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Trace Fossils

© Beverly Eschberger

Generally, when people think about paleontologists and paleontology, they imagine scientists working with the actual bones of extinct animals. There is more to the science of paleontology than just bones, however. When paleontologists discover fossils, they carefully examine the area of the find for what are called "trace fossils". Sometimes, trace fossils may be found when bones are not preserved.

Trace fossils, also called "ichnofossils," are structures left behind in sediments or in other living organisms by animal activities such as crawling, swimming, running, eating and dwelling, resting or hiding. The study of trace fossils is called paleoichnology.

Trace fossils include molds and casts of shells, burrows and borings, footprints, eggs and eggshells, coprolites and bite marks on plant material or on the bones of other animals. Trace fossils are divided into two groups: exogenic trace fossils, which are found on the surface, such as footprints, and endogenic trace fossils, which are found within layers of sediment, such as burrows. Footprints and coprolites are perhaps the most famous and most often studied of trace fossils, but other types of trace fossils can also tell us a lot about how animals lived.

Trace fossils are known mostly from marine and freshwater sediments, where soft earth was burrowed in or walked on, then preserved as the sediments became rock. Surface trails in shallow marine environments are the least likely to be preserved because of wave and current action that would obliterate them. Trails in quiet, deep-water environments are more likely to be preserved, often with fine details.

Trace fossils are often preserved in clastic rocks (a type of sedimentary rock) where body fossils may be rare. Because they are made on or preserved in sediments, we do not have to worry that they have been moved from their point of origin, while bones can be washed away by rivers.

In my article "Paleobotany II," I discussed how bite marks on plants can tell us what sort of animals and insects fed on specific plants, and thus what animals lived in a particular area at a particular time. Insect burrows in plants can also tell us about the insects that lived while the plants were growing, or perhaps after their death, in the case of trees.

Bite marks on bones can tell us what carnivorous (meat-eating) animals lived in a particular area at a particular time. If teeth are left behind in bones, we can determine the species of predator, and what prey animals they liked to feed on.

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