by Tim Flannery
If you have ever had a pond you will know Azolla, a.k.a. pondweed, duckweed fern or fairy moss. Its tiny, crinkly leaves often first appear as a minute speck of floating green that seems to increase slowly. But by the time it has covered 10 per cent of the pond’s surface, it’s only days away from a complete takeover. Given warmth and the right nutrients, Azolla can double its mass every three to ten days.
The evidence that Azolla once grew in the Arctic Sea is today buried beneath thousands of metres of frigid sediments and water below a skin of ice. It may have lain unrecognised forever were it not for some very expensive cores punched deep into the Arctic sediments in 2004 by drill-rig crews searching for oil. The last thing they expected to find was evidence of pondweed. But there it was—in layers of varying thickness distributed through at least eight vertical metres of sediment. The fossils were soon dubbed Azolla arctica.4 The presence of Azolla has now been confirmed in more than 100 drill cores taken from throughout the Arctic region, with the greatest concentrations being in cores drilled from the Arctic Sea itself.
At least five species of Azolla were growing in and around the Arctic Sea 49 million years ago.5 Warmth, fresh water and the nutrients brought in from rivers provided all that the weeds required. At its height, the Azolla bloom covered about 30 million square kilometres of ocean—an area the size of Africa.6 The weed grew so vigorously, sucking in atmospheric CO2 in the process, that it reduced the global atmospheric concentration of CO2 from at least 1000 parts per million to 650. And all that captured carbon would go on to form the Arctic oil reserves that the petro-giants are so keen to get at today.
The Azolla blooms eventually extinguished themselves, for the lack of CO2 lowered global temperatures so substantially that rainfall declined at the poles, causing inflows of freshwater and nutrients to taper off, which starved the weed.* As temperatures continued to drop, a layer of ice formed over the Arctic Sea. Thus was a new icehouse world initiated by a minute weed. Initially, however, the lowering of CO2 concentrations had remarkably little effect on Europe—it was almost as if the preconditions for a major change had been established, but the trigger had yet to be pulled.
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* As remarkable as it now seems, the idea of extinction was opposed on the basis that supposedly extinct species surely survived at some location—perhaps in the unexplored American West—and on the theological grounds that God would not extinguish his own creations.
** A much easier argument to make prior to the publication of Darwin’s Origin. Cuvier died in 1832.
*** The gypsum used to create the famous plaster was formed around 50 million years ago, when vast lagoons dried out, leaving the mineral in thick deposits.
* Lamarck is best known for his theory of evolution, in which he posited that a creature’s experience during its lifetime could be passed on to its offspring.
* This is an excellent example of Gaia’s self-regulation: a negative feedback loop that prevents life from pushing Earth’s climate out of the habitable zone.
CHAPTER 13
La Grande Coupure
As the twentieth century dawned, bringing such wonders as powered flight, electricity and the motor car, the Swiss palaeontologist Hans Stehlin remained glued to his microscope, sitting in his office at the Basel Natural History Museum in Switzerland, pondering old bones. He had become something of a legend for his dogged pursuit of palaeontology, but it seems there was more to his dedication than scientific interest. According to museum folklore he had been thwarted in love, and to forget his misfortune had poured all his energy and passion into his work. Handsome, with a Freud-like beard and piercing eyes, it was also said that he had perfected the death stare. Whenever he required the skeleton of some exotic beast to compare with his fossil bones, he would visit Basel Zoo and stare at the appropriate animal, which would soon thereafter shuffle off its mortal coil.
Sometime around 1910 Stehlin realised that a dramatic change had occurred in Europe’s fauna about 34 million years ago. During a period of turbulent climate change, many species that had survived for millions of years suddenly became extinct, and a host of new species arrived. Stehlin labelled the event la grande coupure—the great cut. And scientists have been arguing about its precise causes and timing ever since. La grande coupure, now dated at 34 million years ago, has been widely hailed as marking the end of that inordinately long tropical epoch, the Eocene, and the beginning of the colder, drier Oligocene. In general terms this is true, for la grande coupure marks a fundamental reorganisation of climate—from a predominantly greenhouse world to an icehouse.*
The cause of the climatic shift this time appears to have been the parting of South America from West Antarctica. The Drake Passage, as the seaway separating these landmasses is known, was initially shallow and remained so for millions of years, but water flow was sufficient to allow the establishment of an ocean current that encircled Antarctica. This allowed cold water to build up and an ice cap to form, which led to a fundamental reorganisation of ocean currents and winds, bringing much cooler conditions.
In Europe, this shift was accompanied by changes in the hydrological cycle. The story of precisely what happened is eloquently told in snail shells—specifically in the fossilised shells of the freshwater snail Viviparus lentus, which once thrived in a coastal floodplain where the Solent now lies, off the Isle of Wight.1 Lister’s river snail (Viviparus contectus)—a large freshwater species with a striped shell, which survives in lakes in Britain today—gives us a good idea of how its ancient relative looked and lived. Isotopic studies of the fossilised snail shells reveal that cold water exported into the north Atlantic from Antarctica caused air temperatures in southern Britain to drop by 4–6° Celsius. But in summer, which is when the snails grew, temperatures plummeted by nearly 10° Celsius. Along with the changing climate, something else very important was occurring. The Turgai Strait, which stretched from the Tethys Sea via what is now the Caspian Sea and on to the Arctic Sea, would in parts disappear. As a result, Europe and Asia were finally connected. And, at about the same time, Europe and North America were joined by a land bridge for one final, brief, moment.
In 2004 the redoubtable Jerry Hooker and his colleagues took another look at Stehlin’s big cut. Examining sediments exposed around the Solent estuary and in northern France and Belgium, they showed that things were, as usually proves to be the case, far more complex than they had first appeared. There was among the fossils they examined evidence for two quite separate extinction events—a smaller one, which coincided with a change in climate, and a larger one a few hundred thousand years later, which coincided with the arrival of new mammalian invaders.2
One of the few surviving lineages was the dormouse. Dormice are not actually mice but members of the family Gliridae, ancient rodents whose ancestors arrived in Europe 55 million years ago from North America. They proliferated, adapting to European conditions and expanding into a wide variety of ecological niches. For more than 40 million years they remained restricted to Europe, before spreading to Africa some time after 23 million years ago, and only much later to Asia. They are Europe’s oldest and most venerable mammals, though their current diversity is the merest vestige of their former glory.
The Oligocene extended from the la grande coupure of 34 million years ago to about 23 million years ago. Despite the cooler conditions, much of Europe’s vegetation remained largely subtropical to tropical. The Turkish coast abounded in mangroves, nipa palms (which cannot tolerate temperatures below 20º Celsius) and other vegetation today associated with tropical southeast Asia.3 About 28 million years ago sea levels dropped once more, and even cooler conditions prevailed. Yet the Turkish plant fossils indicate that rattans and cycads persisted distant from the sea in a forest that was largely made up of flowering plants, including Engelhardia (today restricted to southeast Asia), hickory (no longer found in Europe), and an ancestral hornbeam.4 Though forests continued to hold sway over much of the land, deserts and grasslands w
ere gaining footholds in places like Iberia, allowing for a greater diversity of animal species.
Meanwhile, the land itself was undergoing momentous changes, not the least of which was the uplifting of Europe’s most majestic mountain range, the Alps. The origins of the Alps go back to the age of dinosaurs, but that early uplift was followed by a period of quiescence, which ended at the beginning of the Oligocene when a downwards bending portion of the European plate broke off and started to make its way towards the surface, forcing the modern Alps to rise.
The development of Europe’s current topography involved much subsequent folding, faulting and thrusting. Fragments of land were moving in all sorts of ways and in different directions. Some appear to have shot from one side of the Mediterranean to the other with (by geological standards) great alacrity. Others were shoved deep into Earth’s mantle, becoming melted or deformed in the process, and sheets of rock that originated in Africa, known as nappes, were pushed over rocks of European or oceanic origin. One such piece of African geology was destined to become the peak of the Matterhorn, often called the ‘African mountain’.* As Africa and Europe closed in on each other, vast expanses of the ancient Tethys Sea floor were uplifted then eroded away, giving rise to some of the spectacular limestone landscapes that can be seen today in the foothills of the Alps.
The driving force was Africa. Originally drifting north-northeast, from about 16 million to seven million years ago, it swerved slightly in a north-northwesterly direction. Thereafter it shifted again and began moving northwest—the direction it continues in today.5 This counterclockwise twisting severed the Tethys Sea, temporarily blocked up the Strait of Gibraltar, and thrust the Alps skywards. Indeed, as Africa makes its way north, the Alps continue to rise—at the rate of between one millimetre and one centimetre per year, though they are weathering away almost as fast as they grow.
I wouldn’t be surprised if George Orwell took inspiration from the Oligocene for his novel Animal Farm. Whatever the case, as in Orwell’s cautionary tale, the Oligocene’s signature species were an unsavoury group of pigs and pig-like creatures, foremost among which were the entelodonts, more popularly known as hell pigs, or terminator pigs. The ancestors of these cow-sized creatures had migrated from Asia about 37 million years ago. Palaeontologists will tell you that they were not pigs but relatives of hippos and whales. But had you met one, your first impression would have been of a gigantic, hyper-carnivorous warthog.
Perhaps the entelodonts’ most unlovely feature was their oversized heads. They were garishly ornamented with bony warts the size and shape of a human penis, and their almost crocodilian jaws bore savage tusks and grinding molars. Unlike modern pigs, which aren’t averse to flesh-eating but mostly subsist on vegetable matter, the entelodonts were apex carnivores. And they were fast, their long, slender legs carrying their 400 or more kilograms with a speed that would leave wild boars, and humans, in the dust.
Their efficiency as carnivores is attested to by the discovery of caches of fossilised remains of their victims. One North American cache consisted of the skeletons of several sheep-sized ancient camels.6 It is thought that the entelodonts ran down herds of smaller beasts and slashed and battered them en masse, a practice that compelled them to store their leftovers. Or perhaps rotting flesh was easier on their guts, which had evolved from that of herbivorous ancestors, so they buried their victims to soften up the bodies.
As if hell pigs were not enough of a blight on the Oligocene landscape, Europe was home to two more groups of pig-like creatures. The anthracotheres (meaning coal-beasts, because the first fossils were discovered in coal seams) thrived during the Oligocene. Although related to hippos they were finer, and most appear to have had a pig-like lifestyle. One mysterious anthracothere known as Diplopus (named for its two digits at the end of each very slender leg) entered Europe just before la grande coupure, presumably by swimming the Turgai Strait. Many of its bones are known, including the delicate scapula, but no trace of the skull has ever been found—a fact that causes Jerry Hooker to shake his head in disbelief. The most common remains of other creatures he finds are jaws and teeth, and the lack of Diplopus heads is a deep mystery. Incidentally, the natural history museum in Berlin is the proud owner of an anthracothere turd. It is black and resembles an oversized dog dropping. I was delighted to learn that it appears to be made up largely, if not entirely, of vegetable matter.
Today’s pigs are classified into two families: New World peccaries and Old World pigs, including the wild boars and warthogs. Neither group existed in Oligocene Europe, though there were plenty of creatures that looked like both peccaries and pigs. Instead Europe’s pig-like creatures belonged to an extinct group known as Old World peccaries (family Palaeochoeridae).7 Arising from Asian migrant ancestors, Palaeochoerus (which rather prosaically means ‘ancient pig’) was a typical example. Smaller than a modern wild boar, with a compact body and short limbs, its trotters were unmistakeably pig-like. Moreover, its molars were rather cuspy, which suggests that it had already embarked on an omnivorous diet.
Having seen the depredations that Australia’s wild pigs can inflict on lambing sheep, I find pigs hard to love. My dislike was reinforced in 2016 when researchers discovered that more than three-quarters of the domestic boars and 40 per cent of wild boars they examined had bite injuries to their penises. The photos are horrific. Just who the biters were remains a mystery, but I think something goes wrong when a fundamentally herbivorous creature acquires a taste for flesh.8
Although they may have arisen in Europe, today’s peccaries are exclusively American. They are social, with herds as large as 2000 occasionally reported, and they can attack people. On the night of 30 April 2016, a woman from Fountain Hills, Arizona, was out walking her dogs when she was attacked by six collared peccaries. They knocked her to the ground and tore at her neck and upper body with their teeth, causing serious injuries. Fortunately, the woman was rescued by her husband.9
The Oligocene was not all pigs. Beavers, marmots and hedgehogs entered Europe from Asia, as did the true tapirs and rhinos as well as ruminants. Today the ruminants—species that cheweth the cud and are cloveth of the hoof—are a particularly important group that includes cattle and sheep, deer, giraffes and antelopes. The very earliest European ruminants were small and resembled musk deer, with sabre-like canines and no horns.10
Some immigrant carnivores may have come from North America rather than Asia. One such was Eusmilus, a sabre-toothed killer known as a nimravid. Short of limb and long of canine, this rather unlovely creature was, confusingly, not at all related to later types of sabre-tooths (which are members of the true cat family). The dog-bears (hemicyonines) looked like bulky, short-tailed dogs, but were in fact related to bears, and if this is not confusing enough, there was also a group of bear-dogs (amphycyonids). These close relatives of the canines had come to resemble bears, the largest of which weighed 600 kilograms. They evolved in North America, but by the Oligocene had spread to Eurasia, where they pursued omnivorous and carnivorous lifestyles.
There were also many rodents in Europe during the Oligocene, including a proliferation of dormice, squirrels, voles and beavers. One small creature of note is the ancestor of the desmans. Europe’s most distinctive mammals and members of the mole family that have taken to life in rivers, streams and ponds, there are only two species—one found in the Pyrenees and the other in eastern Russia. The Russian desman, which can weigh half a kilogram, is by far the largest member of the mole family—large enough to be sought for its fur.
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* The Eocene’s end is, in fact, marked by the demise of the obscure single-celled planktonic foraminiferal family known as the Hantkeninidae. But la grande coupure occurred in two pulses about 350,000 years apart, just after the last hantkeninids found their eternal rest.
* To be precise, it is the portion of the Mattterhorn lying above 3400 metres that is African. The lower slopes are composed of marine sediments or European rocks.
The African rock is part of the Apulian Plate, itself originally part of Africa, most of which underlies the Adriatic Sea. It was thrust above the other rocks of the Alps as the Apulian and Eurasian plates collided.
CHAPTER 14
Cats, Birds and Olms
About 25 million years ago, as the Oligocene was nearing its end, the first member of the cat family, known as Proailurus, emerged from Asia and entered Europe. Roughly the size of a domestic cat, its fossils have been found in Germany, Spain and Mongolia. For 10 million years after they arrived in Europe, despite the great numbers of small rodents present, cats showed no sign of prospering, let alone attaining the dominance among ambush predators they have today.
Bird bones make miserable fossils, and the incomplete record from the Oligocene in Europe leaves much room for conjecture. A few scraps found in England and France have been trumpeted as evidence that New World vultures (a group that includes the condors) once soared in Europe’s skies. But a reassessment suggests that they are the bones of an Oligocene cuckoo-roller.1 The bones of Paracygnopterus have been found in southern England, which some claim to be the oldest member of that most ornamental group of waterfowl, the swans. Others, however, opine that it is merely a goose.2
The evidence is better that loons, aquatic diving birds, abounded on lakes in Oligocene England and Belgium, chasing fish in much the same manner as existing species, though whether they uttered anything like the haunting cry of their surviving relative the great northern diver cannot be known. Less expectedly, a species of secretary bird—a creature familiar today on the savannas of Africa—stalked the grasslands of France. But the most important Oligocene arrival was that of Europe’s first songbirds—perhaps at the time of la grande coupure. These early migrants subsequently became extinct in Europe, replaced by later songbird migrants.3