Editing Ammonoidea (section)

=== Basic shell anatomy ===
{{Unreferenced section|date=May 2024}}
[[Image:Ammonite Jeletzkytes.jpg|thumb|''[[Jeletzkytes]]'', a Cretaceous ammonite from South Dakota, US]]
[[Image:Haeckel Ammonitida.jpg|thumb|left|A variety of ammonite forms, from [[Ernst Haeckel]]'s 1904 ''Kunstformen der Natur'' (Art Forms of Nature)]]
[[File:Polished_fossil_ammonite_005.jpg|thumb|Polished fossil ammonite]]
The chambered part of the ammonite shell is called a [[phragmocone]]. It contains a series of progressively larger chambers, called [[camera (cephalopod)|camera]]e (sing. camera) that are divided by thin walls called [[Septa (biology)|septa]] (sing. septum). Only the last and largest chamber, the [[body chamber]], was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. Where the outer [[Whorl (mollusc)|whorl]] of an ammonite shell largely covers the preceding whorls, the specimen is said to be ''[[wikt:involute|involute]]'' (e.g., ''[[Anahoplites]]''). Where it does not cover those preceding, the specimen is said to be ''[[wikt:evolute|evolute]]'' (e.g., ''[[Dactylioceras]]'').

A thin living tube called a [[siphuncle]] passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a [[hyperosmotic]] active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control the [[buoyancy]] of the shell and thereby rise or descend in the water column.

A primary difference between ammonites and nautiloids is the siphuncle of ammonites (excepting [[Clymeniina]]) runs along the ventral periphery of the septa and camerae (i.e., the inner surface of the outer axis of the shell), while the siphuncle of nautiloids runs more or less through the center of the septa and camerae.