A line in the stone
At this unassuming rock face on Klonk Hill, a global standard was set. In 1972, geologists ratified this spot as the world's first Global Boundary Stratotype Section and Point (GSSP). This location formally defines the boundary between two major periods of Earth's history: the Silurian and the Devonian. The boundary, set at 419.2 million years ago, marks the beginning of the Devonian Period, a time when fish evolved into the first four-limbed vertebrates that walked on land.
The term "Golden Spike" is a nickname for a GSSP. While no actual gold is used, a bronze plaque identifies the internationally agreed-upon point in the rock layers. The boundary at Klonk is located in Bed 20 of a sequence of alternating grey limestone and calcareous shale. This specific spot was chosen because it shows continuous, undisturbed sedimentation across the boundary and is full of well-preserved fossils that allow for precise dating and worldwide correlation. This geological area, known as the Barrandian, contains a complete and accessible record of Early Paleozoic life.
The primary marker for the start of the Devonian is biological. The boundary line is defined by the first appearance of a specific species of graptolite, an extinct colonial sea creature, named Monograptus uniformis (also referred to as Uncinatograptus uniformis uniformis). The moment this fossil appears in the rock record, the Silurian ends and the Devonian begins.
The age of fishes begins
During the Silurian and Devonian periods, this part of the Czech Republic was a deep marine basin. The rocks here do not contain the fossils of early tetrapods, which were evolving in shallower, freshwater environments elsewhere on the planet. Instead, the Klonk GSSP is the official starting pistol for the geologic period in which this great evolutionary transition took place.
The layers at Klonk are filled with the fossils of the creatures that lived in this Paleozoic sea. Alongside the index graptolite Monograptus uniformis, scientists have identified numerous species of conodonts (tiny tooth-like fossils from an extinct eel-like vertebrate), trilobites, and crinoids. These fossil assemblages provide secondary markers that help geologists in other parts of the world identify the Silurian-Devonian boundary in their own local rock formations.
The establishment of the Klonk GSSP was an important step for the earth sciences. It solved a long-standing debate known as the "Hercynian Question" about where to draw the line between the two periods. By creating a single, physical reference point, the International Commission on Stratigraphy created a universal language for discussing this chapter of deep time. Klonk is the first of now over 77 GSSPs that define the entire geologic timescale.