Organism–sediment interactions and the evolution of a unique trilobite morphology

During the Ordovician, trilobites of the order Harpetida and the superfamily Trinucleioidea evolved unusual cephalic brims, quite unlike any structure known in modern arthropods. Brimmed trilobites were diverse and widespread, but we still do not fully understand why the harpiform brim evolved or what role it played in their success. Many authors have speculated about the brim's function, generating many untested hypotheses. We tested the hypothesis that the brim evolved to prevent sinking in soft sediments by calculating the depth to which brimless trilobites would sink and showed this explanation to be untenable. We then three-dimensionally printed model cephala with various brim shapes and moved them through natural sediments, testing the hypothesis that the brim evolved as a sediment plough. We found that increased horizontal brim width allowed a cephalon to efficiently displace more sediment, while increased brim height merely impeded the cephalon's progress. However, there is no clear evolutionary trend among harpetids or trinucleids towards wider or flatter brims, indicating that ploughing did not exert a strong selective pressure on brimmed trilobites. This work also serves as a case study in researching taxa without modern biomechanical analogues, demonstrating that many functional hypotheses can be adequately tested by straightforward experimental methods.