Nature of Apatite Crystals in the Tooth of <i>Eusthenopteron</i> from Devonian

Eusthenopteron come under the rhipidistians. Little information is available regarding the ultrastructure and properties of tooth in Eusthenopteron. The purpose of the present study is to examine the nature of apatite crystals in the tooth of Eusthenopteron. Backscattered electron image of SEM revealed the tooth consisted of two layers, tentatively named as the bright surface layer and the dark inner dentin layer, respectively. The surface layer was more calcified than the inner dentin layer. The incremental lines were not observed in the surface layer. Narrow dentinal tubules were confirmed in the inner dentin layer. TEM study demonstrated the crystals of surface layer were not bearing the central dark lines (CDL-free type) in its structures. By contrast, the crystals of the inner dentin layer possessed the central dark lines (CDL-bearing type). X-ray diffraction analysis suggested that the crystal was fluorapatite in the surface layer, and a mixture of hydroxyapatite and fluorapatite in the inner dentin layer. The presence of fluorapatite in the dentin was estimated to be the influence of the fossilization. Using EPMA, F, Al, Si, Ca, and P were detected in the surface layer, and F, Na Mg, Si, Ca, and P were detected in dentin layer. The weight % F of the surface layer was 3.07, and 3.35 in the inner dentin layer. Raman spectrum analysis demonstrated that the phosphate peaks of 965 cm-1 assigned for hydroxyapatite in the inner dentin layer and 967 cm-1 assigned for fluorapatite in the surface layer were detected, respectively. Taking the crystallographic viewpoint and histological feature into consideration, the surface layer was regarded as enameloid and the inner dentin layer was orthodentin including plicidentin.