Ammonoids are found in rock layers from six different geologic periods yet ammonoids are considered excellent index fossils explain why this is possible

Ammonoidia is a phylum (maybe subphylum I don't remember exactly) and includes a great number of individual species which can each be tied to specific geologic time periods. Due to their calcitic and aragonitic shells they hold excellent preservation potential and are commonly found.

Abay where the river empties into the sea