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.

Reassessment of the Generic Attribution ofNumidotherium savageiand the Homologies of Lower Incisors in Proboscideans

“…In addition to bearing the same field number as the associated lower dentition described by Court (1995) and the mandibular symphysis described above (BMNH M. 82398; Court 1995: fig. 5H), its enamel displays the same three layered Schelzmuster (Rodolphe Tabuce, per− sonal communication 2006) observed on that of the lower molars of A. savagei (Tabuce et al 2007). This enamel struc− ture, unique among early proboscideans, is observed only in more derived elephantiforms and elephantoids (Tabuce et al 2007), and therefore is a key character in identifying A. savagei and in establishing its phylogenetic relationships.…”

“…5H), its enamel displays the same three layered Schelzmuster (Rodolphe Tabuce, per− sonal communication 2006) observed on that of the lower molars of A. savagei (Tabuce et al 2007). This enamel struc− ture, unique among early proboscideans, is observed only in more derived elephantiforms and elephantoids (Tabuce et al 2007), and therefore is a key character in identifying A. savagei and in establishing its phylogenetic relationships. This tooth is bilophodont and rectangular in outline, and shares with the M1 the presence of a developed lingual and mesial cingulum, associated with a weak entoflexus.…”

“…This was carried out following the same protocol that Tabuce et al (2007) used when they investigated the microstructure of the enamel of the lower teeth of N. savagei described by Court (1995). Without reducing the gross morphological information con− tent, the enamel fragments were, when available, removed from an identified part of the tooth and their positions re− corded.…”

“…The similarity of the M3 of A. savagei with that of Moeritherium is so striking that this isolated tooth was first recognised as be− longing to an unknown, enormous, lophodont Moeritherium (personal observation). Finally, analysis of the enamel micro− structure of A. savagei revealed that this taxon does not dis− play the typical Schmelzmuster composed of 3−D enamel that is shared exclusively with the other strictly lophodont pro− boscideans N. koholense, B. grave or the deinotheres (Remy 1976;Bertrand 1989;Tabuce et al 2007). Instead it displays a three−layered Schmelzmuster with 3−D enamel limited near the enamel−dentine junction, overlain by HSB (a specific mode of decussation in which prisms decussate in transversely oriented bands) then radial enamel in the outer zone, which is shared by all elephantoids (Bertrand 1989;Pfretzschner 1994;Ferretti 2003), and is therefore is even more derived than that of some early elephantiforms (Tabuce et al 2007).…”

Characteristics of the transverse 2D uniserial arrangement of rows of decussating enamel rods in the inner enamel layer of mouse mandibular incisors

“…Other investigators have used rod organizational patterns to help define possible evolutionary interrelationships among predecessors across specific mammalian orders (Kawai, ; von Koenigswald, ; Sahni, ; Martin, , , ; Stefen & Rensberger, ; Tabuce et al. ; von Koenigswald et al. ; Alloing‐Séguier et al.…”