Editing Nautiloid

{{Short description|Extant subclass of cephalopods}}
{{Automatic taxobox
| name = Nautiloids
| fossil_range = {{fossil range|495|0}}[[Late Cambrian]] – Recent
| image = Nautilus-JB-01.jpg
| image_caption = ''[[Nautilus pompilius]]''
| image2 = Cameroceras trentonese.jpg
| image2_caption = the [[Endocerida|endocerid]] ''[[Cameroceras]]'', shown feeding on the [[Tarphycerida|tarphycerid]] ''[[Aphetoceras]]'', while a quartet of [[Ellesmerocerida|ellesmerocerid]] ''[[Cyclostomiceratidae|Cyclostomiceras]]'' swim by.
| taxon = Nautiloidea
| authority = [[Louis Agassiz|Agassiz]], 1847
| subdivision_ranks = Major subdivisions
| subdivision = *†[[Plectronoceratoidea]] <small>([[paraphyletic]])</small>
*†[[Endoceratoidea]]
*Order [[Nautilida]]
*†[[Multiceratoidea]] <small>( possibly paraphyletic)</small>
*†[[Orthoceratoidea]] <small>(paraphyletic)</small>
}}

'''Nautiloids''' are a group of [[cephalopod]]s ([[Mollusca]]) which originated in the [[Late Cambrian]] and are represented today by the living ''[[Nautilus (genus)|Nautilus]]'' and ''[[Allonautilus]]''. [[Fossil]] nautiloids are diverse and species rich, with over 2,500 recorded species. They flourished during the early [[Paleozoic]] era, when they constituted the main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms ([[orthocone]]s). No orthoconic and only a handful of coiled species, the [[nautilus]]es, survive to the present day.

In a broad sense, "nautiloid" refers to a major cephalopod [[Subclass (biology)|subclass]] or collection of subclasses ('''Nautiloidea ''sensu lato'''''). Nautiloids are typically considered one of three main groups of cephalopods, along with the extinct [[Ammonoidea|ammonoids]] (ammonites) and living [[Coleoidea|coleoids]] (such as [[squid]], [[octopus]], and kin). While ammonoids and coleoids are [[Monophyly|monophyletic]] [[clade]]s with exclusive ancestor-descendant relationships, this is not the case for nautiloids. Instead, nautiloids are a [[Paraphyly|paraphyletic]] [[Evolutionary grade|grade]] of various early-diverging cephalopod lineages, including the ancestors of ammonoids and coleoids. Some authors prefer a narrower definition of Nautiloidea ('''Nautiloidea ''sensu stricto'''''), as a singular subclass including only those cephalopods which are closer to living nautiluses than they are to either ammonoids or coleoids.

==Taxonomic relationships==
Nautiloids are among the group of animals known as [[cephalopoda|cephalopods]], an advanced class of [[Mollusca|mollusks]] which also includes [[Ammonoidea|ammonoids]], [[Belemnoidea|belemnites]] and modern [[coleoidea|coleoids]] such as octopus and squid. Other mollusks include [[Gastropoda|gastropods]], [[Scaphopoda|scaphopods]] and [[Bivalvia|bivalves]].

Traditionally, the most common classification of the cephalopods has been a four-fold division (by Bather, 1888), into the [[Orthoceratoidea|orthoceratoids]], nautiloids, [[ammonoid]]s, and [[coleoid]]s. This article is about nautiloids in that broad sense, sometimes called Nautiloidea ''sensu lato''.

[[Cladistics|Cladistically]] speaking, nautiloids are a [[paraphyletic]] assemblage united by shared primitive ([[Plesiomorphy|plesiomorphic]]) features not found in derived cephalopods. In other words, they are a [[Evolutionary grade|grade]] group that is thought to have given rise to orthoceratoids, ammonoids and coleoids, and are defined by the exclusion of those descendent groups. Both ammonoids and coleoids have traditionally been assumed to have descended from [[Bactritida|bactritids]], which in turn arose from [[orthocone|straight-shelled]] [[Orthoceratoidea|orthoceratoids]]. The ammonoids appeared early in the [[Devonian]] period (some 400 million years ago) and became abundant in the [[Mesozoic]] [[Geologic time scale#Divisions of geologic time|era]], before their extinction at the end of the Cretaceous.

Some workers apply the name Nautiloidea to a more exclusive group, called Nautiloidea ''sensu stricto''. This taxon consists only of those orders that are clearly related to the modern nautilus to the exclusion of other modern cephalopods. In this restricted definition, membership is somewhat variable between authors, but it usually includes Tarphycerida, Oncocerida, and Nautilida.

==Shell==
[[File:Nautilus pompilius - Fernbank Museum of Natural History - DSC00294.JPG|left|thumb|250x250px|A cross-section of a ''[[Nautilus Pompilius|Nautilus pompilius]]'' shell, showing the large body chamber, shrinking camerae, concave septa, and septal necks (partial siphuncle supports)]]
All nautiloids have a large external shell, divided into a narrowing chambered region (the [[phragmocone]]) and a broad, open [[body chamber]] occupied by the animal in life. The outer wall of the shell, also known as the conch, defines its overall shape and texture. The chambers ([[camera (cephalopod)|camera]]e) of the phragmocone are separated from each other by thin curved walls ([[Septum (cephalopod)|septa]]), which formed during growth spurts of the animal. During a growth spurt, the rear of the [[Mantle (mollusc)|mantle]] secretes a new septum, adding another chamber to the series of shell chambers. At the same time, shell material is added around the shell opening ([[Aperture (mollusc)|aperture]]), enlarging the body chamber and providing more room for the growing animal. Sutures (or suture lines) appear where each septum contacts the wall of the outer shell. In life, they are visible as a series of narrow wavy lines on the outer surface of the shell. Like their underlying septa, the sutures of the nautiloids are simple in shape, being either straight or slightly curved. This is different from the "zigzag" sutures of the goniatites and the highly complex sutures of the ammonites.

The septa are perforated by the [[siphuncle]], a fleshy tube which runs through each of the internal chambers of the shell. Surrounding the fleshy tube of the siphuncle are structures made of [[aragonite]] (a polymorph of calcium carbonate – which during fossilisation is often recrystallized to calcite, a more stable form of calcium carbonate [CaCO<sub>3</sub>]): septal necks and connecting rings. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers (called ''cameral deposits'') or within the siphuncle (''endosiphuncular deposits''), a process which may have been connected with controlling [[buoyancy]]. The nature of the siphuncle and its position within the shell are important in classifying nautiloids and can help distinguish them from ammonoids. The siphuncle is on the shell periphery in most ammonoids whereas it runs through the center of the chambers in some nautiloids, including living nautiluses.<ref>{{cite web|url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php|title=The Cephalopoda|publisher=The University of California Museum of Paleontology|access-date=2014-05-11}}</ref>

The subclass Nautiloidea, in its broader definition, is distinguished from other cephalopods by two main characteristics: the [[Septum (cephalopod)|septa]] are smoothly concave in the forward direction, producing external sutures which are generally simple and smooth. The siphuncle is supported by septal necks which point to the rear (i.e. retrosiphonate) throughout the [[ontogeny]] of the animal.

Modern nautiluses have deeply coiled shells which are involute, meaning that the larger and more recent [[Whorl (mollusc)|whorls]] overlap and obscure older whorls. The shells of fossil nautiloids may be either straight (i.e., [[orthocone|orthoconic]] as in ''[[Orthoceras]]'' and ''[[Rayonnoceras]]''), curved (as in ''[[Cyrtoceras]]'') coiled (as in ''[[Cenoceras]]''), or rarely a helical coil (as in ''[[Lorieroceras]]''). Some species' shells—especially in the late Paleozoic and early Mesozoic—are ornamented with spines and ribs, but most have a smooth shell. The shells are formed of aragonite, although the cameral deposits may consist of primary calcite.<ref>{{cite journal | vauthors = Stehli FG | title = Shell Mineralogy in Paleozoic Invertebrates | journal = Science | volume = 123 | issue = 3206 | pages = 1031–1032 | date = June 1956 | pmid = 17800970 | doi = 10.1126/science.123.3206.1031 | bibcode = 1956Sci...123.1031S | jstor = 1750042 }}</ref> The coloration of the shell of the modern nautilus is quite prominent, and, although somewhat rarely, the shell coloration has been known to be preserved in fossil nautiloids. They often show color patterns only on the [[Dorsum (biology)|dorsal]] side, suggesting that the living animals swam horizontally.

==Modern nautiloids==
{{Main article|Nautilus}}
[[Image:Nautilus belauensis profile.jpg|thumb|250px|''[[Nautilus belauensis]]'']]
Much of what is known about the extinct nautiloids is based on what we know about modern [[nautilus]]es, such as the [[chambered nautilus]], which is found in the southwest [[Pacific Ocean]] from [[Samoa]] to the [[Philippines]], and in the [[Indian Ocean]] off the coast of [[Australia]]. It is not usually found in waters less than {{convert|100|m|ft|0|abbr=off|sp=us}} deep and may be found as far down as {{convert|500|to|700|m|ft|-1|abbr=off|sp=us}}.

Nautili are free swimming animals that possess a head with two simple [[Evolution of the eyes|lens-free eyes]] and arms (or tentacles). They have a smooth shell over a large body chamber, which is divided into subchambers filled with an inert gas (similar to the composition of atmospheric air, but with more [[nitrogen]] and less [[oxygen]]) making the animal neutrally buoyant in the water. As many as 90 [[tentacles]] are arranged in two circles around the mouth. The animal is predatory, and has jaws which are horny and beak-like, allowing it to feed on [[crustacean]]s.

Empty nautilus shells may drift a considerable distance and have been reported from [[Japan]], [[India]] and [[Africa]]. Undoubtedly the same applies to the shells of [[fossil]] nautiloids, the gas inside the shell keeping it buoyant for some time after the animal's death, allowing the empty shell to be carried some distance from where the animal lived before finally sinking to the seafloor.

Nautili propel themselves by jet propulsion, expelling water from an elongated funnel called the [[hyponome]], which can be pointed in different directions to control their movement. Unlike the [[Belemnitida|belemnites]] and other cephalopods, modern nautili do not have an ink sac, and there is no evidence to suggest that the extinct forms possessed one either. Furthermore, unlike the extinct [[ammonoids]], the modern nautilus lacks an [[aptychus]], a biomineralized plate which is proposed to act as an operculum which closes the shell to protect the body. However, aptychus-like plates are known from some extinct nautiloids, and they may be homologous to the fleshy hood of a modern nautilus.<ref>{{cite journal | vauthors = Stridsberg S |date=1984 |title=Aptychopsid plates - jaw elements or protective operculum |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1984.tb00670.x |journal=Lethaia |language=en |volume=17 |issue=1 |pages=93–98 |doi=10.1111/j.1502-3931.1984.tb00670.x |bibcode=1984Letha..17...93S |issn=0024-1164|url-access=subscription }}</ref>

==Fossil record==
[[Image:Nautiloid trilacinoceras.jpg|thumb|Fossil nautiloid ''[[Trilacinoceras]]'' from the [[Ordovician]] of [[China]].]]
[[Image:OrdNautiloidInternalMold.jpg|thumb|Fossil orthoconic nautiloid from the [[Ordovician]] of [[Kentucky]]; an internal mold showing siphuncle and half-filled camerae, both encrusted.]]
[[Image:OrhtocerasNautiloid092313.jpg|thumb|Cross-section of an ''[[Orthoceras]]'' nautiloid from the Siluro-Devonian of Erfoud, Morocco.]]
Nautiloids are often found as [[fossil]]s in early [[Palaeozoic]] rocks (less so in more recent strata).

The rocks of the [[Ordovician]] period in the [[Baltic Sea|Baltic coast]] and parts of the United States contain a variety of nautiloid fossils, and specimens such as ''[[Discitoceras]]'' and ''Rayonnoceras'' may be found in the [[limestone]]s of the [[Carboniferous]] period in [[Republic of Ireland|Ireland]]. The marine rocks of the [[Jurassic]] period in [[United Kingdom|Britain]] often yield specimens of ''Cenoceras'', and nautiloids such as ''[[Eutrephoceras]]'' are also found in the Pierre Shale formation of the [[Cretaceous]] period in the north-central United States.

Specimens of the [[Ordovician]] nautiloid ''[[Endoceras]]'' have been recorded measuring up to {{convert|5.7|m|ft|abbr=off|sp=us}} in shell length, and there is a description of a specimen estimated to have reached {{convert|9.1|m|ft|abbr=off|sp=us}}, although that specimen is reported as destroyed.<ref>{{Cite journal |last1=Klug |first1=Christian |last2=De Baets |first2=Kenneth |last3=Kröger |first3=Björn |last4=Bell |first4=Mark A. |last5=Korn |first5=Dieter |last6=Payne |first6=Jonathan L. |date=2015 |title=Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change |url=https://onlinelibrary.wiley.com/doi/10.1111/let.12104 |journal=Lethaia |language=en |volume=48 |issue=2 |pages=267–288 |doi=10.1111/let.12104|bibcode=2015Letha..48..267K |url-access=subscription }}</ref> These large nautiloids would have been formidable predators of other marine animals at the time they lived.

In some localities, such as [[Scandinavia]] and [[Morocco]], the [[fossil]]s of [[orthocone|orthoconic]] nautiloids accumulated in such large numbers that they form limestones composed of nonspecific assemblages known as ''cephalopod beds'', ''cephalopod limestones'', ''nautiloid limestones'', or ''Orthoceras limestones'' in the geological literature. Although the term ''[[Orthoceras]]'' now only refers to a [[Baltic Sea|Baltic coast]] [[Ordovician]] genus, in prior times it was employed as a general name given to all [[orthoconic|straight-shelled]] nautiloids that lived from the Ordovician to the [[Triassic]] periods (but were most common in the early [[Paleozoic]] era).

==Evolutionary history==
Nautiloids are first known from the late Cambrian Fengshan Formation of northeastern [[China]], where they seem to have been quite diverse (at the time this was a warm shallow sea rich in marine life). However, although four orders have been proposed from the 131 [[species]] named, there is no certainty that all of these are valid, and indeed it is likely that these taxa are seriously oversplit.

[[File:Cyrtoceras NT small.jpg|thumb|Reconstruction of ''[[Cyrtoceras|Cyrtoceras sp]]'']]
[[File:Gyronaedyceras eryx.jpg|thumb|''[[Gyronaedyceras|Gyronaedyceras eryx]]'', an oncocerid from the Middle Devonian of Wisconsin]]
[[File:Acleistoceras whitfieldi.jpg|thumb|''[[Acleistoceras|Acleistoceras whitfieldi]]'', an oncocerid from the Middle Devonian of Wisconsin]]
[[File:A coiled cephalopods imprint from Dane county, Wisconsin.jpg|thumb|''[[Trocholites]]'', an [[Tarphycerida|tarphycerid]] from the Upper Ordovician of Wisconsin]]

Most of these early forms died out, but a single family, the [[Ellesmeroceratidae]], survived to the early [[Ordovician]], where it ultimately gave rise to all subsequent cephalopods. In the Early and Middle Ordovician the nautiloids underwent an evolutionary radiation.<ref name="KR2008">{{Cite journal | vauthors = Kroeger B, Landing E | journal = Geological Magazine| volume = 145 | title = Onset of the Ordovician cephalopod radiation – evidence from the Rochdale Formation (middle Early Ordovician, Stairsian) in eastern New York| doi = 10.1017/S0016756808004585| year = 2008 | issue = 4 | pages = 490–520| bibcode = 2008GeoM..145..490K| s2cid = 129441156| url = http://edoc.hu-berlin.de/18452/22517}}</ref><ref name="Kr2009">{{cite journal | vauthors = Kroeger B, Yun-Bai Z | title = Pulsed cephalopod diversification during the Ordovician | journal =  Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 273 | issue = 1–2| pages = 174–201 | year = 2009 | doi = 10.1016/j.palaeo.2008.12.015 | bibcode = 2009PPP...273..174K}}</ref> Some eight new orders appeared at this time, covering a great diversity of shell types and structure, and ecological lifestyles.

Nautiloids remained at the height of their range of adaptations and variety of forms throughout the Ordovician, [[Silurian]], and [[Devonian]] periods, with various straight, curved and coiled shell forms coexisting at the same time. Several of the early orders became extinct over that interval, but others rose to prominence.

Nautiloids began to decline in the Devonian, perhaps due to competition with their descendants and relatives the Ammonoids and [[Coleoid]]s, with only the [[Nautilida]] holding their own (and indeed increasing in diversity). Their shells became increasingly tightly coiled, while both numbers and variety of non-nautilid species continued to decrease throughout the [[Carboniferous]] and [[Permian]].

The massive extinctions at the end of the Permian were less damaging to nautiloids than to other [[taxon|taxa]] and a few groups survived into the early [[Mesozoic]], including [[Pseudorthocerida|pseudorthocerids]], [[Bactritida|bactritids]], nautilids and possibly [[Orthocerida|orthocerids]]. The last straight-shelled forms were long thought to have disappeared at the end of the [[Triassic]], but a possible orthocerid has been found in [[Cretaceous]] rocks. Apart from this exception, only a single nautiloid suborder, the [[Nautilina]], continued throughout the [[Mesozoic]], where they co-existed quite happily with their more specialised ammonoid cousins. Most of these forms differed only slightly from the modern nautilus. They had a brief resurgence in the early [[Tertiary]] (perhaps filling the niches vacated by the ammonoids in the [[Cretaceous–Paleogene extinction event|end Cretaceous extinction]]), and maintained a worldwide distribution up until the middle of the [[Cenozoic]] Era. With the global cooling of the [[Miocene]] and [[Pliocene]], their geographic distribution shrank and these hardy and long-lived animals declined in diversity again. Today there are only six living species, all belonging to two genera, ''[[Nautilus (genus)|Nautilus]]'' (the pearly nautilus), and ''[[Allonautilus]]''.

The recent decrease in the once worldwide distribution of nautiloids is now believed to have been caused by the spread of [[pinniped]]s.<ref name=":1">{{Cite web |date=2022-10-15 |title=How seals made Nautilus a 'Living Fossil' |url=https://journalofbiogeographynews.org/2022/10/15/how-seals-made-nautilus-a-living-fossil/ |url-status=live |archive-url=https://web.archive.org/web/20221023145840/https://journalofbiogeographynews.org/2022/10/15/how-seals-made-nautilus-a-living-fossil/ |archive-date=2022-10-23 |access-date=2023-03-29 |website=Journal of Biogeography |language=en-US}}</ref> From the [[Oligocene]] onward, the appearance of pinnipeds in the geological record of a region coincides with the disappearance of nautiloids from that region.<ref name=":2">{{Cite journal |last1=Kiel |first1=Steffen |last2=Goedert |first2=James L. |last3=Tsai |first3=Cheng-Hsiu |date=2022-09-22 |title=Seals, whales and the Cenozoic decline of nautiloid cephalopods |journal=Journal of Biogeography |language=en |volume=49 |issue=11 |pages=1903–1910 |doi=10.1111/jbi.14488 |s2cid=252576418 |issn=0305-0270 |doi-access=free |bibcode=2022JBiog..49.1903K }}</ref> As a result, nautiloids are now limited to their current distribution in the tropical Indo-Pacific Ocean, where pinnipeds are absent.<ref name=":1" /> The genus ''[[Aturia]]'' seem to have temporarily survive regions where pinnipeds were present through adaptations to fast and agile swimming, but eventually went extinct as well.<ref name=":2" /> Predation by short-snouted whales and the development of [[Oxygen minimum zone|OMZs]], preventing nautiloids from retreating into deeper water, are also cited as other potential causes of extinction.<ref name=":2" />

===Timeline of orders===

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  id:ordovician   value:rgb(0,0.57,0.44)
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  id:devonian   value:rgb(0.8,0.55,0.22)
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 color:paleozoic bar:NAM6 from:-470     till:-323.2 text:[[Oncocerida]]
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</timeline>

==Classification==
{{See also|List of nautiloids}}

=== Older classification systems ===
A consensus on nautiloid classification has traditionally been elusive and subject to change, as different workers emphasize different fundamental traits when reconstructing evolutionary events. The largest and most widely cited publication on nautiloid taxonomy is the [[Treatise on Invertebrate Paleontology]] Part K by Teichert et al. 1964, though new information has rendered this volume outdated and in need of revision. Treatise Part K was based on previous classification schemes by Flower & Kummel (1950) and the Russian ''Osnovy Paleontologii Vol. 5'' (1962) textbook.

Other comprehensive taxonomic schemes have been devised by Wade (1988), Teichert (1988), and Shevyrev (2006). Wade (1988) divided the subclass Nautiloidea (''sensu lato'') into 6 superorders, incorporating orders that are phylogenetically related. They are:
*†[[Plectronoceratoidea]] = †[[Plectronocerida]], †[[Protactinocerida]], †[[Yanhecerida]], and †[[Ellesmerocerida]].
*†[[Endoceratoidea]] = †[[Endocerida]]
*†[[Orthoceratoidea]] = †[[Orthocerida]], †[[Ascocerida]], and †[[Pseudorthocerida]] (the Orthoceratoidea of Kröger 2007)
*[[Nautilitoidea]] = †[[Tarphycerida]], †[[Oncocerida]], and [[Nautilida]].
*†[[Actinoceratoidea]] = †[[Actinocerida]]
*†[[Discosoritoidea]] = †[[Discosorida]]

Three of these superorders were established for orders of uncertain placement: Endocerida, Actinocerida, and Discosorida. The other three unite related orders which share a common ancestor and form a branch of the nautiloid taxonomic tree: Plectronoceratoidea, which consists mostly of small Cambrian forms that include the ancestors of subsequent stocks; Orthoceratoidea, which unites different primarily orthoconic orders (including the ancestors for Bacritida and Ammonoidea); and Nautilitoidea, which includes the first coiled cephalopods, Tarphycerida, as well as Nautilida, which includes the recent ''Nautilus''. Another order, [[Bactritida]], which is derived from [[Orthocerida]], is sometimes included with Nautiloidea, sometimes with [[Ammonoidea]], and sometimes placed in a subclass of its own, [[Bactritoidea]].

Recently some workers in the field have come to recognize Dissidocerida as a distinct order, along with Pseudorthocerida, both previously included in [[Orthocerida]] as subtaxa.

=== Early cladistic efforts ===
[[Cladistics|Cladistic]] approaches are rare in nautiloid systematics. Many nautiloid orders (not to mention the group as a whole) are not [[Monophyly|monophyletic]] [[clade]]s, but rather [[Paraphyly|paraphyletic]] [[Evolutionary grade|grades]]. This means that they include some descendant taxa while excluding others. For example, the paraphyletic order Orthocerida includes numerous orthocerids stretching through the Paleozoic, but it excludes colloids, despite colloids having a well-established ancestry among the orthocerids. Interpretations by Engeser (1996–1998) suggests that nautiloids, and indeed cephalopods in general, should be split into two main clades: '''Palcephalopoda''' (including all the nautiloids except Orthocerida and Ascocerida) and '''[[Neocephalopoda]]''' (the rest of the cephalopods). Palcephalopoda is meant to correspond to groups which are closer to living nautilus, while Neocephalopoda is meant to correspond to groups closer to living coleoids. One issue which this scheme is the necessity of establishing a firm ancestry for nautilus, to contextualize which cephalopods are closer to which of the two living end members. On the basis of morphological traits, Nautilida is most similar to coiled early nautiloids such as the Tarphycerida and Oncocerida. However, these orders diverged from coleoid ancestors in the early Ordovician at the latest, while genetic divergence estimates suggest that Nautilida diverged in the Silurian or Devonian.<ref name=":0" />

A more recent phylogenetic study by Lindgren ''et al.'' (2004), which supports the monophyly of cephalopods, does not bear on the Palcephalopod/Neocephalopod question, since the only cephalopods included were ''Nautilus'' and coleoids.<ref>{{cite journal | vauthors = Lindgren AR, Giribet G, Nishiguchi MK | title = A combined approach to the phylogeny of Cephalopoda (Mollusca) | journal = Cladistics | volume = 20 | issue = 5 | pages = 454–486 | date = October 2004 | pmid = 34892953 | doi = 10.1111/j.1096-0031.2004.00032.x | url = http://faculty.uml.edu/rhochberg/hochberglab/Courses/InvertZool/Cephalopod%20phylogeny.pdf | access-date = 2014-12-02 | url-status = dead | s2cid = 85975284 | citeseerx = 10.1.1.693.2026 | archive-url = https://web.archive.org/web/20160304022922/http://faculty.uml.edu/rhochberg/hochberglab/Courses/InvertZool/Cephalopod%20phylogeny.pdf | archive-date = 2016-03-04 }}</ref>

=== Recent revisions ===
For an in-process revision of Treatise Part K, King & Evans (2019) reclassified nautiloids ''sensu lato'' into five subclasses. Major groups were primarily defined by variation in their muscle attachment types. Other traits referenced during this reclassification include [[protoconch]] morphology, connecting ring structure, and the extent of cameral and endosiphuncular deposits. While most previous studies referred to subclasses with the suffix '-oidea', these authors instead opted for the suffix '-ia', to prevent confusion between group levels. For example, Nautiloidea ''sensu stricto'' was renamed to Nautilia, to differentiate it from the informal broader definition of "nautiloid". In addition, they used the unsimplified names for orders, with the suffix '-atida' rather than the common simplified form, '-ida'.<ref>{{Cite journal | vauthors = King AH, Evans DH |date=2019 |title=High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K |journal=[[Swiss Journal of Palaeontology]] |language=en |volume=138 |issue=1 |pages=65–85 |doi=10.1007/s13358-019-00186-4 |s2cid=133647555 |issn=1664-2384|doi-access=free |bibcode=2019SwJP..138...65K }}</ref>

* Subclass †[[Plectronoceratia]] (formerly Plectronoceratoidea)
** Order †[[Plectronoceratida]]
** Order †[[Yanheceratida]]
** Order †[[Protactinoceratida]]
* Subclass †[[Multiceratia]] (formerly Multiceratoidea)
** Order †[[Ellesmeroceratida]]
** Order †[[Cyrtocerinida]]
** Order †[[Bisonoceratida]]
** Order †[[Oncoceratida]]
** Order †[[Discosorida]]
* Subclass †[[Tarphyceratia]]
** Order †[[Tarphyceratida]]
** Order †[[Ascoceratida]]
* Subclass Nautilia (formerly Nautiloidea ''sensu stricto'')
** Order [[Nautilida]]
* Subclass †[[Orthoceratia]] (formerly Orthoceratoidea)
** Order †[[Rioceratida]]
** Order †[[Dissidoceratida]]
** Order †[[Orthoceratida]]
** Order †[[Pseudorthoceratida]]
** Order †[[Actinoceratida]]
** Order †[[Astroviida]] (suborders †[[Lituitina]] and †[[Pallioceratina]])
** Order †[[Endoceratida]]
[[File:Pohle et al. 2022 nautiloid relationships.png|thumb|277x277px|Nautiloid relationships and body types according to Pohle et al. (2022)]]
Traditional nautiloid classification schemes emphasize certain character traits over others, potentially involving personal bias as to which traits are worth emphasizing according to different authors. This issue may be resolved by sampling all morphological traits equally through [[Bayesian inference in phylogeny|bayesian phylogenetic inference]]. The first cephalopod-focused paper to use this technique was published by Pohle et al. (2022). They recovered several previously hypothesized groups, though many orders were determined to be paraphyletic. The study was focused on early cephalopod diversification in the Late Cambrian and Ordovician, and did not discuss in detail the origin of post-Ordovician groups. The following is a simplified version of their [[cladogram]], showing early cephalopod relationships to the order level (although various isolated [[Family (biology)|families]] also originated during this diversification event):<ref name=":0">{{cite journal | vauthors = Pohle A, Kröger B, Warnock RC, King AH, Evans DH, Aubrechtová M, Cichowolski M, Fang X, Klug C | display-authors = 6 | title = Early cephalopod evolution clarified through Bayesian phylogenetic inference | journal = BMC Biology | volume = 20 | issue = 1 | pages = 88 | date = April 2022 | pmid = 35421982 | pmc = 9008929 | doi = 10.1186/s12915-022-01284-5 | doi-access = free }}</ref>

{{clade|style=font-size:100%; line-height:100%;
|label1='''[[Cephalopoda]]'''
|1={{clade
 |1=[[Plectronocerida]]
 |2={{clade
    |1=[[Yanhecerida]]
    |2={{clade
        |1=[[Ellesmerocerida]] ([[paraphyletic]] to Endoceratoidea, Multiceratoidea, and Orthoceratoidea)
        |2={{clade
            |1={{clade
               |label1=[[Endoceratoidea]]
               |1={{clade
                  |1=[[Endocerida]]
                  |2=[[Bisonocerida]] }}
               |label2=[[Multiceratoidea]]
               |2={{clade
                  |1=[[Cyrtocerinida]]
                  |2={{clade
                     |1=[[Tarphycerida]] (possibly paraphyletic to [[Nautilida]])
                     |2={{clade
                        |1=[[Oncocerida]] (paraphyletic to Ascocerida and Discosorida)
                        |2={{clade
                           |1=[[Ascocerida]] 
                           |2=[[Discosorida]] }}}}}}}}}}
            |label2=[[Orthoceratoidea]]
            |2={{clade
               |1=[[Riocerida]] (possibly paraphyletic to later orthoceratoids)
               |2={{clade
                  |1=[[Dissidocerida]] (paraphyletic to later orthoceratoids)
                  |2={{clade
                     |1=[[Lituitida]]
                     |2={{clade
                        |1=[[Actinocerida]]
                        |2={{clade
                          |1=[[Pseudorthocerida]]
                          |2=[[Orthocerida]] (paraphyletic to [[Bactritida]], [[Ammonoidea]], and [[Coleoidea]])
}}}}}}}}}}}}}}}}}}}}

==Gallery==
<gallery mode="packed">
File:Nautilus pompilius 3.jpg|The [[Nautilus]], one of only two surviving nautiloid genera
File:Allonautilus scrobiculatus.jpg|''[[Allonautilus]]'', the other surviving nautliod genus
File:Endoceras_sp.png|''[[Endoceras]]'' was one of the largest nautiloids to have ever lived
File:Alaskoceras sewardi.jpg|''[[Alaskoceras]]'' was a member of the [[Tarphycerida]], the first cephalopods to evolve coiled shells
File:Plectronoceras.png|''[[Plectronoceras]]'' was one of the earliest known nautiloids, existing in the late Cambrian
File:Diorama of a Devonian seafloor - Goldringia nautiloid cephalopod eating a trilobite (45654169671).jpg|''[[Goldringia]]'' was a [[Rutoceratidae|Rutoceratid]] nautiloid from the middle [[Devonian]] of the [[United States]]
File:Lorieroceras lorieri.jpg|''[[Lorieroceras]]'' was a bizarre [[oncocerid]] nautiloid from the Devonian that possessed a [[Helical symmetry|helically]] coiled shell
File:Cenoceras NT.jpg|A reconstruction of the [[Mesozoic]] nautiloid ''[[Cenoceras]]''
File:Hoko River Nautilids.jpg|Two nautiloids from the [[Eocene]] aged [[Hoko River Formation]], ''[[Nautilus cookanum]]'' and ''[[Aturia alabamensis]]''
File:Diorama of a Permian seafloor - coiled cephalopod, sponges, brachiopods (43887749560).jpg|''[[Cooperoceras]]'' was a [[Tainoceratidae|Tainoceratid]] nautiloid from the Late [[Permian]] that possessed hollow recurved spines running along its shell
</gallery>

== See also ==
* [[Ammonoidea]]
* [[Belemnoidea]]
* [[Lituites]]

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php (retrieved on May 11, 2014)
* {{cite journal | vauthors = Doguzhaeva L | date = 1994 | title = An Early Cretaceous orthocerid cephalopod from north-western Caucasus. | journal = Palaeontology | volume = 37 | issue = 4 | pages = 889–899 }}
* {{cite web | vauthors = Engeser T | date = 1997–1998 | url = http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautpalneocephalopoda.htm | title = The Palcephalopoda/Neocephalopoda Hypothesis | archive-url = https://web.archive.org/web/20050411043421/http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautpalneocephalopoda.htm | archive-date = 2005-04-11 }}
* {{cite book | vauthors = Teichert C | date = 1988 | chapter = Main Features of Cephalopod Evolution | title = The Mollusca | volume = 12 | series = Paleontology and Neontology of Cephalopods | veditors = Clarke ME, Trueman ER | publisher = Academic Press, Harcourt Brace Jovanovich }}
{{refend}}

== External links ==
* [http://www.tonmo.com/science/fossils/nautiloids.php Nautiloids: The First Cephalopods (TONMO.com)]
* [https://web.archive.org/web/20050328205726/http://www.palaeos.com/invertebrates/Molluscs/Cephalopoda/Nautiloidea.htm Palaeos]
{{CephBase Subclass|Nautiloidea}}
* {{cite web | title = Lophotrochozoa : Mollusca | url = http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php }}

{{fossil cephalopods}}
{{Taxonbar|from=Q840536}}

[[Category:Nautiloids| ]]
[[Category:Mollusc subclasses]]
[[Category:Paleozoic cephalopods]]
[[Category:Mesozoic cephalopods]]
[[Category:Cenozoic cephalopods]]
[[Category:Transitional fossils]]
[[Category:Furongian first appearances]]
[[Category:Extant Cambrian first appearances]]
[[Category:Paraphyletic groups]]