Ammonoid

Schematic of the internal (left) and external (right) architecture expected in planispiral ammonoids.

Sketch of different suture line types for each order of ammonoids.

Sketch of different whorl configurations in ammonoids.

Taxonomy

Phylum: Mollusca

Class: Cephalopoda

Subclass: Ammonoidea

Diagnostic features

[See labelled diagram above, left]

Aragonite shell

Septa

Suture (different for goniatites, ceratites and ammonites, see diagram above, centre)

Chambers

Siphuncle

Planispiral shape, involute or evolute (see diagram above, right)

Stratigraphic range

Devonian to Cretaceous

Way of life

Nektonic (jet propelled)

Predatory

Marine

Common preservation

Internal mould

Advanced notes

Ammonoids could passively remove or add water to chambers via the siphuncle. This allowed them to change their density, and maintain neutral buoyancy with the surrounding seawater. This meant that they were able to swim as efficiently as possible.

Ammonoids are extremely good index fossils on account of their diverse morphologies, good preservation potentials and widespread fossil locations.

Specimens

Browse ammonoids

External links

Telling planispiral fossils apart