Tooth development in the Early Devonian sarcopterygian Powichthys and the evolution of the crown osteichthyan dentition

Abstract

In osteichthyans (bony fishes) the dentition is characterized by marginal tooth rows replaced by basal resorption. Basal resorption was present in the stem osteichthyan Andreolepis, which also possessed overgrowing tooth-shaped odontodes, a possible intermediate state between external odontodes and teeth. Andreolepis, however, lacked the enlarged marginal tooth rows found in more advanced osteichthyans, and how these evolved from the stem osteichthyan condition is unknown. Here we present computed tomography and synchrotron-based x-ray phase-contrast imaging of the jaws and teeth of the Early Devonian sarcopterygian Powichthys. We reveal the presence of fields of overgrowing tooth-like odontodes, resembling the dentitions of stem-group osteichthyans, alongside the marginal tooth row. In the inner dental arcade, we show a continuous variation between regions of tooth-like and external odontode-like replacement, apparently driven by variations in the relative rates of bone growth, odontode addition and odontode size. These results suggest that there is a degree of plasticity in oral odontode development. We propose that the tooth rows of crown osteichthyans develop in an essentially similar manner to the teeth of stem osteichthyans, with differences explained by increased resorption activity linked to the larger size of the teeth. Therefore, the external odontodes and teeth of both stem and crown osteichthyan lie on the same developmental continuum.