Horned Armadillos and Rafting Monkeys
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Horned Armadillos and Rafting Monkeys

The Fascinating Fossil Mammals of South America

Darin A. Croft, Velizar Simeonovski

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Horned Armadillos and Rafting Monkeys

The Fascinating Fossil Mammals of South America

Darin A. Croft, Velizar Simeonovski

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About This Book

A thrilling guide to the Cenozoic mammals of South America, featuring seventy-five life reconstructions of extinct species, plus photos of specimens and sites. South America is home to some of the most distinctive mammals on Earth—giant armadillos, tiny anteaters, the world's largest rodent, and its smallest deer. But the continent once supported a variety of other equally intriguing mammals that have no close living relatives: armored mammals with tail clubs, saber-toothed marsupials, and even a swimming sloth. We know of the existence of these peculiar species thanks to South America's rich fossil record, which provides many glimpses of prehistoric mammals and the ecosystems in which they lived. Organized as a "walk through time" and featuring species from fifteen important fossil sites, this book is the most extensive and richly illustrated volume devoted exclusively to the Cenozoic mammals of South America. The text is supported by seventy-five life reconstructions of extinct species in their native habitats, as well as photographs of fossil specimens and the sites highlighted in the book. An annotated bibliography is included for those interested in delving into the scientific literature. "Well-written and easy for the nonspecialist to understand, this is also a most needed updating of this subject, much in the line of classic works such as Simpson's The Beginning of the Age of Mammals in South America and Patterson and Pascual's The Fossil Mammal Fauna of South America." — Richard Fariña, coauthor Megafauna: Giant Beasts of Pleistocene South America "This handsome book, written by a leading expert in South American paleontology, is profusely illustrated with maps, time charts, color photographs of fossils, and exquisite life reconstructions. The book... will appeal to any individual, young and old alike, interested in the fossil record, as well as to students and scholars of paleontology who work in other parts of the globe." — Choice

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1Time and Geography

Geologic Time

THE CENOZOIC ERA, THE MAIN FOCUS OF THIS BOOK, IS DIVIDED into three periods: the Paleogene, Neogene, and Quaternary (Fig. 1.1). Each of these, in turn, is divided into smaller units known as epochs. The Paleogene includes, from oldest to youngest, the Paleocene, Eocene, and Oligocene epochs. The Neogene includes the Miocene and Pliocene, and the Quaternary includes the Pleistocene and Holocene. Since epochs can span up to 18 million years, it is often useful to subdivide them using the terms early, middle, and late. These descriptors are technically defined, and all epochs except the shortest ones, the Pleistocene and Holocene, have early and late subdivisions. (Formal subdivisions of the Pleistocene are currently being discussed.) Three of the longer epochs, the Paleocene, Eocene, and Miocene, also have a middle interval. One can therefore speak of a middle Miocene but not of a middle Oligocene. (Epochs are also subdivided into smaller units known as stages or ages, but those are not addressed here.)
Paleomammalogists (paleontologists who study mammals) use another set of terms to refer to geologic time in addition to epochs: land-mammal ages. Unlike geologic units, land-mammal ages and similar systems vary from continent to continent; their divisions do not correspond in name, age, or duration. Consequently, I will mainly use geological units when discussing geological time and will mention corresponding South American Land-Mammal Ages (SALMAs) only in passing. SALMAs have long been used when speaking of extinct South American mammals, and although they are not emphasized here, it is necessary to have a general understanding of them in order to read the scientific literature on this topic.

Present and Past Geography of South America

South America is a long, relatively narrow continent mainly located in the Southern Hemisphere (see p. 5). Its northern edge lies just north of 12° N latitude and its southern tip sits at 56° S latitude, a distance of nearly 5,000 miles (8,000 km). Its broadest point is a bit south of the equator, where it is more than 3,000 miles wide (5,000 km). The continent narrows to the north and south of this point, with the southern extension being much longer and more gradually tapered. Thus, much of the continent is relatively narrow, especially the part in temperate latitudes known as the Southern Cone. The westernmost point of South America is located almost due south of eastern Ohio, meaning that the bulk of South America lies to the east of North America (other than Greenland).
The most conspicuous geographic feature of South America is the Andes Mountains. Aconcagua, in western Argentina, rises to an elevation of nearly 23,000 feet (7,000 m), making it the highest mountain outside the Himalayas. The Andes are the longest mountain chain in the world and run along the entire western edge of the continent, following the curve of the northern coastline of South America to the east. This huge mountain chain and a large latitudinal span that straddles the equator have created a wide variety of climates and habitats in South America. In the southern part of the continent, known as Patagonia, a narrow strip of temperate rainforest exists west of the Andes, while dry plains predominate to the east. This trend is reversed farther north, where deserts predominate west of the Andes in northern Chile and Peru, and the eastern slope of the Andes and the great Amazon Basin receive abundant moisture. Most of the northern third of South America receives abundant precipitation. On top of this general pattern, regional variation in soils and topography, proximity to the ocean, and seasonal variation in energy from the sun combine to create nearly every type of habitat one can imagine. Such habitats range from nearly lifeless deserts that have received no precipitation within recorded history to the most species-rich rainforests in the world, some of which receive up to 30 feet (10 m) of precipitation each year.
South America is presently connected to North America via the Isthmus of Panama, but this continuous land connection between the two continents has existed in its present form only for a few million years. Prior to this time, most groups of South American mammals were found nowhere else. Visiting South America would have been like visiting present-day Australia or Madagascar. Some mammals might have resembled those found in North America or Africa but would have been only distantly related. Other mammals would be truly unique. This uniqueness was characteristic of South American mammal communities as early as the mass extinction at the end of the Cretaceous Period (66 million years ago). South America was likely connected by land to both North America and Antarctica at this time; Antarctica was not yet covered by ice and was also connected to Australia. As a result, early Cenozoic mammal communities in South America included a mixture of Northern Hemisphere mammals as well as Southern Hemisphere ones. The exchange of mammal species among these continents around the time of the Cretaceous-Paleogene extinction has recently been termed the First American Biotic Interchange or FABI. This event set the stage for the evolution of South American mammals over the next 66 million years. The term FABI parallels the name coined several decades earlier for an equally important event in the history of South America, the Great American Biotic Interchange (GABI)–the large-scale exchange of mammals and other animals that was facilitated by the formation of the Panamanian land bridge in the late Cenozoic.
Between the FABI and the GABI, there was no major exchange of mammals between South America and any other land mass. This is because both of South America’s land connections were severed during the early Cenozoic. However, two mammal groups appeared in South America near the middle of the Cenozoic: rodents and primates (monkeys). Fossils indicate that both of these groups had an African origin. The most likely scenario to explain their appearance in South America is that they traversed the Atlantic Ocean on a floating island of vegetation expelled from the mouth of an African river after a storm. Upon landing in South America, they were able to found new populations of rodents and primates. Incredible as this might seem, it is not unprecedented. Such floating islands are known to occur today, and African mammals evidently reached the island of Madagascar in a similar fashion. What makes these South American dispersals so unusual is the distance involved; some 870 miles of ocean (1,400 km) separated Africa and South America at the time. Based on the fossil record, rodents made the crossing at least 40 million years ago. The arrival time of primates is less certain, but the oldest South American primate fossils are probably 35–30 million years old. The timing and duration of what could be called the Trans-Atlantic Dispersal Interval, or TADI, continues to change as new fossils are discovered, particularly in tropical areas.
Map of South America.
Using the FABI, TADI, and GABI as guides, the history of South American mammal communities can be divided into three intervals. These three intervals have been recognized in one form or another for many decades, though their boundaries have shifted based on new discoveries and remain somewhat blurry. During the first interval, the Early South American Phase, South American mammal communities consisted of endemic South American mammals as well as mammals with links to Australia-Antarctica and North America. The vast majority of groups with strong links to other continents declined and went extinct during the Paleocene and Eocene. Therefore, Oligocene and Miocene communities consist almost entirely of endemic South American mammal groups–including rodents and primates, which diversified there during this time. The interval spanning the Oligocene and Miocene has been termed the Late South American Phase. The beginning of this phase originally coincided with both the TADI and the Eocene-Oligocene boundary, but as the start of the TADI has been pushed into the Eocene, the initiation of this phase has become linked to the beginning of the Oligocene. Nevertheless, TADI mammals (rodents and primates) are only characteristic of Late South American Phase mammal communities. The GABI initiated the third interval of South American mammal communities, which is marked by major mixing of North American and South American groups. As a result, many groups of mammals are found in South America for the first time at sites of this age. Even though the GABI began in the Miocene, I include only mammal communities of Pliocene and Pleistocene age in the Interamerican Phase, since this is when North American mammals are abundant and diverse in South American faunas.
A fourth and final interval can be added to the traditional three-phase scheme described above, the Hypoamerican Phase. This interval is characterized by an extinction that eliminated the last representatives of many groups of South American mammals that had existed there since the Early South American Phase. This extinction coincides with the arrival of humans (Homo sapiens) in South America, and various lines of evidence suggest humans were at least partly responsible for it. For better or worse, the character of South American mammal communities changed dramatically during the Hypoamerican Phase, resulting in the mammal communities we know today.

2Introduction to Mammals

Mammal Classification

IF YOU HAVE TAKEN A BIOLOGY CLASS, YOU ARE PROBABLY ALREADY familiar with the seven main ranks in the hierarchical classification of life: kingdom, phylum, class, order, family, genus, and species. Many other ranks fall in between these main ranks, most formed by adding a prefix to indicate whether it is slightly more inclusive (higher in the hierarchy) or less inclusive (lower in the hierarchy). For example, a subclass is below a class, and a superorder is above an order. This book will focus on the class Mammalia (mammals) but will also discuss members of a few other classes. Within Mammalia, the most important ranks are orders and families. Orders are the “main types” of mammals, such as rodents, primates, and bats. Families are the primary subdivisions of orders and often correspond to familiar groupings. For example, cats (family Felidae), dogs (family Canidae), and bears (family Ursidae) are a few of the families that comprise the order Carnivora. Similarly, deer (family Cervidae), antelope (family Bovidae), and hippos (family Hippopotamidae) are a few of the families that comprise the order Artiodactyla. In mammals, family names usually end in the suffix -idae.

Body Size

Body size (typically measured as body mass) is one of the most fundamental characteristics of any mammal. Lifespan, metabolic rate, number of offspring, and home range size are just a few of the many attributes that correlate with body mass. Estimating the body mass of an extinct species is generally done by measuring a feature that correlates with body mass in living mammals, such as the length of the second molar or the diameter of the upper leg bone (femur), and inserting that value into a regression equation based on data from living species. This often generates a reasonably accurate body mass for the extinct species (one that is thought to be within 30% of its true body mass). Although this is straightforward in principle, it can be challenging in practice. For example, if the extinct species is one that belongs to a group that has no living representatives and/or whose closest living relatives are quite different in size or overall form, it is often uncertain to which modern mammals it should be compared. If the extinct species (or group) is known only from fragmentary remains, this further obscures the situation. On the other hand, having a complete skeleton of an extinct mammal can also make calculating body mass difficult, because estimates based on different parts of the body can differ significantly from one another. In such cases, estimates based on total (head-body) length are often most accurate. Another approach that can be used is creating a physical or digital scale model of an extinct mammal and calculating its mass based on its volume. (Nearly all mammals have roughly the same density.) However, this method requires reconstructing skin, muscles, and other soft tissues that are almost never preserved as fossils. In sum, the body mass of an extinct species can usually be estimated using a few simple measurements, but the accuracy of this estimate can vary greatly depending on the species in question and the material that is available.

Main Groups of South American Fossil Mammals

What follows is a brief summary of most orders of mammals found at Cenozoic fossil sites in South America. Many of these groups still have living representatives, and these are indicated in parentheses. The groups are arranged in outline form so it is clear which groups are subgroups of others. The three main divisions correspond to the three main types of living mammals: marsupials, placentals, and monotremes. Additional details about mammal classification can be found in appendix 2.

A. Marsupialia (Metatheria: marsupials)

Many features distinguish marsupials from other mammals; some are associated with reproduction and others are found in the skull and teeth. Thus, it is possible to determine whether fossil remains pertain to a marsupial even though no soft tissues or other evidence of their reproductive system is preserved. For example, marsupials typically have three premolars and four molars, whereas the reverse is true of placentals. Similarly, marsupials often have holes in their palate known as palatal vacuities, and the lower back edge of their jaw is usually turned inward, which is called an inflected mandibular angle. Metatheria is the group that includes marsupials and their extinct relatives. Although these two terms are not equivalent, all metatherians will be called marsupials for the purposes of this book.
A common misconception is that marsupials exist only in Australia. This is not true now and certainly was not true in the past. Marsupials most likely originated in the Northern Hemisphere and dispersed to southern continents only during the Cenozoic; from North America, marsupials were able to pass through South America and Antarctica to arrive in Australia. Marsupials were still living in North America, Eurasia, and Africa until about 15 million years ago.
Of the roughly 330 marsupial species alive today, more than 100 of them–between one-quarter and one-third of all extant species–live in Central or South America. One of these, the Virginia opossum (Didelphis virginiana), ranges throughout the eastern United States and into southern Canada. Most of these New World marsupials are easily overlooked because they are small, nocturnal (active at night), and/or difficult to see in the wild. Additionally, their body plans are not as varied or distinctive as well-known Australian marsupials like kangaroos (family Macropodidae), wombats (family Vombatidae), or the koala (Phascolarctos cinereus). Despite their more generalized body plans, they are quite versatile and fill important roles in modern mammal communities, especially in tropica...

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