The 2025 Phanerozoic Index Fossil Timescale: An Academic Teaching Tool for Paleontology and Stratigraphy

The Phanerozoic Eon is characterized by profound biological, climatic, and tectonic changes that are recorded in the stratigraphic record through both fossil successions and physical Earth-system signals. Index fossils play a central role in defining and correlating stratigraphic boundaries, yet their distribution and stratigraphic significance are often presented separately from the broader chronostratigraphic framework. This contribution presents the 2025 Phanerozoic Index Fossil Timescale, a reference chart designed to integrate key biostratigraphic markers with formally defined stage boundaries throughout the Phanerozoic.

The chart compiles approximately 70 globally significant index fossil taxa representing major fossil groups, including trilobites, graptolites, conodonts, ammonoids, planktonic foraminifera, and calcareous nannofossils. These taxa are directly linked to Global Boundary Stratotype Sections and Points (GSSPs) and calibrated using the most recent (2024/12) International Chronostratigraphic Chart. In addition to fossil first and last appearances, the reference chart incorporates complementary stratigraphic criteria such as magnetic polarity reversals, stable isotope excursions, geochemical anomalies, and major climatic transitions.

By combining biological and non-biological stratigraphic markers within a single visual framework, the timescale provides an integrated view of Phanerozoic biostratigraphy and chronostratigraphy. The chart is intended to serve both as a practical research reference for stratigraphic correlation and as an academic teaching tool that illustrates the temporal distribution of index fossils and the evolution of life through deep time. This integrated approach highlights the multidisciplinary foundations of modern stratigraphy and facilitates cross-disciplinary communication within the geosciences.