Editing Ammonoidea (section)

=== Septa and suture patterns ===
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 [[siphuncle]]s.

Ammonoid [[Septum|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.<ref>{{Cite web |title=The Cephalopoda |url=https://ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php |url-status=live |archive-url=https://web.archive.org/web/20220324010712/https://ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php |archive-date=March 24, 2022 |access-date=September 24, 2019 |website=ucmp.berkeley.edu}}</ref> The septal curvature in nautiloids and ammonoids also differ in that the septa curves towards the opening in nautiloids, and away from the opening in ammоnoids.<ref>[https://www.geology.arkansas.gov/docs/pdf/geology/invertebrate_fossils/ammonoidea.pdf Phylum Mollusca Class Cephalopoda]</ref>[[File:Ammonites suture terminology english.png|thumb|left|Regions of the suture line and variants in suture patterns]]
[[File:Ammonite-fossil-25-45mm.jpg|thumb|Ammonite clean cut]]While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles ("peaks" that point towards the aperture) and lobes ("valleys" which point away from the aperture). The suture line has four main regions.
[[File:Placenticeras sp. (fossil ammonite) (Pierre Shale, Upper Cretaceous; Meade County, South Dakota, USA) 1.jpg|thumb|[[Placenticeras|''Placenticeras'' sp.]] showing sutures. ]]
The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet. The external (or ventral) saddle, when present, lies directly on the lower midline of the shell. As a result, it is often called the median saddle. On suture diagrams the median saddle is supplied with an arrow which points towards the aperture. The median saddle is edged by fairly small external (or ventral) lobes. The earliest ammonoids lacked a median saddle and instead had a single midline ventral lobe, which in later forms is split into two or more components.

The lateral region involves the first saddle and lobe pair past the external region as the suture line extends up the side of the shell. The lateral saddle and lobe are usually larger than the ventral saddle and lobe. Additional lobes developing towards the inner edge of a whorl are labelled umbilical lobes, which increase in number through ammonoid evolution as well as an individual ammonoid's development. In many cases the distinction between the lateral and umbilical regions are unclear; new umbilical features can develop from subdivisions of other umbilical features, or from subdivisions of lateral features. Lobes and saddles which are so far towards the center of the whorl that they are covered up by succeeding whorls are labelled internal (or dorsal) lobes and saddles.

Three major types of suture patterns are found in the Ammonoidea:
* '''Goniatitic''' – numerous undivided lobes and saddles. This pattern is characteristic of the Paleozoic ammonoids (orders Agoniatitida, Clymeniida, Goniatitida, and Prolecanitida).
*'''Ceratitic''' – lobes have subdivided tips, giving them a saw-toothed appearance. The saddles are rounded and undivided. This suture pattern is characteristic of Triassic ammonoids in the order [[Ceratitida]]. It 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]].

<gallery mode="packed" heights="175px">
File:Ammonite Goniatites plebeiformis Goniatitic suture.jpg|''[[Goniatites|Goniatites plebeiformis]]'' showing Goniatitic suture
File:Ammonite Protrachyceras pseudoarchelonus Ceratitic suture.jpg|''[[Protrachyceras|Protrachyceras pseudoarchelonus]]'' showing Ceratitic suture
File:Ammonite Lytoceras sutile Ammonitic suture.jpg|''[[Lytoceras sutile]]'' showing Ammonitic suture
</gallery>