Modern cephalopods are unique among molluscs in ancestrally possessing chambered, gas- and fluid-filled shells, the internal septa of which are penetrated by a siphuncle. Combined with jet propulsion, the siphuncle enables cephalopods to engage in a nektonic and mostly predatory lifestyle through long-term regulation of their bouyancy. 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 Eoceras 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 along with a peripherally situated segmented tube that appears to bridge the septa through minute canals, indicating that the soft body migrated aperturally to form successive chambers during growth while maintaining contact with earlier chambers via the tube. Collectively, these features identify the segmented tube in Eoceras gen. nov. as a candidate primordial cephalopod siphuncle, thus 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.

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