Geochemical analysis of fossil foraminifera is a routine approach to paleoceaneanographic reconstruction. Certain stable isotopes and minor elements substituted into the CaCO3 lattice during calcification vary at predictable rates in response to changes in ambient seawater such as temperature, glacial ice volume, salinity, water column structure, or productivity. Such rates vary, as shell development is influenced by species-specific offsets in isotopic fractionation during growth. Above all, paleo-reconstruction necessarily assumes the foraminifera assessed represent uniform species, incorporating seawater chemistry consistently through time. Interpretation of parameters that caused chemical or morphological changes in fossil shells is then extrapolated from living analogues. As foraminiferal species differ evolutionarily, and in physiological response to local environmental influences, their shape and chemistry is unsurprisingly diverse. The greater the extent of morphological plasticity, the more vulnerable the species is to biased or erroneous interpretation. A prime example is the Globigerinoides ruber-elongatus plexus. G. ruber (d'Orbigny 1839) is a common planktonic foraminifer used in studies of the upper mixed layer of low-latitude oceans since the Miocene. Taxonomic revisions since 1839 resulted in the inclusion of numerous transitional morphologies now known to be unique biological species. At least two of these (G. ruber & G. elongatus) substitute isotopes dissimilarly. Whether these morphospecies ought to be separated when used as a paleo-proxy – and how – is cause for consideration. Various criteria exist to differentiate morphologies, but their efficacy has not been tested quantitatively and they yield inconsistent geochemical data in different ocean basins. Here, we present morphometric and stable isotope analyses of coretop and downcore G. ruber and G. elongatus from the Atlantic and Pacific, identified using only three criteria: 1) final chamber compression, and 2) asymmetry, and 3) aperture compression. Manual classification, superimposed on objective measurements of shell morphology, allows us to assess to what degree classification reflects morphospace discontinuity, how reliable it is, and what implications this may have for paleoceanographic research.