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Ammonites - Index To The Past

Ammonites are an extinct cephalopod that lived from the Devonian Period through the Cretaceous Period. They were most abundant during the Mesozoic Era. These fossils have been known for thousands of years. The name comes from the god ammon who looked like a man with the horns of a ram. It was called Ammon’s stone.

The basic classification of ammonites is:

  • Phylum Molluska - along with snails, clams, oysters, octopus, and squid.
  • Class Cephalopoda - with living members including squid, octopus and the chambered nautilus.
  • Subclass Ammonoidea - there were a huge number of genus and species

Ammonite fossils have been found from less than a centimeter to 2 meters in diameter.

Generally ammonites had flat spiral shells; some were partial spirals and a few were non spiral these were called heteromorphs.

The Parts of an Ammonite
As the ammonite grew they added on to their shell. They sealed off the old living section with a wall called a septa . The spaces between the septa are called chambers or camera. In ammonites septa are folded.

The chambers were joined by a tube called the siphuncle. The siphuncle was used to fill the chambers with gas. This gave the ammonite control of its bouyancy and allowed the ammonite move up and down in the water column

agadir ammonite

They All Look Alike To Me
Ammonites are classified by the shape, structure, and ornamentation of their shells. The suture patterns along with the positioning of the siphuncle are also considered in classification. The suture patterns mark where the septa joins the outer shell or phragmocone.

ammonite suture patterns
Suture Patterns
ammonite drawing

Ammonite suture patterns All of these species fall into 3 orders based on the shape of the septa, Goniatitida, Ceretida, and Ammonitida.You can see examples of these 3 septe shapes at the right.

The ammonite shell had sections, with the living animal occupying only the section of the shell closest to the head. As the soft-bodied ammonite got larger, it grew a new shell section and sealed off the old one with a layer called the septa.

The shape of the septa varied from species to species. They grew more complex shapes over time from a smooth curve in the early Paleozoic Era to complexly convoluted in the Jurassic and Cretaceous Periods.

The old sections were connected by a tube called a siphuncle. Using the siphuncle the ammonite could adjust its bouyancy to move higher or lower in the water table. It did this by pumping gases or liquids into the old chambers. Because all ammonites were carnivores this helped them find better hunting grounds.

There are three orders of ammonites:

  • Goniatitida – the goniatites
  • Ceratida – the cerritites
  • Ammonitida- the ammonites

The ammonites became extinct at the same time as the dinosaurs about 65 million years ago.

Ammonites were so plentiful that paleontologists use them To date layers of rock.They have been found all over the world.

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