Jungle
Page 6
Better-dated and finer-grained records of climatic and environmental change for our current geological period, the Holocene (11,700 years ago to the present), show us that the broad changes in tropical forest structure and extent noted above for time periods where resolution is limited to 1,000 years or even more miss a variety of finer-scale variabilities that would have confronted tropical forest environments. Study of the changing operation of climatic systems today and in the recent historically recorded past reveals that parts of the tropics can rapidly switch from cold and dry to warm and wet and vice versa. For example, variations in how winds and sea surface temperatures interact over the Pacific Ocean (as part of the El Niño Southern Oscillation climate system) have led to extreme and sometimes unanticipated droughts and flooding in the recent past. Although the broader planetary changes seen on geological timescales undoubtedly influenced evolutionary processes, these short-term shifts would have shaped the experiences of individual organisms and communities. Feedbacks between vegetation and climate systems are also important for these high-intensity processes. This is acutely visible when we take the dramatic example of the “Green Sahara.” Stretching from the Red Sea in the east to the Atlantic Ocean in the west, the Sahara is today the world’s largest nonpolar desert, encompassing much of North Africa. Between, 15,000 and 5,000 years ago, however, conditions were very different, and this gigantic field of dunes was covered in vegetation and water. Climate models have demonstrated that alterations to the Earth’s orbit are not enough to produce this effect alone. In fact, only when earth scientists factor increasing precipitation plus feedbacks from expanding forest and plant life into their computer software can they reproduce a vast oasis, with vegetation likely increasing water in the atmosphere through evaporation from its green surfaces, while also stopping the usual buildup of dust. Certainly the Holocene subtropical and tropical forests—which enable increases in regional air moisture, stabilize landscapes to avoid runoff, act as a buffer against flooding, and absorb solar energy—were critical players in the translation of climate system changes into on-the-ground environmental dynamics. This is something we will see later in the book, as our own impacts on these remarkable environments, and thus the planet’s climate, come to the fore.17
AS WE HAVE seen, the history of tropical forests on Earth is one of upheaval, not timelessness. Extreme geological processes saw the first tropical trial runs begin to disappear from c. 300 million years ago to a point, c. 250 million years ago, when there were basically no forests on Earth. This seeming calamity paved the way for new forests, eventually composed of a new form of pioneering, flowering plant life that came to dominate global vegetation between 140 million and 60 million years ago. By around 60 million years ago, tropical forests closely resembling their twenty-first-century descendants had appeared in the tropics (and even pushed out beyond into a warm world as megathermal forests). But this was not the end. From 10 million years ago they were pushed back by new grassy adversaries to roughly their current equatorial distribution. Still the world was not done with them. Ongoing cycles in the availability of the Sun’s energy and changes in climate circulation systems have buffeted these subtropical and tropical forests, causing them to wax and wane, open and close, ever since. However, tropical forests were never just victims of external change. As geology and climate shaped and moved them, they slowly but surely inserted their green tendrils into the Earth’s systems. While the Sun dominated planetary climate change, this new vegetation cover decided how this solar energy manifested on the ground, whether it made local temperature and precipitation changes more dramatic or buffered local landscapes against them. Furthermore, tropical forests controlled the amount of CO2 in the atmosphere and its distribution, providing some of the most significant land-based storage areas, or “sinks,” of carbon on Earth.
This dynamism, both in terms of evolutionary change and the increasing role played by tropical forests in climate systems, gives us a new appreciation of their significance for life on Earth. Broadly considered, they were the sites of the most significant evolutionary events in the evolution of plants: the rise and diversification of the gymnosperms and angiosperms. They were also changing their distribution and structure as some of the most famous cast members of the animal kingdom arrived, evolved, and left the planetary stage. At the beginning, this involved insects and amphibians, but by the end it included the appearance of our species, its migration across the global tropics, and its formation of a vast variety of social, economic, and political systems that have varied radically until the twenty-first century. Throughout the rest of this book, we will meet the hugely diverse types of tropical forest—from the seasonally dry forests of Mexico and Guatemala to the often frosty forests of New Guinea, from the isolated island forests of Wallacea and the Pacific Islands to African tropical forest-savannah mosaics—encountered by our species, which are the ultimate products of this vibrant evolutionary history. Before we examine Homo sapiens and its closest relatives specifically, however, the significance of tropical forest change to our world as we know it is vividly visible when we look at two of the most evocative groups of animal life to walk the Earth. The next two chapters explore how the dynamics of tropical forests, from the Permian to the Pleistocene, described above, influenced the evolution and fates of, first, the dinosaurs (Chapter 3) and, later, their eventual inheritors, the mammals. Captivated by the fossils of these fascinating creatures, we have perhaps too rarely explored or given credit to the leafy, canopied companions that escorted them on their biological journeys. It is to these stories that we now turn.
Chapter 3
“GONDWANAN” FORESTS AND THE DINOSAURS
I will always remember visiting the National Museum in Cardiff, the capital city of Wales, as a young boy with my family. Unlike many museums, which value academic silence and regular processions around their secure glass cases, this museum created a cinematic experience for its visitors—dramatically using sound and atmospheric visual displays to form the most vivid picture possible of our planet’s history. The continental plate exhibit featured a darkness that focused your eyes onto the flowing red-orange lava of volcanoes and your ears onto the loud, crunching sounds of earthquakes. The exhibit of some of the earliest humans to reach a then icy Wales involved a trip into a cave-like chamber to meet moving, trumpeting mammoths. Most stunning of all, however, was the hall of the dinosaurs. Here, fossil casts of giant reptiles emerged out of shadows, illuminated by well-placed lighting and a chorus of roars and brays. This room has stayed with me as one of my earliest, and perhaps also scariest, encounters with the evolution of life on Earth. However, looking back now, something major was missing from this drama. In fact, it has been missing from almost all of the museum exhibits of dinosaurs I have encountered since. While we are staring at monstrous jaws, reconstructions of colorful feathered or hairy skin, or artistic impressions of vicious hunting scenes, we always seem to forget one of the most key parts of the dinosaurs’ existence: their environments.1
Approximately 240 million years ago, the dinosaurs began as a predominantly carnivorous group of initially small “dinosaurmorph” reptiles. They bucked the trend of other sprawling and shuffling Triassic life and stood upright—adopting a very different way of living and moving. Indeed, when many of you think of dinosaurs, your thoughts will quickly turn to the hulking bipedal carnivores such as Spinosaurus and Tyrannosaurus that have haunted dreams since Hollywood pushed them onto our screens in the early 1990s. Yet these gigantic killers ultimately needed something to eat. From the Jurassic (201.4 million to 143.1 million years ago) into the Cretaceous (143.1 million to 66.0 million years ago), the dinosaurs began to form some of the largest land-based food chains by mass that have ever existed. Herbivorous dinosaurs accounted for around 95 percent of vertebrate biomass in some ecosystems and included the long-necked, long-tailed quadrupedal “sauropod” dinosaurs, such as Argentinosaurus, Diplodocus, and the aptly named Supersaurus and Patagotitan, which were the la
rgest organisms to ever walk on solid ground: with a weight maximum of seventy tons, these sauropods would have dwarfed even the largest of African elephants recorded at ten tons. Researchers estimate that the biggest sauropods would have needed to consume a staggering two hundred kilograms of vegetation every day. For reference, that’s nearly seven hundred cans of baked beans. Amazingly, we still rarely consider the key role that plants must have played in the evolution and sustenance of dinosaur ecosystems or the combined impacts that herds of herbivorous dinosaurs must have had on ancient vegetation—with plants portrayed as a convenient camouflage or as passive backdrops rather than a crucial part of life.2
This is perhaps even more remarkable when we think about the spectacular geological and environmental processes that happened from the Permian (298.9 million to 251.9 million years ago) to the Cretaceous (143.1 million to 66.0 million years ago). Major changes in vegetation accompanied the collapse of Carboniferous rainforests, the rise of the coniferous and cycad gymnosperms, and, later, the flowering angiosperms. Warm, wet forests at times blanketed the Earth before shrinking to more restricted distributions. Meanwhile, the plates of the Earth kept drifting as first Pangaea and then Gondwanaland were rent asunder to form the current continental layout of the tropics. Throughout, new forms of plants were emerging, moving, and evolving in novel, increasingly isolated ecosystems in the tropics. Today we accept that climate and environmental change associated with such significant geological and biological shifts would have massive implications for animal life (and in this chapter, we will take a look at the science that enables us to be more confident in thinking that). Nevertheless, while frequently used for dramatic effect in films, the vegetation of the dinosaurs has often been cut out of their evolutionary story, from their emergence and diversification through cataclysmic disaster to their persistence as birds today. Is this due to the poor preservation of fossil plants or simply a result of plants lacking the media appeal of the reptiles they supported? Either way, it’s high time we inserted plants into the history of these “terrible lizards.” Today leading researchers are using state-of-the-art investigation of reptile (including dinosaur) fossils, fossil pollen and wood records of past environments, and, incredibly, even the occasional preserved last meals of particular dinosaurs to do precisely that.
Figure 3.1. Comparison of selected giant sauropods. Several length estimates have been proposed for different taxa, with varying degrees of accuracy. Adapted from Wikipedia / KoprX
IT MIGHT BE hard to imagine, but the first “true” dinosaurs, defined by a series of particular features seen in their fossilized skulls, hips, and limbs, did not immediately dominate the Earth when they appeared in the Middle-Late Triassic approximately 240 million to 230 million years ago. Scampering alongside large amphibians, other diversifying reptiles, and tiny primitive ancestors of mammals, the dinosaurs were just one of a number of organisms trying to survive on the surface of a planet that had been through the wringer of extreme change. Although things looked bleak for animal life on land following the massive plant extinction event at the end of the Carboniferous and the volcanic-induced extinction of animals and plants at the end of the Permian, a number of researchers, including Dr. Emma Dunne, believe that in these cataclysmic shifts the dinosaurs grasped their chance to rise to the top of the animal kingdom. The changes to the Earth’s surface that began with the collapse of the Carboniferous rainforest, whose canopies had coated practically all exposed equatorial land approximately 310 million years ago, would gradually, but fundamentally, alter the course of evolution. In time, they were to pave the way for all of the main groups of land-based animals we know today. The first key beneficiaries were, however, the reptiles and, more specifically, the dinosaurs.
Emma, a vertebrate paleobiologist at the University of Birmingham, has been interested in fossils for as long as she can remember, though she sees them in a different way to many people. Perhaps unsurprisingly, there is often a fascination with individual fossils, with people wanting to find the first type of a certain animal or particular body shape. However, a lot of information can be missed if these remarkable traces of life are treated as single objects. Emma prefers to think of them together as “parts of a living, breathing ecosystem.” In this way she can try to reconstruct not just what they looked like but also the type of environment they might have lived in. This is essential if we are to go from looking at a stationary bone to determining what these animals might have eaten, what climatic and environmental conditions might have surrounded them, and what impacts they themselves might have had on the world around them. Emma is particularly interested in how land-animal communities changed between the Carboniferous and the early Mesozoic—an age that includes the Triassic, Jurassic, and Cretaceous periods—taking us right up to the dawn of the dinosaurs. To look at this change, she and her colleagues have tried to find a way to properly measure the number of different types (or “diversity”) of animals living in different parts of the world, as well as across the world as a whole, at a given point in time. This, in turn, allows them to investigate how and why this diversity might have changed across space and time. This is not as easy as it might sound, however. The fossil record from this time period reflects huge biases. Most of our knowledge, like that about the ancient forests we have already encountered, comes from particular sites, mainly in North America and Europe. This is a product of the longevity of geological and paleontological study in Euro-American societies, as well as the availability of funding for such enterprises in different social, political, and economic situations—something, as we will see in Chapters 10 and 11, often linked to historical global imbalances in wealth distributions.
To try and get around this problem, Emma and her team made use of a remarkable tool that is actually available to every single one of us: the Paleobiology Database, a free-to-use record of fossil discoveries around the world, from the Proterozoic (2,500 million to 538.8 million years ago) to the Holocene. I can guarantee you that the beautiful software and easy-to-access data will give you hours of distraction, should you let it—allowing you to discover long-extinct creatures and the locations in which they have been found. Playing around with its functions, I went on an evening-long search for the find-spots of some of the most iconic creatures we commonly hear about from paleontologists, from the gigantic Megalodon shark to Triceratops. Most importantly, however, it represents part of a growing push to make scientific data freely available, or “open access,” so that they can be used and combined to answer larger, planetary-scale questions. In this case, as paleontologists repeatedly update the database, adding the location and age of all finds from across the world, some of the more glaring geographical biases can be increasingly overcome. Instead of simply counting species for which there are fossils, larger databases such as this enable the application of novel statistical approaches so that we can get as complete a picture as possible of the entire animal fossil record, spanning the earliest tetrapods (four-legged land animals) to the first dinosaurs and beyond.3
The results of Emma and her team’s research are fascinating. They demonstrate that the collapse of the tropical Carboniferous “coal forests” had a dramatic impact on land-animal diversity. Those affected most were the large, diverse amphibian communities, whose footprints were preserved in the rainforests of Birmingham approximately 310 million years ago. Although early dark, wet, and warm forest ecosystems had provided a haven for animals that needed water to lay eggs vulnerable to drying out, more arid conditions initiated the dramatic decline in these habitats. What happened next is critical. The small reptilian footprints, which were very much in the minority in Carboniferous Birmingham, suddenly proliferated, growing larger. Those tetrapod species that did survive the rainforest collapse began to disperse more freely across the planet, colonizing a number of new environments and moving further away from the tropics. Their laying of eggs resistant to drying unleashed a massive potential for expansion and gave them a number of advantages over the
amphibians as forests continued to decline through the Permian and into the Triassic. In summary, then, the disappearance of tropical forests evidently, at first, actually aided the rise of the reptiles and, in turn, the dinosaurs. The changes in these ecosystems reshuffled the hands of the different animal groups and set out a new field of ecological competition.4
Dinosaurs have often been portrayed as “tropical” animals, existing within a framework of stable, warm climates from the Triassic to the Cretaceous. Yet, from the above, it seems that they could both be affected by and benefit from climatic and environmental instability. This is certainly true when we look at the early evolution and expansion of the first “true” dinosaurs from 230 million years ago. Although dinosaurs rapidly divided into three distinct successful lineages (Theropoda, Sauropodomorpha, and Ornithischia) and had achieved a remarkable global distribution by 206 million years ago, why it took nearly 30 million years from their first appearance for them to enter the tropics and complete their global conquest has remained something of a mystery. Furthermore, even when they did arrive, they did so in a tentative rather than a sweeping manner. The early theropod dinosaurs, which were to later include the infamous velociraptor, made it to the tropics and subtropics by the Late Triassic. This clade, characterized by animals with hollow bones and three-toed limbs, surviving in the form of modern birds, was initially dominated by predatory carnivores hunting small animals scurrying along the ground. By contrast, the other two main dinosaur clades, the increasingly large herbivorous sauropodomorphs and ornithischians, were restricted to the higher latitudes, being somehow “kept out” of the tropics until the Jurassic (201.4 million years ago). Although this division has often been attributed to bad fossil preservation caused by high humidity in the tropics or to potential fights for survival with crocodile-like reptiles that already inhabited the equatorial regions, a growing collection of fossils and the uncanny patterning of dinosaur types by latitude suggests that a deeper underlying geographical trend was at play.5