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How do Global Stratotype Section and Point markers establish Silurian boundaries?

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Silurian paleogeography — Silurian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the middle part of the Silurian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Global Stratotype Section and Point (GSSP) markers, often referred to as a "golden spike," serve as the international standard for defining the boundaries of the Silurian System and its internal chronostratigraphic subdivisions.1

These markers are established by the International Commission on Stratigraphy (ICS) and its associated working groups under the International Union of Geological Sciences. By designating specific points in the rock record where time and rock coincide, geologists create calibration points that function as the primary reference for global correlation.12

For the Ordovician-Silurian boundary specifically, the GSSP is fixed at a horizon in Dob’s Linn, Scotland. Rather than relying on physical phenomena or climatic changes, this boundary is defined by the base of the Parakidograptus acuminatus biozone, a specific group of concurrent graptolite species. This method ensures a precise, internationally recognized definition for the start of the Silurian Period.31