The Common Descent Podcast

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As organisms live, grow, and move in their environments, they leave all sorts of evidence behind on their surroundings. When this evidence becomes preserved in the geologic record, it can be provide some of our best clues to understanding lifestyles and evolution of ancient life. This episode, we talk about Trace Fossils.

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Fossilized Traces

Generally speaking, fossils come in two forms: body fossils are the remains of parts of an organism’s body, including leaves, wood, bone, teeth, and more; trace fossils are fossilized traces, records of the activities or behaviors of organisms, which can include footprints, burrows, bite marks, and beyond. The study of traces is called ichnology, sometimes split into neoichnology (for modern traces) and paleoichnology (for trace fossils or ichnofossils).

Traces come in many forms, and there are differing opinions over what actually counts as a trace. Many classic types of traces are formed when organisms interact with a substrate or surface: worms dig through mud and form burrows, roots leave behind cavities in the soil, predatory snails drill holes in clam shells. When these disturbed sediments become rock, or when the damaged shells become fossils, the traces are fossilized with them. Sometimes the term “trace fossil” also includes objects produced by organisms, such as coprolites (fossilized poop) or fossil eggs. Also sometimes included are layered sediments formed by microbial mats, called stromatolites, which are among the earliest signs of life on Earth.

Ichnofossils are often classified by the behavior that produced them. For example, repichnia is a term for traces of locomotion, which can include footprints and trackways. Other categories include cubichnia (resting traces), domichnia (dwelling traces like burrows), praedichnia (predation traces like bite marks), and calichnia (nesting traces), among many others.

At a finer scale, specific types of ichnofossils are given scientific names, similar to names given to organisms. These are ichnotaxa (including ichnospecies and ichnogenera). For example, Diplichnites is the name for parallel rows walking tracks produced by arthropods; Skolithos are vertical burrows that can be made by worms, crustaceans, and other organisms; and Grallator refers to three-toed footprints left by a variety of small theropod dinosaurs. It’s important to remember that trace classification is different from organism classification; a single type of trace might be made by many different organisms, and one organism can leave many traces. Trace fossils can be as variable as the behavior that creates them, and some ichnofossils include evidence of multiple behaviors at once, or traces left by multiple organisms together.

Tales from Traces

Trace fossils are often direct evidence of behavior, and they can tell us a lot about the lifestyles and activities of ancient organisms. Feeding traces and coprolites can reveal past animals’ diets; damage on shells or bones can be clues to the presence of ancient parasites; traces of nests or trackways can hint at extinct organisms’ social behavior. Trace fossils have even been important for understanding our own ancestors: the famous footprints of Laetoli provide some of the earliest evidence of bipedal posture in human evolution.

Individual trace fossils can be valuable, but ichnology often focuses on communities: ichnofossil assemblages can include evidence of many behaviors from many organisms of an ancient ecosystem. By analyzing these communities, paleontologists and geologists can identify ancient environments: near-shore marine deposits commonly have lots of Skolithos burrows, for example, and Coprinisphaera (dung beetle nests) are often found alongside other insect traces in grassland habitats.

And as we explore geologic time, ichnofossils are valuable indicators of major changes. The Cambrian Explosion is marked, in part, by an increase in complexity in burrows, feeding traces, and other ichnofossils; ancient footprints provide some of our earliest evidence of animals moving onto land, including our tetrapod ancestors; and changes in trace fossil diversity are an important factor when studying the effects of catastrophes like the Permian and Cretaceous mass extinctions.

Dig Deeper

Overviews of the topic:
Ichnology : organism-substrate interactions in space and time, Luis A. Buatois and M. Gabriela Mángano (Book)
The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis (technical review paper)

A History of Ideas in Ichnology (technical)

Principles of Ichnoarchaeology (technical)

Find lots more images of trace fossils here.

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 137 – Fossilization
• Episode 62 – Amber
• Episode 34 – Ancient DNA
• Episode 89 – The Burgess Shale
• Episode 67 – The La Brea Tar Pits

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