Fossils are the remains, traces or impressions of prehistoric organisms. There are wholly intact animals which have been preserved such as the wooly mammoth (discussed below) or the thousands of species of insects caught up in the resins of ancient pine trees and preserved in the yellow-orange translucent fossil-bearing material amber. Prehistoric humans which fell into bogs have been preserved intact. Other fossils are produced by replacement of organic matter by inorganic matter. The inorganic replica is preserved in rock (petrified). The petrification process may be so subtle that microscopic examination, especially of petrified plants, reveals fine cellular structure. Footprints and preserved fecal material, while not actual remains of preexisting animals, are considered to be fossils because they were made by living animals. Footprints reveal something of the weight of the animal, length of stride, and whether it was bipedal or if it moved on all four feet...