Firefly Guide to Fossils

Excerpted from the Introduction What is a Fossil? Fossils are the remains of animals and plants more than 10,000 years old.Fossils usually represent the harder parts of organisms, since these are the most resistant to decay and erosion. Most fossils therefore consist of the bones and shells of animals, or the leaves, seeds, and woody parts of plants.Even the original hard parts may not be preserved intact, because biological processes like scavenging, and geological processes like waves and storms, tend to disarticulate, break and scatter all the various parts of the original organism. This is one reason why it is more common to find separate vertebrate bones than an entire skeleton, and individual crinoid plates are far more frequent than entire crinoids. Another reason is that animals or plants may shed parts of themselves in the normal course of their life. For instance, leaves fall from trees, seeds float or a blown away from the parent plant, and many arthropods, like trilobites, molt their hard carapaces as they grow. Trace fossils are the footprints, burrows, impressions, and borings into rock left by organisms.Fossils are found in the majority of sedimentary rock types. They are paricularly common in limestone, marls, clays, siltstones, mudstones, and shales, and they are less common in sandstones, conglomerates, and graywackes. Fossils can also be preserved in sedimentary ironstones and in volcanic ash.The majority of fossils are aquatic animals or plants (i.e. they live in seas, rivers, lakes, or estuaries), because conditions for preservation are usually better in aquatic environments than on land. Even terrestrial animals and plants are more likely to be preserved in aquatic sediments, either through drowning (in a flood, for example), or because they fell into water, or swept into it by floods and other sudden events. In this way, fossil land mammals are often found in the same deposits as the remains of fish, crocodiles, or turles.Occasionally, entire organisms are preserved in frozen soil (such as mammoths), peat bogs, and asphalt lakes, or trapped in hardened resin (such as insects in amber). Fossil Formation Fossils represent only a tiny fraction of the total number of animals and plants that have existed, because there are several conditions that must be fulfilled before fossilization is possible. The chances that an organism will be fossilized are greatly increased if it has hard parts, such as shell, bones, or a test.The ability of an organism to survive destructive surroundings often depends on the size of the animal or plant, and the strength or percentage of hard parts. For this reason, a large mollusk is more likely to survive than a tiny, more fragile shell. Even so the future of an organism can depend on the speed with which it becomes buried in the sedimentary soil. Given the right conditions, a soft-bodied jellyfish may be preserved quickly enough to leave an imprint in the soil, despite the fleshy nature of its body. These types of soft body fossils are extremely rare.Once a fossil has been formed, conditions within the rock itself can continue to threaten the fossil's preservation. In the course of millions of years the fossil may have been encased in rock, gigantic changes will have taken place in the Earth's crust. Changes in the form and structure of the rock itself may have forced the shape of the fossil to change, or may have crushed it beyond recognition.Firefly Books Staff is the author of 'Firefly Guide to Fossils' with ISBN 9781552978122 and ISBN 1552978125.