Meeting Abstract
Our understanding of the evolution of skull kinesis in fishes requires detailed analysis of feeding in living polypterids, a key lineage due to their phylogenetic position as the earliest-branching extant actinopterygian group. Polypterus skulls contain an upper jaw fused to the neurocranium, eliminating one axis of cranial kinesis utilized by teleosts and Amia in generating suction. However, their skulls remain highly kinetic, using dorsal, ventral, and lateral expansion to generate suction. Additionally, mechanisms of lower jaw depression and the degree to which pectoral girdle and ceratohyal retraction are transferred to the lower jaw through the mylohyoid ligament are poorly understood in this species. This study describes the major patterns of 3D mechanics driving buccal expansion and suction feeding in Polypterus bichir using X-Ray Reconstruction of Moving Morphology (XROMM). Cranial elevation peaks early in the strike, followed shortly by maximal lower jaw rotation, then cleithral and hyoid rotation, which occur nearly synchronously when variables are measured relative to the body axis. As in many other suction feeding fishes, opercular abduction undergoes the final kinematic peak of the cranial elements, maintaining the anterior to posterior movement of water into and through the mouth. Results show substantial cleithral rotation, largely concurrent with ceratohyal rotation and jaw opening, suggesting a central role of the pectoral girdle in jaw opening and suction generation. This pectoral girdle – hyoid bar – lower jaw depression mechanism is likely driven by the hypaxial musculature and may be the ancestral condition for ray-finned fishes.
