Endoceratina

Endoceratina is an extinct superfamily of cephalopods that lived during the Ordovician and Silurian periods. These ancient marine invertebrates are characterized by their large, conical shells, which could grow to be quite substantial in size. The shells of Endoceratina were typically straight or slightly curved and divided into numerous chambers by thin walls called septa. The animal lived in the largest, rearmost chamber, while the others were filled with fluid and gas to control buoyancy.

A distinctive feature of Endoceratina is the presence of a large, internal tube called an endosiphon that ran through the center of the shell, connecting all the chambers. This endosiphon is believed to have played a crucial role in regulating the animal's buoyancy by allowing it to move fluid between the chambers. The exact function and composition of the endosiphon are still subjects of scientific research.

Fossils of Endoceratina are found in marine sedimentary rocks dating from the Early Ordovician to the Late Silurian. They are particularly common in North America and Europe. The group is known for some of the largest fossil cephalopods ever discovered, with some specimens estimated to have reached lengths of over 10 meters. Their presence in the fossil record indicates they were significant components of Paleozoic marine ecosystems, likely occupying a predatory niche. The classification of Endoceratina within cephalopod phylogeny has been debated, but they are generally considered an early offshoot of the nautiloid lineage.