Cyclothonidae

Cyclothonidae is a family of extinct cephalopods belonging to the subclass Nautiloida. These animals lived during the Devonian period, approximately 419 to 359 million years ago. They are characterized by their coiled shells, which are often evolute, meaning the whorls gradually increase in size and do not completely cover the earlier whorls. The coiling in Cyclothonidae shells can vary, with some exhibiting a more tightly coiled gyroconic shape while others are more loosely coiled.

The shell aperture in Cyclothonidae is typically wide and may possess a sinuous form. Like other nautiloids, they possessed a siphuncle, a tube that ran through the camerae (chambers) of the shell, allowing them to control their buoyancy by regulating the amount of gas or liquid within the chambers. Fossil evidence suggests that Cyclothonidae were likely nektonic, meaning they were active swimmers rather than bottom-dwellers. Their diet is presumed to have consisted of smaller marine invertebrates.

The taxonomic classification of Cyclothonidae has been subject to revision over time as new fossil discoveries have been made and analytical techniques have improved. They are generally considered to be a distinct family within the Nautiloida, though their precise relationships to other nautiloid groups are still a subject of ongoing research. The presence of unique shell morphology, particularly the degree and style of coiling, distinguishes them from other nautiloid families of the Devonian.