siphunculi

Siphunculi, plural of siphunculus, are slender tubular features associated with the siphuncle of some shelled cephalopods, notably fossil nautiloids and ammonoids. The siphuncle is a living-tissue tube that runs through the stacked gas-filled chambers (camerae) of the shell, linking each chamber to the animal’s soft body. The siphunculi are projections or related structures that extend into the chambers as part of this system.

Functionally, siphunculi are involved in buoyancy regulation. They participate in the exchange of fluids and gases between chambers and the interior of the animal, enabling adjustments in the volume of gas within the camerae. By altering the gas-to-liquid ratio in the chambers, the animal can change its overall buoyancy and, to some extent, its vertical positioning in the water column.

Anatomically, siphunculi vary in appearance and placement across taxa. They are most commonly discussed in the context of the siphuncle and cameral septa architecture seen in fossil cephalopods, where their presence and morphology aid in systematic and phylogenetic analysis. The study of siphunculi contributes to understanding how extinct groups achieved buoyancy control and how their shells evolved over time.

In paleontological literature, the siphuncle and its associated structures, including siphunculi, are used as diagnostic features for classifying ammonoids and nautiloids and for interpreting their paleobiology, particularly buoyancy mechanics and shell evolution.