SUBSTRATE RELATIONSHIP GUIDED MOLLUSCAN BODY-SIZE RESPONSE TO CLIMATIC FLUCTUATION DURING THE PAST 66 MA

Body size is a key factor in dictating the fate of interaction between an organism and its surrounding environment. A negative temperature-size relationship has been suggested as one of the universal responses to climatic warming. To confirm the generality of this relationship and to evaluate its dependence on existing body-size and latitudinal-range of marine families, we compiled the relationship between body-size and global temperature trends over Cenozoic using a database of marine benthic molluscs of class gastropoda and bivalvia resolved to temporal stages. We evaluated the dependence of climate induced body-size response to existing size and geographic range via correlating the first-difference coefficient of temperature-size relationship with maximum size and latitudinal range of family respectively.

Mean body-size and temperature estimates appear to track each other negatively when binned by geologic stages; however, it is not observable when the mean body-size and temperature time series are detrended by a first-differences transformation implying a lack of size-reduction due to climatic warming during Cenozoic. There exists a strong negative correlation between maximum body-size and ρ^1st _(size-temp) (the coefficient of correlation between 1^st difference of mean body-size vs temperature) for infaunal families implying that families with smaller body-size tend to show higher magnitude of change with climatic fluctuation compared to bigger ones. Epifaunal families do not show such “existing size dependence”. A significant negative correlation for infaunal families also exists between ρ^1st _(size-temp) and latitudinal range implying that families with limited latitudinal range show higher magnitude of body-size change compared to widespread taxa. Such “latitudinal range dependence” is absent among epifaunal families. This difference in response between infauna and epifauna is due to the dissimilar thermal-sensitivity of these groups evolved due to their non-uniform exposure to thermal fluctuation. Our results highlight the dependence of climate induced morphological response on ecological character indicating the complexity in generalizing the biotic outcome of future climatic fluctuation.