Final answer:
The cavities left in rocks when a shell or bone dissolves are called molds. If these molds get filled in with new sediment or minerals, they create casts, which replicate the original organism's form. Molds and casts are valuable for understanding past life on Earth.
Step-by-step explanation:
The cavities left in rocks when a shell or bone dissolves are called molds. These form when the original skeletal material of an organism, such as a shell, dissolves and leaves behind an impression in the sediment. This impression is what we refer to as a mold. If the mold then fills with new sediment or mineral deposits, it forms a cast, which is essentially a replica of the original organism's outside structure. The internal molds occur when sediment fills the interior of a shell before it dissolves, providing a 3-dimensional representation of the organism's inside.
It's noteworthy that molds and casts are common types of fossilization and are key in studying paleontology. Discovering a mold or cast can provide valuable insights into the morphology and lifestyle of extinct species, helping us to reconstruct past environments and biological communities.