Editing Nautilus (section)

==Evolutionary history==
[[File:Allonautilus vs Nautilus.png|thumb|Shell characters of the genera ''Nautilus'' and ''Allonautilus'']]
[[File:Section cut Nautilus.jpg|thumb|Section cut of a nautilus shell]]
[[Fossil]] records indicate that nautiloids have experienced minimal morphological changes over the past 500 million years. Many were initially straight-shelled, as in the extinct genus ''[[Lituites]]''. They developed in the Late [[Cambrian]] period and became a significant group of sea [[predator]]s during the [[Ordovician]] period. Certain species reached over {{cvt|2.5|m|ft|0}} in size. The other cephalopod subclass, [[Coleoidea]], diverged from the nautiloids long ago and the nautilus has remained relatively unchanged since. Nautiloids were much more extensive and varied 200 million years ago. The ancestors of all [[Coleoidea]] (shell-less Cephalopods) once possessed shells, and many early cephalopod [[species]] are only known from shell remains. Following the [[K-Pg extinction event]] most [[nautiloid]] species went [[extinct]], while members of Coleoidea managed to survive. Following the mass extinction, the nautilus became the only extant species of nautiloids.<ref>{{Cite journal|url=https://www.academia.edu/93538950|title=Comparative morphology among representatives of main taxa of Scaphopoda and basal protobranch Bivalvia (Mollusca)|first=Luiz Ricardo L.|last=Simone|date=January 25, 2009|journal=Papéis Avulsos de Zoologia|volume=49|issue=32|pages=405–457|doi=10.1590/S0031-10492009003200001 |via=www.academia.edu|doi-access=free}}</ref><ref>{{Cite journal|url=https://sjpp.springeropen.com/articles/10.1007/s13358-016-0112-7|title=Nautilus: biology, systematics, and paleobiology as viewed from 2015|first1=Peter|last1=Ward|first2=Frederick|last2=Dooley|first3=Gregory Jeff|last3=Barord|date=March 25, 2016|journal=[[Swiss Journal of Palaeontology]]|volume=135|issue=1|pages=169–185|via=sjpp.springeropen.com|doi=10.1007/s13358-016-0112-7|bibcode=2016SwJP..135..169W |s2cid=87025055 }}</ref>

The family Nautilidae has its origin in the [[Trigonocerataceae]] ([[Centroceratina]]), specifically in the [[Syringonautilidae]] of the [[Late Triassic]]<ref name=Kummel /> and continues to this day with ''[[Nautilus (genus)|Nautilus]]'', the type genus, and its close relative, ''[[Allonautilus]]''.

===Fossil genera===
[[File:Eutrephoceras dorbignyanum (Forbes in Darwin, 1846) - Santiago specimen.jpg|thumb|''[[Eutrephoceras|Eutrephoceras dorbignyanum]]'']]
The fossil record of Nautilidae begins with ''[[Cenoceras]]'' in the Late Triassic, a highly varied genus that makes up the [[Jurassic]] ''Cenoceras'' complex. ''Cenoceras'' is evolute to involute, and globular to lentincular; with a suture that generally has a shallow ventral and lateral lobe and a siphuncle that is variable in position but never extremely ventral or dorsal. ''Cenoceras'' is not found above the [[Middle Jurassic]] and is followed by the [[Upper Jurassic]]-[[Miocene]] ''[[Eutrephoceras]]''.

''[[Eutrephoceras]]'' is generally subglobular, broadly rounded laterally and ventrally, with a small to occluded umbilicus, broadly rounded hyponomic sinus, only slightly sinuous sutures, and a small siphuncle that is variable in position.

Next to appear is the [[Lower Cretaceous]] ''[[Strionautilus]]'' from [[India]] and the European ex-[[USSR]], named by Shimankiy in 1951. ''Strionautilus'' is compressed, involute, with fine longitudinal striations. Whorl sections are subrectangular, sutures sinuous, the siphuncle subcentral.

Also from the Cretaceous is ''[[Pseudocenoceras]]'', named by Spath in 1927. ''Pseudocenoceras'' is compressed, smooth, with subrectangular whorl sections, flattened venter, and a deep umbilicus. The suture crosses the venter essentially straight and has a broad, shallow, lateral lobe. The siphuncle is small and subcentral. ''Pseudocenoceras'' is found in the [[Crimea]] and in [[Libya]].

''[[Carinonautilus]]'' is a genus from the [[Upper Cretaceous]] of [[India]], named by Spengler in 1919. ''Carinonautilus'' is a very involute form with high whorl section and flanks that converge on a narrow venter that bears a prominent rounded keel. The umbilicus is small and shallow, the suture only slightly sinuous. The siphuncle is unknown.

''[[Obinautilus]]'' has also been placed in Nautilidae by some authorities, though it may instead be an [[Argonautidae|argonautid]] [[octopus]].<ref>Teichert, C. & T. Matsumoto (2010). The Ancestry of the Genus ''Nautilus''. In: W.B. Saunders & N.H. Landman (eds.) ''Nautilus: The Biology and Paleobiology of a Living Fossil''. Springer. pp.&nbsp;25–32. {{doi|10.1007/978-90-481-3299-7_2}}</ref><ref name=LA>{{cite journal | last1 = Saul | first1 = L.R. | last2 = Stadum | first2 = C.J. | year = 2005 | title = Fossil argonauts (Mollusca: Cephalopoda: Octopodida) from Late Miocene siltstones of the Los Angeles Basin, California | journal = Journal of Paleontology | volume = 79 | issue = 3| pages = 520–531 | doi = 10.1666/0022-3360(2005)079<0520:FAMCOF>2.0.CO;2 | bibcode = 2005JPal...79..520S | s2cid = 131373540 }}</ref>