Fossils are the remains of organisms from the distant past embedded and preserved in the earth's crust. Fossils may be any sort of organism, from a single-celled bacteria to a tree leaf to a dinosaur, and may be anywhere from ten thousand to several billion years old. They usually consist of parts of the organism which were mineralized during the organism's lifetime, such as bones and exoskeletons.
Apologists claim there there are missing transitional fossils between known species which allegedly disproves evolution. However, the existence of some "gaps" in the fossil record would be expected based on the changing distribution of species and the rarity of fossilization.