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Biology - Differences between mammals of North America

Differences between mammals of North America

The differences between the mammals of North America and Eurasia are less
than between those of the continents of the Southern Hemisphere. North
America and Asia are separated by the shallow Bering Strait about 100 km
wide. There is geological evidence that they were connected in the past.
The land bridge allowed migration of animals (and plants) and therefore
gene flow which reduced the opportunity for divergence. This was not the
case between the continents of the Southern Hemisphere.

* _Mesosaurus_ is a fossil reptile which is found only on the eastern
coast of S America and the western coast of S Africa; the shapes of S
America and S Africa are complementary suggesting they were once joined.

The distribution of both present and extinct species can be explained by
the theory of continental drift. As the southern land masses separated from
Gondwana they carried early forms. Over time, speciation occurred on each
continent to give rise to the present fauna and flora. Examination of
fossils and other evidence such as analysis and comparison of mitochondrial
DNA supports the theory that current groups of species may have shared a
common origin. Marsupial fossils, for example, have been found in
Antarctica.

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