Almost all species that have ever existed are extinct. The animals and plants that surround us today are only a small sample of the variety that has lived on Earth since life became abundant in the oceans, about 542 million years ago, and later conquered land. We know of some of these extinct species as fossils, whether they are large dinosaurs like Tyrannosaurus, or animals from ancient oceans, like trilobites. Extinction is an important biological process, and it has shaped our world. Our own actions may be causing extinction of other species, so it is important that we understand what led to extinctions in the past.