Ammonites are a group of fossil marine animals in the class of cephalopod molluscs.
The shell is generally the only part found as a fossil. They have a planispiral shape with jointed whirls. Some forms have a non-jointed spiral or helical spiral or completely unwound spiral. These are called heteromorph ammonites. The shell is subdivided into a phragmocone divided into chambers (separated by suture lines characteristic of each species) and a residential chamber where the animal's soft body is located. All of the chambers communicate through a ventral siphuncle.
The term Ammonite, sensu stricto, applies only to species that lived during the Mesozoic Era (Triassic-Jurassic-Cretaceous).
The animal living today that most closely resembles ammonites, in terms of morphology, is the Nautilus.