Did shell-crushing predators drive the evolution of ammonoid septal shape?

For centuries, paleontologists have sought functional explanations for the uniquely complex internal walls (septa) of ammonoids, extinct shelled cephalopods. Ammonoid septa developed increasingly complex fractal margins, unlike any modern shell morphologies, throughout more than 300 million years of evolution. Some have suggested these morphologies provided increased resistance to shell-crushing predators. We perform the first physical compression experiments on model ammonoid septa using controlled, theoretical morphologies generated by computer-aided design and 3D printing. These biomechanical experiments reveal that increasing complexity of septal margins does not increase compression resistance. Our results raise the question of whether the evolution of septal shape may be tied closely to the placement of the siphuncle foramen (anatomic septal hole). Our tests demonstrate weakness in the centers of uniformly thick septa, supporting work suggesting reinforcement by shell-thickening at the center of septa. These experiments highlight the importance of 3D reconstruction using idealized theoretical morphologies that permit the testing of long-held hypotheses of functional evolutionary drivers by recreating extinct morphologies once rendered physically untestable by the fossil record.

Methods
Figure S1. Materials test of rigid resin. Load versus displacement curved for models cured at 80°C for (A) 15 minutes, (B) 30 minutes, (C) 45 minutes, (D) 60 minutes, (E) 120 minutes, (F) 180 minutes.

Figure S2. Generalized depiction of the box method of fractal analysis modified from Peterman and Barton 2019. For methodological details and original figure see Peterman and Barton 2019.

Figure S3. Septal shape and shell strength with genera grouped by shape. Peak load sustained by each genus with (A) ammonitic septa, (B) ceratitic septa, (C) goniatitic septa, (D) nautiloid septa. Red triangles represent average peak load for each genus. Red dotted line represents average peak load for septal shape type.