Pre-Quaternary Biogeography

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I. Wallace's Realms

One important point in history occurred at the K/T boundary, approximately 65 million years ago. During this time, the dinosaurs are becoming extinct and mammals begin to rise in importance. This period marks the beginning of what we might consider the modern world. Figure 7.4 in the text shows the rise of importance of angiosperms occured somewhat before the K/T boundary, approximately 100 million years ago (in the middle of the Crecateous).

Alfred Russell Wallace

Wallace was a British naturalist who proposed the idea of natural selection at roughly the same time period as Darwin. He also spent a good deal of time classifying the basic distributions of various taxa in realms across the globe. (see fig. 8.1 in the text). For terrestrial mammals, these realms include: Palaearctic, Ethiopian, Oriental, Australasian, Nearctic, and Neotropical. It was noted by Wallace that some of these areas tend to have higher rates of endemism than others, and that the higher rates of endemisn tend to occur in the southern continents. In the case of the marsupials, it is thought that they developed first in the Americas. During the Cretaceous, those in North America spread to Europe and Northern Asia. Those in South American crossed Antarctica and moved into Australia. On the other hand, placental mammals were thought to have developed first in Asia, and from there they spread into the Americas and Australia.

These patterns of distribution can be explained by considering the positioning of the continents since the Cretaceous. Approximately 200 million years ago, the continents were all connected and formed a super-continent known as Pangaea. Pangaea begins to breaks up about 180 million years ago, into the Northern continents (Laurasia) and Southern continents (Gondwana) with a large body of water (Tethys Sea) separating the two continents. This spread continued between the northern and southern continents at 135 million years ago. In addition, the southern continents begin to separate from each other. By 65 million years ago, the southern continents continue to separate and the northern continents had begun to break up and the earth looked more similar to the modern distribution of continents we see today. At this time Europe had separated from North America and the Atlantic Ocean had formed. Also, India was moving north, on a collision course with Asia. However, Australia remains connected to Antarctica until 50 million years before the present. Becuase the southern continents began to separate from each other before the northen continents, we generally see higher rates of endemism in the southern continents.

II. South American Mammals
In Simpson's book on the Splendid Isolation of South America, he notes that North and South America were connected during the Paleocene /Eocene (65 mbp), then again in the Oligocene, and finally from the Pliocene/Pleistocene to the present. In the early Tertiary South America was dominated by some marsupial groups, including possums, armadillos, sloths, ant-eaters, and also some other strange, now extinct mammals. In the mid-Tertiary, monkeys came in from Africa along with some rodents. At the end of the Tertiary, a land bridge was formed that connected North and South America. This land bridge allowed for the movement of animals between the two continents, a moved termed The Great American Interchange. During this migration, mammals from North America were much more successful at invading South America than vice versa. Some North American mammals that moved south include the horse, pig, camel, deer, shrew, rabbit, squirrel, mouse, dog, bear, racoon, weasel and cat. Only a few South American species moved into North America. These included the possum, armadillo, porcupine, and sloth. Some have suggested that the differential success of the North American mammals was a consequence of their aggressiveness which led to a competitive advantage. However, competition probably doesn't explain the entire story. One theory is that the Northern species were more efficient grazers. Another theory emphasizes that the formation of the Andes combined with glaciation and global cooling put pressure on the South American species and this gave an advantage to the North American species which were already adapted to the cooler climate.