Editing Siphuncle (section)

== Morphology ==
[[File:Nautilus pompilius - Fernbank Museum of Natural History - DSC00294.JPG|thumb|A shell of ''[[Chambered nautilus|Nautilus pompilius]]'' in cross section. Septal necks are preserved, but the thin connecting rings have been degraded and lost.]]
The siphuncle of [[fossil]]ised cephalopods is assumed to have worked in the same general way as in living [[Nautilus|nautiluses]]. The siphuncle itself is only rarely preserved, but its shape can be inferred from hardened structures which lie around it. Many fossils show the holes where the siphuncle passes through each septum. Around these holes, the rim of the septum is bent into a stout [[Aragonite|aragonitic]] tube known as a '''septal neck''' (or siphuncle notch).<ref name=":0" /><ref name=":1">{{Cite journal |last1=King |first1=Andy H. |last2=Evans |first2=David H. |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 |issn=1664-2384|doi-access=free |bibcode=2019SwJP..138...65K }}</ref>
[[File:Orthoceras---orthocère.jpg|left|thumb|These polished [[Orthocerida|orthocerid]] nautiloid cephalopod fossils from [[Morocco]] have fully preserved septal necks and connecting rings outlining the shape of the siphuncle.]]
In each chamber of the shell, the siphuncle is encased by a tubular structure known as a '''connecting ring'''. In living nautiluses, the connecting ring is a simple, thin-walled cylinder, with organic or thinly [[Calcite|calcitic]] layers secreted from the tissues of the siphuncle. This fragile and poorly-mineralized form is known as a ''nautilosiphonate'' morphology. Many extinct cephalopods have a much more prominent connecting ring, with a very thick and porous inner [[Calcite|calcitic]] layer. This more strongly-mineralized form is known as a ''calciosiphonate'' connecting ring. Connecting rings are strongly variable in morphology, from narrow homogenous tubes to bulbous, segmented cavities. Some are infolded, sending lobes or blades of calcite into the siphuncle. Connecting rings are typically continuous with the septal necks, and are difficult to distinguish without close examination. However, their developmental origin is wholly separate from the shell and septa, and they utilize calcite rather than aragonite as a biomineralized reinforcement.<ref name=":0" /><ref name=":1" />

Biomineralized structures which develop within the siphuncle are known as '''endosiphuncular deposits''' (or simply siphonal deposits). These may include horizontal partitions (''diaphragms''), stacked conical structures (''endocones''), longitudinal rods, and various other concretions. Endosiphuncular deposits are typically thin structures which may be homologous to parts of the septae or connecting rings.<ref name=":0" /><ref name=":1" />

In most fossil nautiluses, the siphuncle runs more or less through the center of each chamber, but in ammonites and belemnites it usually runs along the ventral edge of the shell. In some fossil straight shelled nautiloids, cylindrical calcareous growths ("siphuncular deposits") around the siphuncle can be seen towards the apex of the shell. These were apparently counterweights for the soft body at the other end of the shell, and allowed the nautilus to swim in a horizontal position. Without these deposits, the apex of the buoyant shell would have pointed upwards and the heavier body downwards, making horizontal swimming difficult. The siphuncle of the [[Endocerida]] also contained much of the organisms' body organs.<ref name="Kroger2009">{{cite journal
 | doi = 10.1016/j.palaeo.2008.12.015
 | title = Pulsed cephalopod diversification during the Ordovician
 | year = 2008
 | author = Kroger, B
 | journal =  	Palaeogeography, Palaeoclimatology, Palaeoecology
 | last2 = Yun-Bai
 | first2 = Zhang
 | volume = 273
 | issue = 1–2
 | pages = 174–183 }}</ref>