Ammonoidea

Ammonoids Temporal range: 409–66  Ma PreꞒ Ꞓ O S D C P T J K Pg N Devonian-Late Cretaceous
Specimen of Pleuroceras solare, from the Lower Jurassic of Bavaria, Germany
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	†Ammonoidea Zittel, 1884
Orders
†Agoniatitida †Ammonitida †Ceratitida †Clymeniida † Goniatitida †Prolecanitida

Ammonite clean cut

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 rams' horns.^[2] Often, the name of an ammonite genus ends in -ceras, which is from κέρας (kéras) meaning "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 join the outer shell wall, and in general by their siphuncles.

Septa

Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen 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.^[3]

Suture patterns

Regions of the suture line and variants in suture patterns

• 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.

• 

  Goniatites plebeiformis showing Goniatitic suture

• 

  Protrachyceras pseudoarchelonus showing Ceratitic suture

• 

  Lytoceras sutile showing Ammonitic suture

Siphuncle

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".

Classification

An ammonite shell viewed in section, revealing the internal chambers and septa

. Large polished examples are prized for both their aesthetic and scientific value.

An ammonitic ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles.

Iridescent ancient ammonite fossil on display at the American Museum of Natural History

, New York City, around 2.5 feet in diameter

• Agoniatitida, Lower Devonian – Middle Devonian
• Clymeniida, Upper Devonian
• Goniatitida
  , Middle Devonian – Upper Permian
• Prolecanitida, Upper Devonian – Upper Triassic
• Ceratitida, Upper Permian – Upper Triassic
• Ammonitida, Lower Jurassic – Lower Paleocene

In some classifications, these are left as suborders, included in only three orders:

The Treatise on Invertebrate Paleontology (Part L, 1957) divides the Ammonoidea, regarded simply as an order, into eight suborders, the Anarcestina, Clymeniina, Goniatitina and Prolecanitina from the Paleozoic; the Ceratitina from the Triassic; and the Ammonitina, Lytoceratina and Phylloceratina from the Jurassic and Cretaceous. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea.

Life

Asteroceras

, a Jurassic ammonite from England

Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks.

Many ammonoids probably lived in the open water of ancient seas, rather than at the sea bottom, because their fossils are often found in rocks laid down under conditions where no

The soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated above the larger sections.[6]

Many ammonite shells have been found with round holes once interpreted as a result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, is more likely evidence of the bite of a medium-sized mosasaur preying upon ammonites.

Some ammonites appear to have lived in cold seeps and even reproduced there.^[7]

Shell anatomy and diversity