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South American Mammals
 The Effects of Paleogeography

       For much of the past 65 million years (the Cenozoic Era or the “Age of Mammals”), South America was an “island continent,” having no land connections with any other major landmasses. (It was connected to Antarctica for quite a while, but this link was severed about 35 million years ago, and the terrestrial mammals on Antarctica all went extinct.) So what does this have to do with fossil mammals? Because South America was geographically isolated during the period of time when today’s major mammal groups were evolving on other continents, the mammals that were living in South America – like those on Australia – evolved in their own unique directions. And until the isthmus of Panama formed a connection with North America a few million years ago, very few mammals were able to emigrate to South America, and very few were able to disperse from it.

 Cretaceous Paleogeography
Eocene Paleogeography
Oligocene Paleogeography
Between the late Cretaceous Period (approx. 75 mya; left map) and the early Eocene Epoch (approx. 52 mya; middle map), Antarctica lost its connection to Australia, but maintained its connection to South America.  During the same period of time, South America lost its connection to North America. By the late Eocene (approx. 35 mya; right map), South America and Australia also became separated, isolating both continents. Figures modified from Vizcaíno et al., 1998.

South America’s “Ancient Inhabitants”

       The lineages of mammals that were on South America when it became geographically isolated are the topic of these pages. Although the descendants of some of these lineages – such as opossums, armadillos, and tree sloths – are still around today, the vast majority of these ancient lineages went extinct without leaving any living representatives. For that reason (and maybe a few others), they often don’t garner more than a mere mention in most textbooks, and discoveries of new fossil species certainly don’t make it into newspapers or onto CNN. So this is one corner of the internet where they’re going to get the attention they deserve.
       But first things first: who are these forgotten inhabitants of the southern half of the New World?  MostAncient Inhabitants specialists recognize at least seven major groups of "endemic" South American mammals (groups that have been mostly or entirely restricted to South America). The descendants of two of these groups were mentioned above – the xenarthrans and the New World marsupials (Ameridelphia). Xenarthra is a very unusual group of mammals that includes sloths, armadillos, anteaters, and their extinct relatives; they all share the presence of a unique type of connection between the vertebrae (the bones of the spine). The New World marsupials constitute a variety of mammals generally called “opossums” and their extinct relatives. (Despite their name, most South American marsupials lack a marsupium/pouch.) All the remaining ancient inhabitants of South America were herbivorous ungulates; in other words, they ate plants and had feet with hooves on them (more or less). They are generally sorted into five main groups: notoungulates, litopterns, pyrotheres, astrapotheres, and xenungulates. Odds are these names don’t mean anything to you now, but hopefully they will after you’ve had a chance to learn a little bit  about them.
       At this point, I should probably mention that I’m NOT planning to discuss two groups of mammals whose ancestors somehow made it onto South America (through a process known as "waif" or "sweepstakes" dispersal) when it was still geographically isolated more than 30 million years ago: Waif Dispersers platyrrhine primates (New World monkeys and marmosets) and caviomorph rodents (a variety of generally un-mouselike New World rodents such as porcupines, guinea pigs, chinchillas, and the ever-popular capybara). Primates in general tend to get more attention than they probably deserve and caviomorph rodents, though a very interesting group, are beyond what I currently want to deal with here.  (Perhaps they will make their way onto these pages in the future.)  These are certainly fascinating groups that continue to be very important components of South America’s fauna, so if you’re interested in learning more about them right now, check out the Wikipedia article on caviomorphs, this nice little page on New World monkeys, or browse the primate families Cebidae and Callitrichidae and the many families of hystricognaph rodents (except the top four, which aren't South American) at the Animal Diversity Web.
       Just to make things entirely clear, the groups of living land mammals you’re probably familiar thatNorthern Immigrants WEREN’T in South America until a few million years ago include: carnivorans (cats, dogs, bears, raccoons, weasels), artiodactyls (“even-toed” hoofed mammals such as bison, deer, camels, peccaries) perissodactyls ("odd-toed” hoofed mammals such as horses, rhinos, tapirs), proboscideans (elephants), lagomorphs (rabbits), and various “insectivores” (shrews, moles). Members of these groups are all recent immigrants to South America who arrived there mainly by walking over the Panamanian Land Bridge from North America (even animals like llamas that we think of as being characteristically South American). The groups  of living land mammals that you might or might not be familiar with that have NEVER been in South America include aardvarks, hyraxes, tree shrews, and "flying lemurs."  

Time and “Land Mammal Ages”

       You can’t talk about fossils without talking about time, and mammalian paleontologists (also known as paleomammalogists) generally have two different ways to talk about how long ago an animal lived.
       The first way is to discuss age in terms of absolute time: saying a fossil is 25 million years old, for instance. The only way to determine a fossil’s age in terms of millions of years is to directly date the rocks in which the specimen was found using radiometric dating techniques. A slightly less precise age can be determined by dating the rocks above and below a fossil, giving a range of possible ages: older than 25 million years but younger than 30 million years, for instance.
       The problem with dating rocks directly is that the method only works for certain types of rocks, and fossils are seldom found in those rock types. More frequently, a rock layer above and below a fossil can be dated, yielding an age range. Talking about age ranges can be a bit cumbersome, however, so paleontologists often instead say the fossil pertains to a specific part of the geologic time scale: the Late Jurassic Period or the early Oligocene Epoch, for instance. Most paleontologists have an idea of how these periods of time are arranged and their rough ages, so it makes for a convenient way to talk about time.  Also, since all paleontologists (not just
SALMA sequence paleomammalogists) use the same time scale, it’s easy to discuss the age of events with scientists from other parts of the world or those that are studying different types of organisms.
       In some instances, the resulting age range is so large as to be essentially meaningless. (A range of 5 to 25 million years old is not very helpful for a paleomammalogist.) At other times, the rocks above and below a fossil can’t be dated at all, or datable rocks are present, but the researchers don’t have the time, money, and/or expertise to do radiometric dating. So then what do you do?
       What paleomammalogists often do is refer the fossil (or the fauna) to what is known as a land mammal “age”. Land mammal ages can be thought of as an alternate system for describing how old something is; the system is based on the observation that faunas of different ages are comprised of different species of mammals. Each land mammal age tends to be characterized by one or more species of mammals (or a combination of species of mammals) that are unique to that period of time. By examining a large number of faunas of different ages, an entire time scale can be erected based on the succession of mammal species. Each unit in the time scale is known as a land mammal age.
       As an example, think about finding a bunch of bones that includes an arm bone from a Columbian mammoth (Mammuthus columbi), the tooth from a sabertooth cat (Smilodon fatalis), and the skull of a dire wolf (Canis dirus).  We know that these animals are extinct now, but that they were alive not too long ago. Without having to date these bones radiometrically, we can fairly confidently say that these belong to the Rancholabrean Land Mammal "Age" (about 50,000 to 10,000 years ago); this period of time is exemplified by the species of mammals that are preserved in the La Brea Tar Pits in California.  Similarly, if we go out to the badlands of South Dakota and find the fossilized jaw of a big titanothere (Brontops sp., also known as a brontothere) we can be fairly confident it comes from the Chadronian Land Mammal "Age" (about 37 to 34 million years ago); this age is characterized by, among other things, the presence of some of the largest, most impressive species of these titanotheres. 
       The only problem with land mammal ages is that they aren’t applicable among all continents. Think about Australia, for instance.  There isn’t one mammal native to Australia that is also native to North America, and it has been that way for tens of millions of years.  Accordingly, there is no way to tell if two faunas from these continents are the same age based solely on the mammal species that are represented. Because of this geographic problem, a separate system of land mammal ages exists for each continent. The set of land mammal ages we’re most concerned with, of course, is the South American Land Mammal "Age" (SALMA) sequence.
       In South America, the Cenozoic fossil record has been divided into approximately 20 of these land mammal "ages" (see figure to the right; click on it for a larger version). Most of these traditionally have been based on fossil localities in an area of southern South America known as Patagonia. The large latitudinal range of South America and the large proportion located in the equatorial tropics (about 70% of the continent's area) have made integrating fossil localities from northern South America into the traditional SALMA sequence challenging; with different species of animals living in the northern and southern parts of the continent, estimating time based on faunal similarity is difficult. This correlation problem has been compounded by a lack of radiometric or paleomagnetic dates (i.e., “absolute” dates) for most faunas and the presence of significant temporal gaps in the fossil record. Recent advances in radiometric dating techniques (e.g.,  laser fusion 40Ar/39Ar dating) and the discovery of important new Cenozoic fossil localities within datable rock layers (especially the Tinguiririca Fauna of Chile) have helped improve our understanding of the relationships among these SALMAs.

Endemic South American Mammals
Visit the links below to explore the world of endemic South American mammals.  
This is a work in progress.

Notoungulates are the most diverse group of endemic South American ungulates. They include animals similar to rhinos, hippos, rabbits, and rodents; others do not closely resemble any living mammals. Many have high-crowned cheek teeth.
Litopterns are the second most diverse group of South American ungulates. They include forms similar to antelopes, horses, and camels, all with relatively low-crowned cheek teeth.
Pyrotheria, Xenungulata
Astrapotheres are large, semi-aquatic mammals with tusks and a short proboscis. Pyrotheres are large, elephant-like mammals with tusks and bilophodont cheek teeth. Xenungulates are large, poorly known Paleocene mammals.
Ameridelphia includes the New World "pouched" mammals (though most lack pouches) such as extant opossums and various extinct forms resembling jumping rats, "insectivores," and carnivorans.
Xenarthrans are a peculiar group that includes sloths (both tree sloths and ground sloths, some giant), anteaters, armadillos, glyptodonts (large armored mammals) and pampatheres.
This page was last updated on July 25, 2008.