Fossils.
Early in the 20th century, Alfred Wegener, primarily, established the continental drift theory. According to Wegener, continents are mobile on the surface of the Earth and were previously connected to form a single supercontinent. Scientists did not think that the continents could migrate when Wegener was alive.
In rocks that are the same age but on continents that are currently far apart, ancient fossils of the same species of extinct plants and animals can be found. Wegener claimed that the species had coexisted, but that once they died and become petrified, the lands had separated. According to his theory, the creatures couldn't have crossed the seas.
1. Glossopteris seed fern fossils were too heavy for the wind to carry them that far.
2. Mesosaurus was a reptile that could swim, although it could only do it in freshwater.
3. The terrestrial reptiles Cynognathus and Lystrosaurus were incapable of swimming.
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