Original: For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous - Tertiary extinction event, which marked the demise of the dinosaurs and of many marine species.
Simplified: For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event, known as the Cretaceous - Tertiary extinction event, that marked the demise of the dinosaurs and of many marine species.