Endoceratidae

Endoceratidae is an extinct family of cephalopods belonging to the subclass Endoceratoidea. These marine invertebrates existed during the Ordovician period, from the Early Ordovician to the Late Ordovician epochs. Endoceratoidea, and by extension Endoceratidae, are characterized by their large, straight, and generally conical shells, known as orthocones. A distinctive feature of this group is the presence of a large siphuncle, which was a tube connecting the camerae (chambers) of the shell. In endoceratids, this siphuncle was typically very large and centrally located within the shell, often occupying a significant portion of the internal space.

The endoceratids were among the largest invertebrates of their time. Fossil evidence suggests that some species could reach lengths of several meters, making them formidable predators or scavengers in their ancient marine environments. Their anatomy, inferred from fossilized shells, indicates they were likely nektonic, meaning they were active swimmers rather than sessile. The presence of a large siphuncle is believed to have played a crucial role in buoyancy control, allowing them to adjust their position in the water column. The function and exact composition of the material within the siphuncle are subjects of ongoing scientific research. Endoceratidae represents a significant lineage within the early evolution of cephalopods, showcasing early adaptations for size and mobility in the Paleozoic seas.