Evolutionary history biases inferences of ecology and environment from δ¹³C but not δ¹⁸O values

Closely related taxa are, on average, more similar in terms of their physiology, morphology and ecology than distantly related ones. How this biological similarity affects geochemical signals, and their interpretations, has yet to be tested in an explicitly evolutionary framework. Here we compile and analyze planktonic foraminiferal size-specific stable carbon and oxygen isotope values (δ¹³C and δ¹⁸O, respectively) spanning the last 107 million years. After controlling for dominant drivers of size-δ¹³C and size-δ¹⁸O trends, such as geological preservation, presence of algal photosymbionts, and global environmental changes, we identify that shared evolutionary history has shaped the evolution of species-specific vital effects in δ¹³C, but not in δ¹⁸O. Our results lay the groundwork for using a phylogenetic approach to correct species δ¹³C vital effects through time, thereby reducing systematic biases in interpretations of long-term δ¹³C records-a key measure of holistic organismal biology and of the global carbon cycle.