by Peter Ward
The late Paleocene tropics remained about the same (hot) temperature, but the Arctic and Antarctic regions warmed markedly. In the Paleocene the difference in seawater temperature between equator and pole was a hefty 17°C (it is an even heftier 22°C now). By early Eocene times, however, the difference had shrunk to only 6°C. And as the high latitudes warmed, the heat exchange between the two regions slowed, reducing both the number and ferocity of storms. The world went calm and got very hot, just as it did so many times before. This was yet another greenhouse mass extinction.
The carbon isotope record across the Paleocene-Eocene boundary in two cores also yielded a surprise. They showed a short-lived negative excursion—the kind of record that occurs when the amount of plant life is reduced—a hallmark of mass extinction. Other paleontologists began looking at the survival record of bottom dwellers from the region, looking specifically at the common benthic, or bottom-dwelling, Foraminifera—and found evidence of a catastrophic mass extinction on the bottom. Was it simply sudden warming of the deep that wiped out the cold-adapted species in short order? These results were published in the early 1990s, and soon after a Japanese paleontologist named K. Kaiho published studies inferring that the fate of the benthic forms was decided not by rising temperature in the great depths of the sea, but by falling oxygen levels on the bottom. This made a lot of intuitive sense, for warm water can often become eutrophic and oxygen poor.
A deep bottom warming and a lowering of bottom oxygen, even a warming of the surface waters. What was the ultimate cause? The K-T asteroid impact event caused sufficient havoc in shallow waters to kill off almost all of the surface and upper water column plankton, but left the deep relatively unscathed but for the loss of nutrients from above. Warming the deepest part of the ocean could conceivably happen if the large parts of the sea bottom quickly warmed, but this would require an entirely new kind of deep ocean volcanism. The sea bottom does have areas of high heat flow, but these are confined to the relatively narrow mid-ocean mountain chains where seafloor spreading—the ocean bottom growth phase of plate tectonics—takes place. Even much faster plate movement by increased rate of volcanism along these mid-ocean rift systems would not do the trick. It was correctly surmised that the entire warm bottom had come from the warm, tropical surface waters where evaporation would make the surface waters saltier and denser. This warm and saline water was then transported along the sea bottom, even as far as the cold, high-latitude sites of Paleocene age.
Some aspect of ocean currents and the normal export of cold, oxygenated surface water down onto the deep-sea bottoms was not working in the Paleocene ocean. The deep, thermohaline circulation system—the main way that the ocean stays mixed—was just the opposite of how such currents work in our current ocean. The first victims were the tiny organisms requiring oxygen, the benthic forams of the deep sea. Many of these species died out, and did so relatively quickly in an event that lasted about four hundred thousand years. Still, to count as a mass extinction at all, it would have to be shown that it was not just the ocean that was affected, but land fauna as well. So the search was on for events on land.
Wholesale changes among oceanic organisms because of this greenhouse event occurred on the land as well.10 The newly discovered extinction in the deep sea stimulated paleontologists to look anew (and collect anew as well) at the fossil record of Paleocene land animals, to see if there was an extinction on land at the end of the Paleocene epoch. It did not take long to see that a great turnover had occurred among the mammals. Accurate dating soon showed that the extinctions on land and sea took place simultaneously.
In terms of the fossil record on land, the event itself seemed to mark nothing less than the start of our modern-day mammalian fauna. While there were numerous kinds of mammals by the latter part of the Paleocene (thirty distinct families are recognized from the collected fossils), many of these were small, and some belonged to groups no longer present, including survivors of small and rodent-like forms, many kinds of marsupials, some raccoon-like ungulates (a strange paradox, having the new entirely herbivorous ungulates taking on a meat-eating role in the Paleocene). There were also true insectivores and the first primates (like the insectivores, still at small size). But by late Paleocene time there were larger forms as well, and some of these were truly bizarre.
Dog- to bison-sized forms called pantodonts were leaf eaters that branched out into living a semiaquatic lifestyle like hippos, or living in trees, as well as having larger forms moving about on all fours on the forest floors. In general they were stout of body, with short legs, and one cannot help but surmise that, at least compared to modern herbivores, they were very clumsy and inelegant walkers. Yet large as they were, by the end of the Paleocene they were joined by even larger herbivores, the giant Dinocerata, which looked like huge rhinos even to the strange sets of knobs and horns on their skulls.
In the piles of strata marking the transition from Paleocene to Eocene, a reduction of species occurs, and over time—not instantly—new kinds of bones appear. Many come from kinds more familiar to us. The first even- and odd-toed ungulates appeared; more modern carnivores related to current groups soon evolved to eat the new herbivores, and all had to take into account an event that changed the very climate of the world. The lesson from past mass extinctions is that new occurrences would not have evolved as they did unless substantial extinction had opened the door to the possibility of new morphologies. This too happened at the end of the Paleocene.
Our colleague Francesca McInerney has given us a wonderful summary based on her work in the North American West that can help us describe the PETM. First, she noted that this event is highly relevant to us humans, as the amount of carbon released into the atmosphere, about 12,000 to 15,000 gigatons, is roughly equivalent to what we humans are releasing over time by our industries and energy use. The temperature change caused elevated greenhouse gases during the PETM made the world 5 to 9 centigrade warmer than it is now. The actual event lasted on the order of 10,000 years. Plants before and after were different from those during the event, when all the gymnosperms, the pines and their kind, disappeared. The plants that were present in her field area, as discovered by paleobotanist Scott Wing of the Smithsonian, were mainly plants that until the PETM lived in lower latitudes and thus at higher temperatures. After the event the old plants came back, as did the insects that were present prior to the 10,000 years of literal hell on Earth. But not so the mammals. This event caused a wholesale change in the North American mammalian fauna.
A final note. Had there been large ice sheets such as we have today they would have rapidly melted. That causes sea level to rise. In our view this is the single most dangerous aspect of human-caused warming: we are melting Antarctic and Greenland ice that will over the coming centuries inundate huge areas of current human farmland. The highest known rate of sea level rise is currently on the south China coast, one of the most heavily populated areas on Earth with sea-level rice farms.
GRASSLANDS AND MAMMALS OF THE COOLING CENOZOIC WORLD
From the Eocene to the start of the 23.5–5.3-million-year-ago Miocene epoch, the world slowly began to cool. At first, during the Eocene, this was almost imperceptible, and in fact there was a global tropical forest with crocodiles living inside the present-day Arctic Circle. But in the Oligocene this cooling accelerated, creating a different kind of major climate, and changing what had been a near uniform global climate to one with extreme seasonality. At the same time, giant continental ice sheets began to form on Antarctica, and probably Greenland as well. These swelling ice sheets caused a rapid and dramatic fall in sea level. At higher latitudes, forests gradually gave way in many places to grassland meadows and savannas. But other changes were taking place as well, changes in the atmosphere that would prove to have enormous consequences to the history of life.
Plants need carbon dioxide. Yet the history of carbon dioxide through the billions of years on Earth has been one of short-term rises and falls that in fact are
only minor variations in a much longer-term trend—the long-term reduction in this gas. With this long decline, our planet is gradually cooled, especially over the last 40 million years. Yet it is far more than the change in temperature that affected the evolution of plants during the Cenozoic era. Perhaps even more important has been an evolutionary formation of a more efficient form of photosynthesis, called C4 photosynthesis, which in many plants supplanted the more archaic mechanism, named C3 (the 3 and 4 in these terms is derived from different chemical changes taking place as sunlight and carbon dioxide are combined to form living plant cells and tissue). C4 photosynthesis, in fact, has shown an extraordinarily rapid rise in importance in terms of the number of plants using one over the other.
Plants that use the C3 pathway leave a different carbon isotope signature than those that use C4. Not only do the plants show this signature, which can be measured when any tissue from the plant is analyzed using a mass spectrometer specialized in looking at living tissue, but any animal eating those plants will leave a trace of it as well. Thus we know from the fossil record whether given herbivorous species ate C3 or C4 plants (or even a combination of the two).
We have two lines of evidence demonstrating when C4 plants first arose. The first is the molecular clock. By comparing the genomes of C4 to C3 plants, geneticists deduced that the differences were large enough that the C4 mechanism could not have arisen less than 25 million years ago (or any earlier than 32 million years ago as well). However, the fossil record yields a quite different answer to the question of when the C4 photosynthetic method first appeared, for the first fossils of C4 plants are only 12 to 13 million years of age.
The evolution to the C4 pathway was not a breakthrough that was then passed down to ever-greater numbers of plant species. In fact, it may have separately evolved more than forty times in the past, by that many separate lineages of plants. The eventual C4 plants are fire and desiccation-resistant plants adapted to heat and dry climates.
The most important C4 plants are grasses because of the dominance of the grass diet to so many kinds of herbivores, large grazing mammals as well as many kinds of birds, including the ubiquitous geese now found on most urban lawns near bodies of water. The reduction in carbon dioxide, especially over the last 20 million years, greatly abetted the expansion of C4 grasslands.11 Most grasses cannot live on forest floors, where the cooler, shadier conditions do not favor their growth.
Deforestation, however, creates a more open habitat, and therefore one that is far better for grasses. While the main idea has long been that the long-term drop in carbon dioxide sparked the evolution to dominance of C4 grasses, an alternative and newer idea is that a change in forest cover of the planet was as important as or perhaps even more important than a drop in carbon dioxide levels. But what would have caused radical reduction in forestation? The answer seems to be forest fires.
A dramatically underappreciated aspect of a planet with plants is the effect of forest fires. Fire, of course, is affected by oxygen levels. In times of higher oxygen, especially during the Carboniferous period of around 320 to 300 million years ago, forest fires may have been ongoing. A view from space during this interval would have shown an atmosphere darkly smudged and thick with smoke, so that there would have been a world-covering global haze that would have made a clear sunny day a rarity. But such smoke covering much of the continents itself would have had a highly significant effect on global temperatures, because much of the smoke from a forest fire can be light in color when viewed from above. The global haze and smoke would have reflected more sunlight back into space than would otherwise have happened, thus changing the albedo (the degree of reflectivity of the sun’s rays hitting the planet).
All of this would have created a chain of events radically changing not only global climate but also the entire history of life from that point onward. The rise of oxygen concentration and its prolonged high for more than 30 percent of all the Carboniferous period would have caused more forest fires. As noted above, this caused global temperature to drop, setting off a chain of events ending in one of the most prolonged polar glaciations in all of Earth’s history. Although it was not global in extent like the snowballs, it was nearly as long as some of them. That time of ice may have lasted more than 50 million years, a time interval coincident with some of the most important of all events in Earth history, including the conquest of land by animals, the evolution of new and advanced (for the time) land plants that were capable of colonizing upland regions of the continents previously uninhabitable by plants, and the first appearance of some of the most important of all vertebrate groups—including the earliest reptiles, and soon after the ancestors of the mammals. But there is another aspect of fire that would’ve affected the history of plant life, and therefore the history of life in general.
New studies on Amazon basin fires have demonstrated that wildfires can greatly influence climate, and not just in the tropics. David Beerling, in his book The Emerald Planet, noted that during April 1988, smoke from fires may have inhibited cloud formation over parts of North America—which in turn affected rainfall patterns. This interval of time, in fact, was one of severe drought—and resulted in one of the driest months of the twentieth century. This spring drought followed some of the most extensive wildfires ever, two of them present in North America during July of 1988, a year when gigantic areas around Yellowstone National Park extensively burned. Beerling invokes a new way of understanding the spread of C4 grasslands—very positive feedback system may have been put in place.12
Positive feedbacks are those that increase environmental change within one particular direction. In our world today, the warming atmosphere causes ever more of the Arctic ice pack to melt, so that there is an ever-smaller percentage of highly reflective white ice in the northern hemisphere. The white, ice-covered oceans reflect sunlight back into space, but when the ice melts and is replaced by dark-colored, open water, the oceans absorb much more heat—and the seas warm. As the seas warm, more ice is melted and the cycle continues. The positive feedback is that the warming causes more warming.
David Beerling suggested that there is a positive feedback in forest fires causing ever more forest fires. The fires change the climate, causing more drought, which makes ever greater areas susceptible to burning, causing a greater extent of fire damage. And so the cycle goes—burning causes more burning.
We enter a time when global temperatures are rapidly rising. The eventual effects this will have on the planet is not entirely unknown. Less predicable is the effect a new, warmed, high-sea-level world will have on human industry, population, and civilizations.
CHAPTER XVIII
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The Age of Birds: 50–2.5 MA
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The history of life as often first taught to us as children is broken down thusly: fish began in what we call the Age of Fish; some crawled ashore to start the Age of Amphibians, which then began what was once called the Age of Reptiles or sometimes the Age of Dinosaurs. Things finished off with an Age of Mammals. It is not hard to see why this has become the common knowledge: humans like to pigeonhole things, and a succession of “ages” is pigeonholing at its best. But among the many, many problems with this account is one of many other truths: there are no pigeons at all in this succession. Let us change that here and consider what we might call an Age of Birds.1
The evolution of birds is a major topic of research.2 It has been a controversial area of research as well, with two major schools of “belief”—one, that birds evolved from a nondinosaur diapsid, something akin to one of the many reptilian-like forms that gave rise to the dinosaurs themselves, or two, that dinosaurs were the direct ancestors of birds. This school even invokes the methodology of cladistics to reinforce the claim that what we call birds are in fact dinosaurs, just highly modified.3
A host of fossils have shown that not only did many smaller bipedal, carnivorous dinosaurs resemble birds in the way they laid their eggs but that these eggs also loo
ked like the eggs of birds. Even more striking has been the new discoveries that many dinosaurs both before and after the first appearance of Archaeopteryx even showed evidence of winglike arms, with feathers, suggesting a second attempt by dinosaurs to gain the ability to fly. The question was whether or not this famous fossil was even a dinosaur.4
The dispute goes back to about 1996, when paleobiologist Alan Feduccia investigated the then newly discovered fossil of what he interpreted to be an intriguing bird that lived about 135 million years ago, just after Archaeopteryx. The bird, Liaoningornis, did not look like a dinosaur bird at all.5 It had massive flight muscles attached to a breastbone similar to modern birds. Yet it was it was found alongside fossils of ancient birds not unlike Archaeopteryx. How could such advanced evolution have taken place so quickly? Instead, Feduccia concluded, birds may have been already very widespread by the time that Archaeopteryx first appeared in the time interval roughly from 140 to 135 million years ago, and that by that time they were already occupying a variety of habitats. While more “advanced” than Archaeopteryx, they were still very primitive by modern bird standards. So where are they? Feduccia believes that most of them died out with the dinosaurs, about 65 million years ago, and that the ancestors of all today’s birds evolved later, between 65 and 53 million years ago, independently of the dinosaurs. This is the so-called big bang theory of birds.6 Feduccia and his colleagues view any similarity between birds and dinosaurs as simply due to convergent evolution, where natural selection independently produces similar morphologies.
This school of thought has the modern birds appearing late—either coincident with the 65-million-year-ago K-T extinction—or some tens of millions of years later. This is certainly not mainstream understanding about bird evolution anymore.7 Within the last decade a large number and variety of birds have been found in Cretaceous rocks ranging from 130 to 115 million years ago, most from China. Some of these fossils show that a great diversity of birds with long, bony tails preceded the evolution of birds with the familiar short, bony tail.8 However the dinosaur-to-birds theory was further supported by the discovery of two species of feathered dinosaurs in China, dating from between 145 million and 125 million years ago, followed by younger, early Cretaceous birds.