Abstract
Cetaceans are unusual mammals that have modified their morphological, physiological and behavioural systems to an aquatic existence. Access to new food sources and ecological niches may have been the trigger for this transition. The skull and feeding apparatus underwent drastic changes from the typical mammalian bauplan. Odontocetes show marked craniofacial changes associated with the ability to echolocate together with the development of elongated rostra and jaws bearing a large, yet simplified dentition. The teeth of odontocetes are peculiar when compared to most mammals, as monophyodonty, homodonty and polydonty are the trend for the group. Teeth have long been used to elucidate aspects of the ecology, functional morphology and systematics of fossil and recent mammal species. The aim of this research was to investigate the morphology, structure and biomechanics of odontocete teeth, in an effort to explore the functional implications and the evolution of the cetacean feeding apparatus. The multidisciplinary approach used here involved morphological description and basic morphometric measurements, as well as more refined techniques such as scanning electron microscopy, nanoindentation testing, geochemical analyses and micro-CT. The focus was in the Delphinida and Platanistoidea, to characterize a formerly speciose clade (Platanistoidea) versus an extant diverse clade (Delphinida). Some archaeocetes and early odontocetes were also analyzed. Results show that there was a high degree of morphological experimentation during the evolution of cetaceans, presumably to explore diverse niches and food sources. A more plastic configuration of the feeding apparatus in delphinoids may have been among the key aspects for the present diversity of the group when compared to a more fixed and conserved morphology in platanistoids and relict extant inioids. The morphological adaptation to an aquatic lifestyle was also reflected in the ultrastructure and mechanical properties of dental tissues in odontocetes. In early cetaceans, enamel was more complex and organized in Hunter-Schreger bands, which suggested a biomechanical adaptation related to food processing. Conversely, most living delphinoids showed a simpler radial or even prismless enamel structure. The lack of occlusion and food processing in most extant Delphinida was also reflected in the hardness and elastic modulus values for dolphin enamel and dentine, which were relatively lower than other mammals. Chemical analyses of odontocete enamel and dentine revealed a conserved tooth chemistry as in many other mammals, with calcium and phosphate being the main components, and minor elements such as magnesium, chlorine, sodium and fluoride. Chemical differences between enamel and dentine evoked different proportions of mineral versus organic content. While Micro-computed tomography was demonstrably useful as a non-destructive method for morphological analysis of extant teeth, it was less so in fossil teeth due to diagenetic alteration. Finally, massive levels of dental erosion, a process resulting in loss of enamel and dentine caused by acid demineralization, was reported and described for some extant delphinids. The multidisciplinary approach of this research allowed a broader characterization of the evolutionary changes and constraints in the structure of the teeth and feeding apparatus of odontocetes, contributing to the understanding of their functional morphology, general biology and evolution.