Ammonites (lat. Ammonis cornu: horns of Ammon) are an extinct group of marine inventebrate animals in the subclass Ammonoidea of the class Cephalopoda.
In body shape they are mostly like Nautilus. Usually have symmetrical spiral bent homes, diameter 0.19 inch to 8.20 feet.
The house is divided into an inhabited chamber (ventricle), which is considerably longer than that of Nautilid, and the air chamber phragmocon filled with a gas, so that the animals that lived only in the last chamber (ventricle), would swim easier. By regulation of the gas animals would repress at a certain depth, while ranged by throwing jets of water through a funnel.
Air chambers are reconstructed with septa and through them sifo passes which is more commonly along the outer edge of the bend. Unlike Nautilid having concave septa, ammonites have convex.
Ammonites homes are three-layered , made of aragonite .
They also have lids (aptychus) that are made from calcite.
We distinguish evolute and involute homes.
Involute homes sometimes have a belly button (umbilicus).
Inside the home there are septi witch are accrue with to home with seams, visible on the outside of homes.
We call them suture. By their folding, we can follow the evolution of ammonites.
They lived from the Devonian (408 million years ago) untill the end of the Cretaceous (65 million), They were quickly developed and spread, and are very good transmissible fossils.
Today we know at least 2,000 genera with more than 6,000 fossil species.