Ammonoids are found in rock layers from six different geological periods. Yet Ammonoids are considered excellent index fossils. Explain why this is possible.

Ammonoids are good as index fossils because of the following characteristics;

1. Their abundance and vast distribution across most shallow marine waters during the periods

2. They have easily recognizable features, such as shells and the highly crenulated and complex suture, that are well preserved in rocks

3. Their rapid evolution that can assist determine changes and variations in the then environment