WHY WERE AMMONITE SUTURES SO COMPLEX? AN INVESTIGATION OF LIQUID RETENTION IN THE SEPTAL MARGINS OF AMMONOID SHELLS

Ectocochleate cephalopods have external shells filled with divider walls called septa. Ammonoid cephalopods had complex, marginally frilled septa that have been subjected to many hypotheses regarding possible biological functions. Among these hypotheses is the retention of chamber liquid via surface tension. The relative amount of liquid retained by the septa was investigated with 3D printed models of theoretical ectocochleate shells. A species of nautiloid, and two species of goniatites, ceratites, and ammonites were chosen to represent seven septa falling on a continuum of increasing complexity. These septa were 3D printed at 6 cm and 9 cm to assess the influence of scaling on liquid retention. For both scales, complex septa retain more liquid via surface tension. However, the fractal-like margins of ammonitic septa retain much more liquid than their simple counterparts due to the recesses created by higher-order frilling. At larger scales, these same shapes retain less liquid per camera, suggesting this behavior is scale dependent.

While experiments with theoretical models offer an understanding of the relative differences in liquid retention, the chosen sutures belong to different morphologies with different terminal sizes. Liquid retention over natural scales was investigated by isolating a single camera from a CT scanned specimen of Damesites. These experiments suggest that capillary retention heavily depends upon scale. At smaller sizes, lower order corrugations of the septum hold single “droplets” of water around the margins. As size increases, higher order frills retain liquid, suggesting that ammonitic septa are functional for this purpose at multiple scales. Beyond the terminal size of Damesites, liquid retention significantly decreases. These scaling properties also support a functional mechanism for the relationship between sutural complexity and terminal size in ammonoids in addition to constructional constraints.

The cameral liquid held by surface tension could have served as liquid ballasts used to reduce positive buoyancy. Furthermore, these liquid reserves could have facilitated greater ranges of slow, passive buoyancy adjustments for vertical migration in the water column. Finally, binding the liquid in the chambers could have reduced disruptive “sloshing” during locomotion.