Editing Ammonoidea

{{short description|Extinct subclass of cephalopod molluscs}}
{{Redirect|Ammonite}}
{{distinguish|Amniote}}
{{Automatic taxobox
| name = Ammonoids
| fossil_range = {{fossil range|409|65|latest=Danian}} Early [[Devonian]]  ([[Emsian]]) - earliest [[Paleocene]] ([[Danian]])
| image = Pleuroceras solare, Little Switzerland, Bavaria, Germany.jpg
| image_caption = Specimen of ''[[Pleuroceras solare]]'', from the Lower Jurassic of Bavaria, Germany
| taxon = Ammonoidea
| authority = [[Karl Alfred von Zittel|Zittel]], 1884
| subdivision_ranks = Orders
| subdivision = * †[[Agoniatitida]]
* {{extinct}}[[Ammonitida]] 
* {{extinct}}[[Ceratitida]]
* {{extinct}}[[Clymeniida]]
* {{extinct}}[[Goniatitida]]
* {{extinct}}[[Prolecanitida]]
}}
'''Ammonoids''' are extinct, (typically) coiled-shelled [[cephalopod]]s comprising the subclass '''Ammonoidea'''. They are more closely related to living [[octopus]]es, [[squid]], and [[cuttlefish]] (which comprise the clade [[Coleoidea]]) than they are to [[nautilus]]es (family [[Nautilidaa]]).<ref>{{cite book|title=Ammonoid Paleobiology: From macroevolution to paleogeography|pages=3–24|chapter=Ancestry, Origin and Early Evolution of Ammonoids|date=August 2015|series=Topics in Geobiology 44|doi=10.1007/978-94-017-9633-0_1|publisher=Springer |editor=Christian Klug |editor2=Dieter Korn |editor3=Kenneth De Baets |editor4=Isabelle Kruta |editor5=Royal H. Mapes |first1=Christian|last1=Klug|first2=Björn|last2=Kröger|first3=Jakob|last3=Vinther|first4=Dirk|last4=Fuchs|volume=44|isbn=978-94-017-9632-3}}</ref> The earliest ammonoids appeared during the [[Emsian]] stage of the Early [[Devonian]], with the last [[species]] vanishing during or soon after the [[Cretaceous–Paleogene extinction event]]. They are often called '''ammonites''', which is most frequently used for members of the order [[Ammonitida]], the only remaining group of ammonoids from the [[Jurassic]] up until their extinction.<ref>{{Cite web |title=What is an ammonite? |url=https://www.nhm.ac.uk/discover/what-is-an-ammonite.html |access-date=2023-12-21 |website=www.nhm.ac.uk |language=en}}</ref>

Ammonoids exhibited considerable diversity over their evolutionary history, with over 10,000 species having been described.<ref>{{Cite web |title=Ammonite |url=https://www.sdnhm.org/exhibitions/fossil-mysteries/fossil-field-guide-a-z/ammonite/ |access-date=2025-01-13 |website=The Nat: San Diego Natural History Museum |language=en}}</ref> Ammonoids are excellent [[index fossils]], and they have been frequently used to link rock layers in which a particular species or genus is found to specific [[Geologic time scale|geologic time periods]]. Their [[fossil]] shells usually take the form of [[planispiral]]s, although some helically spiraled and nonspiraled forms (known as [[heteromorph]]s) have been found, primarily during the [[Cretaceous]] period.

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 [[sheep|rams]]' horns. [[Pliny the Elder]] ({{abbr|d.|died}} 79 AD near Pompeii) called fossils of these animals {{lang|la|ammonis cornua}} ("[[horns of Ammon]]") because the Egyptian god Ammon ([[Amun]]) was typically depicted wearing rams' horns.<ref>''[[Natural History (Pliny)|NH]]'' 37.40.167</ref> Often, the name of an ammonite genus ends in -''ceras'', which is from ancient Greek {{lang|grc|κέρας}} ({{Transliteration|grc|kéras}}) meaning "horn".

==Classification==
{{see also|List of ammonite genera}}
[[Image:Ammoniteplit.jpg|thumb|An ammonite shell viewed in section, revealing the internal chambers and [[Septum (cephalopod)|septa]]: Large polished examples are prized for both their aesthetic and scientific value.]]

=== Orders and suborders ===
[[Image:1212amma.jpg|thumb|An ammonitic ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles.]]
[[Image:Iridescent Ammonite Fossil.jpg|thumb|[[Iridescence|Iridescent]] ancient ammonite fossil on display at the [[American Museum of Natural History]], New York City, around 0.61 metres (2 feet) in diameter]]

The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived:
* [[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: [[Goniatitida]], [[Ceratitida]] and [[Ammonitida]].  The classification of ammonoids is based in part on the [[Biological ornament|ornamentation]] and structure of the [[Septum (cephalopod)|septa]] comprising their shells' gas chambers.

=== Taxonomy of the ''Treatise on Invertebrate Paleontology'' ===
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.

==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.

==Soft tissues, life appearance and ecology==
[[Image:Ammonite Asteroceras.jpg|thumb|''[[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.
[[File:Ammonite soft tissue diagram.png|thumb|330x330px|Diagram of ammonite soft tissue anatomy based on other cephalopods along with a cross section (right). Note that shape and length of tentacles are speculative.]]

Although ammonites do occur in exceptional [[lagerstatten]] such as the [[Solnhofen Limestone]], their soft-part record is surprisingly sparse. Beyond a tentative ink sac and possible digestive organs, no soft parts were known until 2021.<ref name="Wippich2004">{{Cite journal |last1=Wippich |first1=M. G. E. |last2=Lehmann |first2=J. |year=2004 |title=''Allocrioceras'' from the Cenomanian (mid-Cretaceous) of the Lebanon and its bearing on the palaeobiological interpretation of heteromorphic ammonites |journal=Palaeontology |volume=47 |issue=5 |pages=1093–1107 |bibcode=2004Palgy..47.1093W |doi=10.1111/j.0031-0239.2004.00408.x |doi-access=free}}</ref><ref name="Klug et al 2021">{{cite journal |last1=Klug |first1=Christian |last2=Schweigert |first2=Günter |last3=Tischlinger |first3=Helmut |last4=Pochmann |first4=Helmut |date=December 2021 |title=Failed prey or peculiar necrolysis? Isolated ammonite soft body from the Late Jurassic of Eichstätt (Germany) with complete digestive tract and male reproductive organs |journal=[[Swiss Journal of Palaeontology]] |volume=140 |issue=1 |pages=3 |bibcode=2021SwJP..140....3K |doi=10.1186/s13358-020-00215-7 |pmc=7813712 |pmid=33505352 |doi-access=free}}</ref> In this year an isolated specimen showing some of the internal soft anatomy including organs was described.<ref name="Klug et al 2021" /> When [[neutron imaging]] was used on a fossil found in 1998, part of the musculature became visible and showed they were able to retract themselves into the shell for protection, and that the retractor muscles and hyponome that work together to enable jet propulsion in nautilus worked independently in ammonites.<ref>{{Cite web |title=Neutron imaging reveals never-before-seen 3D muscle structure in rare Jurassic ammonite fossil |url=https://www.isis.stfc.ac.uk/Pages/SH21_Ammonites.aspx |publisher=Science and Technology Facilities Council}}</ref><ref>{{Cite news |last=Begum |first=Tammana |date=7 December 2021 |title=Exceptionally preserved ammonite shows its inner soft tissue in 3D |url=https://www.nhm.ac.uk/discover/news/2021/december/new-soft-tissue-analyses-show-how-ammonites-lived-in-jurassic-oc.html |publisher=[[Natural History Museum, London|Natural History Museum]]}}</ref> 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.<ref>{{cite web |title=Introduction to Ammonoidea |url=http://www.bbm.me.uk/portsdown/PH_232_Ammonites.htm |url-status=dead |archive-url=https://web.archive.org/web/20070502202153/http://www.bbm.me.uk/portsdown/PH_232_Ammonites.htm |archive-date=2 May 2007 |access-date=2007-04-26 |work=The Geology of Portsdown Hill}}</ref> The reproductive organs show possible traces of spermatophores, which would support the hypothesis that the microconchs were males.<ref>{{cite journal |last1=Klug |first1=Christian |last2=Schweigert |first2=Günter |last3=Tischlinger |first3=Helmut |last4=Pochmann |first4=Helmut |date=2021 |title=Failed prey or peculiar necrolysis? Isolated ammonite soft body from the Late Jurassic of Eichstätt (Germany) with complete digestive tract and male reproductive organs |journal=[[Swiss Journal of Palaeontology]] |volume=140 |issue=1 |page=3 |bibcode=2021SwJP..140....3K |doi=10.1186/s13358-020-00215-7 |pmc=7813712 |pmid=33505352 |doi-access=free}}</ref> They likely bore a [[radula]] and [[Cephalopod beak|beak]], and marginal siphuncle.<ref name="Landman1996">{{cite book |last1=Landman |first1=Neil H |url=https://books.google.com/books?id=hKbkB4MzUIkC&pg=PA14 |title=Ammonoid paleobiology |last2=Tanabe |first2=Kazushige |last3=Davis |first3=Richard Arnold |publisher=Springer |year=1996 |isbn=978-0-306-45222-2}}</ref> They operated by direct development with sexual reproduction, were carnivorous, and had a [[Crop (anatomy)|crop]] for food storage. They are unlikely to have dwelt in fresh or brackish water.<ref name="Landman1996" />  Many ammonites were likely [[filter feeder]]s, so adaptations associated with this lifestyle like sieves probably occurred.<ref name="Kruta2011">{{cite journal |last1=Kruta |first1=Isabelle |last2=Landman |first2=Neil |last3=Rouget |first3=Isabelle |last4=Cecca |first4=Fabrizio |last5=Tafforeau |first5=Paul |date=Jan 2011 |title=The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation |journal=Science |volume=331 |issue=6013 |pages=70–72 |bibcode=2011Sci...331...70K |doi=10.1126/science.1198793 |pmid=21212354 |s2cid=206530342}}</ref>[[File:Reconstruction-of-two-Late-Volgian-ammonites-from-Craspeditidae-family-Kachpurites.png|thumb|Speculative life restorations of the [[Craspeditidae|craspeditid]] ammonites ''[[Garniericeras |Garniericeras catenulatum]]'' (left) and ''[[Kachpurites|Kachpurites fulgens]]'' (right)|320x320px]][[File:Rhaeboceras reconstruction.png|thumb|Life restoratios of ''[[Rhaeboceras]],'' which had a pair of tentacles tipped with hooks, largely uniquely among ammonites]]A 2021 study reported specimens of the [[Scaphitidae|scaphitid]] ammonite genera ''[[Rhaeboceras]]'' and ''[[Hoploscaphites]]'' with mineralised hooks, which were likely present on the ends of a pair of enlarged tentacles. However, these mineralised hooks appear to be present only in scaphitids and were not typical of ammonites as a whole.<ref>{{cite journal |last1=Smith |first1=C. P. A. |last2=Landman |first2=N. H. |last3=Bardin |first3=J. |last4=Kruta |first4=I. |date=4 June 2021 |title=New evidence from exceptionally "well-preserved" specimens sheds light on the structure of the ammonite brachial crown |journal=Scientific Reports |volume=11 |issue=1 |pages=11862 |bibcode=2021NatSR..1111862S |doi=10.1038/s41598-021-89998-4 |pmc=8178333 |pmid=34088905}}</ref> The number of arms has been subject considerable speculation, with different artists either opting for a nautilus-like restoration with many arms, or a more squid-like restoration with much fewer arms, with a 1996 study suggesting that they probably had 10 arms like modern squid, cuttlefish and octopuses, but that nothing could be said for certain.<ref name=":5">{{Cite journal |last=Allmon |first=Warren D. |date=2017-01-01 |title=Life-restorations of ammonites and the challenges of taxonomic uniformitarianism |url=https://meridian.allenpress.com/esh/article/36/1/1/205099/Liferestorations-of-ammonites-and-the-challenges |journal=Earth Sciences History |language=en |volume=36 |issue=1 |pages=1–29 |doi=10.17704/1944-6178-36.1.1 |issn=0736-623X}}</ref> Paleontologist [[Mark Witton]] has stated that "The basic details of ammonite life appearance are far from clear . . . While we can be certain that a squid-like organism lived in the last chamber of their shells . . . little else can be said with certainty about their appearance. ... Despite being creatures which occur so commonly as fossils that it seems like we should know everything about them, ammonites are creatures fraught with uncertainty for artists and palaeontologists alike. Until new data comes to light, all life reconstructions of ammonites should be taken as extremely tentative, almost speculative renditions of their actual appearance."<ref name=":5" />

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 [[benthic zone|bottom-dwelling]] life is found. In general, they appear to have inhabited the upper {{Convert|250|m}} of the water column.<ref>{{cite journal |last1=Lemanis |first1=R. |date=2020 |title=The ammonite septum is not an adaptation to deep water: Re-evaluating a centuries-old idea |journal=Proceedings of the Royal Society B: Biological Sciences |volume=287 |issue=1936 |doi=10.1098/rspb.2020.1919 |pmc=7657852 |pmid=33049174}}</ref> Many of them (such as ''[[Oxynoticeras]]'') are thought to have been good swimmers, with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were likely to have been slow-swimming bottom-dwellers. [[Synchrotron]] analysis of an [[aptychus|aptychophoran]] ammonite revealed remains of [[isopod]] and [[mollusc]] larvae in its buccal cavity, indicating at least this kind of ammonite fed on [[plankton]].<ref name="Kruta2011" /> They may have avoided predation by squirting [[Cephalopod ink|ink]], much like modern cephalopods; ink is occasionally preserved in fossil specimens.<ref>{{Cite book |author=Doguzhaeva, Larisa A. |url=https://archive.org/details/cephalopodsprese00land |title=Cephalopods Present and Past: New Insights and Fresh Perspectives |author2=Royal H. Mapes |author3=Herbert Summesberger |author4=Harry Mutvei |publisher=Springer |year=2007 |isbn=978-1-4020-6806-5 |editor=N. H. Landman |location=Dordrecht |pages=[https://archive.org/details/cephalopodsprese00land/page/n234 221]–238 |chapter=The Preservation of Body Tissues, Shell, and Mandibles in the Ceratitid Ammonoid ''Austrotrachyceras'' (Late Triassic), Austria |doi=10.1007/978-1-4020-6806-5_11 |display-editors=etal |url-access=limited}}</ref>

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 seep]]s and even reproduced there.<ref>{{cite journal |last1=Rowe |first1=Alison J. |last2=Landman |first2=Neil H. |last3=Cochran |first3=J. Kirk |last4=Witts |first4=James D. |last5=Garb |first5=Matthew P. |date=26 March 2020 |title=Late Cretaceous Methane Seeps as Habitats for Newly Hatched Ammonites |journal=PALAIOS |volume=35 |issue=3 |pages=151–163 |bibcode=2020Palai..35..151R |doi=10.2110/palo.2019.105 |s2cid=214718487}}</ref>

==Size==
{{further|Cephalopod size}}
[[File:Parapuzosia seppenradensis 3.jpg|thumb|Fossil of ''[[Parapuzosia seppenradensis]],'' one of the largest known ammonites]]
The smallest ammonoid was ''[[Maximites]]'' from the [[Upper Carboniferous]]. Adult specimens reached only {{cvt|10|mm|in}} in shell diameter.<ref>{{cite web|author=Nishiguchi, M.K.|author2=R. Mapes|date=2008|url=https://bobtailsquid.ink/wp-content/uploads/2020/05/Nish2008_Book_PhylEvolMolusca_Cephlopoda.pdf|title=Cephalopoda|work=University of California Press|pages=163–199|access-date=2022-09-23|archive-date=2022-09-23|archive-url=https://web.archive.org/web/20220923144033/https://bobtailsquid.ink/wp-content/uploads/2020/05/Nish2008_Book_PhylEvolMolusca_Cephlopoda.pdf|url-status=live}}</ref> Few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size exceeding {{cvt|23|cm|in}} in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as ''[[Titanites]]'' from the Portland Stone of Jurassic of southern England, which is often {{cvt|53|cm|ft}} in diameter, and ''[[Parapuzosia seppenradensis]]'' of the Cretaceous period of Germany, which is one of the largest-known ammonites, sometimes reaching {{cvt|2|m|ft}} in diameter. The largest-documented [[North America]]n ammonite is ''[[Parapuzosia bradyi]]'' from the Cretaceous, with specimens measuring {{cvt|137|cm|ft|1}} in diameter.
[[File:Ammonoidea Iran.JPG|thumb|An ammonoid from [[Iran]]|left]]

==Evolutionary history==
Ammonoids are widely thought to have originated from straight-shelled ([[orthocone]]) "nautiloids" belong to [[Bactritida]] during the early [[Devonian]] ([[Emsian]]), with transitional fossils showing the transition from a straight shell, to a curved (cyrtoconic) shell to a relaxed (gyroconic) spiral and finally to a tight spiral.<ref>Klug, C. and Korn, D. 2004. [https://www.app.pan.pl/archive/published/app49/app49-235.pdf The origin of ammonoid locomotion]. Acta Palaeontologica Polonica 49 (2): 235–242</ref> The [[Kellwasser event|Kellwasser Event]] at the end of the [[Frasnian]] let to a dramatic decline in ammonoid diversity, with only a handful of lineages belong to [[Tornoceratina]] (a subgroup of [[Goniatites bohemicus|Goniatites]]) surviving, becoming ancestral to all later ammonoids. Ammonoids rediversified during the following [[Famennian]], which also saw the radical shift of the [[siphuncle]] from a lower (ventral) to upper (dorsal) position. Ammonites were nearly completely exterminated by the [[Hangenberg Event]] at the end of the Devonian, with only a handful of lineages surviving, with one of the surviving goniatite lineages becoming ancestral to all post-early Carboniferous and later ammonoids. Ammonoids again rediversified during the Early Carboniferous. During the Carboniferous ammonoids underwent alternating periods of diversification and decline, and during the late Carboniferous ammonioid diversity became concentrated in a few geographical regions.<ref name=":6">{{Citation |last=Korn |first=Dieter |title=Taxonomic Diversity and Morphological Disparity of Paleozoic Ammonoids |date=2015 |work=Ammonoid Paleobiology: From macroevolution to paleogeography |volume=44 |pages=431–464 |editor-last=Klug |editor-first=Christian |url=https://link.springer.com/10.1007/978-94-017-9633-0_16 |access-date=2025-01-11 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-017-9633-0_16 |isbn=978-94-017-9632-3 |last2=Klug |first2=Christian |last3=Walton |first3=Sonny A. |editor2-last=Korn |editor2-first=Dieter |editor3-last=De Baets |editor3-first=Kenneth |editor4-last=Kruta |editor4-first=Isabelle}}</ref>

During the Permian, the [[Capitanian mass extinction event]] severely reduced the diversity of Goniatitida and [[Prolecanitida]],<ref name=":6" /> while the [[Ceratitida]], which originated during the Middle Permian, likely from the [[Daraelitidae]],<ref name=":0" /> was largely unaffected<ref name=":6" /> and radiated in the Late Permian,<ref name=":0">{{cite journal |last1=McGowan |first1=Alistair J. |last2=Smith |first2=Andrew B. |date=May 2007 |title=Ammonoids Across the Permian/Triassic Boundary: A Cladistic Perspective |journal=Palaeontology |volume=50 |issue=3 |pages=573–590 |bibcode=2007Palgy..50..573M |doi=10.1111/j.1475-4983.2007.00653.x |doi-access=free}}</ref> becoming the dominant group of ammonoids in this period represented by two groups, the [[Araxoceratidae|araxoceratids]] and [[Xenodiscidae|xenodiscids]]. The [[end-Permian mass extinction]] again reduced ammonoids to the verge of extinction, though both main ceratitd lineages lineages survived, though the xenodiscids were more successful and ancestral to all later ammonoids.<ref name=":6" />

Ammonites were devastated by the [[Triassic–Jurassic extinction event|end-Triassic extinction]], with only a handful of genera belonging to the family [[Psiloceratidae]] of the suborder [[Phylloceratina]] surviving and becoming ancestral to all later Jurassic and Cretaceous ammonites. Ammonites explosively diversified during the Early Jurassic, with the orders Psiloceratina, Ammonitina, Lytoceratina, Haploceratina, Perisphinctina and Ancyloceratina all appearing during the Jurassic.<ref>{{cite journal |last1=Page |first1=Kevin N. |title=The evolution and geography of Jurassic ammonoids |journal=Proceedings of the Geologists' Association |date=January 2008 |volume=119 |issue=1 |pages=35–57 |doi=10.1016/S0016-7878(08)80257-X |bibcode=2008PrGA..119...35P }}</ref> Heteromorph ammonites (ammonites with open or non-spiral coiling) of the order [[Ancyloceratina]] became common during the [[Cretaceous]] period.<ref>{{cite journal |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 |title=Recent advances in heteromorph ammonoid palaeobiology |journal=Biological Reviews |date=April 2021 |volume=96 |issue=2 |pages=576–610 |doi=10.1111/brv.12669 |pmid=33438316 |s2cid=231593832 |doi-access=free }}</ref>

At least 57 species of ammonites, which were widespread and belonged to six superfamilies, were extant during the last 500,000 years of the Cretaceous, indicating that ammonites remained highly diverse until the very end of their existence.<ref name="Ending diversity">{{Citation|last1=Landman|first1=Neil H.|title=Ammonites on the Brink of Extinction: Diversity, Abundance, and Ecology of the Order Ammonoidea at the Cretaceous/Paleogene (K/Pg) Boundary|date=2015|url=http://link.springer.com/10.1007/978-94-017-9633-0_19|work=Ammonoid Paleobiology: From macroevolution to paleogeography|volume=44|pages=497–553|editor-last=Klug|editor-first=Christian|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-017-9633-0_19|isbn=978-94-017-9632-3|access-date=2021-10-26|last2=Goolaerts|first2=Stijn|last3=Jagt|first3=John W.M.|last4=Jagt-Yazykova|first4=Elena A.|last5=Machalski|first5=Marcin|series=Topics in Geobiology |editor2-last=Korn|editor2-first=Dieter|editor3-last=De Baets|editor3-first=Kenneth|editor4-last=Kruta|editor4-first=Isabelle|archive-date=2023-03-16|archive-url=https://web.archive.org/web/20230316184812/https://link.springer.com/chapter/10.1007/978-94-017-9633-0_19|url-status=live}}</ref> All ammonites were wiped out during or shortly after the [[K-Pg extinction event]], caused by the [[Chicxulub impact]]. It has been suggested that [[ocean acidification]] generated by the impact played a key role in their extinction, as the larvae of ammonites were likely small and [[plankton]]ic, and would have been heavily affected.<ref name=":2">{{Citation |last1=Landman |first1=Neil H. |title=Ammonites on the Brink of Extinction: Diversity, Abundance, and Ecology of the Order Ammonoidea at the Cretaceous/Paleogene (K/Pg) Boundary |date=2015 |work=Ammonoid Paleobiology: From macroevolution to paleogeography |volume=44 |pages=497–553 |editor-last=Klug |editor-first=Christian |url=http://link.springer.com/10.1007/978-94-017-9633-0_19 |access-date=2021-10-26 |archive-url=https://web.archive.org/web/20230316184812/https://link.springer.com/chapter/10.1007/978-94-017-9633-0_19 |archive-date=2023-03-16 |url-status=live |series=Topics in Geobiology |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-017-9633-0_19 |isbn=978-94-017-9632-3 |last2=Goolaerts |first2=Stijn |last3=Jagt |first3=John W.M. |last4=Jagt-Yazykova |first4=Elena A. |last5=Machalski |first5=Marcin |editor2-last=Korn |editor2-first=Dieter |editor3-last=De Baets |editor3-first=Kenneth |editor4-last=Kruta |editor4-first=Isabelle}}</ref> [[Nautiloid]]s, exemplified by modern [[nautilus]]es, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a [[bolide]] strike, and thus survived.<ref>{{cite book | last=Ward | first=Peter | chapter=Ammonoid Extinction | title=Ammonoid Paleobiology | publisher=Springer | series=Topics in Geobiology | volume=13 | year=1996 | pages=815–823 | doi=10.1007/978-1-4757-9153-2_20 | isbn=978-1-4757-9155-6}}</ref> Many ammonite species were filter feeders, so they might have been particularly susceptible to marine faunal turnovers and climatic change.<ref name="Kruta2011" /> Some reports suggest that a few ammonite species may have [[Paleocene ammonites|persisted]] into the very early Danian stage of the [[Paleocene]], before going extinct.<ref>{{Cite journal |last1=Machalski |first1=Marcin |last2=Heinberg |first2=Claus |date=2005-12-31 |title=Evidence for ammonite survival into the Danian (Paleogene) from the Cerithium Limestone at Stevns Klint, Denmark |url=https://2dgf.dk/publikationer/bulletin/bulletin-volume-52-2005/#13 |journal=Bulletin of the Geological Society of Denmark |language=en |volume=52 |pages=97–111 |doi=10.37570/bgsd-2005-52-08 |issn=2245-7070 |doi-access=free |access-date=2023-01-08 |archive-date=2021-11-27 |archive-url=https://web.archive.org/web/20211127052236/https://2dgf.dk/publikationer/bulletin/bulletin-volume-52-2005/#13 |url-status=live }}</ref><ref>{{Cite journal |last1=Landman |first1=Neil H. |last2=Garb |first2=Matthew P. |last3=Rovelli |first3=Remy |last4=Ebel |first4=Denton S. |last5=Edwards |first5=Lucy E. |date=2012 |title=Short-Term Survival of Ammonites in New Jersey After the End-Cretaceous Bolide Impact |url=http://www.app.pan.pl/article/item/app20110068.html |journal=Acta Palaeontologica Polonica |language=en |volume=57 |issue=4 |pages=703–715 |doi=10.4202/app.2011.0068 |issn=0567-7920 |doi-access=free |access-date=2023-01-08 |archive-date=2023-01-07 |archive-url=https://web.archive.org/web/20230107161245/https://www.app.pan.pl/article/item/app20110068.html |url-status=live }}</ref>

==Cultural significance==
{{see also|Ammolite}}
In medieval Europe, fossilised ammonites were thought to be [[petrified]] coiled [[snakes]], and were called "snakestones" or, more commonly in medieval England, "serpentstones". They were considered to be evidence for the actions of saints, such as [[Hilda of Whitby]], a myth referenced in Sir [[Walter Scott]]'s ''[[Marmion (poem)|Marmion]]'',<ref name="Lovett">{{cite journal | url=http://en.wikisource.org/wiki/Folk-Lore._Volume_16/The_Whitby_Snake-Ammonite_Myth | title=The Whitby Snake-Ammonite Myth | author=Lovett, Edward | journal=Folk-Lore |date=September 1905 | volume=16 | issue=3 | pages=333–4 | author-link=Edward Lovett | doi=10.1080/0015587x.1905.9719966}}</ref> and [[Saint Patrick]], and were held to have [[spiritual healing|healing]] or [[divination|oracular]] powers. Traders would occasionally carve the head of a snake onto the empty, wide end of the ammonite fossil, and then sell them as petrified snakes. In other cases, the snake's head would be simply painted on.<ref>{{Cite book |last=Cadbury |first=D. |title=The Dinosaur Hunters |publisher=Fourth Estate |year=2000 |isbn=1-85702-963-1 |page=7}}</ref><ref>{{Cite book|url=https://archive.org/details/folklore16folkuoft/page/333/mode/1up|title=Folklore |volume=16 |page=333|date=1905}}</ref>

Others believed ammonites, which they referred to as "salagrana" were composed of [[Coprolite|fossilized worm dung]], and could be used to ward off witches.<ref>{{Cite book |last=Leland |first=Charles Godfrey |url=https://books.google.com/books?id=eGh884SrW3kC&dq=for+it+is+the+dung+of+the+animals+called+ronbrigoli+%28lombrici%2C+or+earthworms%29%2C+which+only+eat+earth+and+throw+up+little+hills+which+take+the+form+of+a+stone%2C+or+rather+of+a+sponge%2C+which+petrify.&pg=PA336 |title=Etruscan Roman Remains in Popular Tradition |date=1892 |publisher=T. F. Unwin |language=en}}</ref>

Ammonites from the [[Gandaki River]] in [[Nepal]] are known as [[Shaligram]]s, and are believed by [[Hindu]]s to be a concrete manifestation of [[Vishnu]].<ref>{{cite news |url=http://news.independent.co.uk/sci_tech/article2259490.ece |title=Fossils: myths, mystery, and magic |work=[[The Independent]] |date=2007-02-12 |access-date=2010-04-23 |url-status=dead |archive-url=https://web.archive.org/web/20071111061251/http://news.independent.co.uk/sci_tech/article2259490.ece |archive-date=2007-11-11 }}</ref>

The [[Ammonite order]], developed by [[George Dance the Younger]], is an [[Classical order|architectural order]] featuring ammonite-shaped [[volute]]s that has seen a handful of uses in [[Architecture of the United Kingdom|British architecture]].<ref>{{cite book|last1=Dziekan|first1=Vince|title=Virtuality and the Art of Exhibition: Curatorial Design for the Multimedial Museum|date=2012|publisher=Intellect Books|isbn=9781841504766|page=178|url=https://books.google.com/books?id=DMc9UrW1gDIC&q=column+-+Ammonite+Order&pg=PA178|access-date=2 August 2016}}</ref><ref name="oxford22">{{cite book|last1=Curl|first1=James Stevens|last2=Wilson|first2=Susan|title=Oxford Dictionary of Architecture|date=2016|publisher=Oxford University Press|isbn=978-0-19-967499-2|page=22}}</ref>

==See also==

* [[Belemnoidea]]
* [[Coleoidea]]
* [[Geologic time scale]]

==References==
{{reflist|2}}

==Further reading==
* {{cite book |last1=Larson |first1=Neal L. |title=Ammonites and the Other Cephalopods of the Pierre Seaway: Identification Guide |date=1997 |publisher=Geoscience Press |isbn=978-0-945005-25-4 }}
* {{cite book |last1=Lehmann |first1=Ulrich |title=The Ammonites: Their life and their world |date=1981 |publisher=Cambridge University Press |isbn=978-0-521-23627-0 }}
* {{cite book |last1=Monks |first1=Neale |last2=Palmer |first2=Philip |title=Ammonites |date=2002 |publisher=Smithsonian Institution Press |isbn=978-1-58834-024-5 }}
* [[Cyril Walker (paleontologist)|Walker, Cyril]] and [[David Ward (paleontologist)|Ward, David]]. ''Fossils''. Dorling, Kindersley Limited, London, 2002.
* [https://web.archive.org/web/20050307100033/http://is.dal.ca/~ceph/TCP/evolution.html A Broad Brush History of the Cephalopoda] by Dr. Neale Monks, from The Cephalopod Page.
* [https://web.archive.org/web/20050405181106/http://is.dal.ca/~ceph/TCP/ammonage.html Ammonite maturity, pathology and old age] By Dr. Neale Monks, from The Cephalopod Page. Essay about the life span of Ammonites.
* [https://web.archive.org/web/20051201091554/http://www.cretaceousfossils.com/invertebrates/ammonites/ammonites_index.htm Cretaceous Fossils Taxonomic Index for Order Ammonoitida]
* [https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=100280&org=NSF Deeply Buried Sediments Tell Story of Sudden Mass Extinction]

==External links==
{{wikispecies|Ammonoidea}}
{{Commons category|position=right|Ammonoidea|<br>Ammonoidea<br>(Ammonites)}}
* [http://www.fossilmuseum.net/Fossil_Galleries/Ammonites.htm Descriptions and pictures of ammonite fossils]
* [http://www.goniat.org/ goniat.org, a palaezoic ammonoid database]
* [https://web.archive.org/web/20060404024923/http://paleozoic.org/gallery.htm paleozoic.org: gallery of ammonite photographs]
* [https://web.archive.org/web/20170306235239/http://y2u.co.uk/%26002_Images/Lyme%20Regis%20Fossils%20%2001.htm photos of ammonites at Lyme Regis, UK]
* [http://taxonconcept.stratigraphy.net/source_main.php?doctaxid=618&doctaxname=Ammonoidea TaxonConcept's data on cretaceous ammonites]
* [http://thinkingwithpictures.blogspot.com/2008/06/ammonites-of-peacehaven.html The ammonites of Peacehaven - photos of giant cretaceous ammonites in Southern England]
* [http://www.tonmo.com/science/fossils/fossilsjump.php tonmo.com: The octopus news magazine online], Cephalopod fossil articles.
* William R. Wahl * [https://web.archive.org/web/20130509085058/http://www.wyodino.org/wp-content/uploads/2011/01/BittenAmmonite.pdf Mosasaur Bite Marks on an Ammonite. Preservation of an Aborted Attack? ]
* [http://palaeoart.blogspot.it/2011/10/mosasaur-diets-everhart-2004.html Mosasaur diet]

{{fossil cephalopods}}

{{taxonbar|from=Q228002}}
{{Authority control}}

[[Category:Ammonites| ]]
[[Category:Mollusc subclasses]]
[[Category:Devonian first appearances]]
[[Category:Paleocene extinctions]]
[[Category:Cephalopods described in 1884]]
[[Category:Taxa named by Karl Alfred Ritter von Zittel]]
[[Category:Neocephalopoda]]