Ammonites
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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’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.

Ammonites first appeared in the late Silurian to early Devonian Periods (~400 million years ago). Through the remainder of the Paleozoic and through the Mesozoic, ammonites underwent repeated and large radiations only to decline in several extinction events. Ammonites were especially abundant in the Mesozoic marine environment due to rapid evolution and diversification, leading to widespread distribution. Only some 10% of species survived the Permian Extinction, and their ultimate demise coincided with that of the dinosaurs in the K-T extinction event that closed the Cretaceous Period.

Cretaceous Heteromorph Ammonite	Massive 80 Pound Procheloniceras Ammonite Fossil	Large Polished Devonian Goniatite Ammonite	Rarely Seen Ammonite Assemblage with Four Species from Nevada