IS EXTINCTION SIZE SELECTIVE? A CASE STUDY OF BRACHIOPODS THROUGH THE PHANEROZOIC

Ecological theory predicts that larger-bodied organisms should be at greater risk of extinction due to smaller population sizes, longer generation times, and greater resource requirements than their smaller-bodied relatives. This prediction is consistent with data from Pleistocene and living terrestrial mammals, but the extent to which this prediction holds across other times and other taxa remains poorly documented. In this study, we examine the relationship between body size and extinction risk over the past 500 million years using a global, genus-level database of brachiopod sizes. Brachiopods were among the most diverse and abundant marine animals during Paleozoic time and are an important component of Sepkoski’s Paleozoic Fauna. We found no persistent relationship between body size and extinction risk. Extinction victims are neither consistently larger not consistently smaller than survivors. Moreover, most stages exhibit no statistically significant difference in mean size between victims and survivors. Similarly, there is also no clear relationship between extinction intensity and the direction or significance of body size differences between victims and survivors. These findings suggest that size-selective extinction is not a general property of animal evolution; rather, when and where it occurs, size selective extinction must reflect either the particular causes of extinction or the ecological and physiological properties of the taxon.