Functional Adaptations for Load-Bearing in a Dermal Bone: The Pectoral Fin Spine of the Russian Sturgeon (Huso Gueldenstaedtii).

Dermal bone, which forms a variety of skeletal structures and persists in a wide range of extant vertebrates, evolved prior to endochondral bone which forms all mammalian load-bearing long bones. Sturgeons (Acipenseriformes) are an early-diverging extant lineage of actinopterygians that retain prominent post-cranial dermal skeletal elements. They retain a long robust spine at the leading edge of the pectoral fin, called the pectoral fin spines (PFS). PFS are bone elements that are present in many extinct and extant species of non-tetrapod jawed fish. In this study, we characterize the structure (light, polarized, micro-computed tomography and scanning electron microscopy), composition (FTIR, TGA, BMD), and mechanical properties (3-point bending and microindentation) of the PFS of the Russian sturgeon (Huso gueldenstaedtii). The microstructure of the PFS is highly organized as it is formed by dermal osteonal bone and parallel fibered bone. Its microarchitecture, along with high material toughness, anisotropy, and substantial ash content, enables the PFS to bear loads and function in both locomotion and protection. In addition, we show an interconnected network of neurovascular canals and ornamentations, features also found in PFS of other non-tetrapod jawed fish. Collectively, these findings demonstrate that dermal bone can form structurally organized, mechanically competent load-bearing elements and provide new insights into PFSs in ray-finned fish.