Megafauna: Giant Beasts of Pleistocene South America

3 Geological and Ecological History of South America during the Cenozoic Era

73 3 Geological and Ecological History of South America during the Cenozoic Era In this chapter, we provide a broad outline of the tectonic, climatic, and biotic changes that occurred in South America over the course the Cenozoic , focusing on the mammals, given that they have served as the main basis for establishing the biostratigraphic framework in South America. Our story will extend only through to the Pliocene (because the changes in South America before this time were essentially self-contained, given its long isolation from North America and, through it, other landmasses) so that we may consider, in Chapter 4, the North American mammals and their fate during this earlier period. With the stage thus set for subsequent events, we return, in Chapter 5, to the thread of the South American climate and biota, consider the events and conditions that led to the formation of the Isthmus of Panama, and explore the changes that occurred once the faunas from the two continents began to intermingle during the Pliocene and, especially, the Pleistocene. This earth that we live on, the world we are comfortable with when it can be considered fixed and unchangeable, is our point of reference. Once in a while, reports of an earthquake, tsunami, or other calamity remind us of just how fragile this stability is. In fact, not only is it fragile, it is totally illusory, because, as we have known for decades, the continents move or drift, sliding heavily over deeper layers of the earth at the scanty pace of a few centimeters per year. Over millions of years, which are mere moments in the immense dimension of geologic time, these few centimeters are transformed into hundreds of kilometers. The continents thus unite and separate, like pieces of a giant puzzle. South America and Africa were united until 180 million years ago (Mya), and the outlines of their respective Atlantic coastlines reveal their former union. The meteorologist Alfred Wegener, who first proposed a scientific theory of drifting continents at the beginning of the last century (the idea of moving continents has roots stretching back to the 1500s) and was ridiculed by the contemporary scientific establishment for his efforts, presented this superposition as among his primary evidence. India was also united to Africa. In separating and migrating north, much of India’s former territory was wrinkled against Asia to form the mighty Himalayas. These three landmasses, plus Australia, New Zealand, and Antarctica, were all united until 180 Mya and formed the supercontinent of the south, termed Gondwana (Fig. 3.1). When Gondwana existed, there was a time during the late Paleozoic in which the South Pole was situated near the south of modern-day Brazil, and gigantic glaciers dominated the regions that today are the prairies bordering Continents on the move: plate tectonics Megafauna 74 the Río de la Plata. The northern continents were also united, forming the supercontinent of Laurasia. Both supercontinents, representing nearly all the exposed landmasses, were united into a single unit, termed Pangaea, which started to break apart about 200 Mya. This pattern of union and separation may have occurred several times in the earth’s 4500 million (or 4.5 billion) year history. Clearly, each union and separation greatly affected life on the continents as well as life in the oceans as these were created or connected. Although here we focus on the fauna that lived only hundreds of centuries ago and no appreciable change has occurred since then in this regard, various critical aspects of this history cannot be easily understood except in the context of moving continents. We have been speaking so far about the movement of continents. However, the continents, the earth’s main current portions of land, are really only the emergent parts of larger components of the earth’s surface. These components are known as plates, and it is the plates that move (hence the term plate tectonics), with the continents going along for the ride, in response mainly to seafloor spreading. A plate is a large, mobile slab of rock and may be oceanic, topped with oceanic rocks submerged to form the seafloor, or continental, formed of both oceanic and continental rock (although some of the smaller plates are made entirely of continental rock). Fig. 3.2 shows the earth’s major plates. Adjacent plates move in different ways relative to each other (Fig. 3.3). At a divergent boundary, the plates move away from each another, whereas at a convergent boundary, they move...