A unique dental renewal mode in the extinct Scheenstia (Actinopterygii, Lepisosteiformes)

Abstract

The pattern of tooth development is genetically very stable during vertebrate evolution, while dental renewal mechanisms are much diversified. The Mesozoic fish Scheenstia shows a novel mode of tooth replacement not reported in any other taxon and implying a peculiar feature: the amelogenesis of replacement teeth occurs intraosseously in an upside‐down position. Based on microCT‐scan reconstruction, we describe the internal anatomy of a left lower jaw of Scheenstia sp. from the Late Jurassic of the Swiss Jura, comprising replacement teeth and their crypts, and cavities corresponding to nervous and vascular canals. We interpret this one‐to‐one, and likely simultaneous, renewal process as follows: amelogenesis, initiation of the 180° rotation of the enamel caps, ascension through individual replacement pores, dentinogenesis, completion of the tooth rotation and finally ankylosis to the bone through a pedicel. The reconstruction of the lower jaw also shows that it is strongly innervated, indicating a highly sensitive chin region in Scheenstia with the possible presence of sensitive dermal organs such as hypertrophied lips, or barbels. This replacement mode probably enhanced the protective effect of intraosseous tooth development but raises the question of its energetic cost.