Morphometric changes have been investigated in the two groups of calcareous nannofossils, Cribrosphaerella ehrenbergii and Microrhabdulus undosus across the Campanian to Maastrichtian of the Zagros Basin of Iran. Results reveal a common episode of size increase at c. 76 Ma, with a sudden increase in the size of C. ehrenbergii and with the emergence of a newly defined, larger species Microrhabdulus zagrosensis n.sp. An even larger species emerges at c. 69 Ma within the Microrhabdulus lineage, Microrhabdulus sinuosus n.sp. The timing of these size changes and origination events matches global changes in nannoplankton diversity and/or in diversity of other planktonic organisms and marine invertebrates. Comparison with long-term global climate change supports that these two distinct episodes of morphological change coincide respectively with the late Campanian carbon isotope event and acceleration of cooling and with climatic instability across the mid-Maastrichtian event.