study of fossils is known as palaeontology. fossils are the remains or impressions of past organisms on sedimentary rocks or other media.
carbon dating introduced by Libby helped in the study of fossils.
in evolution paleontological evidence is acquired by fossils,
eg: 1) by studying the structure of archeopteryx which acts as a missing link
(organisms which are extinct) it can be extracted that they had both reptilian and avian characteristics possessing claws, teeth etc. as in reptiles and wings, sternum etc. as in birds
similarly, pteridophytes resemble both gymnosperms
this helped scientists conclude that birds evolved from reptiles and are hence known as over glorified reptiles
2) we can study the strata of soil in which those organisms were found to know the time period in which they existed.