Fossil Dating: Stacking Up Rock Layers

Paleontologists, scientists who study fossils, have found many fossil remains in rocks. Scientists rely on the law of superposition to help them determine the ages of these fossils. This law states that younger layers of rocks are deposited on top of older layers. This relative dating technique enables paleontologists to infer the ages of rocks and thus the fossils in those rocks. This activity shows how the law of superposition works.

Materials

Scissors

Pictures of organisms (see figure below)

Activity

1. Cut out the pictures of different organisms. The pictures represent fossils of these organisms.
2. Without referring to the table below, stack these pictures in the order in which you think they appeared on Earth. Put the oldest organisms on the bottom of the stack and the youngest on the top of the stack.
3. Compare your stack to the dates in the table. If necessary, rearrange your stack to make it accurate.
4. Add the dates these organisms appeared on Earth to the pictures.

Follow-Up Questions

1. What were the first living things?
2. How much time passed between the appearance of the first living things and the appearance of insects?
3. If you found one fossil embedded in the top of a rock layer and another one embedded in the bottom of the layer, which fossil would be the oldest? Explain your answer.

Answers

1. Bacteria.
2. 2,900 million years.
3. Fossils in the bottom of the stack are the oldest, because over time newer rock layers with more recent fossils are deposited on top of the older layers.

Extension

Carry out research to learn how fossils are formed. Use clay to demonstrate what you learn.