Question

How could fossils be clues to the continental drift

Answer 1

Because if that species is only found on a specific continent but the fossils of that same species were found on a different continent, an explanation could be that the continents were once connected. 

Answer 2

the animals living on pangea were found on every single continent all over the world. for example, if a bird were to live on pangea, and once the continents drifted, the bones/ fossils we're found on each continents