Prehistoric life
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.
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Super-snails
The rise of the gas-filled nautiloids gave way to the rolled up shells of ammonites.
The rise of the gas-filled nautiloids gave way to the rolled up shells of ammonites.
Jurassic coast
The sea eats away at Dorset's Jurassic coast, scattering clues to the past on the beach.
The sea eats away at Dorset's Jurassic coast, scattering clues to the past on the beach.
Dorset's ancient animals
Simon King shows off Dorset's coastal fossil riches including a huge ichthyosaur.
Simon King shows off Dorset's coastal fossil riches including a huge ichthyosaur.
Fossil detection
Lyme Regis beach reveals the creatures that once dominated sea life.
Lyme Regis beach reveals the creatures that once dominated sea life.
Discover what these behaviours are and how different plants and animals use them.
Additional data source: Animal Diversity Web
Discover the other animals and plants that lived during the following geological time periods.
Cretaceous-Tertiary mass extinctionLearn more about the other animals and plants that also form these fossils.
Trace fossils
Ammonites have featured it our folklore - learn more our ancestors beliefs
before we understood fossilisation and evolution.
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 KT extinction event.
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus are 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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Predation defence
Adapted to swimming
Carnivorous
Predator
Ordovician period
Silurian period
Devonian period
Carboniferous period
Permian period
Triassic period
Jurassic period
Cretaceous period