A fossil was the preserved remnants of an organism from the distant past.
In 2526, While Adric was calculating the coordinates to guide the TARDIS back into E-Space, the Fifth Doctor was wandering in a cave examining dinosaur fossils. He said he had always wanted to go back and learn what was the cause for their extinction. (TV: Earthshock)
Permians appeared to be animated skeletons of theropod dinosaurs. An archaeological expedition in Alaska in 1964 unearthed a Permian fossil, which revived and attacked them. (AUDIO: The Land of the Dead)