Pharyngognathy - a key innovation?
Pharyngognathy consists of a series of modifications to the pharyngeal arches - (i) the two 5th ceratobranchials are sutured together into a single structure (i.e., the lower pharyngeal jaw; LPJ), (ii) the muscular sling, which operates the biting mechanism of the pharyngeal jaws, inserts directly into the neurocranium (from the lateral processes of the LPJ), and (iii) the paired upper pharyngeal jaws articulate directly against the neurocranium via a synovial joint. Many bony fishes have pharyngeal arches that are modified to assist with prey processing (for example, minnows and sunfishes), but pharyngognathy is hypothesized (originally by Karel Liem in 1973) to represent a key innovation that improved functional capacity, versatility, and efficiency by freeing the oral jaws from demands associated with prey processing and subsequently permitting their diversification in terms of prey acquisition (the decoupling hypothesis). Today, four-and-a-half decades later, pharyngognathy is still considered a major innovation among fishes despite that we haven't made much progress testing, much less verifying, predictions that follow from the hypothesis.
Allow me to back up. We do know quite a bit - pharyngeal jaws improve efficiency (i.e., handling time) while consuming processing-intensive prey, facilitate the exploitation of prey that would be impractical for oral jaws (i.e., hard-shelled prey that require crushing via the generation of biting force), and when processing demand is low, the pharyngeal jaw bones and associated musculature are often reduced (e.g., among piscivores and filter feeders). Those patterns strongly suggest that pharyngognathy (or usually specific components of pharyngognathy) provide an adaptive function. But, there is tons that we don't know much about - does pharyngeal jaw diversification alter the trajectories of clades across the adaptive landscape? Are oral and pharyngeal jaw evolution decoupled? Do pharyngeal jaws provide anything useful beyond the comparatively well-studied associations with molluscivory and piscivory? Can cichlids rupture algae cells by grinding them in their pharyngeal jaws like carp? The list goes on.