NO GENERAL RELATIONSHIP BETWEEN BODY SIZE AND EXTINCTION RISK IN THE FOSSIL RECORD OF MARINE INVERTEBRATES AND PHYTOPLANKTON

Large body size is frequently cited as a general correlate of extinction risk, but empirical support for this association comes primarily from Quaternary-recent terrestrial vertebrates. It is unclear whether extinction risk is similarly size-biased in other taxa. We conducted a comprehensive examination of the association between body size and extinction risk for a phylogenetically and ecologically broad range of benthic invertebrate and planktonic groups through the Phanerozoic. In none of the groups examined did we find a consistent linear or nonlinear positive relationship between size and extinction risk (e.g., larger species experience higher extinction risk), either for the clade as a whole or among subtaxa within clades. One major group (gastropods) exhibits a weak but marginally significant inverse association between size and extinction risk that is consistent with findings from some previous studies, but taphonomic or collecting biases cannot be ruled out as the source of this relationship. Although other groups show occasional evidence of size-selective extinction at different times, these instances are uncommon and include approximately equal numbers of positive and inverse size-risk associations. Our results demonstrate that selective extinction of large-bodied taxa cannot be considered the normal macroevolutionary regime for marine invertebrates; limited evidence for such a bias in the modern is likely indicative of anthropogenic stresses. Selective extinctions of large species in other taxa during the Quaternary-recent may reflect ecological and life-history differences between these taxa and marine invertebrates, but may also be in part attributable to human impacts.