Notoceras Cretaceous Heteromorph Ammonite from Madagascar 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 geologic time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs
Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell as pictured above. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs. and textures
Ammonites are an extinct group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more closely related to living coleoids than they are to shelled nautiloids such as the living Nautilus species.