The Geologic Time Scale (GTS) in invertebrate paleontology is a chronological system that organizes Earth's history based on major geological and biological events, particularly the appearance and disappearance of fossil groups. Invertebrate fossils are fundamental to the GTS because they are abundant, widespread, and evolved rapidly, making them excellent index fossils for defining and correlating specific time intervals (e.g., periods, epochs). Paleontologists use these fossils to establish biostratigraphy, which helps to date rock layers and understand the evolutionary history and distribution of ancient invertebrate life.