One of the reasons why fossils are common in dreams is that they represent ancient time. This could be a time in your life, the past, or something that feels like it is left behind.
Fossils in a dream can also represent things you have forgotten about or lost. It can signify challenges and struggles you overcame to get to this point.
If you are dreaming about looking for fossils on an island then it may represent a need for exploration. There is the need for discovery and adventure in your life and this might mean it is time for you to find new things out about yourself.