Fossil Formation. When dinosaurs or other ancient animal or plant life dies, a gradual process begins. (A) Then the dead dinosaur is eventually covered by dirt and mud(B) This leaves the hardest parts of the dinosaur-bones and teeth. (C) After thousands of years, the chemicals in the buried dinosaur's body go through a series of changes. (D) As the bone slowly decays, groundwater gets inside the bone. The minerals in the groundwater are replaced with the chemicals in the bone. These minerals are the same as the surrounding rock. As the dinosaur bone turns into a fossil, it becomes a heavy, rock-like version of the original dinosaur. It is now officially called a fossil. When scientists look for dinosaur fossils, they look for specific rock types. Fossils are usually found in either shale, siltstone, mudstone, or sandstone.