A New History of Life (32 page)

This trend of changing size also occurred among the mammal-like reptiles as the Permian Period came to a close. The largest therapsids of all time, the dinocephalians of the Middle Permian, evolved at the peak of oxygen abundance. As oxygen began to drop in the mid-Permian, successive taxa assigned to various therapsid groups, and most important the dicynodonts, showed a trend toward smaller skull sizes. While some relatively large forms still lived in the latest Permian—the genus
Dicynodon
and even the carnivorous gorgonopsians come to mind—by this time many of the dicynodonts were smaller. The latest Permian taxa
Cistecephalus, Diictodon
, and a few others were very small. Research in 2007 showed that the Late Permian through Early Triassic genus
Lystrosaurus
was smaller in the Triassic than it was in the Permian, and the various cynodonts of the late Permian and early Triassic, as oxygen levels were precipitously falling,
were all small in size. There are exceptions—a few giants in the Triassic named
Kannemeyeria
and
Tritylodon
are examples—but in general the therapsids of the Triassic are much smaller than those of the Permian. A recent paper by our colleague (and now at the University of Washington) Christian Sidor has confirmed the drop in size. Thus there is a strong correlation between terrestrial animal size and oxygen levels from the latest Permian into the Triassic. In high oxygen, tetrapods grew large, and then they grew smaller as oxygen levels diminished.

Antonio Lazcano, origin of life specialist and humanist, in Galapagos Islands contemplating a “lower” life form. (Photo by Peter Ward.)

Yale University’s great Peabody Museum is home to one of the largest collections of fossils in the world. It also is home to the greatest paleontological paintings ever done.

There are two gigantic murals gracing an immense wall in the Peabody Museum of Natural History on the Yale University campus. For generations of Americans these two murals—
The Age of Reptiles
, painted over three years (1943–1947), and
The Age of Mammals
, painted over six years (1961–1967)—have been the iconic views of land life’s journey through time.

The first,
The Age of Reptiles
, begins in dark swamps and ends with exploding volcanoes towering over
T. rex
. The second also begins in the jungle, but one with very different and very familiar vegetation. Combined, they tell us that amphibians begat reptiles, which begat mammals. But our view would now require two very different murals to correctly show the vertebrate assemblages in these deep time periods pictorially represented. In fact, we advocate here that there were three separate “ages” of mammals (knowing, of course, that an “age” is nothing more than informal labeling and categorizing, without scientific validity).

The first age of mammals was in the Permian period, the heyday of the therapsids and their ancestral synapsids.
Technically
they are not yet mammals. But they were close. It was a species-rich as well as numerically abundant assemblage. In South Africa, there were as many as fifty genera at a time (and since a normal genus normally contains several [to many] species, the actual diversity at the species level was higher yet; perhaps 150 species is a conservative estimate).

South Africa today is not so different latitudinally and perhaps even climatically from the South Africa of southern Gondwanaland, some 255 million years ago. Today there are 299 species; we can imagine the African veldt of today, but stocked with
Dicynodon
instead of the large herbivores, and many kinds of carnivores from the lion-sized gorgonopsians to the weasel-sized theriodonts. Vast herds grazing not on grass but the low, bushy
Glossopteris
and ferns. Africa of the first age of mammals.

The second age of mammals can be thought of as the time between the late Triassic and the end of the Cretaceous: mammals chained. Held in check by the dinosaur overlords. Living in the ecological cracks: at night, in burrows, in trees. Never bigger than a house cat, and usually far smaller.

Finally, the third age of mammals. Zallinger’s age of mammals. The post-K-T mass extinction outpouring of species filling the families so well known to us today. This is the story most obvious to us: from ratlike survivors of the Chicxulub asteroid’s wrath to the early giants such as titanotheres and uintatheres (rhino-like beasts) to the mammalian panoply we know so familiarly today.

Until about 2000 we knew the first age of mammals largely from the South African Karoo desert. But in the twenty-first century vast new collections have been made from north central Africa by Christian Sidor, and in Russia another gigantic assemblage is now known, thanks to paleontologist Michael Benton’s work. In this second age, the mammals remained very small. It would not be until the Paleogene that mammals would finally gain ascendancy, and like some long-denied heir, would finally get an “age” named after them.

One could almost believe that the whole age of dinosaurs was a big mistake. That but for one huge flood of basalt there might have been a quite different history. Human intelligence 250 million years ago? It did not take long to go from apes to something more advanced not so long ago.

The Karoo desert of central South Africa can be a bit of a disappointment to its first-time visitors. When the two words “Africa” and “desert” are found in the same phrase, there is often an image of the Sahara desert, Africa’s most famous dry place, or the Kalahari, another vast wasteland with little life because of its shifting sands and harsh conditions of blazing heat by day and freezing temperatures each night. With animals and plants having such a hard time, and existing at such low standing diversity and abundance, it is no wonder that the human populations in the Sahara and Kalahari are limited in size as well. Very little in the way of plant or animal crops can be farmed there.

Unlike these two African deserts, the Great Karoo desert has no shifting sand dunes; it is mainly rock that is often well vegetated, and there seems no place in its vastness where one cannot find sheep dung, evidence for the ubiquity of this introduced species. There are no elephants or giraffes, no hippos or crocodiles or water buffaloes or rhinos; it has animal life, and in places lots of it, but its species are not ones redolent in the memory of Africa. There are also quite a few people, on large ranches. Thus the Karoo is not the place for desert-seeking tourists. What it does have, however, is a hundred-million-year-long accumulation of sedimentary rocks deposited in a time interval from about 270 million years ago to perhaps 175 million years ago.

In the middle of this vast rock heap is the world’s best record of large terrestrial animal life living both before and after that most consequential of all mass extinctions, the Permian-Triassic mass extinction. Generations of paleontologists going back to the middle 1800s have searched the ancient riverbeds and river-valley floors that the Karoo strata were created by and in. Animals often are carried into rivers after death, or are in waterholes where they may have been attacked long ago, leaving bones to fall into mud and become preserved there.
This region was the prime record for this period until very recently, with new work in both eastern Russia by our colleague Mike Benton of Bristol and the north central part of Africa, in the country of Niger, where another of our colleagues, Christian Sidor of the University of Washington, have unearthed important new records.
¹
Yet even these new regions cannot compare the richness and temporal resolution that the Karoo rocks have given us—if “given” should be used at all. In fact, the Karoo has given up its vast store of information about one of the most critical times in life’s history on Earth very grudgingly. It has to be taken, and while the work to do this seems glamorous (who does not dream of being a paleontologist finding a giant leering skull of some ancient predator, such as
T. rex
), it is at best difficult on the humans who pursue this passion.

A drive from Cape Town into the center of the Karoo is an all-day affair. But because the rocks are slightly tilted, while the landscape inexorably rises in altitude as one travels north and east into the Karoo, the entire book of strata that the Karoo holds can be read from its ancient mid-Permian-period cover to its Jurassic, dinosaur-bearing last chapter. It is not only time that changes as one goes upward through the many thousands of aggregate feet that is the Karoo sedimentary record. One starts in a time of ice and icebergs and ends in what may have been one of the hottest times in Earth history, as well as passing through an interval tens of millions of years in length when atmospheric oxygen receded to its lowest level since animals first occurred at all, nearly 600 million years ago. Yet if much can be understood by reading this entire record, there is one interval of rock, representing time, that has been more studied than any other.

These are the several hundreds of meters of strata deposited between 252 and 248 million years ago—rocks deposited in the last millennia of the Permian period (and thus the Paleozoic era, which ends with the end of the Permian) and first few millions of years following the vast mass extinction of 252 million years ago.

For decades now, geoscientists have been asking several principal questions of these rocks, and their rare but often exquisitely preserved skulls and body skeletons: First is the question of how long the mass
extinction took, from the start of extinction rates first exceeding the normal “background” extinction rate, which has been calculated to have been about one extinction each five years. Second, we want to know if the catastrophic extinction on land took place simultaneously with the Permian marine mass extinction. Third, and perhaps most interesting, is the question of what caused the mass extinction. Finally, it is important to discover how quickly terrestrial ecosystems recovered, because these latter clues might give us useful information for surviving any future Permian-like mass extinction, a prospect far more probable than our species seems to realize.

To paraphrase one of the great twentieth-century paleontologists, David Raup of the University of Chicago: Were the surviving species gifted with good genes—or simply good luck?

If intense controversy still exists about the cause or causes of the Permian extinction, on one aspect of that time interval are all in agreement: in the aftermath of the extinction ecosystems were profoundly affected, and extinction recovery was long delayed. It is this latter evidence that readily distinguishes the Permian extinction from the later Cretaceous-Tertiary event. While both caused more than half of the species on Earth to disappear, the world recovered relatively quickly after the “K-T” event. This may have been due to different causes for the two. Asteroid impact on the Earth and the environmental destruction accruing from the impact have for more than a decade been accepted as the cause of the K-T event. But the killing conditions following the impact soon dissipated. This was not the case after the Permian event. As we have seen above, while some Earth scientists believe that the Permian as well as the K-T events were caused by large-body impacts on the Earth, it seems as if the environmental conditions causing the Permian extinction persisted for millions of years after the onset of the extinction. It is not until the Middle Triassic, some 245 million years ago, that some semblance of recovery seemed to be under way.

These results would be expected if some part of the Permian mass extinction were directly or indirectly caused by the reduction in oxygen at the end of the Permian. The newest Berner curves show that oxygen stayed low into the Triassic, and there is even some indication that the oxygen levels did not bottom out and begin rising until near the end of the lower Triassic, which might account for the long delay in the recovery. This evidence suggests that the environmental events producing extinction just kept persisting. If so, and if animals were capable of any sort of adaptation in the face of these deleterious conditions, we would predict that the Triassic would show a host of new species not only in response to the many empty ecological niches brought about by the mass extinction, but might also show new species arising in response to the longer-term environmental effects of the prolonged extinction event itself. This is the pattern that is observed during the Triassic; the world was refilled with many species looking and acting like some of those going extinct (therefore an ecological replacement), but a host of novel creatures also appeared, especially on land. In the next chapter we will postulate that many of the latter new species evolved to counter the continued low oxygen that had begun near the end of the Triassic, but that continued right into the Jurassic, a period of more than 50 million years. The Triassic was truly the crossroads of animals adapted to two different worlds, one of higher oxygen and one of low.