What happens to the origenal shell from which a mold fossil is formed?

When a mold fossil is formed, the original shell typically breaks down or dissolves over time due to environmental conditions such as water, sediment pressure, or chemical processes. As the shell decays, it leaves behind a cavity or impression in the surrounding sediment, creating the mold. This mold can later be filled with minerals or sediments that harden, resulting in a cast fossil. Thus, the original shell is lost, while its shape is preserved in the mold.