Many authors have noted the apparent “decoupling” of the taxonomic and ecological severity of mass extinction events, with no widely accepted mechanistic explanation for this pattern having been offered. Here we test between two key factors that potentially influence ecological severity: biosphere entropy (a measure of functional redundancy), and the degree of functional selectivity (in terms of deviation from a pattern of random extinction with respect to functional entities). Our results demonstrate that while the Shannon entropy of the biosphere prior to a mass extinction event determines the expected outcome following a major perturbation of a given magnitude, variation in biosphere structure between major extinction intervals is insufficient to explain the observed variation in ecological severity. Within this information-theoretic framework, we show that it is the degree of functional selectivity which is expected to primarily determine the ecological impact of a given perturbation when biosphere entropy is not substantially different.