Answer:
Fossils
Step-by-step explanation:
By using fossils we can not only know that extinct animals once roamed the planet, but also get an estimate of when they did so. We can use fossils to pin down their location (which we can use to see what might have killed them, resource loss, climate change, human interaction, etc.), to see when they appeared and went extinct (thanks to rock layers and knowledge of geology), and even what may have been in their diets (based on location or.. well, other pieces of evidence, if you know what I mean.) Fossils also give you their size and shape which can help paleontologists determine the type of animal, ex. birdlike, reptile, mammal, etc.