A new look at Ecologic Evolutionary Units (EEUs)

Ecologic Evolutionary Units (EEUs) were long intervals of Phanerozoic time during which marine communities maintained stable ecologic structures. Boucot (1983) recognized 12 Ecologic Evolutionary Units and numbered them EEU I through EEU XII. The Ecologic Evolutionary Units are revised, and three are eliminated because they were intervals of community recovery following extinction events. Community structures were controlled by the composition of the Evolutionary Faunas of Sepkoski (1981), and communities became increasingly complex during each successive Evolutionary Fauna. To reflect their connections with the EFs the nine EEUs are renamed using a prefix designating the Evolutionary Fauna in which the EEUs occur. C1 and C2 are from the Cambrian EF, P1–4 are from the Paleozoic EF, and M1–3 are from the Modern EF. each Evolutionary Fauna communities in similar environmental setting in different EEUs have similar ecologic structures, and are recognized as congruent communities. Extinction events destroyed community organizations and removed evolutionary constraints, which allowed many species to move into new ecologic settings during the recovery intervals.