Absence of general rules governing molluscan body-size response to climatic fluctuation during the Cenozoic

Body size is a key factor in predicting the outcome of organisms’ interaction with their environment. A negative temperature–size relationship (TSR) is claimed to be one of the universal responses to climatic warming. Studies on extant biota also predicted that groups with narrow latitudinal range, tropical affinity, and higher body size would show higher sensitivity to climatic fluctuation. To confirm the generality of these relationships among marine ectotherms, we compiled the data on body size and global temperature over the Cenozoic using a global database of marine molluscs of Class Gastropoda and Bivalvia. Molluscan species do not show any signature of TSR for any taxonomic, regional, or ecological category during the Cenozoic. We did not find any evidence supporting heightened response in groups with limited latitudinal spread or with large body size. The body-size response of tropical species is not different from those of temperate species. Response of infauna and epifauna does not show any difference either and hence, refutes the predicted variation due to difference in their thermal-specialisation. Our results highlight the limited validity of ‘universal rules’ in explaining the climate-induced morphological response of marine communities in deep time and underscore the complexity in generalising the biotic outcome of climatic fluctuation.