What is the symbolism of the fossil from the beach in the novel, The Lie Tree?

When Faith was very young, she found a fossil on the beach, and that fossil symbolizes Erasmus's duplicity. It is only after Faith sees the other lies Erasmus has told that she realizes he put that shell on the beach for Faith to find. In some ways, that fossil symbolizes the relationship between Faith and Erasmus. For Faith as a child, the find was fantastic because she felt it gave them something in common. When she learns that her father planted it for her to find, she realizes that all the positive memories she has of their relationship were built on her beliefs, which were not the truth.