Abstract
Among medium- to large-sized terrestrial ‘ungulates,’ there is often a relationship between increasing body size, correlated changes in diet, and increased complexity of the enamel microstructures [notably the development of Hunter-Schreger bands (HSB)]. An exhaustive survey of the enamel microstructures of living and extinct Hyracoidea demonstrates, however, that the Schmelzmuster within this order of mammals is generally one-layered and formed by radial enamel despite a large range of body sizes and dietary adaptations; HSB are remarkably absent. Radial enamel is characteristic of early diverging hyracoids, as well as more derived members of the extinct families Geniohyidae and Pliohyracidae, and the extant Procaviidae. Only some large ‘Saghatheriidae,’ and all members of the family Titanohyracidae, developed a more complex enamel microstructure (i.e., with prisms decussating), a unique condition among Mammalia that we name ‘bundled enamel’ (BE). This structure is reminiscent to some degree of both the ‘Pyrotherium enamel’ and the ‘3D enamel’ of proboscideans. Hyracoids with BE represented a major component of the diversity of mid- to large-sized herbivores during the Paleogene in Africa. Like HSB, which are developed by most other ‘ungulates,’ the BE is regarded as a device for resisting propagation of cracks during mastication. Hyracoids never developed however the ‘modified radial enamel’ that is characteristic of most large and hypsodont perissodactyls and artiodactyls that entered Africa during the Miocene.
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Acknowledgments
We wish to thank François Pujos (Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Mendoza) and Pierre-Olivier Antoine (Institut des Sciences de l’Évolution, Montpellier) who invited us to contribute to this volume. We are also very grateful to the many curators and colleagues who provided access to material in their care, or allowed us to study material for this project, particularly Gregg Gunnell (Division of Fossil Primates, Duke University Lemur Center, Durham), Joséphine Lesur, Sevket Sen, and Pascal Tassy (Muséum National d’Histoire Naturelle, Paris), Jean Remy (Institut des Sciences de l’Évolution, Montpellier), Mohammed Mahboubi (Université d’Oran), Daniel Brinkman and Christopher Norris (Yale Peabody Museum of Natural History, New Haven), Elmar Heizmann and Reinhard Ziegler (Staatliches Museum für Naturkunde Stuttgart) and (late) Karl Alban Hünermann (Paläontologisches Institut der Universität Zürich). We also thank Mikael Fortelius (Department of Geosciences and Geography, University of Helsinki) for providing several images of Titanohyrax enamel microstructure, and Chantal Cazevielle (CRIC, Montpellier) and Didier Cot (IEM, Montpellier) for SEM assistance. Finally, we would like to thank the two anonymous reviewers for their valuable comments and suggestions to improve the quality of this paper. ERS thanks the U.S. National Science Foundation (grants BCS-0416164, BCS-0819186, BCS-1231288), the Leakey Foundation, and Ann and Gordon Getty for supporting fieldwork in Egypt. RT thanks the French National Research Agency (ANR-ERC PalAsiAfrica Program/ANR-08-JCJC-0017 grant).
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Tabuce, R., Seiffert, E.R., Gheerbrant, E. et al. Tooth Enamel Microstructure of Living and Extinct Hyracoids Reveals Unique Enamel Types Among Mammals. J Mammal Evol 24, 91–110 (2017). https://doi.org/10.1007/s10914-015-9317-6
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DOI: https://doi.org/10.1007/s10914-015-9317-6