There are two aspects to learning what fossils can teach us. The first comes from the direct observation of fossil evidence. The second is less direct because it is concerned with the relative position of fossils in the rocks where they are found.

Direct observation of fossils. Except for rare cases in which organisms are immediately preserved, e.g., the frozen mammoth or, another case, insects trapped in amber, fossil remains present us only with a part of some organism. So, obviously, one problem in interpretation is the reconstruction of the whole organism. Perhaps the most famous practitioner in this area was Cuvier, the father of paleontology. A fossil tooth was a fascinating clue to Cuvier. From it he could make educated guesses as to the size of the original organism and its age and its food. The latter then allowed cautious speculation as to the nature of such internal organs as the digestive system. This led to more information on body size and shape. And so on. Comparable work is being done today and the reconstruction of whole humans from parts of their skull is a case in point. Such reconstructions can be extremely useful in casting light on the creatures of bygone times. In some cases specially preserved fossils, such as the impression of dinosaur skins, allow us to reconstruct details that we would otherwise have had no knowledge of.

More difficult to interpret are the traces left by organisms, such as worm tracks left in the mud of ancient seas or the molecular fossils, mentioned earlier. In these instances we may never know the exact nature of the organism that left such evidence of its existence. Nonetheless the information we do have is useful. It helps complete the picture of what type of organism was alive at a certain time in the past history of this earth. Worm tracks, for example, document, at the least, that worms were present. Even though we may not be able to say which species or class or maybe even which phylum, we do know that crawling things of a certain approximate size were part of the ecosystem under study. That enhances, though admittedly in a limited way, our understanding of past life.