Paleontological Research Vol. 6 No.3 September 2002 The Palaeontological Society 0 pan Co-Editors Kazushige Tanabe and Tomoki Kase Language Editor Martin Janal (New York, USA) Associate Editors Alan G. Beu (Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand), Satoshi Chiba (Tohoku University, Sendai, Japan), Yoichi Ezaki (Osaka City University, Osaka, Japan), James C. Ingle, Jr. (Stanford University, Stanford, USA), Kunio Kaiho (Tohoku University, Sendai, Japan), Susan M. Kidwell (University of Chicago, Chicago, USA), Hiroshi Kitazato (Shizuoka University, Shizuoka, Japan), Naoki Kohno (National Science Museum, Tokyo, Japan), Neil H. Landman (Amemican Museum of Natural History, New York, USA), Haruyoshi Maeda (Kyoto University, Kyoto, Japan), Atsushi Matsuoka (Niigata University, Niigata, Japan), Rihito Morita (Natural History Museum and Institute, Chiba, Japan), Harufumi Nishida (Chuo University, Tokyo, Japan), Kenshiro Ogasawara (University of Tsukuba, Tsukuba, Japan), Tatsuo Oji (University of Tokyo, Tokyo, Japan), Andrew B. Smith (Natural History Museum, London, Great Britain), Roger D. K. Thomas (Franklin and Marshall College, Lancaster, USA), Katsumi Ueno (Fukuoka University, Fukuoka, Japan), Wang Hongzhen (China University of Geosciences, Beijing, China), Yang Seong Young (Kyungpook National University, Taegu, Korea) Officers for 2001-2002 Honorary President: Tatsuro Matsumoto President: Hiromichi Hirano Councillors: Shuko Adachi, Kazutaka Amano, Yoshio Ando, Masatoshi Goto, Hiromichi Hirano, Yasuo Kondo, Noriyuki Ikeya, Tomoki Kase, Hiroshi Kitazato, Itaru Koizumi, Haruyoshi Maeda, Ryuichi Majima, Makoto Manabe, Kei Mori, Hirotsugu Nishi, Hiroshi Noda, Kenshiro Ogasawara, Tatsuo Oji, Hisatake Okada, Tomowo Ozawa, Takeshi Setoguchi, Kazushige Tanabe, Yukimitsu Tomida, Kazuhiko Uemura, Akira Yao Members of Standing Committee: Makoto Manabe (General Affairs), Tatsuo Oji (Liaison Officer), Shuko Adachi (Finance), Kazushige Tanabe (Editor in Chief, PRJ, Tomoki Kase (Co-Editor, PRJ, Kenshiro Ogasawara (Planning), Yoshio Ando (Membership), Hiroshi Kitazato (Foreign Affairs), Haruyoshi Maeda (Publicity Officer), Ryuichi Majima (Editor, "Fossils"), Yukimitsu Tomida (Editor in Chief, Special Papers), Tamiko Ohana (Representative, Friends of Fossils). 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Phone: (978)750- 8400, Fax: (978)750-4744, www.copyright.com Cover: Typical Pleistocene fossils from the Japanese Islands. Front cover: Sinomegaceros yabei (Shikama). Back cover: Paliurus nipponicum Miki, Mizuhopecten tokyoensis (Tokunaga), Neodenticula seminae (Simonsen and Kanaya) Akiba and Yanagisawa and Emiliania huxleyi (Lohmann) Hay and Mohler. All communication relating to this journal should be addressed to the PALAEONTOLOGICAL SOCIETY OF JAPAN c/o Business Center for Academic Societies, Honkomagome 5-16-9, Bunkyo-ku, Tokyo 113-8622, Japan Visit our society website at http://ammo.kueps.kyoto-u.ac.jp/palaeontl Paleontological Research, vol. 6, no. 3, pp. 239-258, September 30, 2002 © by the Palaeontological Society of Japan Enamel microstructure of some fossil and extant murid rodents of India RAJEEV PATNAIK Centre of Advanced Study in Geology Panjab University, Chandigarh-160014, India (e-mail: [email protected] ) Received July 7, 2000; Revised manuscript accepted April 9, 2002 Abstract. Spatial arrangement of various enamel types (schmelzmuster) present in the incisors and molars of some fossil and extant murid rodents of India was studied from both the functional and phylogenetic points of view. Hunter-schreger bands (HSBs) along with radial enamel (RE) in mice molars have been found to oc­ cupy the entire height of the enamel crown (from the base to the top) on the anterior and the posterior por­ tions. These HSBs tend to be horizontal around the base and inclined apically around the top. A clear distinction between the leading and the trailing edges of chewing surfaces based on the difference in the ori­ entation of prisms has been observed in hypsodont murid molars. On the leading edges, the long axes of prisms originating from the enamel-dentine junction tend to be oriented towards the load, whereas those on the trailing edges turn away from the load. The schmelzmuster in molars of Mus, indicate an omnivorous diet, whereas that in Golunda, Millardia, and Bandicota points to adaptation for an abrasive diet. The Indian bandicoot rat (Bandicota) with its large, hypsodont molars has developed horizontally oriented (relative to the occlusal surface) HSBs at the base of the enamel crown. These HSBs have been found in enamel layers ori­ ented both almost parallel and perpendicular to the occlusal surface, an observation that corroborates the presence of horizontal tension at the base of the tooth due to vertical load on the occlusal surface. In the light of the observations made here, a model depicting changes in schmelzmuster in murid rodents through Late Miocene and Plio-Pleistocene times is suggested. Key words: enamel microstructure, functional morphology, India, Muridae, phylogeny, rodent Introduction arranged in layers or zones, and when prisms of alternate layers run in different directions, a decussating structure is The enamel microstructure can be studied at various produced called 'Hunter-Schreger bands' (HSBs) (Korven­ hierarchical levels (Koenigswald and Clemens, 1992; kontio, 1934). Complexly interwoven bundles of prisms Carlson, 1990). Dental enamel in all mammals is made up that are not arranged in discrete layers are called 'irregular of hydroxyapatite 'crystallites'. These fiber-like crystal­ enamel' (Koenigswald and Clemens, 1992; Koenigswald, lites are arranged almost parallel to each other to form bun­ 1997) and prisms with a strong lateral deviation relative to dles called 'prisms' and these in tum are surrounded by an the enamel surface are termed 'tangential enamel' 'interprismatic matrix' (lPM) which is also made up of (Koenigswald, 1977). The three-dimensional arrangement crystallites (Wahlert and Koenigswald, 1985). The crys­ of different enamel types within a tooth define its tallites of IPM mayor may not run parallel to the prism 'schmelzmuster' (Koenigswald, 1980) and the variation of they surround (Martin, 1990). Because of the difference in schmelzmuster from tooth to tooth defines the 'dentition' the orientation of prism and interprismatic crystallites, dis­ level of the enamel microstructure hierarchy. tinct prism boundaries called as 'prism sheathes' are The enamel microstructure of rodent incisors is quite formed. The path of a prism can be traced from the different from that of their molars due to their special enamel-dentine junction (EDJ), through the entire thickness functional requirements (Koenigswald et aI., 1987, 1994). of the enamel, terminating at the outer surface. 'Enamel Rodent incisors are ever-growing, and their main function types' are defined by the spatial arrangement of groups of is to cut and dig. In the majority of rodents, incisor prisms. The part of the enamel in which prisms run paral­ enamel, which covers only the labial portion, is made up of lel to each other is termed 'radial enamel' (RE) outer radial enamel and inner HSBs oriented transversely to (Koenigswald, 1977). In many mammals, prisms are the long axis. Usually HSBs occupy at least 50% of the 240 Rajeev Patnaik enamel thickness of rodent incisors, but in murid and many Transverse section caviomorph rodents HSBs can occupy up to 80% (Martin, 1992). The thickness of individual bands of prisms also ,Longitudinal Section differs among taxa. A uniserial HSB is 1-2 prism thick, pauciserial 2-4 prisms thick, and multiserial more than 4 4L~---"'-~~4M -section parallel prisms thick (Korvenkontio, 1934; Martin, 1993). Murid .." to the occlusal rodents have only uniserial HSBs. surface The main objectives of the present work are 1) to docu­ ment the complexity of the enamel microstructure in some B of the Plio-Pleistocene murids and their extant counterparts collected from India, and 2) to trace the functional adapta­ A tion back in time based on comparison of fossil and Recent forms. There are certain morphological characters (such OPE as hypsodonty, relative width of molars and orientation of IPE transverse crests) in murid molars which can be correlated with a grazing diet (Crabb, 1976; Meulen and Musser, 1999). This grazing property of mammalian molars is also reflected at the enamel microstructure level (Rensberger, 1973 1975; Pfretzchner, 1994; Janis, 1988; Fortelius, 1985; D Weijs, 1994). Therefore, an attempt has been made here Transverse section to discover the dietary adaptations (grazing or browsing) of I angle of inclination .