A comment on our original paper ( Davydov and Lucas, 2026 ) provided two examples (Devonian/Carboniferous and Permian/Triassic boundaries) to support the assertion that a volcanic ash and its radioisotope date would be the best primary marker for GSSP proposals and correlation. We demonstrate that precise correlation of the base Carboniferous and base Triassic requires the use of all stratigraphic markers including radioisotope dates where available. We reply that there is no practical way to correlate a numerical age in many sections, and that attempting to do so conflates the essential separation of rock and time. However, volcanic ash beds and their ages, are recognized as essential tools for calibration and to test correlations. • The comments on Harper et al. (2026) are rebutted. • The utility of the Global Stratotype Section and Point (GSSP) is confirmed for the definition of boundaries of chronstratotype units. • A definition using volcanic ash beds and their ages, while useful tools for calibration and correlation, lack the factual basis of a point in rock.
Harper et al. (2026) studied this question.