The fossil record, however, is quite incomplete. Here's one major reason why: Sediment has to cover an organism's remains in order for the long fossilization process to begin. Most organisms decompose before this can happen. Fossilization odds increase if the organism happened to exist in large numbers or lived in or around sediment. For example, trilobites, ancient marine arthropods, met both criteria, so they're rather common fossils. The Tyrannosaurus rex, however, is far rarer. It was large and land-dwelling, and as a top predator made up a far smaller percentage of the population.
