Osteology of the Small‐Sized <i>Hyphessobrycon piabinhas</i> Fowler 1941 (Characiformes, Characidae) With a Discussion on Developmentally Truncated Characters in the Family

Evolutionary body size decrease has profound consequences for the morphology of an organism. In the evolution of the Characidae, the most species-rich family of Neotropical fishes, a prominent trend is the reduction of body size. The most typical effect is the simplification and reduction of morphological features through terminal deletion processes, resulting in the loss of skeletal elements and structures. To provide further information on the matter, we present a detailed description of the skeleton of Hyphessobrycon piabinhas, a poorly known, small representative of the largest genus of Characidae. We further discuss the identity and phylogenetic relationships of H. piabinhas. It belongs to the subfamily Stethaprioninae and exhibits considerable morphological similarity to other congeners from neighboring drainage systems. We identify several morphological simplifications in H. piabinhas and discuss them based on ontogenetic data available for Characiformes. These developmentally truncated elements are also present in many other small representatives of the family and seem to be among the first morphological modifications to occur in the context of body size reduction of Characidae. We argue that structural losses are not strictly correlated with sizes below 26 mm SL, although the most notable simplifications are typically observed in the miniatures.