Modular skull traits map ecological and evolutionary diet axes in ungulatesModular skull traits map ecological and evolutionary diet axes in ungulates

Understanding how herbivore skull morphology reflects diet is central to interpreting both extant ecological strategies and fossil feeding adaptations. Previous studies rely on coarse feeding guilds that obscure continuous variation in the mechanical and architectural properties of plants consumed. Here, we combine extensive skull measurements from artiodactyls with quantified diet composition and plant functional traits to test how skull morphology responds to specific dietary constraints across evolutionary scales. Our analyses reveal functionally interpretable associations between individual skull traits and plant traits related to leaf toughness and size, and architectural defence. Traits involved in food acquisition, mastication, and comminution respond to distinct dietary axes, supporting a modular view of skull evolution. Comparisons of non-phylogenetic and phylogenetically structured models, reveal that mastication and comminution trait associations with monocot-rich diets persist across lineages. Whereas, access-related trait associations with plant architecture, and mastication and comminution associations with leaf toughness, are shaped by lineage-level sorting. Together, these results show that ungulate skull morphology tracks continuous variation in plant functional traits through modular evolutionary responses operating across multiple temporal scales. This continuous trait-based framework moves beyond traditional feeding guilds and provides a more mechanistic basis for predicting herbivore feeding strategies in both living and extinct systems.