Editing Ammonoidea (section)

==Shell anatomy and diversity==
[[File:Placenticeratidae - Placenticeras whitfieldi.jpg|thumb|left|Fossil shell of ammonite ''[[Placenticeras whitfieldi]]'' showing punctures caused by the bite of a [[mosasaur]], [[Peabody Museum of Natural History]], Yale]]
[[Image:Amonite-portugal.JPG|thumb|''[[Orthosphynctes]]'', a Jurassic ammonite from Portugal]]

=== 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.

=== 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>

=== 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.<ref>{{Cite journal |last1=Lemanis |first1=Robert |last2=Korn |first2=Dieter |last3=Zachow |first3=Stefan |last4=Rybacki |first4=Erik |last5=Hoffmann |first5=René |date=2016-03-10 |title=The Evolution and Development of Cephalopod Chambers and Their Shape |journal=PLOS ONE |volume=11 |issue=3 |pages=e0151404 |bibcode=2016PLoSO..1151404L |doi=10.1371/journal.pone.0151404 |issn=1932-6203 |pmc=4786199 |pmid=26963712 |doi-access=free}}</ref> 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.<ref>{{Cite journal |last=Kröger |first=Björn |date=2003 |title=The size of the siphuncle in cephalopod evolution |url=http://link.springer.com/10.1007/BF03043304 |journal=Senckenbergiana Lethaea |language=en |volume=83 |issue=1–2 |pages=39–52 |doi=10.1007/BF03043304 |issn=0037-2110}}</ref> The word "siphuncle" comes from the [[Neo-Latin]] ''siphunculus'', meaning "little siphon".<ref>{{Citation |title=siphuncle, n. |date=2023-03-02 |work=Oxford English Dictionary |url=https://oed.com/dictionary/siphuncle_n |access-date=2024-06-07 |edition=3 |publisher=Oxford University Press |language=en |doi=10.1093/oed/6104320866}}</ref>

=== Sexual dimorphism ===
[[Image:DiscoscaphitesirisCretaceous.jpg|thumb|''[[Discoscaphites|Discoscaphites iris]]'', Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi, US]]

One feature found in shells of the modern ''[[Nautilus]]'' is the variation in the shape and size of the shell according to the [[sex]] of the animal, the shell of the male being slightly smaller and wider than that of the female. This [[sexual dimorphism]] is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (the ''macroconch'') being female, and the smaller shell (the ''microconch'') being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in ''[[Bifericeras]]'' from the early part of the [[Jurassic]] period of [[Europe]].

Only recently has sexual variation in the shells of ammonites been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, because the dimorphic sizes are so consistently found together, they are more likely an example of sexual dimorphism within the same species.

Whorl width in the body chamber of many groups of ammonites, as expressed by the width:diameter ratio, is another sign of dimorphism. This character has been used to separate "male" (Largiventer conch "L") from "female" (Leviventer conch "l").<ref>{{cite book |doi=10.1007/978-1-4615-4837-9_23 |chapter=Whorl Width in the Body Chamber of Ammonites as a Sign of Dimorphism |title=Advancing Research on Living and Fossil Cephalopods |year=1999 |last1=Sarti |first1=Carlo |pages=315–332 |isbn=978-1-4613-7193-9 }}</ref>

=== Variations in shape ===
The majority of ammonite species feature planispiral shells, tightly coiled in a flat plane. The most fundamental difference in spiral form is how strongly successive whorls expand and overlap their predecessors. This can be inferred by the size of the umbilicus, the sunken-in inner part of the coil, exposing older and smaller whorls. ''Evolute'' shells have very little overlap, a large umbilicus, and many exposed whorls. ''Involute'' shells have strong overlap, a small umbilicus, and only the largest and most recent whorls are exposed. Shell structure can be broken down further by the width of the shell, with implications for [[hydrodynamic]] efficiency.

Major shell forms include:

* '''Oxycone''' – Strongly involute and very narrow, with sharp ventral keels and a streamlined, lenticular ([[lens]]-shaped) cross-section. These ammonoids are estimated to be [[nekton]]ic (well-adapted to rapid active swimming), as their shell form incurs very little drag and allows for efficient, stable coasting even in turbulent flow regimes.<ref name=":3">{{Cite journal |last1=Peterman |first1=David J. |last2=Ritterbush |first2=Kathleen A. |date=2022-07-04 |title=Resurrecting extinct cephalopods with biomimetic robots to explore hydrodynamic stability, maneuverability, and physical constraints on life habits |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=11287 |doi=10.1038/s41598-022-13006-6 |pmid=35787639 |issn=2045-2322|pmc=9253093 |bibcode=2022NatSR..1211287P }}</ref>
* '''Serpenticone''' – Strongly evolute and fairly narrow (discoidal) in width. Historically assumed to be primarily [[plankton]]ic (free-floating drifters),<ref>{{Citation |last=Westermann |first=Gerd E. G. |title=Ammonoid Life and Habitat |date=1996 |url=https://doi.org/10.1007/978-1-4757-9153-2_16 |work=Ammonoid Paleobiology |series=Topics in Geobiology |volume=13 |pages=607–707 |editor-last=Landman |editor-first=Neil H. |access-date=2023-05-15 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4757-9153-2_16 |isbn=978-1-4757-9153-2 |editor2-last=Tanabe |editor2-first=Kazushige |editor3-last=Davis |editor3-first=Richard Arnold}}</ref> a nektonic lifestyle is also plausible for many species.<ref>{{Cite journal |last1=Ritterbush |first1=K. A. |last2=Hoffmann |first2=R. |last3=Lukeneder |first3=A. |last4=De Baets |first4=K. |date=2014 |title=Pelagic palaeoecology: the importance of recent constraints on ammonoid palaeobiology and life history |journal=Journal of Zoology |language=en |volume=292 |issue=4 |pages=229–241 |doi=10.1111/jzo.12118 |issn=0952-8369|doi-access=free }}</ref> Thanks to their flattened shape, these ammonoids accelerate effectively, though their large umbilicus introduces more drag in successive thrusts.<ref name=":3" /> Relative to oxycones, serpenticones take less effort to rotate around the transverse axis ([[Pitch (aeronautics)|pitch]]).<ref name=":4">{{Cite journal |last1=Peterman |first1=David J |last2=Ritterbush |first2=Kathleen A |date=2022-12-12 |title=Stability–Maneuverability Tradeoffs Provided Diverse Functional Opportunities to Shelled Cephalopods |journal=Integrative Organismal Biology |language=en |volume=4 |issue=1 |pages=obac048 |doi=10.1093/iob/obac048 |pmid=36518181 |issn=2517-4843|pmc=9743176 }}</ref> Serpenticone ammonites resemble coiled snakes and are abundant in the Jurassic rocks of Europe. Carved serpenticones fulfill the role of the "snakestones" in medieval folklore.
* '''Spherocone''' – Moderately involute and quite broad, globular (nearly spherical) in overall shape. Their semi-spherical shape is the most efficient for moving in laminar water (with a low [[Reynolds number]]) or [[Diel vertical migration|migrating vertically]] through the water column. Though less hydrodynamically stable than other forms, this may be advantageous in certain situations, as spherocones can easily rotate around both the transverse axis<ref name=":4" /> and the vertical axis ([[Yaw (rotation)|yaw]]).<ref name=":3" />
* '''Platycone''' – Intermediate between serpenticones and oxycones: narrow and moderately involute.
* '''Discocone''' – Intermediate between oxycones and spherocones: involute and moderately broad. The modern ''Nautilus'' is an example of a discocone cephalopod.
* '''Planorbicone''' – Intermediate between serpenticones and spherocones: Moderately broad, evolute to involute. Wider and more involute ammonoids on the serpenticone-spherocone spectrum are termed ''Cadicones''.

Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, resembling that of the modern ''Nautilus''. In others, various patterns of spiral ridges, ribs, nodes, or spines are presented. This type of complex ornamentation of the shell is especially evident in the later ammonites of the Cretaceous.

==== Heteromorphs ====
[[Image:BaculitidArticulated.jpg|thumb|''[[Baculites]]'' ammonite from the Late [[Cretaceous]] of Wyoming, US: The original [[aragonite]] of the outer conch and inner septa has dissolved away, leaving this articulated internal mold.]][[File:Didymoceras stevensoni, Late Cretaceous, Pierre Shale Formation, Weston County, Wyoming, USA - Houston Museum of Natural Science - DSC01926.JPG|thumb|150px|Heteromorph ammonite ''[[Didymoceras stevensoni]]'']]
[[File:Aristonectes2DB.jpg|thumb|Life restoration of the heteromorph ammonite ''[[Diplomoceras]]'' with the plesiosaur ''[[Aristonectes]]'' ]]
[[File:Hyphantoceras orientale.png|thumb|228x228px|Life restoration of the heteromorph ''[[Hyphantoceras]]'']]
Ammonoids with a shell shape diverging from the typical planispiral form are known as [[heteromorph]]s, instead forming a conch with detached whorls (open coiling) or non-planispiral coiling. These types of shells evolved four times in ammonoids, with the first forms appearing already in the Devonian period.<ref>{{cite journal | url=https://onlinelibrary.wiley.com/doi/full/10.1111/brv.12669 | doi=10.1111/brv.12669 | title=Recent advances in heteromorph ammonoid palaeobiology | date=2021 | last1=Hoffmann | first1=René | last2=Slattery | first2=Joshua S. | last3=Kruta | first3=Isabelle | last4=Linzmeier | first4=Benjamin J. | last5=Lemanis | first5=Robert E. | last6=Mironenko | first6=Aleksandr | last7=Goolaerts | first7=Stijn | last8=De Baets | first8=Kenneth | last9=Peterman | first9=David J. | last10=Klug | first10=Christian | journal=Biological Reviews | volume=96 | issue=2 | pages=576–610 | pmid=33438316 }}</ref> In late Norian age in Triassic the first heteromorph ammonoid fossils belongs to the genus Rhabdoceras. The three other heteromorphic genera were Hannaoceras, Cochloceras and Choristoceras. All of them went extinct at the end of Triassic.<ref>{{cite book | url=https://books.google.com/books?id=7SU_DwAAQBAJ&dq=Heteromorph+ammonoids+Norian+Rhaetian&pg=PA252 | title=The Late Triassic World: Earth in a Time of Transition | isbn=978-3-319-68009-5 | last1=Tanner | first1=Lawrence H. | date=16 November 2017 | publisher=Springer }}</ref> In the Jurassic an uncoiled shell was found in the Spiroceratoidea,<ref>{{cite book | url=https://books.google.com/books?id=CfHwDwAAQBAJ&dq=Spiroceratoidea+uncoiled+forms&pg=PA67 | title=Geologic Time Scale 2020 | isbn=978-0-12-824361-9 | last1=Gradstein | first1=Felix M. | last2=Ogg | first2=James G. | last3=Schmitz | first3=Mark D. | last4=Ogg | first4=Gabi M. | date=2020 | publisher=Elsevier }}</ref> but by the end of Cretaceous the only heteromorph ammonites remaining belonged to the suborder Ancyloceratina.<ref>{{cite journal | doi=10.1111/let.12443 | title=The concept of 'heteromorph ammonoids' | date=2021 | last1=Landman | first1=Neil H. | last2=Machalski | first2=Marcin | last3=Whalen | first3=Christopher D. | journal=Lethaia | volume=54 | issue=5 | pages=595–602 | bibcode=2021Letha..54..595L | doi-access=free }}</ref> One example is ''[[Baculites]]'', which has a nearly straight shell convergent with the older [[orthocone]] nautiloids. Still other species' shells are coiled helically (in two dimensions), similar in appearance to some [[gastropod]]s (e.g., ''[[Turrilites]]'' and ''[[Bostrychoceras]]''). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in ''[[Australiceras]]'').

Perhaps the most extreme and bizarre-looking example of a heteromorph is ''[[Nipponites]]'', which appears to be a tangle of irregular whorls lacking any obvious symmetric coiling. Upon closer inspection, though, the shell proves to be a three-dimensional network of connected "U" shapes. ''{{lang|la|Nipponites}}'' occurs in rocks of the upper part of the [[Cretaceous]] in Japan and the United States.

=== Aptychus ===
{{Main|Aptychus}}

[[File:Trigonellites latus.jpg|thumb|A drawing of an aptychus which was mistakenly described as a [[bivalve]] and given the name "''[[Trigonellites|Trigonellites latus]]''", from the [[Kimmeridge Clay]] Formation in England|left]]

Some ammonites have been found in association with a single horny plate or a pair of calcitic plates. In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an [[operculum (gastropod)|operculum]], but more recently they are postulated to have been a jaw apparatus.<ref>{{cite journal | last1=Morton | first1=N | year=1981 | title=Aptychi: the myth of the ammonite operculum | journal=Lethaia | volume=14 | issue=1| pages=57–61 | doi=10.1111/j.1502-3931.1981.tb01074.x | bibcode=1981Letha..14...57M }}</ref><ref>{{cite journal | last1=Morton | first1=N. | last2=Nixon | first2=M. | year=1987 | title=Size and function of ammonite aptychi in comparison with buccal masses of modem cephalopods | journal=Lethaia | volume=20 | issue=3| pages=231–238 | doi=10.1111/j.1502-3931.1987.tb02043.x | bibcode=1987Letha..20..231M }}</ref><ref>{{cite journal | last1=Lehmann | first1=U. | last2=Kulicki | first2=C. | year=1990 | title=Double function of aptychi (Ammonoidea) as jaw elements and opercula | journal=Lethaia | volume=23 | issue=4| pages=325–331 | doi=10.1111/j.1502-3931.1990.tb01365.x | bibcode=1990Letha..23..325L }}</ref><ref>{{cite journal | last1=Seilacher | first1=A | year=1993 | title=Ammonite aptychi; how to transform a jaw into an operculum? | journal=American Journal of Science | volume=293 | pages=20–32 | doi=10.2475/ajs.293.A.20 | bibcode=1993AmJS..293...20S }}</ref>

The plates are collectively termed the [[aptychus]] or aptychi in the case of a pair of plates, and anaptychus in the case of a single plate. The paired aptychi were symmetric to one another and equal in size and appearance.

Anaptychi are relatively rare as fossils. They are found representing ammonites from the Devonian period through those of the Cretaceous period.

Calcified aptychi only occur in ammonites from the [[Mesozoic]] era. They are almost always found detached from the shell, and are only very rarely preserved in place. Still, sufficient numbers have been found closing the apertures of fossil ammonite shells as to leave no doubt as to their identity as part of the anatomy of an ammonite.

Large numbers of detached aptychi occur in certain beds of rock (such as those from the Mesozoic in the [[Alps]]). These rocks are usually accumulated at great depths. The modern ''Nautilus'' lacks any calcitic plate for closing its shell, and only one extinct [[nautiloid]] genus is known to have borne anything similar. ''Nautilus'' does, however, have a leathery head shield (the hood) which it uses to cover the opening when it retreats inside.

There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is often unclear to which species of ammonite one kind of aptychus belongs. A number of aptychi have been given their own genus and even species names independent of their unknown owners' genus and species, pending future discovery of verified occurrences within ammonite shells.