A Toothy Tale of Evolution: Convergence in Tooth Morphology among Marine Mesozoic–Cenozoic Sharks, Reptiles, and Mammals

Abstract

Although mechanisms of niche replacement are discussed thoroughly in the evolutionary paleontological literature (i.e., extinctions, competition, evolution of new adaptive morphologies), actual studies involving quantitative analyses are not common. In this study, morphological features of dentition in Late Cretaceous and Cenozoic marine vertebrate predators were analyzed.The analysis included species of Late Cretaceous and Cenozoic sharks, Late Cretaceous marine reptiles, and Cenozoic marine mammals. Dental characters used in the study were both discrete and continuous. Species included in the analysis were originally collected from Late Cretaceous and Cenozoic rocks from the south-central, southeastern, and the mid-Atlantic regions of the United States, as well as Europe and the Pacific Rim. A morphometric “tooth space” was constructed using the eigenvectors generated from Principal Component Analysis of the dental character data.The results of the analysis show that Mesozoic marine reptiles occupied a small, discrete region of the tooth morphospace, whereas Cretaceous sharks occupied a much larger, diffuse region of the morphospace. During the Paleogene a profusion of shark tooth morphologies occurred and then expanded into new areas of tooth morphospace.Yet, no overlap with the morphospace previously occupied by Mesozoic marine reptiles occurred.A large number of novel tooth morphologies evolved with the evolution of marine mammals during the Cenozoic. Remarkably, many of the tooth forms converged on the Mesozoic marine reptile designs, and hence a major overlap of marine mammal tooth morphospace with the previously occupied Mesozoic marine reptile morphospace occurred.Additionally, the shift from heterodonty (teeth of different types) to homodonty (teeth of similar types) occurred in several members of both the Mesozoic marine reptiles and the Cenozoic marine mammals. Based on dental morphology, this study indicates that following the extinction of the Mesozoic marine reptiles during the Late Cretaceous, Cenozoic sharks failed to occupy the vacated niches, yet Cenozoic marine mammal dentition converged on the previous Mesozoic marine reptile tooth designs.Apparently, Cenozoic marine mammals occupied the vacated Mesozoic marine reptile dietary niches.