Tanabe, K., Shigeta, Y., Sasaki, T. & Hirano, H. (eds.) 2010. Cephalopods - Present and Past Tokai University Press, Tokyo, p. 67-75. Introduction The ecology of extinct animals has primarily been estimated based on fossil morphology and analogy to modern relatives. Even for extant species, the natural lives of organisms are poorly understood due to difficulties in undertaking direct observations of their life habits in nature. The composition of the skeletal remains of such organisms, however, potentially records their ecology as well as their living environment, thereby providing a geochemical approach to the indirect observation of remote and past lives. One of the most successful applications in this regard is stable oxygen isotopic analyses of carbonate mollusk shells, from which the ambient water temperature can be inferred. By determining the oxygen isotopic compositions of different stages of marginal-growing shell carbonate, for example, the life histories of both extant and extinct cephalopods can be reconstructed in terms of their living habitat (e.g., Oba et al., 1992; Landman et al., 1994; Moriya et al., 2003). Analyses of the stable nitrogen isotopes of organic matter within calciied skeletal remains are potentially useful in estimating the nutritional sources during an animal’s lifetime. The stable nitrogen isotopic composition of animal organic tissue is known to relect the animal’s diet, with a systematically increase in δ 15 N values toward the upper part of the hierarchy of the food web (e.g., Hobson and Welch, 1992; Keough et al., 1996; Yoshii et al., 1999; Ogawa et al., 2001; Post, 2002). The mean enrichment factor of δ 15 N in body tissue is ~3.4‰ per trophic step (e.g., DeNiro and Epstein, 1981; Minagawa and Wada, 1984). Calciied Reconstructing the life history of modern and fossil nautiloids based on the nitrogen isotopic composition of shell organic matter and amino acids YUICHIRO KASHIYAMA 1, 2* , NAOKO O. OGAWA 1 , YOSHITO CHIKARAISHI 1 , NAPUSSAKORN KASHIYAMA 1 , SABURO SAKAI 1 , KAZUSHIGE TANABE 3 , AND NAOHIKO OHKOUCHI 1 1 Japan Agency for Marine-Earth Science and Technology 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 2 Present Address: Department of Chemistry, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571 Ibaraki, Japan (*correspond- ing author, e-mail: chiro@chem.tsukuba.ac.jp) 3 Department of Earth and Planetary Science, University of Tokyo 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Received April 30, 2008; Revised manuscript accepted September 22, 2008 Abstract. We determined the nitrogen isotopic compositions of organic matter in the shell material of mod- ern nautilids (Nautilus pompilius) and a mid-Cretaceous cymatoceratid (Cymatoceras sakalavus from the Albian of Madagascar) to reconstruct the trophic ecology in natural habitats and its change over the animals’ lifetimes. The profiles of δ 15 N values along the growth direction exhibited a common pattern in modern Nautilus specimens, suggesting three ontogenetic stages in the trophic ecology of the animal. The Embry- onic Stage, represented by the innermost shell formed during embryogenesis, is characterized by relatively elevated δ 15 N values (12–14‰). The δ 15 N values show a sharp decrease (by approximately 3‰) shortly after hatching of the shell, recorded between the 7th and 12th septa (the Post-hatching Stage). In the following Juvenile-mature Stage, δ 15 N values exhibit a relatively stable pattern in the range 10–12‰. The decrease in δ 15 N values during the transition from the Embryonic Stage to the Juvenile-mature Stage probably relects a change in nutrient source from yolk to a diet. Analysis of shell material formed in an aquarium following capture from the ocean revealed a sudden drop in δ 15 N values, probably relecting the change from natural to artiicial diet. The compound-speciic nitrogen isotopic compositions of amino acids from the shell of N. pompilius suggest that the above isotopic patterns obtained from the bulk material do indeed relect changes in the apparent trophic level throughout its life. In contrast to modern Nautilus, δ 15 N values obtained for C. sakalavus show a gradual increase from 1.2 to 3.8‰ with increasing septum number, suggesting an increase of approximately one trophic step in the food web throughout its life. Key words: Nautilus pompilius; fossil nautiloid; nitrogen isotopes; trophic level; amino acids trophic level; shell organic matter.