Ch 19 Index Fossil

It is easy to compare the ages of fossils found in sedimentary rocks at one location. Fossils found in an upper rock layer will be younger than fossils found in a lower layer, unless the layers have been overturned. It is not as easy to compare the ages of fossils found in rocks at different locations. Scientists use index fossils to determine the relative ages of rock layers. Scientists use radiometric dating to find the actual age of rocks and fossils.

You will use fossils to order the rock layers pictured at the end of this lab. Nine of the layers represent periods from the Paleozoic and Mesozoic Eras. One layer represents the Cenozoic Era.

1. Cut out each drawing of a rock layer. note: The fossils are not drawn to scale.

2. Spread out the layers on a flat surface. Use the Key to Fossils to identify the fossils in each layer. Write the names of the fossils on the drawings. note: Some of the drawings represent one species or one genus. Some represent a higher taxonomic level.

3. The oldest rock layer is from the Cambrian Period. Some organisms in this layer will not be found in any other layer. No organism in this layer still exists. Locate the layer that represents the Cambrian Period.

4. Look for fossils that are found in only two layers. Using this information, pair up layers that must represent consecutive periods in the geologic record.

5. Use other fossils to determine which layer in each pair is older and the order of all the layers from oldest to youngest.

6. Each drawing has a letter in the upper left corner. Use the letters to record the correct sequence of layers in the data table on page 118. Record the letter for the youngest layer in the first row and the letter for the oldest layer in the last row.

To be considered an index fossil, it must meet 3 criteria:

1. The fossilized organism must be easily recognizable. It must be easy to ID and look unique.

2. The fossils have to be geographically widespread, or found over large areas so that we can use them to match layers separated by huge distances.

We can use index fossils and key beds to correlate, or match rock layers that are the same age. By doing this we can then place other layers of rocks in order of their relative ages to find the oldest and youngest rocks in a series of outcrops.