520-Million-Year-Old Fossil Reveals Cephalopod Buoyancy Evolution

Researchers have discovered a 520-million-year-old fossil named Eoceras shaanxiense from the early Cambrian Shuijingtuo Formation in South China, revealing how early cephalopods learned to float. This newly identified species possesses the oldest-known siphuncle, a tiny segmented tube running through its shell chambers that regulated buoyancy by managing water and gas balance. Led by Junfeng Guo of Changan University and Bing Pan of the Chinese Academy of Sciences, the international research team used scanning electron microscopy and micro-computed tomography to analyze 32 small shelly fossils preserved via phosphatization. Their findings bridge a 30-million-year evolutionary gap between molecular clock estimates and the previously earliest known cephalopod Plectronoceras cambria. The anatomical features of Eoceras demonstrate that early cephalopod buoyancy evolved in distinct stages, starting with straight multi-chambered shells before developing a primitive, sealed siphuncle. Because this system was still basic and primitive, Eoceras likely spent most of its life near the seafloor rather than swimming freely in open waters. This groundbreaking discovery provides essential evidence for early animal evolution during the Cambrian Explosion, establishing Eoceras as a critical intermediate form between non-cephalopod mollusks and later sophisticated swimming cephalopods like modern squids, octopuses, and nautiluses.