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The remains are then covered in sediment, or layers of sand and mud.
Over time the soft parts of the animal decay, leaving only the harder parts like teeth and bones.
A paleontologist is a scientist that studies fossils.
Types of Fossils There are two main types of fossils: body fossils and trace fossils.
A body fossil is the physical remains of an animal. Some very small organisms, called microbacteria, leave behind microscopic particles called filaments.
The soil in a bog is very low in nutrients and oxygen.The mold is an empty cavity, and can be filled with minerals that become a cast for the long dissolved bone.There are some rare places where a whole animal can be preserved.A fossil only becomes a fossil when it has reached a certain age.Though there is no one age, a good rule is anything over 10,000 years old can be considered a fossil.
A cast fossil is a fossil made when an organism that is buried in sediment decays and leave behind a mold, or hollow impression in its shape.