Palaeontologists have identified the earliest known cephalopod mollusc equipped with a siphon — a tubular extension of the mantle found in shelled cephalopods. This discovery pushes the dating of this key evolutionary feature back to the Early Cambrian period, around 520 million years ago, according to a study published in Nature.
Cephalopods are a group that today includes octopuses, squids, cuttlefish, and nautiluses, and once also included extinct ammonites. They are unusual among molluscs because their internal structure evolved to support buoyancy.
Chambered shells regulated buoyancy using structures such as septa and, crucially, the siphon — a tube that could be filled with water or gas. The siphon is considered one of the defining innovations that distinguish cephalopods from other molluscs.
However, due to the rarity of Early Cambrian fossils, scientists have only a fragmentary understanding of how this system originally evolved.
“Combined with jet propulsion, the siphon allows cephalopods to adopt a nektonic and largely predatory lifestyle through long‑term buoyancy regulation,” said palaeontologist Zuchen Song of Chang’an University and his colleagues.
The origin of this key innovation remains uncertain, as there is a significant gap between the earliest widely recognised cephalopod, Plectronoceras cambria (from the Late Cambrian), and molecular clock estimates that place the divergence of this lineage from other molluscs in the Early Cambrian.
In the new study, researchers examined 32 specimens of a previously unknown cephalopod species from the Shuijingtuo Formation in China.
This animal, named Eoceras shaanxiense, lived in Early Cambrian oceans around 520 million years ago. Measuring just a few millimetres in size, it had a straight conical shell, an angled aperture, multiple internal walls, and a segmented tube running along the lower side of the shell, connecting to the chambers via tiny channels.
After comparing this structure with known cephalopod siphons, palaeontologists concluded that it matches the siphon morphology, making Eoceras shaanxiense the earliest cephalopod discovered with a siphon. It represents a transitional stage between earlier species with chambered shells and later cephalopods with a more advanced buoyancy system.
“The shell contains many septa, as well as a peripherally located segmented tube that appears to connect the septa through minute channels,” the team explained. “This suggests that the soft body migrated apically, forming successive chambers during growth while maintaining contact with earlier chambers via this tube. Collectively, these features indicate that the segmented tube in Eoceras shaanxiense can be regarded as a possible primitive cephalopod siphon.”
Because Eoceras shaanxiense’s buoyancy system was still quite primitive, it likely spent much of its life on the seafloor rather than swimming freely, although the anatomy of its soft tissues remains unknown.
“This discovery extends the known range of stem cephalopods into the Early Cambrian and reveals the early stages in the evolutionary development of the chambered phragmocone used for buoyancy control,” the scientists concluded.
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