Final answer:
Scientists can determine the existence of an exact species at a particular time by analyzing core samples of sediment and matching the sediment chemistry with known sources. This allows them to identify the species that existed during a specific time period based on the chemical composition of the sediment.
Step-by-step explanation:
Scientists can determine the existence of an exact species at a particular time by analyzing core samples of sediment. When sediment is weathered and eroded from rocks, it retains a distinct chemical makeup. Geologists can analyze the sediment chemistry and match it with known sources on the continent. This allows them to identify the exact species that existed during a specific time period based on the chemical composition of the sediment.