Ammonite Fossils

Also see :
Ammonites
Subclass Ammonoidea

Subclass Ammonoidea
Ammonites are an extinct marine taxon (subclass Ammonoidea) in the Phylum Mollusca and Class Cephalopoda. Their closest living relative is probably the modern nautilus that they closely resemble. Ammonite fossil shells are of particularly beautiful spiral forms, except for some more uncommon forms without spirals that are called heteromorphs. The ammonite name derives from the organism’s resemblance to a coiled ram’s horn, after the god Ammon (that was depicted as a man with ram's horns).

The ammonite’s shell contained a spiraling progression of ever larger chambers divided by thin walls called septa. The animal only occupied the last and largest chamber. A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. The ammonite secreted gas into these shell chambers, enabling it to regulate the buoyancy of the shell. As the ammonite grew, it added newer and larger chambers toward the larger open end of the coil.

While the majority of ammonites have a shell that is a flattened coil, others have a shell that is partially uncoiled, partially coiled and partially straight (as in Australiceras), nearly straight (as in baculites and belemnites), or coiled helically - superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). These partially-to-totally uncoiled forms appeared in the Lower Cretaceous and are known as heteromorphs.