Ammonites were similar in shape and form to the modern Nautilus. They were very common in the oceans of the Paleozoic and Mesozoic eras, 400 to 65 million years ago, evolving in the Devonian Period. Fossil shells as small as 3 mm and as large as 3 m have been found.
Ammonites are, of course, members of the animal kingdom. Since they are without backbones, they are invertebrates. They belong to the phylum Mollusca because of their soft body and ability to create shells. Modern day varieties include snails, clams, and oysters, but the octopus and squid have a small internal shell or no shell at all. They are mollusks that belong to the class cephalopoda as does the ammonite. The tentacled head is the primary feature of the members of this class, so the modern chambered nautilus also belongs to this class.
ancient fossil, the subclass is Ammonoidea.
These are the orders:
Goniatitida- The goniatites ranged from the Devonian to the upper Permian Period. They are characterized by septa with round saddles and pointed lobes.
Ceratida- This order existed from the Carboniferous Period to the Triassic Period. They are characterized by septa with round saddles and serrated lobes.
Ammonitida- They first show up during the Permian Period and lasted through the Cretaceous Period. Their septa have folded saddles and lobes. The shells are decorated with a complex pattern of lines called lirae. The name comes from its appearance: it resembles a rams horn. In Egyptian mythology, the God Ammon looked like a man with horns like a ram. The ancient fossil was considered Ammons stone, thus inheriting the name, ammonite.
Ammonites first appeared in the lower Devonian Period. It is thought by some that they evolved from the older nautiloids.
The septa are the walls that devide the chambers within the shell. Nautiloids had simple septa with a single arc. The ammonites developed septa that had intricate folds called lobes and saddles. They also developed delicate lacey patterns on the outer shell.
There are three
basic patterns for ammonite septa.
irregular zigzags-this is called goniatite
regular wavy-called ceratite
intricate feathery or fern like patterns-ammonite
The pattern of the septa can be reflected on the outside of the shell. These are called sutures.
These patterns along with the shape of the shell and the structure of the septa are how this cephalopod is classified.
distribution of the ammonites;
GONIATITES (Devonian - Permian) had round saddles and mainly undivided pointed lobes.
CERATITES (Carboniferous to Triassic) had round saddles and serrated lobes.
AMMONITES (Permian to Cretaceous) had folded saddles and folded lobes that are finely subdivided in fractal patterns.