Temporal range: 409–66 Ma DevonianCretaceous
Asteroceras BW.jpg
Artist's reconstruction of Asteroceras
Scientific classification e
Zittel, 1884
Orders and Suborders

Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, 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.[citation needed] The earliest ammonites appear during the Devonian, and the last species died out in 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 geologic 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.[1] Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for "horn".

Diagnostic characters

Ammonites (subclass Ammonoidea) can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa joint the outer shell wall, and in general by their siphuncles.


Ammonite interiors showing septa; Pierre Shale, Upper Cretaceous, South Dakota

Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex from the front, distinguishing them from nautiloid septa which are typically simple concave dish-shaped structures. The topology of the septa, especially around the rim, results in the various suture patterns found.

Suture patterns

Three major types of suture patterns are found in the Ammonoidea:

  • Goniatitic - numerous undivided lobes and saddles; typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids.
  • Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites".
  • Ammonitic - lobes and saddles are much subdivided (fluted); subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a biostratigraphical point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids, but extends back all the way to the Permian.


The siphuncle in most ammonoids is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber. This distinguishes them from living nautiloides (Nautilus and Allonautilus) and typical Nautilida, in which the siphuncle runs through the center of each chamber. However the very earliest nautiloids from the Late Cambrian and Ordovician typically had ventral siphuncles like ammonites, although often proportionally larger and more internally structured. The word "siphuncle" comes from the New Latin siphunculus, meaning "little siphon".

Other Languages
Afrikaans: Ammonoidea
العربية: أمونيت
asturianu: Ammonoidea
azərbaycanca: Ammonoidealar
български: Амонити
català: Ammonits
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မြန်မာဘာသာ: အမ်မိုနိုက်
norsk: Ammonitter
norsk nynorsk: Ammonittar
oʻzbekcha/ўзбекча: Ammonitlar (molyuskalar turkumi)
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Simple English: Ammonite
slovenčina: Amonity
slovenščina: Amoniti
srpskohrvatski / српскохрватски: Amoniti
suomi: Ammoniitit
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Tiếng Việt: Phân lớp Cúc đá
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中文: 菊石