The clavobranchialis musculature in sarcopterygian fishes, and contribution to osteichthyan feeding and respiration

Various fossil lungfish taxa preserve distinct depressions on the smooth postbranchial lamina of the dermal pectoral girdle. These depressions are largely unknown in other sarcopterygian fishes, but are present in the rhizodont sarcopterygian Strepsodus. Comparisons with extant actinopterygian fishes suggest these depressions mark the point of origin for the clavobranchialis musculature, extending anterodorsally into the gill chamber to insert on the ventral surface of the ceratobranchial(s). Studios examining feeding and respiratory mechanisms of bony fishes (Osteichthyes) have emphasised the role of mandibular depression in generating negative pressures within the oral cavity to draw in water/air/food via suction. However, phylogenetically basal actinopterygians, fossil lungfish and other fossil sarcoptcrygians (such as Strepsodus) lack the apomorphies that increase suction among bony fishes. In these taxa the clavobranchialis muscles may serve to augment this negative pressure by retracting the ceratobranchials and increasing the size of the oral/ oropharyngeal cavity. A comparable action is performed by the chondrichthyan coracobranchiales muscles, particularly during feeding, and the function of these ventral gill arch muscles is likely to be a synapomorphy of jawed vertebrates (Gnathostomata). This musculature is absent from jawless vertebrates such as the Osteostraci.