Body size, sampling completeness, 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 dataset comprising the body sizes, stratigraphic
ranges, and occurrence patterns of 9,408 genera of fossil marine animals
spanning eight Linnaean classes across the past 485 million years. We find
that preferential extinction of smaller-bodied genera within classes is
substantially more common than expected due to chance and that there is
little evidence for preferential extinction of larger-bodied genera. Using
a capture-mark-recapture analysis, we find that this size bias of
extinction persists even after accounting for a pervasive bias against the
sampling of smaller-bodied genera within classes. The size bias in
extinction also persists after including geographic range as an additional
predictor of extinction, indicating that correlation between body size and
geographic range does not provide a simple explanation for the association
between size and extinction. Regardless of the underlying causes, the
preferential extinction of smaller-bodied genera across many higher taxa
and most of geological time indicates that the selective loss of
large-bodied animals is the exception, rather than the rule, in the
evolution of marine animals.