Abstract
Teeth are important elements in studies of modern and fossil Cetacea (whales, dolphins), providing information on feeding habits, estimations of age and phylogenetic relationships. The growth layer groups (GLGs) recorded in dentine have demonstrated application for aging studies, but also have the potential to elucidate life history phenomena such as metabolic or physiologic events. Micro-Computed Tomography (Micro-CT) is a non-invasive and non-destructive technique that allows 3-dimensional study of mineralized tissues, such as human teeth, and their physical properties. Teeth from extant dolphins (Cetacea: Odontoceti) and some fossil odontocetes were scanned in a Skyscan 1172 Micro-CT desktop system. X-rays were generated at 100 kV and 100 µA for extant samples, and at 80kV and 124 µA for fossils. 0.5 mm thick aluminum and copper filters were used in the beam. Reconstructed images were informative for most extant species, showing a good resolution of the enamel layer, dentine and pulp cavity. Greyscale changes in the dentinal layers were not resolved enough to show GLGs. Visualization of the internal structure in fossil cetacean teeth depended on the degree of diagenetic alteration in the specimen; undifferentiated enamel and dentine regions probably reflect secondary mineralization. However, internal details were finely resolved for one fossil specimen, showing the enamel, internal layers of dentine and the pulp cavity. Micro-CT has been proven to be a useful tool for resolving the internal morphology of fossil and extant teeth of cetaceans before they are sectioned for other morphological analyses; however some methodological refinements are still necessary to allow better resolution of dentine for potential application in non-destructive age determination studies.
Highlights
Teeth are a valuable tool in studies of fossil and extant animals, supplying data on the feeding habits, environmental influences, agonistic and display behaviours, phylogenetic relationships among species, and estimations of age (Ungar 2010)
By studying captive and known-age animals, it was demonstrated that dentinal layers, or growth layer groups, correspond to annual increments, having the potential to provide the age of the animal (Myrick 1991)
This paper aims to demonstrate the potential applications of Micro-CT in dental studies of fossil and living cetaceans, outlining the methodological approach, the advantages, and the limitations of this technique in comparison to other methods
Summary
Teeth are a valuable tool in studies of fossil and extant animals, supplying data on the feeding habits, environmental influences, agonistic and display behaviours, phylogenetic relationships among species, and estimations of age (Ungar 2010). In contrast with the eutherian dental standard, dolphins produce a single set of teeth that remain in place throughout their life (monophyodonty), the teeth are undifferentiated and simplified in shape to cones or pegs (homodonty) and they have a much-increased number of teeth (polydonty) compared to most terrestrial mammals (Flower 1885, Myrick 1991, Ungar 2010). These simplified teeth are covered by a cap of enamel, which is deposited before birth. By studying captive and known-age animals, it was demonstrated that dentinal layers, or growth layer groups, correspond to annual increments, having the potential to provide the age of the animal (Myrick 1991)
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