Cryptic species (genetically isolated and ecologically disparate sibling taxa) are seemingly common in the marine plankton, and both molecular clock and palaeontological methods suggest that some, if not many, of these diverged many millions of years ago. We have morphometrically analysed over 2,000 specimens of the common planktic foraminifers Globigerinella siphonifera and Globigerinoides ruber, extracted from well-dated ODP cores at two tropical sites (926A (Atlantic) and 806 (Pacific)). In both forms, DNA methods suggest the occurrence of deep (7–11 Ma) diverging cryptic species complexes and strong evidence of ecological specialisation—but with minimal concomitant morphological change. Our morphometric dataset (based on a suite of characteristics ranging from ultrastructural assays to multivariate analysis of whole-test form) suggests that the Gl. siphonifera sub-types are resolvable as fossils if the correct characters are extracted, but cryptic species of Gs. ruber appear to be almost entirely resistant to morphological division. In both cases, morphometric and/or molecular clock methods imply the long-term coexistence of ecologically discrete, but morphologically near-identical, reproductively isolated species. From a functional viewpoint, this finding is ambiguous£ either very strong stabilising selection is acting to constrain morphological evolution (suggesting the adaptive primacy of test form), or the locus of selection is centred on other characters (implying that test form is of little or no adaptive significance). Resolving this paradox is of prime evolutionary importance, certainly amongst foraminifers, and potentially throughout the pelagic realm.