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Ammonoids
Ammonoids are related to the modern nautilus. They lived between the Devonian and Cretaceous periods. Ammonoids lived in a chambered, coiled shell, the many chambers connected by a thin tube called a siphuncle. They were able to change the contents
of the chambers to alter their buoyancy and existed at every level of the world’s oceans. They were hunters, able to move quickly in search of prey.
Ammonoids developed a pattern of ribbed shells. The junction between the chamber walls and the outer shell is called the suture.
(Below) A section through an ammonite, showing chambers.
Septa Chamber
Tentacles Eye
Siphuncle
The earliest ammonoids are called
goniatites (Devonian to Permian periods) and
have a very simple suture pattern. They lived
during the Palaeozoic Era. They were followed
by ammonoids with a more complicated suture
pattern, which were called ceratites (Devonian
to Triassic periods). They were replaced by the ammonites during the Jurassic and Cretaceous
periods. Ammonites developed a very complicated suture pattern that makes each species easy to identify.
Ventral margin
Hard, chambered shell is preserved.
Aperture
(Below) Ammonoids.
Soft body parts in living chamber are not preserved.
Siphon
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Swims by expelling water through siphon.