich in small glassy spheres—the remains of rocks that had been melted and shot out of Earth’s atmosphere, before solidifying and raining back down.
Perhaps the most impactful of marine extinctions, at least in Europe, was that of the coccolithophores, whose fossils, deposited by the gigatonne, formed the chalk that gives the Cretaceous Period its name. From the white cliffs of Dover to the chert used for building and to the rocks of the tunnels of World War I battlefields in Belgium and northern France, Europe is filled with evidence of the coccolithophores’ past abundance. With the extinction of many key types, the creta (chalk) would never form again.*
Although most of us are oblivious to the threat, asteroid strikes continue to be a possibility. In December 2016, NASA scientists warned that we are ‘woefully unprepared’ in the event of an asteroid or comet striking Earth.5 Even a far smaller strike than the one of 66 million years ago could devastate our civilisation.
______________________
* In geology, a basin is an area of down-folded or down-faulted rocks that has accumulated a thick layer of sediments.
* Oxygen dropped to near present levels after the impact.
* Some coccolithophores must have survived, because chalk deposition continued in a few places, including England and Denmark, for a few million years after the bolide impact.
CHAPTER 8
A Post-Apocalyptic World
The great bolide extinction event marks the end of the age of dinosaurs and the beginning of the age of mammals. Known as the Cenozoic Era, meaning ‘recent life’, it’s the division of time that we live in. The Cenozoic is divided into epochs, beginning with the Palaeocene, which extends from about 66 to 56 million years ago.1 Meaning ‘old new’, this confusing name was coined in 1874 by Wilhelm Philippe Schimper, a French expert on mosses who also dabbled in palaeobotany.
What was the European Archipelago like once the climate settled down, and life began to reclaim the land? Frustratingly, we face a ghastly blank in the fossil record at this critical moment—a blank that persists for five million years. The chance that fossils of land-based creatures would be preserved was not helped by the fact that much of the European Archipelago was submerged at the time (though large islands did exist). But from evidence elsewhere, particularly in North America, we can assume that for millennia a devastated landscape dominated by ferns existed.* Then, slowly, the surviving trees and bushes emerged from their refuges, perhaps from deep valleys or the seed bank in the soil, or from seeds that drifted across the ocean. But the climate was now altered: Europe was cooler and drier, so new types of plants flourished, while some survivors now found conditions difficult.
Despite the changed climate, how the trees must have grown! For they were not only freed from t
he browsing lips of the dinosaurs, but from the mouths of many leaf-eating insects that, at least in North America, had become extinct as well.2 It seems reasonable to assume a similar impact in Europe, which would have allowed the island forests to grow denser and more quickly than ever before. Reproduction, however, may have been more difficult, for pollinators and seed dispersers were in short supply.
What was life like in those rapidly growing forests? We gain our insight courtesy of a hole, 25 metres deep and just a metre wide, dug in a football field at Hainin, near Mons in Belgium, which intersects sediments laid down about five million years after the bolide impact. The excavation resulted from a chance discovery in the 1970s when geologists drilled several smaller holes hoping to obtain samples of marine sediments. Instead they found something infinitely more valuable: fossils of the earliest European land-based creatures from the age of mammals.3 Subsequently, three other holes were sunk in the football field, each yielding new fossils and new insights into a vanished age.
In a brief moment of glory just before the drilling frenzy, the ROC de Charleroi-Marchienne played in First Division, but today it lingers in Third Division B. I hope that the drilling of the football field had nothing to do with it, but, speaking for myself, I would have dug up half of Brussels itself for the fossils that those drillers got at Hainin. Admittedly, the volume of finds was rather meagre. The 400 fragments—mostly isolated teeth from rat-sized mammals, and a few bones of reptiles, amphibians and fish—would fill a matchbox or two. But what a yield of information! They tell us that the fresh waters at Hainin must have been substantial, for they included the remains of a large predatorial fish known as a bonytongue, or saratoga. Much sought after by game fishers, today they are only found in the rivers of southeast Asia and Australia, but at the time the Hainin deposit was forming they occurred all over the world.4 The bones of ancient alytids—ancestors of the midwife toads—were also present, as were the remains of a salamander.
The albanerpetonids—was there ever a more awkward name? Let’s call them the pert’uns—were newt-like amphibians that burrowed through leaf litter. Their fossils are found in North America, Asia and Europe (including at Hainin), where they occur in sediments formed both before and after the bolide strike. Imagine a pert’un lying in the palm of your hand. An inhabitant of the soil, it is probably dark in colour, and might be mistaken for a knobbly skinned newt. But unlike any newt, pert’uns feel hard, because under their skin they have bony armour. The creature raises its head to look at you, revealing a lithe and flexible neck unlike that of any living amphibian.
Amphibians were the first vertebrate colonisers of the land—back in the Devonian Period, some 370 million years ago. Today we have just three major lineages of amphibians—the anurans (frogs and toads), the newts and salamanders, and the worm-like caecilians, all of which can trace their ancestry back to long before the dinosaurs. The pert’uns were a fourth lineage—one that originated at the very beginning of the amphibian story. Over the generations, the eyes of pert’uns have taken in most of the history of life on land. And we humans almost got to see them. In 2007, fossils dating to a mere 1.8 million years ago and preserved in deposits formed in limestone near Verona, Italy, were recognised.5 To be robbed of the chance of meeting a pert’un by such a small span of time (geologically speaking), after they’d been around for 370 million years, seems tragic. It would be lovely to think that in some obscure valley in Europe, a pert’un survives today.
It seems very odd that the eggshells of two different kinds of turtles were preserved at Hainin, as eggs rarely fossilise. We cannot identify the turtles that produced the eggs, but the fact that three of the four great European turtle lineages became extinct when the bolide hit limits the possibilities. The only survivors were the side-necks, but they were on borrowed time, becoming extinct about 10 million years later. All European turtles living today are descended from immigrants that arrived after the bolide impact.
Two different crocodile-like creatures are represented by a vertebra each, so little can be said about them.6 But two other tiny vertebrae attest to something far more interesting: the presence of a blind snake. Blind snakes are the most primitive of all snakes, and the Hainin bones are the oldest fossils of a blind snake found anywhere on Earth.7 Burrowing creatures, they live like worms, which they closely resemble, and feed on ants and termites. A single species, found in the Balkans and the Aegean Islands, survives in Europe today.
Fossils of amphisbaenids—bizarre, subterranean, worm-like lizards that originated in North America more than 100 million years ago—were also found at Hainin. About 10 centimetres long, they are formidable predators with horrible-looking eyeless heads and powerful, interlocking teeth that can tear hunks off their victims, which are eaten alive. With loose skin that seems to move along of its own accord and drags the body behind it, an amphisbaenid can move forwards or backwards with equal ease. Blind, pale of skin and uncanny, some bear a resemblance to the Seer of Kattegat in the television series Vikings. The amphisbaenids survived the bolide impact in North America, and their presence at Hainin indicates that they migrated to Europe very early on.8 Unlikely seafarers, they appear to have crossed the north Atlantic on drifting vegetation.9 Today, four species of amphisbaenids survive in Europe—two on the Iberian Peninsula and two in Turkey.*
A most striking thing about Hainin’s fauna is how chthonic it is. Salamanders and toads, sightless, burrowing lizards and blind snakes are all creatures of the earth itself. As I think about their world, I’m reminded of images of Europe in the wake of a far more recent catastrophe. Film footage from the end of World War II captures poor, beleaguered creatures emerging from their burrows among the rubble into a devastated and sadly reduced world. It is as if only the bowels of the Earth itself could offer refuge from such destruction.
Sixty-six million years ago, the consequences of the bolide impact lasted not decades but for millions of years. Yet life did eventually recover. In a forest beside the sea, as palaeontologists believe the Hainin site once was, those regrowing groves were enlivened by one group of small survivors. Scrambling over fallen logs and up into the branches was a surprising diversity of rat-sized mammals. The most abundant were fifteen-centimetre-long nocturnal eaters of insects and fruit known as adapisoriculids. They were long thought to be related to hedgehogs, but more recent studies identify them as primitive creatures that did not develop a placenta, but which in other respects were similar to the placental mammals. They looked much like rats, and existed for about 10 million years after the bolide strike. Most species were European.
Among the most intriguing mammals lurking in Hainin’s forests were the kogaionids—initial survivors of the bolide impact, we met them briefly on Hateg. Unique to Europe, their remains abound at Hainin; one kind Hainina, takes its name from the site. Great survivors they may have been, but the kogaionids were very primitive mammals that probably laid eggs. Although not much larger than rats, the kogaionids could never have been mistaken for rodents. Let us imagine that we are in Hainin’s ancient forests. A movement in the umbrageous undergrowth betrays the presence of something leaping from the ferns. It moves about just like a frog but is covered with fur. Behold a kogaionid, the only mammal ever to have developed a means of locomotion akin to that of frogs and toads.10* As it opens its mouth to consume the blind snake it has ambushed, you see large shearing premolars, which it uses to cut up its prey. Strangely, the long lower incisors it has impaled its victim on are blood-red—a result of their enamel being strengthened with iron.11 Rat-sized, primitive ungulates, marsupials and elephant shrews complete the Hainin mammal fauna.12 All could have survived the bolide impact in burrows and, during the dark chill that followed, by eating small invertebrates such as worms, hoppers and insects, or the seeds left behind in the soil.
______________________
* Some ferns are pioneer species, able to quickly colonise bare ground.
* The ancestors of Europe’s amphisbaenids appear to have
arrived across the sea in separate migrations.
* There is still some debate about the locomotion of the kogaionids, Earlier reconstructions depicted them to be squirrel-like creatures, but a recent re-analysis suggests that they moved like frogs.
CHAPTER 9
New Dawn, New Invasions
Ten million years after the extinction of the dinosaurs a new geological epoch was dawning. The onset of the Eocene is marked by a shift in the ratio of two isotopes of carbon—C12 and C13—indicating an eruption of fossil carbon into the atmosphere. The event is one of the most striking in Earth’s history. Within 20,000 years—a mere geological instant—the fossil carbon caused global temperatures to increase by between 5ºC and 8ºC and they remained elevated for 200,000 years. At the same time the oceans, especially the north Atlantic, acidified. Ocean circulation changed radically (in some regions reversing), and deep-sea foraminifera (single-celled organisms) went extinct en masse. On land, rainfall patterns changed, with some regions subjected to biblical deluges and others drying up. Erosion and leaching on an unprecedented scale depleted the soil, laying down vast new sediment beds on river floodplains. Rainforests flourished as far north as Greenland.
Some researchers think that the warming was caused as kimberlite pipes (volcanic vents originating deep in the Earth’s mantle) reached the surface near Lac de Gras in northern Canada and released huge amounts of carbon. Others think that a release of natural gas from the ocean depths was the cause. The extreme acidification of the central and north Atlantic supports this idea, as does the presence of several large crater-like structures on the ocean floor, which have narrow sheets of volcanic rock, known as sills, at their bases. The molten rock in the sills may have ignited vast reserves of shallowly buried natural gas, much like a match applied to a gas barbecue.1 Whatever the cause, it is generally accepted that the warming was triggered by a smaller annual flow of carbon than humanity is currently contributing to the atmosphere.2