AMMONITES

Ammonites are an extinct group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs).