Stable isotope analyses were performed on ontogenetic dissections of four taxa of low latitude Late Cretaceous planktonic foraminifera from DSDP Hole 390A. The species studied include Planoglobulina acervulinoides, Planoglobulina multicamerata, Pseudoguembelina palpebra, and Racemiguembelina fructicosa. Delta18O and delta13C data indicate a deeper surface water paleohabitat for P. multicamerata than the other three taxa, and ontogenetic increases in delta18O values suggest all these taxa underwent vertical migrations from shallow to deeper surface waters. Changes in delta13C values through ontogeny include sharp increases in delta13C composition in the juvenile size intervals, a decrease in the rate of delta13C change through intermediate size intervals, and reversals to a negative trend in delta13C values in terminal size intervals. The intermediate and terminal growth changes in delta13C signals are similar to ontogenetic trends observed in some extant and Paleogene planktonic foraminifera and may result from decreasing metabolic rates through ontogeny or endosymbiont digestion prior to gametogenesis. The ontogenetic delta13C increases of 1.04‰, 0.76‰, 0.83‰, and 0.77‰ in R. fructicosa, P. palpebra, P. acervulinoides, and P. multicamerata, respectively, may indicate the presence of photosymbionts. However, our review and critique of the current literature discussing photosymbiont effects on stable isotope values in living and fossil planktonic foraminifera suggests that conclusions regarding the presence of photosymbionts in fossil taxa may be more equivocal than previously thought.

Supplement to: Houston, Ryan M; Huber, Brian T (1998): Evidence of photosymbiosis in fossil taxa? Ontogenetic stable isotope trends in some Late Cretaceous planktonic foraminifera. Marine Micropaleontology, 34(1-2), 29-46