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Molds
When an organism is trapped in sediment, the surrounding sediment will eventually become firm. The organism inside later dissolves, and if minerals do not fill that cavity, it creates a fossil called a mold. The mold eventually hardens into rock, leaving a detailed impression of the outer surface of the organism. An imprint is a type of mold that is often left by thin organisms such as leaves. Molds are commonly found in soft rocks such as sandstone.
Casts
When molds are filled with sediment or minerals, a replica of the organism is created. This is called a cast fossil. The cast transforms into rock over time, leaving behind a three-dimensional imprint of the organism. Often shells are found in this type of fossil, but other marine life, such as fish, can leave cast fossils. There is typically no organic material left behind in cast fossils. It is washed away or dissolved as the rock forms.
Trails, Tracks, and Burrows
Tracks, trails and burrows are a type of trace fossil often found in rocks. These occur when an animal burrows or walks in some type of sediment, leaving cavities behind. These cavities are preserved as the sediment forms into rock. Tracks or footprints can give information about the speed and length of an animal's stride and whether they walked on two legs or four. They can also show whether an animal held the tail up or dragged it behind. Tracks, trails and burrows might be found in the form of worm trails or dinosaur tracks, but often this type of fossil is hard to identify. They are often classified by the type of activity rather than the exact organism that left them behind.
Corprolites
Corprolite means dung stone, which describes this type of fossil well. It is the fossilized feces of an animal such as a mammal or a dinosaur. Often these are mistaken for rocks, but they contain information within them that palaeontologists use to determine the diet of animals that no longer exist. When analyzed physically and chemically, palaeontologists can determine whether an herbivore or a carnivore left it behind. For example, a carnivore's dropping will contain small bone fragments, while an herbivore's droppings may contain gastroliths, or stomach stones.
Gastroliths
Smooth, polished stones found in the abdominal cavities of skeletal remains are called gastroliths. These stones are used by the animals to aid in digestion by grinding up plant and vegetable material in the stomach. Often they resemble small pebbles, making them hard to identify unless found in or near fossilized remains. Gastroliths aid palaeontologists in answering questions relating to diet and migration routes of the animals in which they're found.