9.6: Trace Fossils

Trace fossils are sedimentary structures formed as the result of the activity of an organism, other than growth (Jackson, 1997, Glossary of Geology). They represent fossilized behavior and thus provide important insight into the conditions that existed during/immediately after the deposition of sediment. The study of trace fossils is called ichnology.

Types of Trace Fossils

Trace fossils can be grouped into several major types of structures based on what the organism was doing:

Bioturbation structures – record disruption of bedding by burrows, tracks, and trails.

Bioerosion structures – record erosion of substrated by boring, scraping, or biting.

Biostratification structures – occur when organisms impart an organization to sediment by making layers or structures. Just watch this soldier crab!

Fecal material – runs the spectrum from mundane to spectacular.

Although they might appear quite similar to trace fossils, there are several types of things that don’t technically count. These would include any sorts of tool marks formed by passive contact between an object (even an animal) and the bed as well as growth structures like stromatolites and roots.

Trace Fossil Taxonomy

Classified into ichnogenus and ichnospecies based on physical traits of the burrow. We do it this way because different trace-makers may produce similar burrows when behaving similarly and because substrate can profoundly influence the appearance of a track, even if from the same organism. Some of the most common trace fossils are described and pictured in XXXX and an extensive list (organized by ichnogenus) is provided at fossilid.info.

Behaviors recorded by trace fossils

Regardless of what you are and when you lived, the same behaviors tend to help you survive and produced some broadly similar traces:

Resting traces – are shallow depressions made when when organisms settle or dig into substrate.

Crawling traces – are linear to sinuous horizontal trails obviously produced by locomotion

Feeding traces – can be branched, single, typically significant horizontal component and complex structure via repeated feeding activity

Dwelling traces – Simple, bifurcated or u-shaped burrows; +/- thick linings; generally perpendicular to bedding; +/- some horizontal components

Grazing traces - bedding plane parallel, made at sediment surface (strip mining), rarely overlapping commonly with complex ornamentation or patterns

Escape/equilibrium structures – nested or stacked vertical resting traces, commonly showing systematic motion either up or down

Please also see the Digital Atlas of Ancient Life's trace fossil webpage, which is organized by behavior and provides 3D models of trace fossils.

Ichnofacies

Trace fossils record behaviors and those behaviors are dictated by environmental conditions. Consequently certain types of trace fossils tend to occur together because those behaviors work in a given environment. These associations (ichnofacies) recur in time and space and include:

Scoyenia Ichnofacies – this is the name for the collection of terrestrial trace fossils. In terms of behavior, anything goes!

Skolithos Ichnofacies – This collection of trace fossils occurs in a high energy environment with wave/tide activity and a constantly shifting substrate. Typically this ichnofacies occurs in shoreface type environments. In terms of behavior, organisms tend to have vertical burrows as they constantly strive to maintain their burrows and maintain equilibrium relative to the sediment surface.

Cruziana Ichnofacies – occurs in permanently submerged, fully marine conditions where you have muddy and poorly sorted substrate. In terms of specific environments it is typically in offshore transition water depths between fair-weather and storm wave base. Conditions are optimal, and in terms of behaviors anything goes.

Zoophycos Ichnofacies – occupies a broad paleobathymetric range, but generally consists of deepwater environments with muddy substrate and modes amounts of oxygen. Things are pretty friendly to invertebrates here as well and behaviors consist of live, eat, and build.

Nereites Ichnofacies – occurs in deepwater, oxygen poor environments with a muddy substrate. Times are tough and that requires efficiency in all things. Consequently, geometric forms are common as organisms try to optimized feeding strategies.