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

Abstract

Microstructural features of the mammalian tooth enamel are rarely used to construct phylogenies, although macromorphological characters of the dentition figure prominently in phylogenetic analysis. In order to test the phylogenetic significance of the enamel microstructures, we investigate here the earliest proboscideans recently found in the Early Palaeogene of Africa (e.g. Phosphatherium, Daouitherium, Khamsaconus, and Numidotherium). The results are discussed in the light of the recent advances concerning the intra- and interordinal relationships of the Proboscidea. We also consider other basal paenungulates such as 'anthracobunids', embrithopods, and hyraxes. The analysed microstructures suggest that the enamel ancestral morphotype of paenungulates was primitive for eutherian 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 developed very complex enamel, the 3-D enamel, which represents an apomorphy for the group. The three-layered Schmelzmuster, typical of the elephantoids (3-D enamel, HSB, and radial enamel), is acquired during the late Eocene with the enigmatic 'Numidotherium' savagei. This species is here considered as an advanced proboscidean along with Moeritherium-Deinotheriidae-Elephantiformes. The peculiar enamel of elephantoids arose step by step. Although homoplasy and mosaic evolution occur, the enamel microstructures represent an important source of new dental characters for phylogenetic reconstructions. As macromorphological characters testified, the diversity of the enamel microstructures observed in the various basal proboscideans illustrates an unexpected early diversity of the order in Africa. © 2007 The Linnean Society of London.