How did the cephalopod shell change during evolution?

Cephalopod shell evolution is characterized by a transition from simple external structures to diverse, often internal, forms. Early cephalopods possessed simple, caplike shells that elongated and developed internal partitions called septa to form the nautiloid shell. To manage the cumbersome length of these straight shells, many lineages evolved coiling.1

The Ammonoidea evolved from straight-shelled Devonian ancestors, eventually developing coiled forms before their extinction in the Cretaceous. In contrast, modern cephalopods generally possess internal, degenerate shells. This modification resulted from the mantle progressively overgrowing the shell, an adaptation likely linked to the development of an active, swimming lifestyle.1

This evolutionary trajectory led to various specialized forms:
* Belemnites: Featured a modified internal shell.1
* Cuttlefishes: Retain calcified phragmoconal septa.1
* Squids: Possess only a horny, internal structure known as a "pen".1
* Octopoda: The shell is reduced to cartilaginous stylets or fin supports.1

While the pearly nautilus maintains an external, chambered shell for buoyancy, the paper nautilus female secretes a unique, unchambered shell for egg protection.2