DEAN, M.N.*; HUBER, D.R.; NANCE, H.; U. Calif. Irvine; U. S. Florida; U. N. Carolina Chapel Hill: Functional morphology of jaw trabeculation in Narcine brasiliensis
Design of minimum-weight structures that retain their integrity under dynamic loading regimes has long challenged engineers and functional morphologists. A solution to this problem found in both human and biological design is weight/strength optimization by hollowing a structure and replacing its core with supportive struts. In animals, this is observed in sea urchin test, avian beak and wing bone, and tetrapod cancellous bone. Additionally, within elasmobranch fishes, mineralized trabeculae (struts) have been reported singularly in durophagous myliobatid stingrays (Elasmobranchii: Batoidea), and are believed to be absent in basal members of the batoid clade. However, this study presents a secondary case of batoid trabeculation in the lesser electric ray, Narcine brasiliensis. While the orientation of myliobatid trabeculae is perpendicular to the occlusal surface of the jaws, high-resolution X-ray images reveal that the trabeculae of N. brasiliensis are arranged in the frontal plane, normal to the jaws’ long-axis and parallel to the occlusal surface. Stingrays use their reinforced jaws to crush bivalves, yet N. brasiliensis feeds by ballistically protruding its jaws into the sediment to retrieve polychaetes. At peak protrusion, the jaw arch is medially compressed such that the trabeculae are positioned to resist excavation forces. These struts are localized to areas likely to experience the highest load: the intrinsic jaw joints, hyomandibular-cranial joint, and the thinnest sections of the jaws immediately lateral to the symphyses. At these locations, the supports are positioned to resist both compression at the jaw joints, and local buckling of the jaws as they contact the sediment. Thus, trabeculation in batoids appears to perform strikingly different ecological functions, and was either independently derived twice or secondarily lost by intermediate members of the Batoidea.
