endocerids

Endocerids were a group of extinct cephalopods that lived during the Paleozoic Era, primarily in the Ordovician and Silurian periods. They are characterized by their large, straight, and often conical shells, known as orthocones. These shells were typically lined with a siphuncle, a tube that connected the various chambers of the shell, allowing the animal to control its buoyancy by filling or emptying it with gas or fluid. The siphuncle in endocerids was often large and centrally located, sometimes occupying a significant portion of the shell's diameter.

The size of endocerid shells varied considerably, with some species reaching lengths of several meters, making them among the largest invertebrates of their time. Their exact lifestyle is still debated, but they are generally thought to have been nektonic, swimming in the open water. Their large size and likely predatory nature suggest they occupied a significant niche in ancient marine ecosystems. Endocerids are important index fossils, meaning their presence in rock layers helps geologists date those formations. Their distinctive shells are commonly found in fossiliferous rocks from the Ordovician and Silurian periods worldwide. The group became extinct by the end of the Silurian, with their ecological roles potentially being filled by other cephalopod groups like nautiloids and ammonoids in later periods.