Novelty Unlocks Morphological Diversity and Functional Opportunity for Fish Jaws

Abstract Morphological novelties that increase mechanical complexity create new axes for functional diversification and expand the evolutionary solutions to common problems. Intramandibular flexion is enabled by a novel joint between the two primary bones of the lower jaw and has spurred several specialized prey capture modes in fishes. We explore morphological, functional, and biomechanical implications of the intramandibular joint (IMJ) and its repeated independent origins across bony fishes. We offer a new scheme for classifying the incredible diversity of IMJ-associated lower jaw functional morphology based on commonly occurring postures. Additionally, we review documented functions permitted by intramandibular flexion and develop a simple biomechanical model to illustrate how changes in jaw morphology impact key functional traits and the capacity to modulate them. The unique attributes of IMJ-bearing fishes, we propose, have important consequences for how these feeding systems evolve. The replicated nature of the novelty further improves our ability to observe how various iterations on a similar adaptive theme may create divergent mechanical and evolutionary outcomes.