There have been five major mass extinctions among the marine biota during the ∼0.6 b.y. history of metazoan life on Earth. These mass extinctions have been ranked from the largest to the smallest by the severity of taxonomic diversity losses, but they have not been ranked by the severity of the ecologic changes that they produced. Here we utilize a system of paleoecological levels that allows for the ranking of ecological degradation or shifts associated with significant taxonomic events, along with an analysis of large-scale paleoenvironmental patterns of two of the great evolutionary faunas, to compare the relative ecologic degradation caused by two major mass extinctions. The Late Ordovician and Late Devonian mass extinctions produced similar taxonomic losses (marine families declined ∼22% and 21%, respectively). However, our analyses show that whereas the Late Ordovician extinction resulted in only minimal permanent ecological change, the Late Devonian extinction resulted in the complete restructuring of many components of the marine ecosystem. Thus, the large-scale taxonomic and ecological significance of these extinction events are decoupled, implying that some taxa are ecologically more critical than others.