EXPLORING THE METABOLIC RATE OF DIFFERENT ECTOCOCHLEATE CEPHALOPODS USING STABLE CARBON ISOTOPES OF SHELL MATERIAL

Reconstructing the metabolic rate of fossil organisms is key to understanding their evolution. The metabolic rate of extant organisms is commonly determined by measuring the oxygen consumption in laboratory experiments. However, direct observations are difficult in extinct organisms. Therefore, paleontologists attempt to reconstruct the metabolic rate of extinct organisms using different approaches. For example, in cephalopods, a recent study suggested that ammonoids possessed a higher metabolic rate than nautiloids based on the pattern of chamber volume development. To test this hypothesis, another approach is needed to determine the metabolic rate of extinct organisms. The carbon isotopic value (δ ¹³C) of the shell is a function of both metabolic carbon in modern cephalopods and the dissolved inorganic carbon (DIC) in the water. Furthermore, another recent study reports that the metabolically derived carbon in modern cephalopods is correlated with metabolism-related factors. In this study, we use the fraction of metabolic carbon (C_meta) as a proxy for the metabolic rate of cephalopods. We analyzed the δ ¹³C of the shell in the modern nautilid Nautilus macromphalus to estimate the fraction of metabolic carbon (C_meta). Additionally, we explore the metabolic rate of extinct ectocochleate cephalopods (ammonoids and nautilids) using published values of δ ¹³C_shell and δ ¹³C_meta to determine which group possessed a higher metabolic rate.