Numidotherium koholense

Tabuce, Rodolphe, Delmer, Cyrille & Gheerbrant, Emmanuel, 2007, Evolution of the tooth enamel microstructure in the earliest proboscideans (Mammalia), Zoological Journal of the Linnean Society 149 (4), pp. 611-628 : 616

NUMIDOTHERIUM KOHOLENSE

The numerous fragmentary dental specimens belonging to N. koholense allowed us to perform an exhaustive analysis of the enamel microstructure of that species. Bertrand (1989) conducted an important unpublished PhD related to this subject; in light of the more recent advances in enamel microstructural studies, we improve Bertrand’s analysis in discussing the Schmelzmuster of N. koholense . In the entire enamel thickness of incisors, canine, premolars, and molars, N. koholense shows thick bundles of prisms that decussate in all directions ( Fig. 5A ), this enamel type is termed 3-D enamel ( Pfretzschner, 1994). Therefore, at the dentition level, N. koholense is characterized by an invariable Schmelzmuster. The 3-D enamel also occurs in DP3–4.

The 3-D enamel is interpreted as resulting from the combination of both horizontal and vertical decussations. In some areas the vertical component is more attenuated than the horizontal one, which generates essentially horizontal decussations; this peculiar microstructure is evidently evocative of HSB ( Fig. 5B ). In a vertical section of a lower molar, the enamel thickness is around 1600 µm. At the prisms level, serial horizontal sections of molars show the following zonation: a thin layer (10–15 µm) of prismless enamel in the outermost part ( Fig. 5B ), thin zones of round–open prisms near both the OES and EDJ, and a thick intermediate zone of prisms showing a keyhole pattern ( Fig. 6A ). This pattern is observed in more than 80% of the enamel thickness. The diameter of the prisms varies from 5.5 to 7 µm.

Figures

Figure 5. A, Numidotherium koholense, early Eocene, El Kohol, Algeria; vertical section of an upper molar with thick bundles of prisms that decussate in all directions, this organization corresponds to the 3-D enamel and results in combinations of both horizontal and vertical decussations. B, same sample as (A): in some places the vertical component is more attenuated than the horizontal one, which generates essentially horizontal decussations; this peculiar and limited microstructure is evidently evocative of Hunter-Schreger bands (HSB). OES, outer enamel surface.

Figure 6. A, Numidotherium koholense, early Eocene, El Kohol, Algeria; vertical section of a lower molar showing zones of transition between areas of decussations; the prisms exhibit a keyhole cross section and the interprismatic matrix (IPM) is very reduced. B, Numidotherium savagei, late Eocene, Dor el Talha, Libya; vertical section of a lower molar with 3-D enamel near the enamel dentine junction (EDJ). C, Anthracobune pinfoldi, either late early or early middle Eocene, Kuldana Formation, Pakistan; horizontal section of a P4 near the outer enamel surface (OES), closed circles within hexagonal structures are typical of poorly mineralized areas of the outer zone; the hexagons represent the area of one ameloblast and the central circles may represent the trace of Tomes’ process. D, Seggeurius amourensis, early Eocene, El Kohol, Algeria; natural vertical section of an upper molar with a Schmelzmuster composed of radial enamel only. E, Crivadiatherium iliescui, late Eocene, Hateg Basin, Romania; vertical section of a molar showing modified radial enamel in the inner first-third of the enamel thickness; in this zone, the prisms are lanceolate in outline and the crystallites of the IPM are directed perpendicular to the prisms long axis.