Development of the caudal fin and supporting skeleton in Lophiosilurus alexandri (Siluriformes: Pseudopimelodidae) and insights into its evolution in the family

Abstract The ontogeny of the caudal skeleton in Lophiosilurus alexandri, a pseudopimelodid endemic to the São Francisco River basin, was examined to clarify the development and homology of caudal elements. Twenty-six specimens, from newly hatched larvae to adults, were analyzed using cleared-and-stained preparations, dried skeletons, and radiographs. Five stages were identified: yolk-sac, flexion, postflexion, juvenile, and adult. The caudal fin first appears as a finfold, with principal and procurrent rays gradually ossifying. Hypurals 1–5 form early as cartilaginous elements, and later fusions generate the compound terminal centrum (PU1+U1+2) and two major hypural plates (PH+H1+2 and H3–5). The epural ossifies late in juveniles and develops a hook-shaped proximal process. Uroneurals fuse to the compound centrum during the juvenile–adult transition, forming the pleurostyle. A detached U3 forms early and later fuses with PU1+U1+2, a pattern typical of Siluriformes and considered plesiomorphic for L. alexandri and Pseudopimelodidae. Variation in principal and procurrent rays during ontogeny likely reflects functional demands. These findings detail the developmental sequence and homologies of caudal elements in L. alexandri, providing a comparative basis for understanding caudal fin evolution in Siluriformes.