9: Fossils

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Fossils are defined as any evidence of past life; for our purposes we will consider something to be a fossil if it is unusual in that it is either ancient or has been altered from its original form in a way that makes it more likely to be preserved. Fossils are very important because they tell us about how organisms changed and evolved through time, how they interacted with each other and their environment, and how Earth’s climate and environments have changed through time.

Learning Objectives

List the major types of fossils and modes of preservation
Describe the morphology of fossils using the appropriate terminology
Identify the major types of invertebrate body fossils in hand sample and thin section
Identify the most common trace fossils and apply the ichnofacies concept to interpret depositional environments

Chapter thumbnail shows details a fossil gastropod (Masha Milshina via Wikimedia Commons; CC BY-SA 4.0).

9.1: Types of Fossils
The three types of fossils are body fossils (preserved parts of the actual organism), trace fossils (tracks, trails, and other types of fossilized behavior), and geochemical fossils (have a unique chemistry caused by living organisms).
9.2: Types of Preservation
Robust hard parts (teeth, bone, shell) and even soft parts (muscle, skin, other tissue) can be preserved in certain situations. Replacement occurs when the original organic material is replaced with another material. Molds and casts are void and infills formed after organisms. Recrystallization occurs when a mineral changes to a more stable form. Carbonization happens when all the volatiles are driven off leaving only a carbon film. Permineralization occurs when mineral precipitate into void
9.3: Describing Fossils
The morphology of fossils can be reasonably described by focusing on the number and size of holes occupied by the organism, the symmetry of the fossil, and the shape and internal structure of shells.
9.4: Major Fossil-Forming Groups (Invertebrates)
The overwhelming majority of fossils come from invertebrates that have hard skeletal material. The most common and easily recognized are sponges, corals, brachiopods, bryozoans, mollusks (bivalves, gastropods, and cephalopods), certain arthropods, and echinoderms.
9.5: Microfossils
We classify a diverse range of organisms as microfossils and count anything that is generally less than 2 mm in size and best studied with a microscope. Common microfossils not covered elsewhere in this chapter include radiolarians, diatoms, foraminifera, ostracods, and conodonts.
9.6: Trace Fossils
Trace fossils include tracks, trails, burrows, and any other fossilized behaviors. Trace fossils are named on the morphology of the feature, not on the interpreted trace maker. The ichnofacies concept groups associations of trace fossils together that likely record behaviors that worked in a particular environment.
9.7: Fossils in Thin Section
Thin sections show a two dimensional cut through fossils which can make identifying them even more challenging than it would be in hand sample. This chapter provides a starting point for identifying fossils by providing several summary diagrams and numerous photographs.