HISTORICAL BIOGEOGRAPHY OF SOUTH AMERICA, PART I: Living Vertebrates

Jim Gibson
Geoscience Research Institute

Geoscience Reports 25:1-6 (Spring 1998).
    Related pages — | EDITORIAL | PORTUGUESE |

Introduction

    Different regions of the earth have different types of plants and animals. While we all recognize this, explaining "why" is not so easy. Somehow, kangaroos got to Australia, but horses only made it with human help. Many plants and insects are found in both South America and Australia, but the mammals on these two continents are different. These and many other similar questions are often asked by researchers in biogeography.
    Biogeography is the study of the distribution patterns of plants and animals. Biogeographers are interested in explaining the processes by which plants and animals became distributed on the earth. No written record exists of how this happened, and one cannot repeat history as an experiment. This makes the questions difficult to study. However, some clues are helpful in attempting to find an explanation. These clues include the geographic locations of fossils, the kinds of organisms able to disperse to oceanic islands, and the geologic features of a region.
    Biogeography is an important subject to those who would like to understand the history of life on this earth. Biogeographic distribution patterns are commonly used to support the theory of evolution. This paper will attempt to outline some of the major points, including strengths and weaknesses, of a creationist understanding of biogeography, using South America as an example.
    One should approach this type of study with realistic expectations. Since not much is known about what actually happened in the past, historical scientists can only attempt to construct an explanation, and then see how well that explanation works. The theory presented in this paper does not work perfectly. One might compare these efforts to a kind of guessing game. The correct answer is not known, but you hope you are getting closer to the correct answer.

Ecology impacts the distribution of fauna. Major ecological and physiographic regions of South America are delineated on this map.
    Temperate regions: cross-hatched;
    Tropical evergreen forest: diagonal;
    Tropical and subtropical non-forest:
        horizontal with savannas in black;
    Mountains: stippled.
(Figure modified from Duellman 1979)

Previous Creationist Studies

    Creationists have long struggled with the problem of explaining present animal distributions as starting with dispersal from the ark. Browne (1983:1-31) has written a history of biogeography, which reviews some of the early debates, going back as far as the early 17^th century. Recent creationist authors who have discussed the question include Whitcomb and Morris (1961:79-88), and Woodmorappe (1990). None of these authors has discussed the fauna of South America specifically.

Assumptions of a Flood Model of Biogeography

    Below is a brief sketch of some of the assumptions of biogeographic interpretation from a creationist world view:
    1. The flood involved worldwide catastrophic destruction. Scripture describes a world-wide catastrophe dominated by flooding that destroyed air-breathing land animals that were not on the ark. Although Noah brought some seeds with him on the ark, many plants apparently survived the flood outside the ark. (The Bible records that an olive leaf was brought to Noah by the dove he released before Noah had a chance to leave the ark and plant any olive trees.) Many other kinds of organisms also survived the flood outside the ark, including all kinds of living sea creatures, from the whale to the clam to the penguin.
    2. Organisms could have been transported long distances by flood water before being buried. During the latter stages of the flood, the surface of the earth was probably divided into many basins, separated by shallow water or intermittently exposed land. Floating animals and plants might wash up on either side of a basin. This means that finding similar terrestrial fossils on continents that are now separated does not necessarily imply that the organisms had to walk on dry land between the two continents.
    3. All the major groups of organisms are assumed to be present at the beginning of the flood; however, not all of the different groups were buried at the same time and place. Those organisms buried first would be found deeper in the sediments than those buried later in the flood. The "first appearance" of a group of organisms in the fossil record (the geologic column) refers to the first burial and preservation of a fossil from a particular group. This does not imply that the group did not exist before the time it was buried. The layer or "timing" in which the first appearance of a group occurs may be a result of its habitat, behavior, mobility, density, or pre-flood biogeographic distribution, or a combination of one or more of these factors.
    4. Terrestrial vertebrates (land animals with backbones) dispersed from the ark to fill the earth after the flood. Therefore, the present distribution of living terrestrial vertebrates should be related to the history of the continents after the flood.

Predictions of a Flood Model of Biogeography

    On the basis of the above assumptions, some biogeographic predictions can be made and are listed below:
    1. Living terrestrial vertebrates should be distributed in a manner that reflects the present continental arrangement. Exceptions may include those groups found on oceanic islands, which have the ability to cross salt water. In general, living groups of strictly terrestrial vertebrates should not be entirely restricted to the southern hemisphere.
    2. Those invertebrate and aquatic groups that survived the flood outside the ark should show some distribution patterns due to wind or oceanic currents. Since the major ocean currents do not cross the equator, it is likely that some plants, insects, and aquatic animals should be distributed only in the southern hemisphere. This should include both the living groups and fossils of these organisms.
    3. Fossil distributions of terrestrial vertebrates should not be constrained in the same way as those of living groups. According to Assumption 2, animals could be washed into the ocean and carried by currents to both sides of the shallow ocean. Thus, similar fossils could be buried in Africa and South America. Oceanic currents flowing around Antarctica could carry potential fossils between Australia, South America and southern Africa.

Biogeographic Relationships of Living South American Vertebrates

1. Terrestrial mammals
    South America has a rich diversity of living terrestrial vertebrates, including some 36 families of terrestrial mammals (excluding bats). Fourteen of these mammal families are widely distributed, and include such familiar animals as dogs, cats, camels, rabbits and squirrels. These 14 mammal families can be explained as animals that migrated from the ark to South America because these animals or their fossils can be found from the Middle East to South America along probable migratory routes.
    The remaining 22 mammal families have much more restricted distributions. Eleven families of marsupials, primates and rodents are restricted to South America. The other 11 families of edentates, primates and rodents are confined to the New World (North and South America). Examples of these restricted groups include opossums, armadillos, monkeys and guinea pigs. The manner in which these 22 families reached South America is not known. The possibility of directed dispersal will be discussed below.
    No family of mammals is restricted only to South America and Africa or to South America and Australia. One group of terrestrial mammals, the marsupials, are nearly restricted to South America and Australia. However, marsupial fossils are found on every continent, including Antarctica, and their biogeographical history, including how they reached South America, is not well understood.

An armadillo (Zaedyus) from Argentina. Armadillos are found primarily in South America, with one species reaching the southern part of the United States. (Photo courtesy Clyde Webster)

2. Terrestrial birds
    South America is famous for its rich diversity of birds. Of about 80 families, 57 families can be considered terrestrial. Twenty-two of these families are widely distributed, indicating strong ability to disperse. Dispersal of these groups could have begun from the ark.
    Thirty-five bird families are restricted either to South America (6 families) or to the New World (29 families). It is not known how these 35 groups of birds reached South America.
    One group of terrestrial birds, the "ratites" is presently restricted to the southern continents. These are the large flightless birds, such as the rheas and tinamous of South America, the ostriches of Africa, and the emus and cassowaries of Australia. Some scientists believe them to be closely related (e.g., Cracraft 1974), while others believe they are an artificial group, lumped together because of their large size and juvenile traits (Olson 1985). They are reputed to be strong swimmers, but their biogeographical history is not clear. Ostrich-like fossils are known from Paleogene deposits in Europe, showing they are not a strictly southern group when fossils are included. No family of terrestrial birds is restricted entirely to South America and Africa. Ostriches may have dispersed to Africa from the north. The origins of the South American rheas and tinamous are unknown.

A toucan from Panama. Toucans belong to the family Rhamphastidae, which is restricted to the tropics of South and Central America. (Photo courtesy Elaine Kennedy)

3. Terrestrial reptiles
    South America has 11 families of lizards and 9 families of snakes. Four lizard families and seven snake families are widely dispersed. Several groups are found on oceanic islands, indicating strong powers of dispersal for these groups of lizards and snakes.
    One family of lizards is restricted to South America, and five more are restricted to the New World. Two families of snakes are restricted to the New World. Some South American groups may be linked to groups in Africa or Madagascar. Rafting across the Atlantic may be the most likely explanation for such links.

A large iguana from Brazil. Large iquanas are found in the New World tropics, including the West Indies, and in the Galapagos Islands, in Fiji and Samoa, and in Madagascar. (Photo courtesy Jim Gibson)

4. Freshwater reptiles
    Only one family of turtles is terrestrial, but it has been able to disperse to the Galapagos Islands and other oceanic islands, so all turtles probably have the capacity for dispersing across water. Of six families of turtles in South America, 1 is endemic (found only in South America), 3 are widespread, and 2 are restricted to the southern continents. Turtles probably do not require dispersal from the ark to explain their distributions, but may have lived through the flood.
    Crocodilians are present in South America, as well as throughout much of the warmer areas of the world. It is doubtful that crocodiles depended on the ark for survival.

A "snake-necked" turtle, family Chelidae, from Brazil. This family is restricted to South America and Australia. (Photo courtesy Jim Gibson)

5. Freshwater amphibians
    Fifteen families of amphibians live in South America. This includes three caecilian (wormlike amphibians), one salamander and 11 frog families. Two families of caecilians and 6 of frogs are endemic to South America or restricted to the New World. One family of frogs is restricted to the southern continents. The remaining six families are widespread. In addition, two groups of South American frogs are thought possibly related to two groups of Australian frogs. It is unlikely that frogs required the ark to survive the flood, since most of them have aquatic stages. Some caecilians also have aquatic stages, and may have survived the flood outside the ark.

A neotropical frog (Eleutherodactylus) from Puerto Rico. This is a member of the family Leptodactylidae, which is widely distributed in the tropics of the New World, with a few species reaching the United States. (Photo courtesy Jim Gibson)

6. Freshwater fish
    South America has 32 families of strictly freshwater fish. One family is shared with North America and Africa. One family is shared with Africa and India. One family is shared with Australia, Africa and Southeast Asia, with some relationships suggesting southern connections. One family is shared only with Africa. The remaining 28 families are endemic to South America. Fish could not have survived inside the ark, so their distributions do not need explaining on the basis of dispersal from the ark.

7. Plants
    Many South American plant families, and even genera, are shared with other southern continents. More than 100 genera, representing some 50 families, of woody plants are common to the lowland tropical forests of South America and Africa. Taxa shared with Australia include Araucaria among the gymnosperms and the angiosperms Nothofagus (Nothofagaceae), Laurelia (Monimiaceae), Beilschmiedia (Lauraceae), Weinmannia (Cunoniaceae), Orites (Proteaceae) and the family Winteraceae. Several plant families, and even genera, are restricted to South America and other southern continents. Such distribution patterns are not the result of dispersal from the ark.

A southern beech forest (Nothofagus) in Australia. Nothofagus trees are restricted to South America and the Australasian region. (Photo courtesy Jim Gibson)

Summary of Part I

    Many families of vertebrates appear to have reached South America from the north, as would be expected as they dispersed from the ark after the worldwide flood. These include all the widespread families. Many other families are restricted to South America. Their biogeographical history is unknown.
    Two groups of South American terrestrial vertebrates appear to be restricted (or nearly so) to the southern continents. These are the marsupials and the "ratites". Their biogeographical histories are unknown, but fossils of both groups have been found in the northern continents.
    A few families are found only on South America and other southern continents. These include two families of "side-necked" turtles, at least one family of frogs and at least one family of freshwater fish. Many taxa of plants are also restricted to the southern continents. None of these groups was dependent on the ark for survival, and their distributions probably reflect geographical changes and ocean currents associated with the flood.

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