Stable isotope analyses (δ¹⁸O, δ¹³C) of Spirula spirula, a mesopelagic cephalopod with a loosely coiled internal calcareous (aragonitic) shell, suggest that δ¹⁸O is precipitated in equilibrium with the surrounding water, recording the temperature of the seawater inhabited by the animal through its life. The δ¹⁸O trends are interpreted to reveal a life that begins in deep waters characterized by cool temperatures before rising to warm surface waters to feed during its juvenile stage. Following this brief period in warmer waters, the isotopes suggest that the remainder of the organism's life is spent in progressively cooler (deeper) waters. The incorporation of isotopically light metabolic carbon, however, significantly affects the stable carbon isotope signal recorded in S. spirula, effectively obscuring the record of δ¹³C of seawater dissolved inorganic carbon archived in the shell carbonate. This may relate to the internal position of the shell, in which the growing margin is anchored in soft tissue and separated from the ambient seawater within the mantle cavity. By analogy, δ¹³C of extinct cephalopod shells may, thus, prove useful as a guide to the amount of soft tissue surrounding the growing margin of the shell. Changes in δ¹³C of the shell may also indicate a change of diet concurrently with the inferred rise to warm surface waters. The results of this study have important implications for understanding ancestors of S. spirula, such as belemnites, in terms of the constraints on equilibrium precipitation of shell carbonate, sought in terms in paleoenvironmental studies.