Heterochrony leads to evolutionary novelty: the catfish pectoral-fin spine (Teleostei: Siluriformes)

Studies of sequence heterochrony have revealed changes in developmental timing during skeletogenesis, which are widespread among major groups of tetrapods and may be associated with major changes in morphology, life history and function. Despite the greater potential for heterochronic shifts to occur in actinopterygian fishes due to their more complex and morphologically diverse skeletons, similar studies within this major vertebrate group are rare. In this study, we provide an example of an association between heterochrony and the appearance of an evolutionary novelty: the pectoral-fin spine of catfishes, the highly modified anteriormost pectoral-fin ray. Using both Sequence ANOVA and PGi analyses, we show that the developmental onset of the pectoral-fin spine of catfishes is greatly pre-displaced in the sequence of ossification compared to the anteriormost pectoral-fin ray of non-siluriform otophysans. The catfish pectoral-fin spine represents a case of peramorphosis linked with a morphological and functional innovation of a highly diverse evolutionary lineage of actinopterygian fishes.