Phosphatherium escuilliei, Gheerbrant, Sudre & Cappetta, 1996

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 : 614

PHOSPHATHERIUM ESCUILLIEI

In a vertical section of an upper molar, the enamel is 500–900-µm thick. The Schmelzmuster is two-layered, with radial enamel in the outer zone and HSB in the inner one ( Fig. 2A ). The HSB represent more than 85% of the enamel thickness, and they start immediately at the EDJ. In some areas, HSB reach the OES. HSB start either perpendicular to the EDJ or with a little inclination; the bands can run straight outwards but they are frequently bent. The HSB are of variable width: they vary from three to more than 20 prisms; the bands are generally larger near the EDJ. Bifurcations of HSB occur in the entire thickness of the enamel. A tangential section of an upper molar shows pronounced undulations of the HSB ( Fig. 2B ). Band bifurcations occur dominantly with the changes of direction of the HSB.

At the prism level, Phosphatherium shows the following zonation: a very thin layer (6–23 µm) of prismless enamel in the outermost part ( Fig. 1B ), thin zones of round prisms near both the OES and EDJ ( Fig. 1D ), and thick intermediate zones of densely packed basally opened prisms that constitute the typical keyhole pattern ( Fig. 1C ). In this prism type, which is observed in 60% of the enamel thickness, the prisms are arranged in horizontal rows and in alternating positions. The prisms can be very compressed in the ‘ginkgo-tree-leaf ’ pattern ( Kosawa, 1978; Koenigswald & Sander, 1997). In this cross-section pattern, open prism sheaths touch each other and the remaining IPM is incorporated into the ‘tails’ of the prisms ( Koenigswald et al., 1993). In inner and outer zones, where prisms sheaths are closed and round, the IPM envelops the prisms; IPM crystallites show the same orientation to the long axis of prisms. The diameter of the prisms varies from 5 to 8 µm.

Figures

Figure 1. A, Khamsaconus bulbosus, earliest Eocene, N’Tagourt 2, Ouarzazate Basin, Morocco; natural vertical fracture of the DP4 (holotype and unique specimen) with radial enamel. B, Phosphatherium escuilliei, earliest Eocene, Ouled Abdoun Basin, Morocco; vertical section near the outer enamel surface (OES), with irregular prism cross sections that vary from open to closed. C, same sample as in (B) showing Hunter-Schreger bands (HSB) and the typical keyhole cross sections of the prisms in the ‘ginkgo-tree-leaf ’ pattern. D, same sample as in (B) and (C) near the enamel dentine junction (EDJ), the interprismatic matrix (IPM) crystallites show the same orientation as the long axis of prisms.

Figure 2. A, Phosphatherium escuilliei, earliest Eocene, Ouled Abdoun Basin, Morocco; vertical section of an upper molar with Hunter-Schreger bands (HSB) that represent more than 85% of the enamel thickness. B, Tangential section in the same species, HSB present pronounced undulations. EDJ, enamel dentine junction; OES, outer enamel surface.