6.3 Fossils & Extinction

Question 1

What are fossils?

Answer 1

A fossil is any trace of an animal or plant that lives long ago. They are most commonly found in rocks.

Question 2

What are the three ways that fossils are formed?

Answer 2

• Gradual replacement by minerals: As body parts like teeth, bones and shells decay really slowly, it means that they can be gradually replaced by minerals and slowly form rock-like substances that are in the exact same shape and size as the original structures. So when we dig them up and separate them from the surrounding rock, we are left with a perfect model of the bone or tusk it was made with.
• Casts + impressions: We get casts when an organism is buried in a soft material like clay. As the clay hardens, the organism decays and we are left with a gap that is the same shape or size as the organism was. Impressions are things like footprints, where organisms leave a mark on the ground that stays there over time.
• Preservation: No decay takes place at all. This occurs when animals get stuck in things like amber or tar pits and because there is no oxygen or moisture, no decay can take place and the organism is completely intact.

Question 3

What are the reasons that a species may become extinct? (6)

Answer 3

• Environment changes too quickly / global warming
• Habitat is cut down
• A new predator arrives and slowly kills them all
• A new disease spreads through the population and kills them all
• A new species outcompetes them
• A catastrophic event may occur, like the asteroid in Mexico that wiped out the dinosaurs 66 million years ago.

Question 4

Why are fossils used as evidence for evolution?

Answer 4

They can tell us a lot about what the organisms looked like and how long ago they existed. Generally, the deeper the rock, the older the fossil. By arranging fossils in chronological order, gradual changes in organisms can be observed. This provides evidence for evolution, because it shows how species have changed and developed over many years.

Question 5

Why might a dead animal be preserved in a tar pit, rather than decay?

Answer 5

Not enough oxygen for decay to take place

Question 6

Why is the fossil record incomplete?

Answer 6

• Some organisms are soft-bodied so do not fossilise well
• Some fossils formed long ago may have been destroyed since