Evolution of the tooth enamel microstructure in the earliest proboscideans (Mammalia)

Abstract

UMR 5143 CNRS, Paleobiodiversite et Paleoenvironnements, CP38, Departement Histoire de la Terre, Museum National d’Histoire Naturelle, 8 Rue Buffon, 75005 Paris, FranceReceived September 2005; accepted for publication June 2006Microstructural features of the mammalian tooth enamel are rarely used to construct phylogenies, although mac-romorphological characters of the dentition figure prominently in phylogenetic analysis. In order to test the phylo-genetic significance of the enamel microstructures, we investigate here the earliest proboscideans recently found inthe Early Palaeogene of Africa (e.g. Phosphatherium, Daouitherium, Khamsaconus, and Numidotherium). Theresults are discussed in the light of the recent advances concerning the intra- and interordinal relationships of theProboscidea. We also consider other basal paenungulates such as ‘anthracobunids’, embrithopods, and hyraxes. Theanalysed microstructures suggest that the enamel ancestral morphotype of paenungulates was primitive for euth-erian mammals, consisting in radial enamel. Some basal proboscideans developed decussations of prisms in Hunter-Schreger bands (HSB), as did most of the medium to large-sized mammals. More evolved proboscideans developedvery complex enamel, the 3-D enamel, which represents an apomorphy for the group. The three-layered Schmelz-muster, typical of the elephantoids (3-D enamel, HSB, and radial enamel), is acquired during the late Eocene withthe enigmatic ‘Numidotherium’ savagei. This species is here considered as an advanced proboscidean along withMoeritherium-Deinotheriidae-Elephantiformes. The peculiar enamel of elephantoids arose step by step. Althoughhomoplasy and mosaic evolution occur, the enamel microstructures represent an important source of new dentalcharacters for phylogenetic reconstructions. As macromorphological characters testified, the diversity of the enamelmicrostructures observed in the various basal proboscideans illustrates an unexpected early diversity of the order inAfrica. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 149, 611–628.