Here is my idea of it. If, say, a fossil that was believed to have been around since 200 mya was discovered again and was dated at around 50 mya, that would not necessarily disprove evolution, as it could have survived through that time. Also, fossils of its type (whatever it is; shell, fern, etc.) were already found before 50 mya, so it would not be a particularly unique species of a new type.
If, however, a rabbit fossil was found and dated back to 500 mya, that would cause some issues, since at that time no mammals (and therefore rabbits) could have been around according to evolution and the current fossil record. As of now, no discovery like this has been found, and evolution has not been refuted from such a discovery.