Fossilized ammonite

Ammonites

Ammonites were free-swimming molluscs of the ancient oceans, living around the same time that the dinosaurs walked the Earth and disappearing during the same extinction event. They came in a range of sizes, from tiny species only a couple of centimetres across, to large ones reaching over two metres in diameter. The animal would have lived in the last and largest of a chain of spiralled chambers. Filling these chambers with fluid or gas allowed the ammonite to sink like a stone to avoid predators, though ammonite shells with toothmarks on them have been found, evidence that it didn't always work. Fossilised shells are usually, but not always, beautiful spirals.

Scientific name: Ammonitida

Rank: Order

Common names:

Snake stones

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Behaviours

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Additional data source: Animal Diversity Web

When they lived

Discover the other animals and plants that lived during the following geological time periods.

What killed them

Cretaceous-Tertiary mass extinction Cretaceous-Tertiary mass extinction
The Cretaceous-Tertiary mass extinction - also known as the K/T extinction - is famed for the death of the dinosaurs. However, many other organisms perished at the end of the Cretaceous including the ammonites, many flowering plants and the last of the pterosaurs.

Fossil types

Learn more about the other animals and plants that also form these fossils.

Trace fossils Trace fossils
It's not only the actual bodily remains of dead animals and plants that can become fossils. Things created or left behind by animals can also fossilise, such as their footprints, burrows and dung.

Fossil Folklore

Ammonites have featured it our folklore - learn more our ancestors beliefs before we understood fossilisation and evolution.

About

Ammonites /ˈæmənaɪts/ are an extinct group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more closely related to living coleoids (i.e. octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event.

Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs).

The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder (d. 79 AD near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for "horn".

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BBC News about Ammonites