Shaping Shell Strength: Testing the Influence of Shell-Crushing Predators on Shell Shape over Geologic Time

Mollusk shells have evolved an incredibly diverse array of shapes. The shapes are well preserved in an extensive fossil record. While some of these geometries are still present in modern fauna, others disappeared when taxa went extinct. Since shells primarily serve a defensive purpose, they are believed to have evolved in conjunction with a wide range of predators. These predators attacked shelled invertebrates by drilling holes through them, smashing, peeling, and crushing them. Many previous investigators have postulated that changes in shell shape over geologic time resulted from changing predation pressures. Some suggest that temporally persistent morphologies are more well adapted to predators. While a wealth of literature exists speculating on the role of shell shape in defending against shell crushing predators, the contribution of shape to shell strength has never been experimentally tested in isolation. Previous investigators have been unable to isolate the influence of shell shape due to unavoidable complicating factors such as shell thickness, microstructure, and taphonomy (the state of preservation). To overcome this challenge, I have used mathematical modeling and recent advances in 3D printing to generate physical models of mollusk shells with carefully controlled parameters. Using this novel approach, I conducted the first experiments exploring how different shell shapes perform under compression while controlling shell thickness, size, and microstructure. I reviewed and utilized a standardized mechanical experiment to analyze external coiling geometries of gastropods and bivalves, as well as internal cephalopod shell morphologies. Using this method, I generated several novel findings: First, that gastropod shell shapes during the Mesozoic had different relative strengths than those implied by the literature. Moreover, the strengths of these shapes are dependent upon the crushing orientation of the shell. Second, that bivalve shell strength has demonstrable tradeoffs with shapes that allow escape behavior. However, these tradeoffs are reflected by multiple aspects of shell shape in specific combinations. Finally, the complexity of the interior walls of cephalopod shells was unlikely to have been driven by crushing predation pressure; however, I found biomechanical constraints for their natural thickness. This work demonstrates the importance of empirical testing of fundamental evolutionary hypotheses