What are Ammonites?
Ammonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods. Ammonites’ closest living relative is probably not the modern Nautilus (which they outwardly resemble), but rather the subclass Coleoidea (octopus, squid, and cuttlefish). Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as “heteromorphs”). Their name came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled rams’ horns. Plinius the Elder (died 79 A.D. 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.
Ammonites were an important group of marine mollusks that were numerous in the Earth’s oceans for 360 million years, from about 425 million years ago, during the Silurian period, to 65.5 million years ago, when they went extinct along with the dinosaurs and many other clades. Ammonites were cephalopods, most closely related to living octopus, squid, and cuttlefish, though all their closest relatives are extinct. They lived in a distinctive spiral shell, which they could fill with gas and use to float at a desired level in the water. A few ammonites had non-spiral shells, including complex twirling patterns. Ammonites were given their name by Pliny the Elder, who called them “horns of Ammon” after the Egyptian god that was frequently depicted with the horns of a ram. At the time they evolved, ammonites would have been one of the most intelligent species on the planet, along with other sophisticated mollusks. They had large eyes used to locate prey and tentacles to grab it with. Some amm
Ammonites are an extinct group of molluscs that flourished in the seas of the Mesozoic Era (from around 245-64 million years ago). They secreted coiled shells made of the mineral aragonite and the soft parts of the animal lived within the shell, much like a snail. In terms of shell shape and probably also the ecology of the living animal, the closest modern analogue of ammonites is the pearly nautilus. This cephalopod mollusc inhabits the Pacific and Indian Oceans, swimming at depths of up to 500 metres and feeding on fishes and crustaceans. Nautilus belongs to a cephalopod group – the nautiloids – more ancient than ammonites. Nautiloids co-existed in the seas with ammonites for hundreds of millions of years. However, whereas the last species of ammonites disappeared during the mass extinction event at the end of the Cretaceous period, nautiloids survived and have persisted as rare animals to the present day.
