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.