South American Fossil Mammals
South America’s Isolation
South America was an island continent for much of the past 66 million years (the Cenozoic Era or the “Age of Mammals”). As a result, its mammals evolved in their own unique directions. If you could travel back in time to see South America 25 million years ago, it would be like visiting Australia today; you would encounter mammals completely unlike those found anywhere else in the world. However, South America was not completely isolated during this interval. Here is a very brief (and rough) chronology of this continent’s land connections since the extinction of the dinosaurs:
- 66 million years ago: South America was connected to both North America and Antarctica. Soon after this point, it lost its connection to North America.
- 66-50 million years ago: South America was connected to Antarctica which, in turn, was connected to Australia. The Antarctica-Australia connection was probably severed near the end of this interval but perhaps as much as 15 million years later.
- 50-34 million years ago: South America was connected to Antarctica, which was not yet covered by ice (glaciated).
- 34 million years ago: The connection between South America and Antarctica was severed and Antarctica became glaciated.
- 34-9 million years ago: South America had no land connections to any other continent.
- 9-3 million years ago: Islands formed between South and North America. A complete Isthmus of Panama most likely formed near the end of this interval
- 3 million years to present: Complete land connection between South and North America.
Native South American Mammals
Unfortunately, many of the mammal groups that evolved in South America prior to and during its extended isolation are now extinct. It is for this reason that they and their names are unfamiliar to most people. These mammals did not include ancient relatives of dogs, horses, cattle, and rabbits, but rather constituted a variety of other branches of the mammal evolutionary tree. They mostly fall into one of three categories:
- Ancient inhabitants: early Cenozoic mammal groups that evolved in South America or arrived there via connections with North America or Antarctica (and Australia) before it was completely isolated.
- Lucky arrivals: mammal groups that apparently reached South America from Africa by accidentally crossing the Atlantic Ocean during the middle Cenozoic.
- Northern immigrants: mammal groups that traveled from North America to South America during the late Cenozoic by “island hopping” or crossing what is now the Isthmus of Panama.
Most of the mammals that were living in South America between 66 and 45 million years fall into three categories: marsupials, xenarthrans, and native ungulates. Other interesting but rather minor groups (in terms of abundance and diversity) included meridiolestidans, gondwanatheres, and monotremes. Bats (order Chiroptera), were also present in South America by the early Eocene, but their fossil record on that content is so poor that relatively little can be said about them.
A wide variety of marsupials called South America home during the Cenozoic and the continent is still home to more than 90 species, more then one-quarter of the total number of marsupial species alive today. In fact, marsupials probably inhabited South America before they were present in Australia. It is thought that marsupials traveled from North America to Australia via Antarctica and South America when all of these continents were connected in the late Mesozoic. Some marsupials probably also went in the opposite direction but did not leave any living descendants.
Didelphimorphia: The vast majority of extant South American marsupials (more than 90%) are opossums belonging to the order Didelphimorphia. Most opossums are small, nocturnal, tree-dwelling species that inhabit tropical to subtropical areas of Central and South America. However, the Virginia opossum (Didelphis virginiana) can be found as far north as southern Canada, and the Patagonian opossum (Lestodelphys halli) can be found in the southern end of Argentina. Possums (without the initial “o”) are Australian marsupials of the order Diprotodontia (suborder Phalangeriformes) that are not closely related to South American opossums.
Paucituberculata: Small marsupials known as shrew-opossums include only a handful of mainly terrestrial species that inhabit higher elevation and higher latitude area near the Andes Mountains. They represent an entirely different group of marsupials, the order Paucituberculata. This group was much more widespread and anatomically diverse in the past than it is today.
Microbiotheria: Only a single lineage of small, opossum-like microbiotheres survives today. For many years, it was thought to be represented by a single species, the monito del monte (Dromiciops gliroides). However, a recent study has argued that two additional species of Dromiciops should be recognized. This group was never particularly diverse, but it does have a long fossil record. Curiously, microbiotheres are more closely related to Australian marsupials than other South American marsupials. This group’s rather inauspicious claim to fame is that it has the smallest geographic distribution of any order of mammals; monitos del monte are only found in a small area of southern Chile and extreme western Argentina.
Sparassodonta: The extinct order Sparassodonta may be the most fascinating group of extinct South American marsupials. (Technically, they may fall just outside of this group, in which case they are more properly called stem marsupials or metatherians). These animals were the mammalian predators of ancient South American ecosystems, since no placental meat-eaters were living there until a few million years ago. Some sparassodonts were as large as a bear, and one group evolved saber-like canines like those that of saber-toothed cats.
Polydolopimorphia: The order Polydolopimorphia is the other major group of now-extinct South American marsupials. Polydolopimorphians were generally small marsupials that varied greatly in their ecological adaptations. Some species lived in trees and may have resembled small primates whereas others were terrestrial hoppers similar to kangaroo rats.
Xenarthrans–sloths, armadillos, and anteaters–are among the most characteristic and charismatic South American mammals. Although they are quite different in their appearance and habits, they share many features that indicate they are closely related to one another. In fact, the group is named for one of these features: extra connections between the vertebrae (bones of the spine) known as xenarthrae (“strange joints”). Some 31 species of xenarthrans still exist today, but they are a mere shadow of the group’s past diversity in terms of number of species and variety of lifestyles.
Cingulata: Armadillos and other armored xenarthrans belong to the order Cingulata (“belted ones”). Cingulates account for about two-thirds of living xenarthran species and also have a larger geographic range than the remaining xenarthrans (pilosans). The nine-banded armadillo (Dasypus novemcinctus) ranges north into the central United States, and the pichi (Zaedyus pichiy) can be found at the southern tip of South America. Several other lineages of armadillos were quite different from those alive today but are now extinct. Two other groups of large to giant cingulates, pampatheres and glyptodonts, were even less armadillo-like and went extinct about 12,000 years ago. Glyptodonts in particular were conspicuous components of South American faunas of the past.
Pilosa: Sloths and anteaters are grouped together in the order Pilosa (“hairy ones”). Anteaters (suborder Vermilingua) have a relatively poor fossil record and apparently were never particularly diverse. Nevertheless, the four modern species vary quite a bit in their body size and habits. The two types of living sloths, two-toed (Choloepus spp.) and three-toed (Bradypus spp.), both hang around in trees (literally) and superficially resemble one another to a great degree but are only distantly related to one another when extinct species are taken into account. The number and variety of extinct sloths is astounding, particularly compared to these few living descendants. Living and extinct sloths belong to the suborder Phyllophaga (sometimes called Tardigrada or Folivora instead).
A variety of mammals referred to as native (or endemic) South American ungulates lived in South America for much of the Cenozoic but are now extinct. The term ungulate is used rather loosely in regards to these groups; it is not entirely clear how closely these mammals are related to modern ungulates such as cows and horses, and although some of these extinct mammals had hooves, others had nails, claws, or something in between. All native South American ungulates were herbivores, and many (but not all) species were of medium to large size.
Notoungulata: Notoungulates were the most diverse group of native South American ungulates and were also abundant. They included animals similar in some respects to rhinos, hippos, rabbits, and rodents; others did not closely resemble any living mammal. At least four groups of notoungulates evolved ever-growing (hypselodont) premolars and molars, and this is a characteristic feature of most Miocene species. The group has traditionally includes more than a dozen families, though some of these are probably no longer valid. Several groups of notoungulates survived into the Pleistocene (within the past 2 million years)
Litopterna: Litopterns were the second most diverse group of South American ungulates. They includes species similar in some respects to antelopes, horses, and camels. In contrast to notoungulates, litopterns had relatively low-crowned (brachydont) premolars and molars. Four families of litopterns are generally recognized and, like notoungulates, some of these survived into the Pleistocene.
Astrapotheria: Astrapotheres were distinctive mammals that were less common and diverse than notoungulates and litopterns. They were tusked mammals that ranged from medium to very large in size, and most species probably had a tapir-like proboscis. At least some species were probably semi-aquatic, and the group went extinct in the late Miocene, about 10 million years ago.
Pyrotheria: Pyrotheres were large, elephant-like mammals that are only found in fossils sites older than about 25 million years. They were only abundant during the last part of this interval (the late Oligocene) and thus are rather poorly known.
Minor ungulate groups: Several other types of native ungulates were present in South America during the early Cenozoic but went extinct during the Eocene epoch or soon thereafter. Perhaps the most prominent of these were the didolodontids, bunodont ungulates of the Paleocene and Eocene whose evolutionary relationships are unclear.
All of the groups below represent ancient lineages that evolved early in the Mesozoic Era, prior to the split between placental and marsupial mammals. As a result, they are collectively known as non-therian mammals. (The group that includes both placentals and marsupials is called Theria).
Monotremes: Modern monotremes, which include the duck-billed platypus (Ornithorhynchus) and several species of echidnas (Tachyglossus, Zaglossus), are restricted to Australia, Tasmania, Kangaroo Island, and New Guinea, but at least one species, Monotrematum sudamericanum, was living in South America during the Paleocene. Monotremes lay eggs rather than give birth to live young, a trait retained from our early mammal ancestors.
Meridiolestidans: Meridiolestidans are quite scarce in the Cenozoic fossil record of South America, but the last surviving member of the group was a remarkable mole-like animal called Necrolestes patagonensis that has traditionally been placed in its own family, Necrolestidae.
Gondwanatheres: Gondwanatheres were so-named because their remains have only been found on land masses derived from the ancient southern supercontinent of Gondwana. They were peculiar, somewhat rodent-like mammals with hypsodont teeth closely related to a very successful group of extinct group of northern continent mammals known as multituberculates.
Near the middle of the Cenozoic, two types of mammals appeared for the first time in South America: rodents and primates. Both of these groups had already been inhabiting other continents for millions of years, so it is clear they simply arrived in South America rather than originated there. Analyses of evolutionary relationships have shown that their closest relatives were living in Africa at the time. Therefore, the most likely explanation is that they somehow crossed the Atlantic Ocean, landed in South America, and founded new populations of rodents and primates. Although this sounds implausible, it is more likely than possible alternative explanations. Additionally, even very improbable events become significantly less so over millions of years. This is far from the only example of transoceanic dispersal of mammals between one land mass and another, though it is certainly one of the more extreme.
Caviomorpha: Rodents arrived in South America during the Eocene (by at least 41 million years ago) and gave rise to a spectacular radiation of some 230 living species known as caviomorph rodents. The name of this group literally means that these are rodents that have the form of a guinea pig (Cavia). In reality, caviomorphs span a wide range of shapes, sizes, and habits and are divided among 12 families. They include the largest living rodent (the capybara, Hydrochoerus hydrochaeris), more than a dozen species of porcupines (family Erethizontidae), the jackrabbit-like Patagonian hare (Dolichotis patagonicum), chinchillas and their relatives (family Chinchillidae), and many species of tuco-tucos (Ctenomys spp.), which are South America’s version of pocket gophers (family Geomyidae). The closest living relatives of caviomorph rodents include Old World porcupines (family Hystricidae) and several other families of Eastern Hemisphere rodents that are grouped together in the suborder Hystricomorpha.
Platyrrhini: Like rodents, the first South American primates gave rise to an impressive evolutionary radiation: more than 120 species in five families. These primates are known as platyrrhine (flat-nosed) primates and are closely related to Old World apes and monkeys (catarrhine primates). Platyrrhines include some of the most popular and acrobatic monkeys such as spider monkeys (Ateles) and capuchins (Cebus spp.), both of which have grasping (prehensile) tails that can be used as a fifth limb. Platyrrhines also include a wide variety of colorful tamarins and marmosets (family Callitrichidae). The platyrrhine primate fossil record is relatively sparse, quite unlike that of caviomorph rodents.
Near the end of the Cenozoic, a few groups of North American mammals began to appear in South America. They probably arrived by “island hopping”: getting from one area to another by crossing relatively short spans of ocean between large islands. The number of mammal groups to cross between the two continents increased as the amount of exposed land increased during the Pliocene, and the two continents probably were fully connected by at least 3 million years ago. Pronounced glacial (cooler) and interglacial (warmer) intervals during the Pliocene and Pleistocene probably affected the types of vegetation covering Central America at different times and thereby the types of mammals that were able to make the crossing. In total, seven orders and 19 (perhaps 20) families of North American mammals became established in South America during what is now termed the Great American Biotic Interchange or GABI. Nearly all of these still have living representatives:
Artiodactyla (even-toed ungulates)
- Camelidae: two wild species (guanaco and vicuña) and two domestic species (llama and alpaca)
- Cervidae: nearly 15 species of deer, many in the genus Mazama
- Palaeomerycidae (extinct): possibly one extinct species
- Tayassuidae: three species of peccaries, each in its own genus
Carnivora (meat-eating mammals)
- Canidae: about a dozen species of dogs and foxes (many of the genus Lycalopex)
- Felidae: about a dozen species of cats (mainly of the genus Leopardus)
- Mephitidae: three species of skunks (genus Conepatus)
- Mustelidae: about a ten species of otters and weasel-like mammals
- Procyonidae: nine species of raccoon relatives
- Ursidae: the spectacled bear (Tremarctos ornatus)
- Leporidae: several species of rabbits (genus Sylvilagus)
Perissodactyla (odd-toed ungulates)
- Equidae: horses (now extinct but subsequently reintroduced)
- Tapiridae: four species of tapirs (genus Tapirus)
- Hominidae: humans
- Gomphotheriidae: extinct relatives of elephants (now extinct)
- Cricetidae: a spectacular radiation of hundreds of species of rats and mice
- Geomyidae: one species of pocket gopher (genus Orthogeomys)
- Heteromyidae: about ten species of pocket mice
- Sciuridae: about 15 species of squirrels
- Soricidae: around a dozen species of shrews (genus Cryptotis)
- Loomis, F. B. 1914. The Deseado Formation of Patagonia. Runford Press, Concord, New Hampshire, 232 pp.
- Paula Couto, C. d. 1952. Fossil mammals from the beginning of the Cenozoic in Brazil. Marsupialia: Polydolopidae and Borhyaenidae. American Museum Novitates 1559:1-27.
- Sinclair, W. J. 1906. Mammalia of the Santa Cruz Beds. Volume IV, Paleontology. Part III, Marsupialia; pp. 333-460 in W. B. Scott (ed.), Reports of the Princeton University Expeditions to Patagonia, 1896-1899. Princeton University, E. Schweizerbart’sche Verlagshandlung (E. Nägele), Stuttgart.
- Vizcaíno, S. F., R. Pascual, M. A. Reguero, and F. J. Goin. 1998. Antarctica as background for mammalian evolution; pp. 199-209 in S. Casadío (ed.), Paleógeno de América del Sur y de la Península Antártica. Asociación Paleontológica Argentina, Publicación Especial 5, Buenos Aires.