Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture.

Area of Science:

• Paleontology
• Functional Morphology
• Biophysics

Background:

• Chambered ammonoid conchs exhibit increasing internal architectural complexity over geologic time.
• The adaptive significance of septal complexity in ammonoids remains debated, with traditional focus on biomechanical stress resistance.
• The role of septal complexity in buoyancy manipulation warrants further investigation.

Purpose of the Study:

• To propose and test the hypothesis that increased septal complexity enhances fluid retention capacity within ammonoid chambers.
• To explore the implications of fluid retention for buoyancy regulation and mass compensation in ammonoids.

Main Methods:

• Utilized 3D-printed archetypes of cephalopod shells with varying septal complexity.
• Measured liquid retention by septa and within entire chambers.
• Applied biomimetic hydrophilic coatings to assess surface tension effects.

Main Results:

• Surface tension significantly regulates water retention capacity, which scales positively with septal complexity and membrane capillarity.
• Water retention capacity scales negatively with shell size.
• Increased liquid retention demonstrates enhanced surface tension potential, supporting improved chamber refilling.

Conclusions:

• Greater liquid retention capacity in complex ammonoid shells likely improved buoyancy regulation and compensated for mass changes.
• Findings suggest ammonoids with complex sutures possessed more active buoyancy control than other ectocochleate cephalopods.
• The relationship between septal complexity, liquid retention, and surface tension offers a functional explanation for ammonoid evolution.