Recent cephalopods are the most intelligent invertebrates and are unique among molluscs in ancestrally possessing chambered, gas-filled shells penetrated lengthwise by a siphuncle that regulates buoyancy that, combined with jet propulsion, enables a nektonic and mostly predatory lifestyle through jet propulsion. The origin of this key innovation remains enigmatic, as major gaps exist between the earliest accepted cephalopod, Plectronoceras cambria from the late Cambrian, and molecular clock estimates placing the lineage’s divergence from other molluscs in the early Cambrian. Here we report Eosceras shaanxiense gen. et sp. nov., a millimetre-sized cephalopod with an orthoconic shell from the Shuijingtuo Formation (Cambrian Stage 3) of South China. Internally, the shell exhibits multiple septa and a peripherally situated segmented tube that appears to bridge the septa through minute canals, indicating that the soft body migrated anteriorly to form successive chambers during growth while maintaining contact with earlier chambers via the tube. Collectively, these features identify the segmented tube in Eosceras as a candidate primordial cephalopod siphuncle, extending the known range of stem cephalopods back to the Early Cambrian and also revealing the early stages in the evolutionary assembly of a chambered phragmocone employed in buoyancy regulation.

Changes after Mar 10, 2026: We changed the files name.