Buoyancy control in ammonoid cephalopods refined

The internal architecture of chambered ammonoid conchs profoundly increased in complexity through geologic time, but the adaptive value of these structures is disputed. Specifically, these cephalopods developed fractal-like folds along the edges of their internal divider walls (septa). Traditionally, functional explanations for septal complexity have largely focused on biomechanical stress resistance. However, the impact of these structures on buoyancy manipulation deserves fresh scrutiny. We propose increased septal complexity conveyed comparable shifts in fluid retention capacity within each chamber. We test this interpretation by measuring the liquid retained by septa, and within entire chambers, in several 3D-printed cephalopod shell archetypes, treated with (and without) biomimetic hydrophilic coatings. Results show that surface tension regulates water retention capacity in the chambers, which positively scales with septal complexity and membrane capillarity, and negatively scales with size.

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FIG: Creation of cylindrical shells models (a–c) and chamber models (d–f) used in liquid retention experiments. (a) Wireframe view showing the suture (orange) wrapped around the internal whorl section. (b) Extruded, virtual model with the cylindrical shell and septum unified together. (c) Final 3D printed model used to measure the liquid retained by surface tension in the septal recesses. (d) Virtual model of a single chamber (camera). (e) Virtual model of the camera subtracted from a bounding volume. (f) Final, 3D printed model with empty chamber inside and holes for drainage on the adoral, adapical, and ventral sides of the model.

A greater capacity for liquid retention in ammonoids may have improved buoyancy regulation, or compensated for mass changes during life. Increased liquid retention in our experiments demonstrate an increase in areas of greater surface tension potential, supporting improved chamber refilling. These findings support interpretations that ammonoids with complex sutures may have had more active buoyancy regulation compared to other groups of ectocochleate cephalopods. Overall, the relationship between septal complexity and liquid retention capacity through surface tension presents a robust yet simple functional explanation for the mechanisms driving this global biotic pattern.

Peterman, D.J., Ritterbush, K.A., Ciampaglio, C.N. et al. Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture. Sci Rep 11, 8055 (2021). https://doi.org/10.1038/s41598-021-87379-5

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