BODY SIZE AND EXTINCTION RISK IN THE MARINE FOSSIL RECORD

Larger body size has long been assumed to correlate with greater risk of extinction, helping to shape body size distributions across the tree of life, but lack of comprehensive size data for fossil taxa have left this hypothesis untested for most higher taxa across the vast majority of evolutionary time. Here we assess the relationship between body size and extinction using a database comprising the body sizes, stratigraphic ranges, and occurrence patterns of 9,408 genera of fossil marine animals spanning four phyla and eight Linnaean classes across the past 485 million years. We find that smaller-bodied animals within classes, orders, and families have been preferentially lost to extinction, even after accounting for the greater completeness of the fossil record for larger-bodied genera. The association between smaller body size and greater likelihood of extinction is statistically significant for all three eras (Paleozoic, Mesozoic, Cenozoic) when all genera are analyzed together, for five of seven phylum-by-era combinations and eight of 14 class-by-era combinations. No combination exhibits the preferential extinction of larger-bodied genera. Larger body size is associated with wider geographic range, even after controlling for interval-level sampling intensity, number of occurrences, and genus age, suggesting that large body size enables wider geographic range for many higher taxa in the marine realm and that this connection may explain the phylogenetically widespread tendency for larger-bodied genera to survive preferentially across geological time.