The fossil record of cephalopods just got a lot older. And weirder.
Paleontologists have identified the earliest known cephalopod to possess a siphuncle. This tubular extension is critical for buoyancy in shelled cephalopods. The discovery pushes the evolutionary timeline of this feature back to the Early Cambrian, approximately 520 million years ago in what is now China.
Cephalopods today include octopuses, squid, and cuttlefish. Nautiluses still carry their shells. Extinct ammonites also belonged here. They share one unusual trait among mollusks: an internal architecture designed to regulate buoyancy.
Shelled species used chambers, septa, and that crucial tube. The siphuncle could be filled with water or gas. It allowed them to adjust their position in the water column. This mechanism separated cephalopods from other mollusk groups. But Early Cambrian fossils are rare. Scientists had a fragmented picture of how this system began.
“Combined with jet propulsion, the saphuncle enables cephalopods to engaging in a nektonic and mostly predatory through long-term regulation of buoyancy,” said Zuchen Song, a paleontologist at Chang’an University, along with colleagues.
There was a gap in the timeline. The earliest accepted cephalopod is Plectronoceras cambria. It dates to the Late Cambrian. Molecular clocks suggest the lineage diverged from other mollusks earlier, in the Early Cambrian. Something was missing.
Examining 32 specimens from the Shuijingtuo Formations
Researchers examined 32 specimens. They came from the Shuijingtuo formation in China. They belong to a previously unknown species. The team named it Eoceras shaanxiense. It lived in Early Cambrian oceans around 520 million years ago*.
The creatures were tiny. Millimeters in size. They possessed straight, cone-shaped shells. The opening was tilted. Inside were multiple internal walls.
A segmented tube ran along the underside. It connected to the chambers via tiny canals. This structure looked like a siphuncle. It was peripherally situated.
After comparing these specimens to known cephalopod features, the scientists concluded this was a true siphuncle. Eoceras shaanxiene is now the earliest siphuncle-bearing ceplaohpod ever found. It represents a transitional step. It sits between earlier chambered species and later cephalopds with advanced buoyancy systems.
“Internally, the shell exhibit multiple septa along with peripherally situated segmented that appears to bridge the septa though minute canals indicating that the soft body migrated apertrually to form successive chambers during growth maintaining contact with earlier chambers via tube,” they wrote.
These features identify the tube as a candidate primordial siphuncle*.
Why buoyancy mattered in the Early Cambrian
The buoyancy system was primitive. Eoceras shaanxienese likely stayed on the seafloor most of its life. It probably did not swim freely. The soft tissue anatomy remains unknown. But the shell structure tells a story of evolution in action.
The discovery extends the known range of stem ceplaopods back to the Early Cambriam. It reveals the early stages in evolutionary assembly of a chambered pragmamocone. This structure was employed for buoyancy regulation*.
The paper was published in Nature this week*.
The find fills a significant gap. How did these animals learn to control their depth so quickly? We don’t have a definitive answer. But this fossil offers the best look yet. It shows that 520 million years ago, life was already figuring out how to leave the mud behind. Or at least, try to*.
