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 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 fossiIs, 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

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),

The T. rex of its time, the gorgon was the largest The drawings here represent four possible renditions looked like.

of the Paleozoic predators. of what this animal might have nine mammal-like reptiles known as dicynodonts (which shared a common ancestor with mammals), three biarmosuchians (a primitive group of reptiles), nine gor-gonopsians (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 Lystro-saurus. 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 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 mate-rial—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

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

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.

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