Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions

There have been five major mass extinctions among the marine biota during t he similar to 0.6 b.y. history of metazoan life on Earth. These mass extinc tions have been ranked from the largest to the smallest by the severity of taxonomic diversity lasses, but they have not been ranked by the severity o f the ecologic changes that they produced, Here we utilize a system of pale oecological levels that allows for the ranking of ecological degradation or shifts associated with significant taxonomic events, along,vith an analysi s of large-scale paleoenvironmental patterns of two of the great evolutiona ry faunas, to compare the relative ecologic degradation caused by two major mass extinctions. The Late Ordovician and Late Devonian mass extinctions p roduced similar taxonomic losses (marine families declined similar to 22% a nd 21%, respectively). However, our analyses show that whereas the Late Ord ovician extinction resulted in only minimal permanent ecological change, th e Late Devonian extinction resulted in the complete restructuring of many c omponents of the marine ecosystem, Thus, the large-scale taxonomic and ecol ogical significance of these extinction events are decoupled, implying that some taxa are ecologically more critical than others.