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Histological Observation of Paleoparadoxia Incisor from the Noto Peninsula,Japan

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Histological Observation of Paleoparadoxia Incisor from the Noto Peninsula,Japan Int J Oral-Med Sci 9(2):154-158,2010 Communication Histological Observation of Paleoparadoxia Incisor from the Noto Peninsula,Japan Kunihiro Suzuki,and Tomoyasu Yamamoto Department of Oral Medicine(Biology),and Research Institute of Oral Science,Nihon University School of Dentistry at Matsudo,Matsudo,Chiba 271-8587,Japan Kuzubukuro Earth Science Research Club,Sayama,Saitama 350-1317,Japan Correspondence to: Kunihiro Suzuki Desmostylian remains including Desmostylus and Paleoparadoxia have E-mail: suzuki.kunihiro50＠nihon- been reported from Miocene (5 to 20 Ma: Mega annum)deposits in u.ac.jp Japan and along the west coast of the North America.In this report, Keywords: we describe the histological characteristics of a Paleoparadoxia incisor. desmostylia, Paleoparadoxia, incisor, histology,Noto Peninsula specimens were observed by reflective light micros- Desmostylia, comprising six genera (Ashoroa, copy and by scanning electron microscopy(Hitachi S Behemotops, Cornwallius, Desmostylus, Paleoparadox- -2700). ia, and Kronokotherium) are extinct marine mam- mals found in late Oligocene and Miocene deposits in the northern Pacific coastal region(1,2).Desmostylus Two fragments of Paleoparadoxia incisor were and Paleoparadoxia have been found in Miocene detected in the marine deposits in 2004.They were deposits in several localities on the Noto Peninsula flat in shape and bilateral enamel around the dentin. in central Japan (3-5). One of the samples had two layers of enamel over the We performed histological analysis of Paleopar- dentin, and had a mesiodistal diameter of 6 mm, a adoxia incisors that were found by one of the authors, labiolingual diameter of 2 mm and length of 16 mm Yamamoto, at two Middle Miocene sites in Shiga (Fig.1-3). The other sample had lost significant Town,Ishikawa Prefecture.The root of an incisor amounts of enamel on one side.It had a mesiodistal was previously collected at another locality(5).The specimens described in this paper are housed in the Department of Oral Medicine (Biology),Nihon Uni- versity School of Dentistry at Matsudo. Two incomplete Paleoparadoxia incisor specimens assumed to be P.tabatai were obtained from the Middle Miocene Sekinohana Calcareous Sandstone (about 14 Ma)in Shiga Town,Ishikawa Prefecture. One of the samples provided for histological observa- tion had lost a significant amount of enamel on one side.It was cut longitudinally and horizontally into two pieces with a diamond cutter for three-dimen- sional observation.After polishing with a diamond paste and slight etching with 1/20 N HCl for 20-30 s, Fig.1. Enamel surface of the labial side.Measurement shown the specimens were sputter-coated with Au-Pd.The in millimeters. Int J Oral-Med Sci 9(2):154-158,2010 Fig.4. Enamel surface of the lingual side. Fig.2. Fractured area on the lateral side of the incisor. Dentin is covered by enamel on both sides. Fig.5. Exposed dentin shows uneven surface. Enamel remains at the edge of the lateral side. Fig.3. Enamel surface of the lateral side. (DEJ)was about 30°upward and about 20°near the diameter of 11 mm,a labiolingual diameter of 2 mm, surface of the enamel (Fig.6).The HSB changed and length of 22 mm.The dentin was exposed with gently in a horizontal direction about 500μm from an uneven surface(Fig.4,5).Shallow depressions of the enamel surface and several incremental lines various sizes were observed on the enamel surface in (striae of Retzius)were observed in this region (Fig. both specimens. 6).Each band was seen to unite and break up in the The maximum thickness of enamel was 1.2 mm in vicinity of 100μm from the DEJ (Fig.7).The bound- the middle part of the specimen. Hunter-Schreger aries of the bands could be observed clearly by light bands (HSB) ran through the whole width of the microscopy.However,they were indistinct on scan- enamel showing a moderate curve(Fig.6).The incli- ning electron microscopy because they changed grad- nation of the HSB to the dentinoenamel junction ually from each band to the adjacent band (Fig.8). Int J Oral-Med Sci 9(2):154-158,2010 Thick inter-prismatic substance filled up the gap and buried the prisms with orthogonal direction of the crystals(Fig.9).In the molar,HSB ran through the whole width of the enamel showing an S-shaped moderate curve(3). The enamel prisms were seen to meander in the tangential section (Fig.10).The enamel prism was almost elliptic or rectangular in shape,6μm along the long axis and 3μm along the short axis. The inter-prismatic substance was distinctive and sur- rounded the prism to a thickness of 1-2μm and the direction of the crystals was almost orthogonal to the prism (Fig.11). Fig.6. SEM image showing Hunter-Schreger bands on the longitudinal section. The dentinal tubules were preserved and thin Fig.7. SEM image showing enamel prisms at the Fig.9. SEM image showing enamel prisms and inter-pris- dentinoenamel junction. matic substance near the enamel surface. Fig.8. SEM image showing the Hunter-Schreger bands in Fig.10. SEM image showing undulating rows of enamel the middle area of enamel. prism on the tangential section. Int J Oral-Med Sci 9(2):154-158,2010 Fig.14. SEM image showing calcification lines and calcos- Fig.11. SEM image showing enamel prisms and inter-pris- pherites. matic substance on the tangential section. peritubular dentin was observed inside(Fig.12).The dentinal fibers were also preserved in the tubules, which included main and lateral branches, with a diameter of 0.2-2μm (Fig.13). The mineralizing layers (calcification lines)of the dentin were obser- ved in the deep area of the dentin (Fig.14). This description of the Paleoparadoxia incisor found on the Noto Peninsula enables comparisons with the molar (6)and premolar (7),and with the teeth of Desmostylus. This study was supported in part by a Grant-in- Fig.12. SEM image showing preserved dentinal fibers in the Aid for Scientific Research (C-20592154) from the dentinal tubules. Peritubular dentin is detected inside the Japan Society for the Promotion of Science and a tubules. Grant for Supporting Project for Strategic Research of Nihon University School of Dentistry at Matsudo by the MEXT (2008-2012). 1. Inuzuka N,Domning DP,Ray CE: Summary of taxa and morphological adaptations of the Desmostylia. The Island Arc,3: 522-537,1995. 2. Inuzuka N : Primitive late Oligocene desmostylians from Japan and phylogeny of the Desmostylia. Bull Ashoro Mus Paleontol,1: 91-123,2000. 3. Inuzuka N: Studies and problems on the order Des- mostylia.Monograph,28: 1-12,1984. 4. Kaseno Y : Occurrence and stratigraphical horizon of the Desmostylian fossils from Noto, Japan. Mono- graph,28: 69-72,1984. Fig.13. SEM image showing thin dentinal fibers in the lateral 5. Inuzuka N, Karasawa H : Some fossils of Paleopar- branches of the dentinal tubules. Int J Oral-Med Sci 9(2):154-158,2010 adoxia from Miocene Calcareous Sandstones on the 1995. Noto Peninsula,central Japan.Earth Science,40: 294 7. Kobayashi I, Kamei T : A histological study on a -300,1986. tooth of Paleoparadoxia. Memories of the Faculty of 6. Suzuki K, Takahashi Y, Fukuda R, Takagi H, Koz- Science,Kyoto Univ.,Series of Geology and Mineral- awa Y : A histological study on molar enamel of ogy,XL: 13-25,1973. Paleoparadoxia. Nihon Univ J Oral Sci, 21: 57-62,.
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