A fossil is the remains or a trace of a living thing from long ago, kept in rock. Most fossils are found in sedimentary rock, because only gently settling sediment can bury a shell or a bone without destroying it.
- Body fossils: hard parts like shells, bones and teeth, often with minerals filling their pores.
- Molds and casts: a shell dissolves and leaves a hollow (a mold); minerals fill the hollow and make a copy (a cast).
- Petrified wood: minerals replace wood bit by bit, copying even its cells.
- Preserved remains: insects in amber, mammoths in frozen ground.
- Trace fossils: footprints, burrows and other signs an animal left behind.
Key idea
Fossils form when remains or traces of life are buried quickly in sediment before they rot or are eaten. They show what lived long ago, what the environment was like, and which rock layers are the same age.
Why it works
A dead animal on the ground is soon eaten, rotted or scattered. Quick burial in mud or sand cuts it off from scavengers and from oxygen, which decay needs. Hard parts last longest, so shells, bones and teeth are by far the most common fossils, and soft creatures like jellyfish are rare. Because the conditions are so special, only a tiny share of all living things ever became fossils.
Worked example: What does it tell us?
Fossil clams and corals are found in limestone near the top of a mountain. What does that tell you?
- Clams and corals live in the sea.
- So the rock formed on a sea floor and was later lifted up into a mountain.
Answer: The area was once under the sea
Worked example: A good index fossil
Which makes a better index fossil for dating layers: a snail species that lived for 200 million years in one small lake, or a sea creature found all over the world that lived for only 2 million years?
- An index fossil must be easy to find in many places: widespread and common.
- It must also have lived for only a short time, so finding it pins down a narrow age.
- The widespread, short-lived sea creature wins.
Answer: The widespread, short-lived sea creature
Common mistake
Thinking most living things become fossils. Only a tiny share do, mostly those with hard parts that were buried quickly in sediment. Check: think how many animals die in a forest each year, and how few bones you ever find lying around.