The carbon stable isotope (δ¹³C) composition of the calcitic tests of planktonic foraminifera has an important role as a geochemical tracer of ocean carbon system changes associated with the Cretaceous/Paleogene (K/Pg) mass extinction event and its aftermath. Questions remain, however, about the extent of δ¹³C isotopic disequilibrium effects and the impact of depth habitat evolution on test calcite δ¹³C among rapidly evolving Paleocene species, and the influence this has on reconstructed surface-to-deep ocean dissolved inorganic carbon (DIC) gradients. A synthesis of new and existing multispecies data, on the relationship between δ¹³C and δ¹⁸O and test size, sheds light on these issues. Results suggest that early Paleocene species quickly radiated into a range of depths habitats in a thermally stratified water column. Negative δ¹⁸O gradients with increasing test size in some species of Praemurica suggest either ontogenetic or ecotypic dependence on calcification temperature that may reflect depth/light controlled variability in symbiont photosynthetic activity. The pattern of positive δ¹³C test-size correlations allows us to (1) identify metabolic disequilibrium δ¹³C effects in small foraminifera tests, as occur in the immediate aftermath of the K/Pg event, (2) constrain the timing of evolution of foraminiferal photosymbiosis to 63.5 Ma, ∼0.9 Myr earlier than previously suggested, and (3) identify the apparent loss of symbiosis in a late-ranging morphotype of Praemurica. These findings have implications for interpreting δ¹³C DIC gradients at a resolution appropriate for incoming highly resolved K/Pg core records.