Phragmocone

 ( Cephalopod Zootomy ) 

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

In most and Ammonoidea, 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 . The early coleoids and Belemnoidea 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 have been preserved within phragmocones, presumably where they crawled in for refuge.

Post Comment