The first fossil considered as evidence for the hypothesis of continental drift was

mesosaurus

Mesosaurus are early aquatic relative of reptiles, elongated and slim,  adapted to shallow brackish water whose skeletons are found in the Early Permian Period (299 million to 271 million years ago). Their fossils were found only in South Africa and Iraver formations of Brazil, the two localities which at present are 4,800 km apart with an ocean in between them.

Because of its being small, it is concluded  that the mesosaurs could not have  traversed broad stretches of open ocean, giving the  evidence that the continents of the Southern Hemisphere were once joined.  The distribution of mesosaurs became the first proof of continental drift. 

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