Ossification of the cranium of Inpaichthys kerri (Characiformes: Acestrorhamphidae) with discussion of paedomorphic traits in the genus.

The study of development provides valuable information on the evolution of morphological traits, enabling the detection of important evolutionary processes and revealing unique ontogenetic characteristics not observed in adult individuals. The Characiformes is one of the largest groups of Neotropical fish and includes several lineages of small species that have undergone a reduction in body size, a phenomenon often associated with paedomorphosis. In this study, we analyzed an ontogenetic series of a small characiform, Inpaichthys kerri, ranging from newly hatched individuals to adults, with the aim of describing in detail the development of the cranial skeleton and establishing its ossification sequence. Seventy-two skull bones were described, from the first signs of ossification to their adult forms, including the complete sequence of appearance of these structures throughout development. We also identified a unique developmental sequence for the infraorbital bones of I. kerri. We highlight the presence of paedomorphic characters found in I. kerri, which are shared by its congeners and by other small Characiform lineages, and discuss them based on ontogenetic and phylogenetic information. The information collected and discussed here is unprecedented and is valuable to the understanding of relationships within Acestrorhamphidae.