Fossil records are commonly used as the basis for evolution theory. These consist of fossilized animals and plants (and their parts). Fossilized remains of plants and animals belonging to different geologic era give us the proof of how these life forms adapted to changing environmental conditions (by changing the habitat or developing new organs, etc.), how some species became extinct, while other dominated. And hence are used as proof of evolution.
The fossil records suffer from 3 types of bias: temporal bias, geographic bias and taxonomic bias. Fossil of certain geologic time may be easier to find as compared to those or other era, such as newer fossils are easier to find than older ones. Plate tectonics causes loss or displacement of fossils. Geographical bias arises from the fact that more fossils are found in lowland habitats, where fossilization was easy. Taxonomic bias arises because marine fossils are prevalent, even though marine species were not the dominant life forms, simply because fossilization of fishes is easier. Besides, fossils are not found everywhere, as they could have been destroyed over time due to environmental or human factors, and hence there are missing links in the chain of evolution of life on Earth.