Background. The pycnodontiform fish Pycnodus is one of the representatives of the highly diverse actinopterygian fish fauna from the early Eocene Bolca Lagerstätte, representing one of the youngest and thus last occurrences of the extinct neopterygian clade Pycnodontiformes. This genus has historically been used as a wastebasket taxon in regards to poorly known pycnodontiform fossils and authors have argued over the specific status of the Bolca Lagerstätte Pycnodus in terms of how many species are contained within the genus with some arguing for multiple species and others suggesting lumping all Bolca specimens together into one species.
Methods. Here, we use a quantitative approach performing biometric and geometric morphometric analyses on 39 specimens of Pycnodus in order to determine if the morphological variability within the sample might be related to inter- or intraspecific variation.
Results. The analyses revealed that the variations of body shape, morphometric and meristic characters are continuous and cannot be used to distinguish different morphotypes. On the contrary, our results show a remarkable link between shape and size, related to ontogeny.
Discussion. Differences in body shape of small (juvenile) and large (adult) individuals is probably related to different microhabitats occupation on the Bolca reef with juveniles sheltering within crevices on the reef and adults being more powerful swimmers that swim above the coral. There is no evidence of nocturnal feeding in this pycnodont as previously hypothesized. Taxonomically, we suggest that the Bolca Pycnodus should be referred to strictly as Pycnodus apodus as this was the name given to the holotype. Additionally, an overview of species assigned to Pycnodus is given.