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Phragmocone

Chambered portion of a cephalopod shell

Summary

Chambered portion of a cephalopod shell

Cutaway of a nautilus shell showing the chambers

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.{{cite journal There are occasions where trilobites have been preserved within phragmocones, presumably where they crawled in for refuge.{{cite journal

References

Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

cephalopod-zootomy

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