FOSSILS, REMAINS OF ANCIENT LIFE, HAVE PLAYED A LARGE role in our exploration of the Earth's past. In antiquity they led to the recognition of sea-level oscillations. During the Renaissance, they raised questions about the tales of Genesis. In the nineteenth century, when paleontology became a science of its own, they served to develop an Earth history and demonstrated the great complexity of the tree of life. In the twentieth century, these concepts were greatly refined.
Part of what makes fossils so useful is their enormous abundance. In many outcrops, shells of clams or snails or brachiopods weather out by the hundreds, even thousands, and many limestone cliffs are replete with corals and bryozoans. A teaspoon of clay or chalk may contain hundreds of foraminifera or millions of coccoliths.
But such simply armored organisms constitute only a fraction of any living fauna. Carcasses with compound skeletons, such as echinoderms and vertebrates, are normally disarticulated, as they are torn apart by scavengers or rotted by bacteria and fungi, and few are the bones or ossicles that are not dragged off by scavengers; distributed by rivers, currents, and waves; or scattered by sediment burrowers. Animals with fairly standardized body plans, such as vertebrates, can still generally be assembled from scattered bones, but in the case of echinoderms, commonly having thousands to tens of thousands of individual ossicles and a baffling variety of body plans, the task is hopeless.
Beyond this, faunas include many animals that have little or nothing in the way of mineralized skeletons that escape bacterial decomposition. How and when did these groups evolve, and what role, if any, did they play in fossil faunas of the past?
Furthermore, extinct groups of animals often pose problems of relationship. Trilobites, for example, clearly were arthropods, but were they crustaceans, merostomes, or an independent and now wholly extinct