The carbon stable isotope (13C) 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 13C isotopic disequilibrium effects and the impact of depth habitat evolution on test calcite 13C 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 13C and 18O 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 18O 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 13C test-size correlations allows us to (1) identify metabolic disequilibrium 13C 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 13C DIC gradients at a resolution appropriate for incoming highly resolved K/Pg core records.