Key innovations permit clades to infiltrate ecological niches, modifying evolutionary constraints on natural selection and engendering rapid diversification. The remarkably diverse Loricariidae family (order Siluriformes) is important to the study of key innovations: these catfish are ‘armored' by bony plates (scutes) and mineralized cones (denticles) which arise before sexual maturity and persist through adulthood, indicating their importance to reproductive success and longevity. Previous work has suggested that denticles are newly expressed teeth. Here, morphological characterization of the developing exoskeleton in three species defined a spatially consistent sequence of maturation and mineralization. Homology of teeth and denticles was corroborated by structural similarities, while genetic analyses suggested that key genes in tooth development are present in the juvenile catfish epidermis. These combined data give a richer understanding of the relationship between denticles and teeth, which has implications for studies of speciation, adaptation, and genomic flexibility.