Early evolution of the gular musculature and its innervation in ray-finned fishes

Gular muscles are an important but often overlooked component of cranial anatomy in bony fishes. They are located on the ventral surface of the head and are derived from the mandibular and hyoid arches. We present a comprehensive review of the gular musculature and its innervation across early diverging actinopterygian lineages. By integrating data from sarcopterygian and chondrichthyan outgroups, we refine existing homology hypotheses, resolve terminological inconsistencies, and uncover a series of previously unrecognized evolutionary innovations that occurred early in ray-finned fish phylogeny. These include the origin of novel muscular subdivisions, the reconfiguration of attachments, and shifts in functional roles. Notably, while ancestral gular muscles functioned exclusively in buccopharyngeal compression, we show that in Neopterygii and Teleostei, parts of this muscle complex were co-opted to also perform buccopharyngeal expansion. This was achieved primarily through the attachment of the interhyoideus to the lower jaw and its anterolateral repositioning relative to the hyoid bar. These transformations represented a significant functional shift toward the sophistication of suction feeding in teleosts. Our analyses also reveal distinct lineage-specific patterns of motor innervation in the gular muscles of fishes: whereas in most cartilaginous fishes the facial nerve extends anteriorly to also innervate mandibular-arch muscles, in bony fishes the trigeminal nerve typically extends posteriorly to also innervate hyoid muscles. These findings provide additional evidence against the widely held assumption of nerve-muscle specificity.