What Is An Ammonite?

Ammonites are Cephalopods, molluscs with an external shell divided by complex partitions called septa.
The Cephalopoda are a class of molluscs.  Cephalopoda means “head footed”, this refers to the arms all Cephalopods have ringing their mouth.  The Cephalopods include the Nautiluses, squids, octopuses, and the extinct Belemnites and Ammonites.  Cephalopods have a world wide distribution.  They are abundant in the shallow waters but are also present at intermediate and abyssal depths.  Thousands of fossil Cephalopod species are known.  Only a few hundred exist in our time, mostly the octopuses, squids, and nautiluses.
Cephalopods have a beak used to bite off pieces of food held by the arms surrounding the mouth.  The food is then passed to a rasping tongue called a radula.   The radula has rows of teeth to further reduce the food particle size.
Another feature of the Cephalopods is existence of a vein called a siphuncle that pierces each septum through the chambered part of the shell.  This vein allows the cephalopod to control the amount of water or gas in each chamber thus controlling buoyancy.  The radula and siphuncle are important factors in determining the animal’s identity.
Paleontologists have divided Cephalopods into three sub classes, the Nautiloidea, Ammonoidea, and Coleoidea.

The Nautiloidea

Illustration of the Nautilus partition.

Nautilus suture/partition.



The sub class Nautiloidea has an external shell divided into chambers by smooth, curving septa.



Illustration of nautilus siphuncle placement.

Nautilus siphuncle placement.


A siphuncle generally pierces each septum near the center. Nautilus has a radula with rows of 13 teeth.  Each row has a large central tooth with 6 smaller teeth on each side.