The mass dying out of the late Permian Period was the greatest of all extinctions. Although some animals, like this burrowing Lystrosaurus, may have tried to escape their fate, eventually 90% of all the animals on Earth disappeared. This scene will repeat itself at the end of the world.

ONE

THE DEEP PAST
A Tale of Two Extinctions

Karoo Desert, South Africa

On a cloudless September day a paleontologist gears up for a collecting trip in the dry Karoo Desert of southern Africa. His route will cover a distance of about 3 kilometers, beginning at the base of a large valley overlooked by a high mountain ridge known as Lootsberg Pass. The traverse will take him through time as well as space. As he climbs up through the bed of an ephemeral creek he will be ascending a stairway made of stacked layers of sedimentary rock, each stratum representing a slice of time, starting with 251-million-year-old strata and ending in 249-million-year-old rocks. Somewhere on this walk he will pass through the remains of a singular biodiversity catastrophe, the single most calamitous mass extinction to have ever savaged the Earth, an event so severe that it has forced geologists to subdivide time around it. He begins his traverse in Permian rocks, representing the last time interval of what is termed the Paleozoic Era, so named because of its archaic assemblage of fossils. He will end up in the Triassic period, the first unit of the Mesozoic Era, or �time of middle life�. The division between these two groups of strata was caused by mass extinction. The various tools of his trade are attached to or slung on the hooks, holsters, belts, and vest he wears; water and food are doled out and stored in the backpack that completes his

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burden. A broad-brimmed hat tops it all off, and he laughs at himself � Halloween in Africa, a geologist in costume. With no more ceremony than locking the vehicle doors, he sets out into a basin tens of kilometers across.
His first impression is of heat, first felt against his rapidly drying skin, then glimpsed as faintly perceived shimmers in the clear air. Great vultures ride these thermals, but otherwise the vista is exanimate. Only a thin ribbon of road brings order to the wide valley floor, and a sense that the living share this place with the fossil dead. The air is so clear that great distances lie visible, as if the landscape is part of another, larger planet, where the very horizon recedes impossibly far, or perhaps this world of dreams is flat. Green fights a losing battle among the low, dispirited shrubs and thorny scrub, fading to relentless brown, a thousand shades of brown; a color elsewhere so monotonous yet here so diversified.
From his vantage point at the base of the valley, the world seems to have encountered a great test, and been found wanting. A test of life failed; failed in the present, failed immeasurably more so in the quarter-billion-year-old past, for Loots-berg Pass is a fossil graveyard, a headstone to Planet Earth�s greatest extinction.
Three hundred million years ago, in a time long before dinosaurs, mammals, or birds had first evolved, the southern part of what we now call Africa was gripped in the glacial deep freeze of a profound ice age. Slowly, the land warmed, and a landscape suitable for life emerged. First low mosses, then higher forms of life colonized the rapidly warming region, eventually creating a lush world of wide river valleys far from the sea. Into this region animals found their way, and thrived. They left their-remains in the ancient river sediments, remains that only now are eroding free in the isolated sedimentary rock banks and outcrops beneath Lootsberg Pass.
The geologist makes his way to low outcrops of greenish sedimentary rock carved into the grass and scrubland making up the wide valley floor. The sedimentary rock beds in this or any other exposure are windows to the deep past, for it is within such strata that information about ancient environments, as well as ancient inhabitants, is entombed. Because of their textures and bed form, these particular sedimentary rocks could have formed only in rivers. The rocks also bear fossils, remains of ancient plants and animals.
The river valleys of 250 million years ago would have looked much like any river valley today, with meandering streams and swamps. But the rich plant life would probably seem exotic and peculiar to us if we could somehow be transported back to this ancient time. While the world today is dominated by flowering plants, the fossils in these greenish river deposits are from species far more ancient: mosses, ferns, club mosses, ancient horsetails, and most commonly, seed ferns of a type called

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Glossopteris (the modern-day ginkgo is a descendant, and gives us a sense of what this plant may have looked like). Giant horsetails in stands like bamboo might have lined the riverbanks. Ferns, mosses, and primitive plants known as lycopods were also common. There might have been savanna-like regions as well (but without grass, a much later innovation). Paleobotanist Bruce Tiffany envisions the Karoo vegetation as �gallery forests�, isolated stands and thickets composed of seed ferns, with conifer trees in areas of moisture, surrounded by regions of true ferns. The ferns may have formed extensive communities, almost like grasslands. All of this richness fringed the watercourses; in more upland regions, away from water, there may have been little vegetation. All in all, it was an ideal place for land life.
At first only squat, belly-dragging amphibians lived in these river valleys. But as eons passed, more advanced land dwellers arrived or evolved: the fully terrestrial reptiles, small at first but rapidly enlarging until a great diversity of spectacular and hulking behemoths waddled and shuffled about the landscape. Several stocks lived in this ancient African splendor. Most common were four-legged creatures called therapsids, or �mammal-like reptiles�. But other reptilian legions spawned here as well, such as the ancestors of turtles, crocodiles, lizards, and, eventually, the dinosaurs. Some were hunters, far more were hunted. All have left a copious fossil record of their presence, for the Karoo strata are packed with bones.
The therapsids are virtually unknown to us in any sort of cultural context; theirs is the true lost world. When in Edwardian times Sir Arthur Conan Doyle wrote his scientific adventure story The Lost World, he recreated an environment known at that time only to academics: the world of the Mesozoic Era, known to us as the Age of Dinosaurs. He created a place lost in the world because of geographic isolation, but he was really painting a picture of scientific isolation, for even in the early twentieth century the great Age of Dinosaurs was still a lost world, so little did science (and the public) know about it. The Age of Dinosaurs is clearly no longer so lost. Every schoolchild knows the dinosaurs� tongue-twisting names, their food preferences, and even their color schemes. Nothing so well known to Hollywood and popular culture can be considered lost. Instead, the true lost world is that of the mammal-like reptiles � a time and place that disappeared from the Earth a quarter billion years ago.

Paleontologists now have a fairly accurate census of the large vertebrate genera living in the Karoo Basin just prior to the great extinction. There were two amphibian genera (and thus at least two, but probably more, species), six types of captorhinids (ancestors of turtles), two eosuchians (ancestors of dinosaurs, crocodiles, and birds),

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The �. rex of its time, the gorgon was the largest of the Paleozoic predators.
The drawings here represent four possible renditions of what this animal might have looked like.

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nine mammal-like reptiles known as dicynodonts (which shared a common ancestor with mammals), three biarmosuchians (a primitive group of reptiles), nine gorgonopsians (all large, fearsome predators), ten therocephalians (another group of now-extinct reptiles), and three cynodonts � doglike predators that are on the direct line of all living mammals. All told, forty-five separate genera of vertebrate creatures are known from this last million years of the Permian Period.
This census demonstrates that life was diverse in the Age before Dinosaurs. To put this number in context, there were fewer large vertebrate genera in the Permian Period than, say, on the plains of modern-day Africa or in the rainforests of the present day. But there were more large animals back then than are found today in the grassy regions of North America, or Australia, or Europe, or Asia. This ancient world was diverse, in some cases more diverse than our own in the category of large, four-legged land life.
Until the highest reaches of Permian-aged rocks, there appears to be no diminution of either the numbers or diversity of the Permian fauna as one approaches the boundary marking the mass extinction. The most common fossil is Dicynodon, name-giver of this highest Permian zone, but many other types are found as well. As in any environment of today, the herbivorous forms far outnumber the predators. Then, very curious changes begin to appear in the rock record.
About halfway up the gully fronting the Lootsberg Pass region, the rocks begin to change color from greenish to red. The green and olive strata first show faint patches of purple, and as successive strata are passed on a journey up through the great stratal column making up this region, more and more of these red to purplish blotches are found within the rocks. Another change occurs as well: the fossils become more rare and far less diverse. Forty feet above the first appearance of reddish strata, only three types of fossils can be found, and two of these were not present in the greenish strata below. Dicynodon is still present, but it is now the only member of the impressive diversity of Permian fauna that was so commonly found in the strata below. The two new fossil types that appear are a small but vicious-looking predator called Moschorhinus and a curious dicynodont genus called Lystrosaurus. Elsewhere in the Karoo, a few other types are known from this interval as well, including a small lizardlike form, some amphibians, creatures looking something like a dog, and a small reptile that turns out to be the ancestor of the dinosaurs.
Dicynodon, Moschorhinus, and Lystrosaurus are found together in beds over a stratal thickness of perhaps 50 feet or so. For the last 10 feet of this interval the beds are pure red; they have lost any semblance of green color. And then a most curious

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sedimentary phenomenon occurs: one last time, green beds appear. The most distinctive of these beds are found in the Lootsberg gully � and, it turns out, everywhere else in the Karoo at this same stratigraphic interval so far examined. These last green beds are very thinly laminated, showing the finest-scale bedding planes and sedimentary structures. They have no burrows, no evidence of plant material � and no fossils of vertebrate animals. They are completely barren, with an aggregate thickness of only 10 feet. They are the signposts of global catastrophe.
The strata immediately above and below these thin, green laminated beds show no bedding planes and are red in color. The lack of distinct bedding in the underlying and overlying strata comes from a process known as bioturbation, caused by the action of burrowing organisms such as insects, worms, and crustaceans, which disrupts the original bedding, making it gradually indistinct. Almost all sedimentary beds are thinly laminated when they are first deposited. But in most environments in our time (and probably in most of the Permian time as well), the action of burrowing animals disrupts this fine-scale bedding. As years and then centuries pass, the fine-scale differences in sediment composition producing the visible bedding planes are destroyed, trampled, ingested, homogenized. The resulting rock is massive, featureless, and free of bedding plane surfaces. Oddly enough, it is the presence of fine bedding planes that alerts the geologist to the fact that something extraordinary has happened, for the presence of such beds indicates that organisms were not present. It tells of a world existing in the absence or near absence of animals. And that is rare indeed.
The sun rises higher in the clear sky; the geologist is halfway through his trek. The heat of the day bares its fangs; sweat emerges on his skin only to dry instantly in the hot wind. He feels like an inverted diver; he drinks from the large water bottles he carries, filling himself with water like some lost fish emerging onto land in a diving suit that pumps water, rather than air, into his body. The vegetation around him is all scrubby, low, and brown; an occasional carnivorous fly buzzes about his face, intrigued by this moving, sweaty heat source. More water, salted nuts, an orange, an apple, and he shrugs on the heavy pack once more and continues upward.
The rocks are very different now. All of the finer rocks are brick red in color. It is like the surface of Mars � perhaps in more ways than one. The geologist comes to a thick ledge of sandstone, and finds pebbles and bones along the undersurfaces of these thick beds. They show features indicating that they were deposited by braided streams, the anastomosing channels that water follows as it first leaves the mountains, or on any other steep slope. There is no evidence of the more meandering

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Inheritors of the post-Permian world, the dinosaurs would quickly dominate in species and individuals.

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rivers of a typical river valley, no indication of the cross-beds and point-bar deposits that all rivers past and present make when they cross a river valley. Such deposits are common in the green beds of the Permian rocks seen earlier on this trek, but have disappeared from the Triassic strata. He wonders, imagines the scene. Perhaps the land suddenly tilted upward, creating a slope where none was before; mountain building could do that. But there is no other evidence that the long-ago region of Lootsberg Pass was affected by rapid mountain building.
He searches his long memory, and Mars and H. G. Wells come to mind. Long ago there was water on Mars, and rivers. But all of the rivers on Mars were braided, leaving behind the same types of deposits, he is sure, that are found in these lowest Triassic strata in the Karoo. The reason the rivers on Mars were braided is that there was nothing to stabilize their banks, no deep roots to hold them in check, for evolution there, if it produced life at all, probably never got beyond bacteria. And the connection clicks. He has a vision of a long-ago Earth, where rivers were always braided � until plant life evolved and introduced a new type of river, the meandering river so familiar to us all in our world, and familiar too in the Permian period. Then, 250 million years ago, a huge mass extinction made this portion of the Earth, and perhaps all of the Earth, suddenly Mars-like, stripped it of all of the Permian trees and bushes that had greened that ancient world and kept its rivers flowing in the sinuous and meandering channels so recognizable and familiar to those of us who live in a tree-filled age. It hits him: this ancient extinction killed off the Permian trees, perhaps most of the Permian plants. And in so doing it changed the way the rivers flowed.
The day is nearly over when he finishes his climb. In the highest rocks he finds numerous fossils, mostly of the pig-sized Lystrosaurus. But he sees other fossils as well, one that will give rise to the mammals, and another that will be the seed stock of an entirely different group: the dinosaurs, those heirs to the Paleozoic world whose own world also ended abruptly in global catastrophe and mass extinction, in an event best studied along a scenic seacoast in France.

Hendaye, France

Long ago, Spain whirled in its continental drifting, made a hard right turn, and ran into France with a tectonic lunge. Rocks crumbled, and the Pyrenees became the zipper uniting these two great blocks. An ancient seafloor was raised in the process. Today a part of that ancient ocean is exposed for all to see, but like Gomorrah and Sodom, that deep-sea bottom and its trove of skeletons has been turned to

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stone. Now it is a scenic park on the border between Spain and France, a coastal bit of the Basque country. On a very hot day a geologist prepares to hike this bit of coast in order to visit one of the world�s most impressive Cretaceous-Tertiary boundary sites, a place where the great catastrophe ending the Age of Dinosaurs is preserved in dramatic fashion. To get there, he has to walk a pathway only the twentieth century could have built, a pathway containing clues not only to the past, but to the future � the future of evolution � as well.
He starts his trek along a busy scenic coastal road lined with �snacks� and open-air cafes, then strides onto a wide sandy beach covered with naked humans. A lone, futile sign proclaims Nudism Interdit! (Nudism Forbidden). It is July, a hot morning, and already throngs from nearby Spain are jostling with the German tourists for the best bits of littoral territory as they lather their naked bodies with sunscreen amid piles of discarded clothing. Every age and form of humanity spreads itself out to fry in the sun, and the geologist is an odd sight as he walks through the sand, at times stepping over and by the prone naked bodies, festooned as he is with the hammers, compasses, water bottles, packs, and other regalia of his trade. It is an odd sight to see a clothed man, let alone an equipped clothed man. Odder still, he is walking to work, while the rest of humanity is here to frolic in the waves, playing the odd game of Spanish paddleball. The tide of humanity washed onto this shore is oblivious to another flood of flotsam floating in from Spain with the tide: the flotillas of garbage caressing their legs and ankles in the warm Bay of Biscay as they unconsciously celebrate their dominion over a thoroughly tamed world. Not a single one of them worries about being eaten by some predator that day. It is a sure sign that a great mass extinction has taken place: only during mass extinctions do the predators disappear.
At the end of the beach a great rocky headland exposes pinstripes of strata, the Upper Cretaceous sedimentary beds he has come to sample. But the rocks rise precipitously and vertically up from the sea, leaving no path for the beachcomber to pass, so he must climb up onto the headland to get to his target site, still a half mile down the coastline. A well-worn path beckons upward near the end of the beach, and he follows it amid the sweet smell of beach and salt air. The neatly groomed track winds through bracken, then brings him next past a large fenced enclosure filled with children. As he passes closer, he sees that in contrast to the frolic and play normally associated with the young, these children are listless, slow-moving, or motionless. Some are wheeled by white-coated attendants. He realizes that this large outdoor reserve is for autistic and retarded children, all helpless and heart-wrenching in their plight. He walks slowly by, staring, but they take absolutely no

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notice of him. France has put its most pitiable next to the sea, in an exquisite setting � these children that in another age would die early, but here will live and in many cases breed, and in some cases perpetuate their disabilities. Natural selection is no longer at work for these, or any other, humans.
He ponders this experiment in future evolution as he finally passes by the manicured lawn, itself some new evolutionary joke of grass bred for looks, and the path begins to rise. Now a different assault on his senses occurs: the cool, sweet salt air is suddenly replaced by a gut-wrenching odor, a choking miasma. The path now runs next to the municipality of Hendaye�s sewage treatment plant, its huge outdoor pools of cess slowly rotating in giant concrete cisterns. Unfortunately, there is no way over the headland except by this path. A littoral territory once the home of a small tribe of humans is now inhabited by tens of thousands of humans and visited each year by that many and more, and their combined fecal output is now so voluminous that it can no longer be simply dumped into the sea. So here it is �treated� and then dumped into the sea, creating a riotous explosion of algal growth in the shallow water around the sewage outfall pipes, an experiment in ecology that is utterly changing the inter-tidal and subtidal communities along the coast as now bountiful phosphates and nitrates amid their rich liquid fertilizers putrefy the region.
Finally he is past this hurdle as well, and he enters a fairyland. High above the beach a great pasture unfolds: acres of manicured grounds, scattered trees, and the magnificent vista of the sea. Sitting above it all is a splendid spired castle, now housing French astronomers by all accounts, although no telescope can be glimpsed. He is now in the reserve called Abbadia, a huge park that was once the fertile fields of the adjoining castle, and he feels transported back to earlier centuries, with even more distant time travels just ahead. He shoulders through a herd of sheep � animals stupid and bizarre compared with their ancestors. Their fecal pellets lie everywhere, and he wonders if they are, after humans, the most common large mammals on the planet. He ponders the process called domestication and how all domesticated animals seem to have lost brainpower as they were sculpted by humans into the species that they have become. He imagines the world of 8,000 years ago as humanity began to populate it with entirely new types of animals and plants in the single greatest evolutionary experiment since the ancient mass extinctions.
As he walks across the high meadow in the sparkling summer sun, the twentieth century and its history once again intrudes. Amid the waving grass, grazing sheep, and linear hedgerows are the scattered remains of huge concrete bunkers, jumbled masses of fractured concrete and twisted rebar. The blockhouses were the

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work of the Nazis, part of the Atlantic Wall they built for defense, now nothing but large ruins of concrete littering the flat grounds like yawning caves or the litter of capricious giants. A movement within the first broken blockhouse he passes startles him; he expects a fox or dog, but a naked man slowly stands, watching him. He passes by, and another man can be seen in the next smashed bunker. Soon he realizes that the field is alive with half-seen men, all silent, many only partially clothed or, like the first, not wearing clothes at all. He understands suddenly that this park is the territory and cruising ground of the local gay community, a meeting place where the vacationers who come here, and the locals who live here, swap microbes and homogenize the world�s infectious diseases. It is a microcosm of what is happening to the world�s animals and plants. He wonders, how much of their behavior is genetic, and will that be a future of evolution?
He tops the crest of the headland and begins to drop down toward the sea. A steep and switchbacked trail makes a precarious path to the water�s edge, where gently tilted strata are now exposed by the low tide. He strides out onto these rocks, inch- to foot-thick limestone layers packed with the most spectacular fossils.
Giant clams lie frozen in the strata. Not the giant clams of our age that are now seen as birdbaths in backyard gardens, but flatter clams, with huge oval shells as much as a yard in length. They are nothing like any clam now alive, yet once these fossils were dominant members of the Mesozoic sea bottom community. They are called inoceramids, and they are hallmarks of a time when dinosaurs ruled the land and ammonites swam the seas. These same ammonites, with coiled shells like that of the nautilus, are also found in the clam-rich strata, although they are never so numerous as the clams. The geologist notes a few, and begins to walk perpendicular to the bedding, and thus up through time.
It is a beautiful walk, with high cliffs of white limestone and reddish marl arching overhead, the sea slapping the rocks, and gulls wheeling about in noisy cacophony; no clouds mar the deep blue sky. When he has walked along the coast for about 40 meters, the most peculiar thing happens: the clam fossils begin to disappear. Soon they are rarely seen, and then they are gone altogether. They and their kind disappear not only from the strata on this seacoast but from all rocks dated at 67 million years old and less, in which they had been common. After a reign of over 170 million years, this type of clam suddenly goes extinct. The strata look the same, but the giant clams are gone.
The geologist continues his walk along the seacoast, moving relentlessly up through time as he crosses the tilted strata on the rocky beach. Fossils are still

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present, but they are relatively few in number. Most are sea urchins, although a few small clams and the rare but beautiful ammonites can be seen on this infrequently visited stretch of coast. He passes into a small bay, and the scenery changes. The tan to olive limestone he has been passing over is superseded by a gigantic wall of bright pink rock. There is a clear point of contact between the olive rock and this thicker, pinker limestone, and he moves into the bay to this contact. It is his goal this day. A thin clay layer several inches thick marks the boundary between the olive Cretaceous rocks and the pink rocks of the overlying Tertiary Period. This layer is also where the last ammonites can be found, while its counterpart on land is the stratum with the last dinosaur fossils. He smashes out a few fragments of this clay stone with his rock hammer and examines them with a powerful loupe. The clay contains a thin, rusty layer, and under magnification he can see that this thin layer is packed with tiny spherules, invisible to the naked eye but clearly visible even under the low magnification of the loupe. He is looking at bits and pieces of Mexico, on an extended European holiday after being blasted into space by the great asteroid impact that ended the Mesozoic Era 65 million years ago. In the warm sun, on this perfect day, he stretches out on the rocks, one hand on the last of the Cretaceous, the other slightly above it, on the oldest rocks of the Tertiary, spanning two eras, and imagines the scene:

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The Age of Dinosaurs ended when an asteroid crashed into the Earth at Chicxulub on Mexico�s Yucatan Peninsula.

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The impact creates great heat, both on land and in the atmosphere. The shock heating of the atmosphere is sufficient to cause atmospheric oxygen and nitrogen to combine into gaseous nitrous oxide; this gas then changes to nitric acid when combined with rain. The most prodigious and concentrated acid rain in the history of the Earth begins to fall on land and sea, and continues until the upper 300 feet of the world�s oceans are sufficiently acid to dissolve calcareous shell material. The impact also creates shock waves spreading outward through the rock from the hole the asteroid punches in the Earth�s crust; the Earth is rung like a bell, and earthquakes of unprecedented magnitude occur. Huge tidal waves spread outward from the impact site, eventually washing ashore along the continental shorelines of North America, and perhaps Europe and Africa as well, leaving a trail of destruction in their wake and a monstrous strandline of beached and bloated dinosaur carcasses skewered on uprooted trees. The surviving scavengers of the world are in paradise. The smell of decay is everywhere.
For several months following this fearsome day, no sunlight reaches the Earth�s surface. After the initial rise in temperature from the blast itself, the ensuing darkness that settles in causes temperatures to drop precipitously over much of the Earth, creating a profound winter in a previously tropical world. The tropical trees and shrubs begin to die; the creatures that live in them or feed on them begin to die; the carnivores that depend on these smaller herbivores as food begin to die. The �middle life� of the Mesozoic Era � a time beginning 250 million years ago � comes to the end of its nearly 200-million-year reign.
Following months of darkness, the Earth�s skies finally begin to clear, but the mass extinction � the deaths of myriad species � is not yet over. The impact winter comes to an end, and global temperatures begin to rise � and rise. The impact has released enormous volumes of water vapor and carbon dioxide into the atmosphere, creating an intense episode of greenhouse warming. Climate patterns change quickly, unpredictably, and radically around the globe before the Earth�s temperatures regain their normal equilibrium. They swing from tropical to frigid,

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Debris from the K-T impact would have created continent-size fireworks before raining ash and darkness on the planet for years.

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then back to even more tropical than before the impact, all in a matter of a few years. These temperature swings produce more death, more extinction. The dinosaurs die out, as do most � but not all � mammals. Most life in the sea is exterminated.
The end-Cretaceous catastrophe was global, immense. It shares many characteristics with the Permian extinction so vividly exposed and expressed in the Karoo: both affected the Earth so much that they changed the nature of sedimentary rocks of the time. In France that change is clear � the latest Cretaceous rocks are green in color; the K-T boundary layer is dark mudstone, and the recovery rocks of the succeeding Tertiary are the thick pink limestone. Such changes occur only in the face of great chemical changes.
The geologist ponders the site. The boundary beds may have been a product of this single calamitous event, the impact of a huge asteroid with the Earth 65 million years ago. But the other victims on this Hendaye beach, the giant clams found in the strata beneath this site, were killed off 2 million years prior to the impact. What killed them? Was their passing (and that of many other creatures at the same time) the result of an Earth already stressed? It appears that the Cretaceous-Tertiary mass extinction, like the great Permian extinction that preceded it, was multi-causal.
The geologist�s reverie is broken by a great rumbling sound, and he notices, for the first time, the giant culvert snaking down from the cliffs above, a pipe three feet in diameter, ending in the small bay he is standing in. A great deluge of brown water belches from the pipe, filling the bay with treated sewage from the plant on the bluff above. The Cretaceous rocks and the overlying Tertiary strata are quickly covered, clues to a long-ago extinction fouled with the last meals of the good people of Hendaye.

Lessons from the Past

Mass extinctions are biological events. But they have been transformed into geologic evidence, and therein lies the problem. Turning flesh into stone means the loss of most biological information, and at best we have only the slenderest of clues to the events of that time. Even so, the transition of creatures during the two mass extinctions profiled above can teach us a great deal about how mass extinctions can affect the nature of evolution on the planet. Not only did the composition of the fauna (and flora) change radically, through the replacement of one suite of species with another, but so too did the body types of the animals and plants involved. There was not only a turnover, but also what we might call a �changeover�.

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The last assemblage of vertebrate animals present on Earth immediately prior to the great Permian extinction was entirely made up of four-legged types � quadrupeds. All of the dicynodonts walked on four legs, as do the majority of reptiles and mammals living on Earth today. By the very end of the Permian many held their legs beneath the body, as all mammals do today. Some of these types of animals survived. In the Triassic Period, soon after the mass extinction, the survivors and the earliest of the new species to evolve also could be typified as quadrupeds. But from that point on things began to change. With the first appearance of dinosaurs in the Triassic Period, a new suite of forms made its appearance: bipeds. While there were indeed many four-legged dinosaurs, the dominant form of the Mesozoic, exemplified by the allosaurs, tyrannosaurs, iguanodons, and duck-billed dinosaurs, was bipedal. Even the four-legged dinosaurs (such as the giant sauropods, stegosaurs, ankylosaurs, and ceratopsians) had body forms different from anything found among the late Permian faunas, for nothing in the late Permian had the long tails or giant sizes found among the dinosaurs. The large animal life on either side of the great Permian extinction is quite dissimilar. The body forms of the Paleozoic land life do not closely resemble those of the dinosaurian fauna that followed.
Would dinosaur body types have evolved even if the Permian extinction had not occurred? This is an unanswerable question, but we do know that the mammal-like reptile faunas of the late Paleozoic were moving toward the mammalian condition. Some investigators even interpret them as having been quite mammalian. In the absence of dinosaurs, would these same animals have produced T. rex or Triceratops clones, with body shapes mimicking those of the dinosaurs? It seems doubtful, for true mammals have never really explored the bipedal or long-tailed body types, kangaroos and some small rodents being among the few exceptions to this rule. We are left with a powerful observation: entirely new types of body forms may be the legacy of a mass extinction.
The world in the aftermath of the Permian extinction was desolate, and not only on land. In the seas the extinction was equally devastating. As on land, the great dying in the Permian seas radically reset the evolutionary agenda. Perhaps the most telling evidence of the extinction�s severity is found in the western United States, in the reddish strata deposited in shallow seas following the extinction. Such warm, sunny seaways are today the sites of rich communities of organisms living above, on, and in their sandy bottoms. Because the continent of North America was farther south 250 million years ago, the shallow seaways of its western portions

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were in equatorial latitudes, and prior to the great mass extinction they were homes to rich and diverse coral reefs � among the most diverse habitats on Earth, then as now. Yet after the extinction, these same geographic sites were virtual biological deserts, barren of all life save a scattering of rare invertebrates and vertebrates. The most common organisms were stromatolites, layered algae that had almost disappeared from Earth more than 500 million years ago for a simple reason: with the rise of herbivorous animals, such layered mats of vegetation could not survive the incessant grazing that resulted. After the extinction, however, stromatolites made a comeback, suggesting that most of the seas were without their usual assortment of herbivores. The seas, like the land, remained impoverished for several million years. The old order passed away; the world of mammal-like reptiles and trilobites, spiky archaic trees, and gorgonopsian predators crumbled, to be replaced by a world of dinosaurs and pines, and ultimately by flowering plants and burrowing clams and bony fishes in the sea.
Eventually, the Mesozoic biota rose up for the Permian ashes, and then it too was struck down in a second great mass extinction. Across the globe, in every ecosystem, the changeover in fauna was spectacular � just as it was in the earlier Permian extinction. Ammonites and their legions of shelled cephalopod relatives disappeared from the seas, to be replaced by bony fishes and a new type of cephalopod � the cuttlefish. The reefs of the time died out, and when reefs eventually reappeared, they were composed of framework-building organisms of entirely different types. The changeover on land is far better known: the complete extinction of the dinosaurs allowed the rise of the many types of mammals we see today. And like the earlier Permian event, the enormous catastrophe ending the Mesozoic was followed by the rise to dominance of evolutionary dynasties quite different from those that came before. The lesson of these two great mass extinctions seems clear: extinction leads to evolutionary innovation. But is this always the case, and is it the only, or even the most important, lesson to be learned from such past global catastrophes?
As it turns out, these two mass extinctions were discovered by accident. In the eighteenth and nineteenth centuries it became imperative to devise some way of determining the age of rocks on the Earth�s surface. By the early 1800s European and American geologists had begun to use fossils as a means of subdividing the Earth�s sedimentary strata into large-scale units of time. In so doing they made an unexpected discovery: they found intervals of rock characterized by sweeping changes in fossil content. Setting out to discover a means of calibrating the age of

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Although their turn would come, mammals did not take a prominent place in the Mesozoic food chain.

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rocks, they discovered a means of calibrating the diversity of life on Earth. And they found intervals of biotic catastrophe, which were named mass extinctions.
The two largest mass extinctions � those examined above � were so profound that they were used by John Phillips, an English naturalist, to subdivide the stratigraphic record � and the history of life it contains � into three large blocks of time. The Paleozoic Era, or time of �old life�, extended from the first appearance of skeletonized life 530 million years ago until it was ended by the gigantic Permian extinction of 250 million years ago. The Mesozoic Era, or time of �middle life�, began immediately after the Permian extinction and ended with the Cretaceous -Tertiary extinction 65 million years ago. The Cenozoic Era, or time of �new life�, extends from that last great mass extinction to the present day. At the time of Phillips�s work, in the middle part of the nineteenth century, the notion that a species could go extinct was still quite new, and his recognition that not only single species, but a majority of species, could and did go extinct in short intervals of time was radical for its day.
John Phillips�s 1860 paper also marked the first serious attempt at estimating the diversity, or number of species, present on the Earth in the past. Phillips showed that, over time, the diversity of life on Earth has been increasing, in spite of the mass extinctions, which are short-term setbacks in diversity. The mass extinctions somehow seemed to make room for larger numbers of species than were present before. Far more creatures were present in the Mesozoic than in the Paleozoic, and then far more again in the Cenozoic. But the mass extinctions did more than just change the number of species on Earth. They also changed the makeup of the Earth.
Mass extinctions are thus one of the most significant of all evolutionary phenomena. The wholesale destruction of animal and plant species in such large numbers opens the floodgates of evolutionary innovation. Far more than initiating the simple formation of new species a few at a time, the violent cataclysms of mass extinction reset the evolutionary clock. The two events profiled in this chapter are only the most severe of more than fifteen such episodes during the last 500 million years and, not coincidentally, the most consequential in bringing about new evolutionary innovation. They literally changed the course of life�s history on this planet. Had the Permian extinction not taken place, there probably would have been no Age of Dinosaurs, and mammals might have dominated the planet by 250 million years ago, rather than 50 million years ago. Did this extinction delay the rise of intelligence by 200 million years? And, in turn, if the dinosaurs had not been suddenly killed off following an asteroid collision with the Earth 65 million years ago,

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there probably would have been no Age of Mammals, since the wholesale evolution of mammalian diversity took place only after the dinosaurs were swept from the scene. While dinosaurs existed, mammals were held in evolutionary check. Mass extinctions are thus both instigators of and obstacles to evolution and innovation. Yet much of the research into mass extinctions suggests that their disruptive properties are far more important than their beneficial ones.
On every planet, sooner or later, a global catastrophe can be expected that could seriously threaten the existence of animal life or wipe it but altogether. Earth is constantly threatened by planetary catastrophe, mainly by the comets and asteroids that cross its orbit, but potentially from other hazards of space. Yet it is not only the hazards of outer space that threaten the diversity of life on this planet � and surely on other planets. There are Earth-borne causes of catastrophe as well as extraplanetary causes.

Do Causes Matter?

In one way or another, all mass extinctions appear to be immediately caused by changes in the �global atmospheric inventory� � changes in the components of the Earth�s atmosphere or in their relative amounts. Such changes can be caused by many things: asteroid or comet impact, releases of carbon dioxide or other gases into the oceans and atmosphere during flood basalt extrusion (when great volumes of lava flow out onto the Earth�s surface), degassing caused by the exposure of oceanic sediments rich in organic material during sea level changes, or changes in ocean circulation patterns. The killing agents arise through changes in the makeup and behavior of the atmosphere or in factors, such as temperature and circulation patterns, that are dictated by properties of the atmosphere. Sudden climate change was probably involved in the Permian extinction, and the Yucatan asteroid impact is the probable cause of the Cretaceous catastrophe. But there may be a further cause of major mass extinction: the emergence of a global intelligence.
The causes listed above all derive from a single source. Yet the history of mass extinction on this planet suggests that more that a single cause is associated with the events we find in the rock record. Sometimes these multiple events occur at the same time; sometimes they are separated by hundreds of thousands of years. Perhaps one perturbation stresses the planet, making it more susceptible to the next. Both the Permian and end-Cretaceous calamities appear to have been brought about by more than a single cause.

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But is �cause� really such an important thing to know? Generations of humans have been inculcated with the notion that each crime must be solved, the �who� in a �whodunit� revealed. For the mass extinctions, we may have to be satisfied with understanding the effect, rather than the cause.

The Anatomy of a Mass Extinction

The typical sequence of events in a mass extinction begins with the extinction phase, when biotic diversity falls rapidly. During this time, the extinction rate (the number or percentage of taxa going extinct in any time interval) far exceeds the �origination rate� (the number of new taxa evolving through speciation). After some period of time, the extinction phase ends and is succeeded by a second phase, often called the survival phase. This is a time of minimal diversity, but no or few further extinctions. During this interval the number of species on Earth levels out, neither increasing or decreasing. The third phase, called the rebound phase, is when taxonomic diversity slowly begins to increase. The final phase is the expansion phase, and it is characterized by a rapid increase in diversity due to the evolution of new species. The latter three phases are grouped together into what is known as a �recovery interval�, which is followed by a long period of environmental stability (until the next mass extinction). The rate of the recovery is usually proportional to the intensity of the extinction that triggered it: the more intense the mass extinction, the more rapid the rate of new species formation.
Three types of taxa are generally found immediately after the mass extinction: survivors, or holdover taxa; progenitor taxa, the evolutionary seeds of the ensuing recovery; and disaster taxa, species that proliferate immediately after the end of the mass extinction. All three types of taxa are generally forms that can not only tolerate, but thrive in, the harsh ecological conditions following the mass extinction event. They are generally small, simple forms capable of living and surviving in a wide variety of environments. We have another term for such organisms: weeds.
The recovery interval is marked by a rise in diversity. This sudden surge in evolution is generally due to the many vacant niches found within the various ecosystems following the mass extinction. Because so many species are lost in a mass extinction, it creates new opportunities for speciation. Darwin once likened the speciation process to a wedge: the modern world has so many species in it that for a new species to survive and compete, it must act like a wedge, pushing out some other already entrenched species. But after a mass extinction no wedging is necessary.

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Early on, virtually any new design will do. Many new species appear with morphologies or designs seemingly rather poorly adapted to their environment and inferior to those of species existing prior to the extinction. Rather quickly, however, a winnowing process takes place through natural selection, and new, increasing efficient suites of species rapidly evolve.
The great mass extinction ending the Permian created a long-term deficit in diversity, but eventually, in the Mesozoic Era, that deficit was made up. In fact, after every mass extinction that has occurred on Earth over �the past 500 million years, biodiversity has not only returned to its former value, but exceeded it. Sometime during the last 100,000 years, biodiversity appears to have been higher than it has been at any time in the past 500 million years. If there had been twice the number of mass extinctions, would there be an even higher level of diversity than there is on Earth now?
Interesting as this question is, it has not yet been tested in any way. The fossil record, however, does yield some evidence that mass extinctions belong on the deleterious rather than the positive side of the biodiversity ledger. Perhaps the best such clue comes from the comparative history of reef ecosystems. Reefs are the most diverse of all marine habitats; they are the rainforests of the ocean. Because they contain so many organisms with hard skeletons (in contrast to a rainforest, which bears very few creatures with any fossilization potential), we have an excellent record of reefs through time. Reef environments have been severely and adversely affected by all past mass extinctions. They suffered a higher proportion of extinctions than any other marine ecosystem during each of the six major extinction episodes of the last 500 million years. After each mass extinction reefs disappear from the planet, and usually take tens of million of years to become reestablished. When they do come back, they do so only very gradually. The implication is that mass extinctions, at least for reefs, are highly deleterious and create net deficits of biodiversity. And whether we are talking about reefs, rainforests, or any other ecosystem, the reality is that for millions of years following a mass extinction the biodiversity of the planet is impoverished.
So, while there are many who would argue that since mass extinctions are sources of innovation, a modern one would not be such a bad thing, as it would be the source, ultimately, of a new age and even greater biodiversity, I will argue in the following pages that this is simply not the case.

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CONTENTS