Ammonites are extinct relatives of modern-day cephalopods such as squids and octopus. Their soft body parts were encased in a spiral-shaped shell.
Ammonites take their name from the ancient Egyptian god Amun: their shells resembled the horns of the solar deity, who was depicted with the head of a ram.
These creatures appeared on Earth about 400 million years ago and were completely extinct by the end of the Cretaceous period, about 65 million years ago. The history of ammonites began with their straight shells turning into spirals, a form they preserved throughout their existence. Only in the Late Triassic, some 180 million years ago, the spirals began to unfold and take on a variety of shapes. Such species are called heteromorph ammonites. A particularly large number of them appeared at the end of the Cretaceous period.
The ammonite shell is formed by spiral twists, with each twist divided into chambers. The body usually occupied the last chamber, with the head carrying the eyes, and tentacles peeking out. The remaining, bigger part of the shell was filled with gas, which made the shell buoyant in the water.
Originally, the ammonite shell was composed of the mineral aragonite. Buried under the layer of sediments on the seafloor, dead ammonites were exposed to mineral-rich water which resulted in fossilization. During this process, trapped in an oxygen-free environment, all organic matter in the ammonite shell was being degraded, and the aragonite in the shell itself was recrystallized and replaced by various minerals depending on physical and chemical conditions.
Shells replaced by calcite (such as the ammonites from deposits in Madagascar) are the most commonly found. In rare cases, aragonite can be replaced by parallel oriented calcite crystals, which produces the optical effect of iridescence (Craspedodiscus). In the presence of hydrogen sulfide acting as a reducing agent, some ammonite shells were replaced by pyrite (Speetoniceras).
