How did Murchison use fossils to identify Silurian rocks?

Scottish geologist Roderick I. Murchison identified Silurian rocks by utilizing fossils as diagnostic tools to establish stratigraphic order. In his seminal work, The Silurian System (1839), he illustrated 656 fossils, defining them as characteristic of the Silurian time interval.1

Murchison’s methodology relied on the observation that specific fossil types possess broad geographic distributions and consistently appear in the same relative stratigraphic order. By identifying these successive fossil assemblages, he created a framework that allowed geologists to classify exposed strata as Silurian if they correlated with fossils diagnostic of the established stratotype in the United Kingdom.1

This approach evolved into the modern use of index fossils, such as graptolites, which are now used to define biozones. Today, the Silurian System is rigorously subdivided into series and stages based on these specific fossil assemblages, where the first appearance of diagnostic species in the stratigraphic column marks the beginning of a particular time interval.23