what is an ammonite

Ammonites were shelled cephalopods that lived in the oceans and are now extinct. They belong to the subclass Ammonoidea of the class Cephalopoda. Ammonites had a coiled external shell divided into chambers separated by walls known as septa, which strengthened the shell and stopped it from being crushed by the external water pressure. The shell usually takes the form of planispirals, although some helically spiraled and nonspiraled forms (known as heteromorphs) have been found. Ammonites are excellent index fossils, and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible. They are classified into species and genera whose names must be Latin words or words that have been ‘Latinised’ . Ammonites evolved rapidly and were quite diverse, ranging in size from just a few millimeters to times bigger, with larger sizes more common from the Late Jurassic onwards. Although ammonites do occur in exceptional lagerstatten such as the Solnhofen Limestone, their soft-part record is surprisingly bleak.