Endocerida

Endocerida is an extinct order of cephalopods that lived during the Ordovician and Silurian periods. These were free-swimming, predatory marine invertebrates characterized by their distinctive endosiphonate shell structure. The shell, or conch, was typically long and conical, often reaching considerable lengths, with some specimens estimated to be over 10 meters long. The most notable feature of the endocerid shell was the large, cone-shaped siphuncle that ran through the interior. This siphuncle was often filled with cameral fluid, which the animal could adjust to control its buoyancy, allowing it to ascend or descend in the water column.

The siphuncle was a prominent feature, extending from the apex of the shell to the living chamber. In many endocerids, the siphuncle was significantly larger than the rest of the internal soft tissues, occupying a substantial portion of the shell's volume. This large siphuncle is the defining characteristic of the order, distinguishing them from other cephalopod groups. Endocerids were a diverse group, with various families and genera exhibiting different shell shapes, siphuncle structures, and likely ecological niches. They were significant predators in their marine environments, possessing tentacles to capture prey. Their extinction occurred by the end of the Silurian period, with their ecological roles potentially being filled by other cephalopod groups like nautiloids and ammonoids that diversified in later periods.