Since the Permian, Earth's aquatic ecosystems have been ecologically dominated by numerous lineages of predatory amniotes. Many of these groups evolved elevated ridges of enamel that run down the apical-basal axis of their teeth, referred to here as apicobasal ridges. This trait is commonly used as a taxonomic tool to identify fossil species and higher groupings, but the function of the ridges and their associated ecological significance are poorly understood. Here, we aim to clarify the phylogenetic distribution of apicobasal ridges among amniotes and to examine how the morphology of apicobasal ridges varies across species. We show that these ridges have evolved independently numerous times and are almost exclusively found in aquatic-feeding species. Ridge morphology varies, including tall, pronounced ridges, low, undulating ridges and interweaving ridges. Their internal structure also varies from tooth crowns with locally thickened enamel to undulating enamel-dentine interface. We assess the relative merits of different hypothetical functions of the ridges and propose that although apicobasal ridges might provide some strengthening of the tooth, their morphology and pattern of evolution do not indicate that this is their primary function. Instead, we suggest that apicobasal ridges serve to increase the efficiency of puncture, grip and/or removal.
- convergent evolution