Pyrrhic Extinction of Planktonic Foraminifera during the K/T Event

Although mass extinctions may in part be random, many appear to have been selective. Most studies of selectivity have focused on identifying morphological or ecological traits that enabled individuals, and consequently their species, to survive a time of crisis. A well-known example is body size of mammals versus dinosaurs during the K/T event. An alternative model of extinction is possible, in which species survive crises not by having superior individuals but by winning a war of attrition, whereby they outlast other species due to high initial abundances or widespread geographic ranges. These two models can be distinguished by examining the pattern of extirpation (i.e., disappearance from a particular region) of the surviving species; that is, whether or not they suffer losses, too. The ubiquity of selectivity by geographic range size, a species-level trait, suggests that this survival by “pyrrhic victory” may be a general feature of mass extinctions.

I tested this hypothesis for the K/T event, using the stratigraphic ranges of 80 planktonic foraminifera species preserved in 18 deep sea cores. If species survived this crisis by pyrrhic victory, then not only should the surviving species show initially higher abundances or more widespread ranges than victims, they should also have experienced high rates of extirpation. Preliminary results show that survivors did in fact suffer heavy losses, on par with the losses of victims from the same region. For example, 73% of species from core 1211B in the western Pacific Ocean became extinct while 67% of the surviving species were extirpated from the region. Similarly, species from core 750A from the Southern Ocean experienced 18% extinction and 9% extirpation. Overall, there was a good correlation between extinction rate and the extirpation rate, suggesting that no species was intrinsically superior to any other in hard-hit areas. Rather, survival was by pyrrhic victory.