an arrangement or distribution of things into zones.
during the paleozoic era, evolutionary innovation moved onto land, as plants and, subsequently, at least nine clades of animals established what would become earth’s most diverse, productive, and biomass-rich ecosystems. age estimation becomes increasingly refined as we move upward in time, with more abundant fossiliferous successions and more highly resolved biostratigraphic zonation. increasingly, the calibration of evolutionary events is based not on the direct dating of key fossiliferous horizons but on radiometric constraints placed on successions that can be correlated with key fossil horizons. for example, husson et al. (71) dated six ash beds in a basal devonian sedimentary succession, yielding an estimate of 421.3 ± 1.2 ma for the beginning of the devonian period. similarly, 377.2 ± 1.7–ma ash beds just below the upper kellwasser horizon in germany constrain a widely correlated biogeochemical event associated with the extinction of reef faunas (72). nih.gov