Hard-object feeding adaptations infer relative predator-prey size relationships in Devonian placoderms.

Devonian placoderms exhibit exceptional diversity in jaw and dental morphology, making them an ideal system for examining early gnathostome feeding mechanisms. Here, we investigate inferognathal (mandible) function in eight eubrachythoracid arthrodire placoderms from Australia's Late Devonian Gogo Formation. A combination of finite element analysis and quantification of dental surface complexity revealed distinct, size-dependent strategies for processing mechanically resistant materials (durophagy). Taxa with broad, flat dental surfaces experienced low strain during simulated bites, consistent with crushing armoured prey that could be engulfed whole. In contrast, the jaw of the largest taxon combined exceptional structural strength with highly complex, reinforced dentition, enabling force concentration to pierce through armour or shell, and tear apart prey items exceeding its oral capacity. Taxa of intermediate size exhibited higher strain and specialised slicing dentition, consistent with generalist diets or feeding strategies focused on shearing through softer tissues. Consumption of armoured prey in placoderms was therefore governed not by predator size alone, but by the relative size of prey to predator. Hard object feeding in arthrodire placoderms was not a result of a single mechanical solution, but involved several distinct solutions influenced by body size and prey-processing constraints.