Science Resources for Learning

How are fossils formed?

There are many ways that living things leave fossils behind. Here are some examples:

  • Petrified fossils
  • Trace fossils
  • Casts
  • Molds
  • Coprolites
  • Imprints

Petrified Fossils

Fossils become petrified when minerals seep into bones, shells, or other hard part hardening these parts into rocks.  Most dinosaur bones are petrified fossils.

Trace fossils

Trace fossils include footprints, tracks, trails, and burrows made by living things. The most common example of a trace fossil is a dinosaur footprint.


A cast forms when minerals or rocks harden in the shape of a mold.


Molds are space that is in the shape of the living thing inside a rock


Coprolites are the petrified remains of animal dung.  Coprolites can reveal what an animal ate, which also shows what other living things shared the animal’s environment.


Imprints are similar to trace fossils. Imprints are impressions of parts of organisms left in soil of sediment before it hardens. Thin objects, such as leaves, fish, and feathers, form imprints.