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The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae.

In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camerae are linked by a siphuncle which determines buoyancy by means of gas exchange.

Despite this benefit, such a large shell adds to the mass of the animal, and hence is disadvantageous in catching fast-moving prey. Some nautiloids, such as the Silurian Ascocerida, dropped the phragmocone upon maturity, presumably to increase speed and maneuverability. They thus became the early Paleozoic equivalent of coleoids. The early coleoids and belemnoids adopted a different approach: the phragmocone was retained but became internal and reduced. In general the shell in cephalopods tends to be vestigial or absent.

Fossil record

Being the only biomineralised part of most cephalopods, the phragmocone is typically the only part to enter the fossil record. It is sometimes infilled with sediment, with sediment presumably getting in through the siphuncle. There are occasions where trilobites have been preserved within phragmocones, presumably where they crawled in for refuge.

References

1. Henderson, ROBERT A.; McNamara, Kenneth J. (1985). "Taphonomy and ichnology of cephalopod shells in a Maastrichtian chalk from Western Australia". Lethaia. 18 (4): 305. Bibcode:1985Letha..18..305H. doi:10.1111/j.1502-3931.1985.tb00710.x.
2. Arnold Davis, R. H. B. Fraaye, Char, Richard (2001). "Trilobites within nautiloid cephalopods". Lethaia. 34 (1): 37–45. Bibcode:2001Letha..34...37D. doi:10.1080/002411601300068251.{{cite journal}}: CS1 maint: multiple names: authors list (link)

v t e Cephalopod anatomy
Shell	Types External Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet Features External Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls
Mantle & funnel	External anatomy Dermal structures Fins (wings) Flotation devices ("secondary fins") Funnel–mantle locking apparatus Tail Internal anatomy Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands
Head & limbs	Brachial crown Arms Hectocotylus Tentacles Carpus Dactylus Manus Suckers and hooks Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Occipital region Nuchal crest (occipital crest) Nuchal folds (occipital folds) Occipital membrane Olfactory organ Nuchal organ Other parts Eyes Statocysts
General	Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)

• Cephalopod zootomy