Ammonite

Ammonite

Ammonites are an extinct group of marine mollusk animals in the subclass Ammonoidea of the class Cephalopoda. These creatures are more closely related to living coleoids (i.e., octopuses, squid, and cuttlefish) than to shelled nautiloids such as the living Nautilus.

Description

Ammonites are known for their spiral shells, which are typically planispirally coiled, although there are some helically coiled and non-spiral forms. The shell of an ammonite is divided into chambers separated by thin walls called septa. The animal lived in the last chamber, with the earlier chambers filled with gas or liquid to aid in buoyancy.

Evolution and Classification

Ammonites first appeared in the Devonian period and became extinct at the end of the Cretaceous period, around 66 million years ago, during the Cretaceous–Paleogene extinction event. They are divided into several orders, including:

• Goniatitida
• Ceratitida
• Ammonitida

Ecology

Ammonites were marine animals and are believed to have been predatory, feeding on small marine organisms. They are thought to have been similar in lifestyle to modern cephalopods, using jet propulsion to move through the water.

Fossil Record

Ammonite fossils are found in many locations around the world and are often used as index fossils, helping to date the rock layers in which they are found. Their widespread distribution and rapid evolution make them excellent markers for the geological time scale.

Cultural Significance

Ammonites have been known to humans for thousands of years and have been used in various cultures as amulets and symbols. Their spiral shape has often been associated with the golden ratio and Fibonacci sequence.

See Also

• Nautiloid
• Belemnite
• Fossil

References

• Arkell, W. J. (1957). A Treatise on the Ammonoidea. Geological Society of America.
• Monks, N., & Palmer, P. (2002). Ammonites. Smithsonian Institution Press.

External Links

• Ammonites.org