Earliest siphuncle-bearing cephalopod from the early Cambrian

Area of Science:

• Paleontology
• Evolutionary Biology
• Marine Biology

Background:

• Modern cephalopods possess a unique chambered shell with a siphuncle for buoyancy control, crucial for their nektonic lifestyle.
• The evolutionary origin of this key innovation is unclear, with a gap between early Cambrian molecular clock estimates and the oldest known cephalopod fossils.

Purpose of the Study:

• To investigate the early evolution of cephalopods and the origin of the siphuncle.
• To identify fossil evidence that bridges the gap in the cephalopod fossil record.

Main Methods:

• Discovery and analysis of a new fossil species, Eoceras shaanxiense gen. et sp. nov.
• Detailed examination of the internal shell structure, including septa and a segmented tube.

Main Results:

• Eoceras shaanxiense, from the early Cambrian of South China, possesses an orthoconic shell with septa and a peripheral segmented tube.
• This tube shows minute canals connecting to the septa, suggesting it functioned as a primordial siphuncle.
• The fossil indicates that cephalopod soft bodies migrated aperturally, forming chambers while maintaining contact via the tube.

Conclusions:

• The discovery of Eoceras shaanxiense extends the known fossil record of stem cephalopods to the early Cambrian.
• It provides evidence for the early stages of siphuncle and chambered phragmocone evolution, essential for buoyancy regulation in cephalopods.