There are four types of fossils: mold fossils, cast fossils, trace fossils and true form fossils. Fossils are divided into two distinct categories: fossilized body parts, such as teeth, claws, bones and embryos, and fossilized traces, including nests, tooth marks, droppings and footprints.
Body fossils are the most common, especially those of the hard parts of the body, such as the bones, teeth and claws, although some soft body tissue fossils have been found. Trace fossils, including dinosaur tracks that were made in fine sand or mud, are quite common. Gizzard rocks, which are known as gastroliths, are also common. These were stones that some types of dinosaurs swallowed to help them digest their food.