Paleontology is a wonderful discipline for gaining some perspective. It illustrates that things haven’t always been like they are today: not only animals and plants, but also biomes, ecosystems, landforms, and even the continents themselves. This is easy to forget because changes in the natural world usually occur at an imperceptible pace, far beyond one’s lifetime. But for a paleontologist who studies remains that are thousands, millions, or even billions of years old, such differences between the present and the past are eminently clear. It is also clear that the present day (broadly speaking, the Anthropocene) is an arbitrary interval that is no more “normal” than any other time period. In fact, it is pretty abnormal from many perspectives! The modern world is simply the interval with which we are most familiar. I am of the opinion that no place on Earth highlights this concept more clearly than South America, though I admit that this comes from the biased perspective of a paleontologist who specializes in mammals.
What makes South America stand out compared to other continents? For one, it still boasts an incredible diversity of terrestrial life. That is no longer true of Antarctica, which has been covered by ice for more than 30 million years. Among the remaining continents, South America is the only one that has been in the “Goldilocks Zone” of faunal exchange since the extinction of the dinosaurs 66 million years ago; enough immigration for the continent to be populated with more than a dozen major mammal groups, but not so much that the species in those groups broadly resemble those on other continents. To see this phenomenon, you only need to scratch the surface, literally and figuratively; the Pleistocene megafaunal extinction that took place some 12,000 years ago hit South America harder than any other continent, which means you don’t have to go back very far in the geological record to uncover species quite unlike anything alive today. As you dig deeper into geological time, you find fewer familiar mammals and more exotic ones. A mere 20 million years ago, South America’s mammal communities were nearly as distinct as those of modern Australia.
This book looks backward from today to trace the origins of South America’s mammals and other animals that call the continent home. As you will see, the histories of the major groups of backboned animals (vertebrates) vary in their ecological adaptations, their dispersal abilities, and how long ago their families, orders, and other major groups originated. Reconstructing such histories relies on many different types of information in addition to the fossil record, but the fossils themselves and their geological contexts (including their ages) are the most important clues for documenting when particular groups reached particular areas. Since most mammals are characterized by sturdy bones and durable teeth, they have left a better fossil record than other groups of terrestrial vertebrates. As a consequence, we can provide more details about their history in South America than we can about other groups. Nevertheless, mysteries remain among the mammal fossil record, and the search for answers is what fuels the fieldwork undertaken by me and many other investigators in South America.
One major unanswered question is how South America’s native ungulates (SANUs) fit into the Tree of Life. Some molecular data suggest that some of them may be most closely related to horses, rhinos, and tapirs (perissodactyls) among mammals alive today, but even if that is the case, it isn’t clear whether this is true of all major SANU groups or just some of them. Moreover, it raises some perplexing biogeographic questions. The oldest SANU fossils are from about 65 million years ago, but current evidence indicates that perissodactyls didn’t reach North America (from Asia) until about 55 million years ago. Is there a 10-million-year gap in the perissodactyl record in North America, or did the ancestors of SANUs get to South America by some other route? Perhaps we aren’t recognizing some ancient mammal remains as the ancestors of SANUs that they are. SANUs were cornerstones of South American terrestrial ecosystems for most of the Cenozoic, but there is still much to be learned about their origin and diversification.
When I first visited the Neotropics, the animal I most wanted to see was a sloth. It looks calm, cool, and collected, and there isn’t anything else like them. I don’t think there is any modern mammal as quintessentially South American as a sloth. It is also characteristic of the continent’s fossil record. For some 30 million years, sloths spanning a wide range of shapes and sizes occupied medium to large herbivore ecological niches throughout South America. Despite that, we have no clue about their diversity or ecology prior to the middle of the Cenozoic; the first half of their evolutionary history is a black box. How is that possible? They either left no fossil record, or the fossils they left are in the sites that remain to be discovered. I very much hope the latter is the case and that we uncover something about these early sloths within my lifetime. They are likely the only example of a group of plant-eating mammals that evolved from an ancestor specialized for feeding on social insects such as ants and termites. Tracing the steps in this unlikely journey requires fossils of the earliest members of the group.
The renowned evolutionary biologist George Gaylord Simpson described the late Cenozoic faunal exchange between the Americas as “one of the most extraordinary events in the whole history of life.” It occurred when a land connection developed between North and South America - for the first time in nearly 60 million years - making it possible for many North American groups to disperse to South America and vice versa. According to the fossil record, most of this exchange took place during the past 3 million years, but some exchange took place much earlier, as early as 9 million years ago. Explaining this pattern is a topic that has been and will continue to be controversial, partly because of the many types of data involved. In addition to the mammal fossil record, there is the marine record to take into account; for marine animals, the great connection between the Americas created a great schism between the Atlantic and the Pacific. Add to that geological data from this tectonically complex area of the globe, and the result is a mixture of perspectives using different tools to look at different aspects of the same phenomenon. It has been a bit like the parable of a group of blind men trying to describe an elephant based only on the part they are touching. However, it is also clear that we are missing big parts of the puzzle. For example, the oldest records of North American mammal groups in South America are from the southern half of the continent. We are undoubtedly missing much of their early history in tropical latitudes, a deficit that may be possible to resolve only through new fossil discoveries.
So, “Where Did They Come From?” You will find many answers to this question in this book, but I think you will find these and other mysteries that remain as compelling as the answers we have. Hopefully, both will inspire you to visit South America to see some of these animals in the flesh as well as some of the myriad fossils that have helped us document their geographic and evolutionary journeys through the millennia.
Darin A. Croft
Department of Anatomy
Case Western Reserve University School of Medicine