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 substrate by boring, scraping, or biting.
- Biostratification structures occur when organisms impart an organization to sediment by making layers or structures.
- Fecal material also counts as a trace fossil! It can tell us about the organism's diet and possibly its biological affinity.
Figure \(\PageIndex{3}\): Sand bubbler crabs eat by processing sand through their mouthparts and discard the sand as small round balls which imparts an organization to the sediment and thus represents biostratification.
Although they might appear quite similar to trace fossils, there are several structures 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.
Behavioral Classification (Ethology)
Regardless of what an organism was and when it lived, certain behaviors are very common and produce trace fossils with comparable features. Some of the most common behaviors include:
- Dwelling traces ( Domichnia ) – are formed when the organism constructs a home in the subsurface. They include a wide range of morphologies including bifurcating or u-shaped burrows that are commonly perpendicular to bedding. Some may have thick linings and a horizontal component.
- Escape ( Fugichnia ) and equilibrium ( Equillibrichnia ) structures – are formed when an organism moves vertically through the substrate to escape burial or maintain it's equilibrium with the sediment surface. They are commonly similar to dwelling burrows except that they appear nested or stacked.
- Farming traces ( Agrichnia ) - are burrows that are formed to allow an organism to grow algae or bacteria for later consumption. They typically consist of complicated geometric patterns.
- Feeding traces ( Fodichnia ) – are formed as an organism moves through sediment in order to process it for food. They often have a significant horizontal component and may be composed of single burrows that branch, bifurcate, or show a slight offset to form a more complicated pattern.
- Grazing traces ( Pascichnia ) - are formed at the sediment surface as an organism moves along as it feeds. They are typically bedding plane parallel, rarely crosscut each other, and may have complex ornamentation or patterns
- Locomotion traces ( Repichnia ) – are linear to sinuous horizontal (bedding plane parallel) footprints, tracks, or trails produced by movement of an organism.
- Predation traces ( Praedichnia ) - form when one organism eats, or attempts to eat, another. They can include bite marks, borings, repair of crushed shells, and a variety of other forms.
-
Resting traces (
Cubichnia
) are generally preserved as shallow depressions made when organisms settle or dig into substrate. They commonly preserve the shape of the organism or parts of the organism that are in contact with the substrate.
Taxonomic Classification
Trace fossils are classified into ichnogenera and ichnospecies based on the 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 and/or noteworthy trace fossils are pictured and described below.
| Image | Name (Ichnogenus) and Description |
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Arenicolites Vertical u-shaped burrow that lacks concentric spriete and may have funnel-shaped tops. Broadly similar to Diplocraterion but without stacked spriete. Interpreted as a feeding/dwelling burrow of worms, crustaceans, beetle larvae, and a variety of other organisms. Image from Gennadi Baranov via Fossilid.info ; CC BY-NC . |
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Asteriacites Star-shaped resting traces that may be preserved as depressions on the top of beds or in relief on the base of beds. Thought to be the resting trace of a starfish. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 4.0 . |
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Chondrites A vertical to horizontal downward branching burrow with a pattern similar to the roots of a plant. Fill can be slightly more coarse-grained than the surrounding sediment. <1 mm in diameter and up to several inches tall. Thought to be a feeding structure of an annelid worm or a broadly similar organism. Image from Gennadi Baranov via Fossilid.info ; CC BY-NC . |
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Cochlichnus Bedding parallel trace that roughly approximtes a sine curve. Interpreted as an invertebrate locomotion trace. Image from Falconaumanni via Wikimedia Commons ; CC BY-SA 3.0 |
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Conichnus Cone-shaped depression with a round bottom. Coarser clasts can be concentrated around the burrow margin. Interpreted as a dwelling or resting trace. Although a variety of tracemkers are possible, many were formed by anemones. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 4.0 . |
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Cosmorhaphe Bedding plane parallel meandering trace that does not cross-cut itself. Interpreted as a grazing trace. Image from Massimilianogalardi via Wikimedia Commons ; CC BY-SA 3.0 |
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Cruziana A horizontal, bilobed burrow with a pronounced medial ridge, commonly with herringbone-like scratches that converge towards the medial ridge. Interpreted as a locomotion trace of an arthropod. Image from PePeEfe via Wikimedia Commons ; CC BY-SA 3.0 . |
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Diplichnites Elongate trackway with two parallel rows of tracks; the tracks themselves consist of numerous small, closely spaced blunt to elongate impressions caused by footfalls. Interpreted as the locomotion trace of an arthropod. Image from Kenneth C Gass via Wikimedia Commons ; CC BY-SA 3.0 . |
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Diplocraterion Vertically oriented, u-shaped burrow that has a stacked appearance caused by curved concentric "spreite" (backfills) connecting the arms of the burrow. In bedding plane view they are commonly preserved as paired circular burrows with a "dumbbell" morphology caused by the connecting spreite. Superficially similar to Skolithos but it is u-shaped with paired limbs. Roughly the diameter of a straw. Interpreted as the dwelling burrow of a crustacean. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Helminthoida Trace consists of tight, closely spaced bedding plane parallel coils that do not crosscut one another. Helminthopsis is very similar but has a more irregular and open pattern of loops/coils. Interpreted as the feeding trace of a worm. Image reproduced with written permission for the educational purposes (NC ND) of this LibreText contribution by ENS de Lyon. Original image by Pierre Thomas - Lithothèque ENS de Lyon; https://planet-terre.ens-lyon.fr/ressource/Img38-2003-04-14.xml |
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Helminthopsis Irregular, bedding plane parallel traces that do not crosscut one another; it appears more like loose coils than meanders or loops. The fill is commonly different than the surrounding material. Helminthoida is very similar but has a more regular curved pattern with tighter loops. Interpreted as the feeding trace of a worm. Image from Mark Wilson via Wikimedia Commons ; public domain . |
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Lockeia An oval or teardrop-shaped trace that is commonly preserved in positive relief on the bottom of a bed. It is bilaterally symmetrical and looks similar to the bottom of a bed. Broadly similar to Rusophycus but lacks the medial ridge. Roughly the size and shape of a kidney bean; commonly very dense occurrences. Interpreted as a bivalve resting trace. Image from James St. John via Flickr ; CC BY 2.0 DEED . |
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Macaronichnus Variably oriented burrows that are longitudinally sinuous and roughly circular in cross section. Burrows are "cleaner" and have less orgnic material than surrounding sediment. Most are <1.5 mm diameter, unlined, and unbranching (although they can cross-cut). Interpreted as feeding traces of polychaete worms. Image from Dafoe, L. T. & Williams, G. L. (2020). Lithological, sedimentological, ichnological, and palynological analysis of 37 conventional core intervals from 15 wells, offshore Labrador (Newfoundland and Labrador) and southeast Baffin Island (Nunavut). Geological Survey of Canada, Bulletin , 613. https://doi.org/10.4095/315362 via NRCan Photo Database ; Open Government Licence - Canada |
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See Figure 7c in https://www.researchgate.net/publication/262559003_Chapter_4_The_Ichnofacies_Paradigm/figures?lo=1 |
Burrows are bedding plane parallel depressions or ridges organized into chaotic loops or circular forms. Interpreted as locomotion traces of worms, midges, or larvae |
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Nereites A bedding plane parellel meandering trace consisting of a central furrow flanked by small lobes on both sides of the furrow. Interpreted as a grazing trace. Image from Richdebtomdom via Wikimedia Commons ; CC BY-SA 3.0 ) |
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Olivellites Sinuous, unbranched, bedding plane parallel trace that has two lobes separated by a medial ridge that runs parallel to the long axis of the trace. The lobes are ornamented with fine curved meniscate-like ornamentation that runs roughly parallel to the long axis of the trace. Interpreted as a gastropod locomotion or grazing trace. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Ophiomorpha Vertical to horizontal cylindrical burrow with a smooth interior wall and and an outer wall that has a knobby or "corn-cob" like appearance (lined with fecal pellets). Tunnels can change diameter and branch. Interpreted as a crustacean dwelling trace. Image from James St. John via Flickr ; CC BY 2.0 DEED . |
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Paleodictyon A bedding plane parallel network of hexagonal burrows organized in a honeycomb-like arrangement; vertical shafts common in the modern but rarely preserved. Interpreted as a gardening trace for cultivating microbes; trace maker uncertain. Image from Falconaumanni via Wikimedia Commons ; CC BY-SA 3.0 . |
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Phycosiphon Looping burrows that extend away from a central tunnel; individual loops build outward via small, subtle spriete. Interpreted as a deposit feeding trace. Image by West Virginia Geologic Survey via https://www.wvgs.wvnet.edu/www/museum/seenfossil/seenfossil.htm |
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Planolites A roughly bedding plane parallel unlined burrow that is typically less than 0.5 cm in diameter. Some specimens branch. Burrows are typically linear, cross-cutting, and preserved in positive relief on the base of sandstone beds. Planolites and Thallasinoides are similar, the main difference is size - the former is more the size of a noodle and the latter is more the size of sausage. Interpreted as the feeding trace of a worm. Image from Gennadi Baranov via Fossilid.info ; CC BY-NC . |
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Rhizocorrallium A horizontal, oblique, or U- or J-shaped burrow that shows muddy spreite between the limbs of the burrow. It is distinguished from Diplocraterion because it is typically bent, inclined, or horizontal. It is interpreted as a dwelling or feeding burrow. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Rosselia A vertically-oriented, downward-tapering funnel-shaped burrow that may be lined with mud. Upper part was originally bulb-shaped but later eroded to form more of a funnel. Interpreted as a dwelling burrow of a worm-like detritus feeder. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Rusophycus A bilobate depression that may have a medial ridge and scratches in the depressions. Looks like a coffee been. Most commonly preserved in positive relief on the base of beds. Most are interpreted as resting traces of arthropods. Image from James St. John via Flickr ; CC BY 2.0 DEED . |
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| You can view an image at https://ichnology.ku.edu/invertebrate_traces/tfimages/scoyenia.html |
Scoyenia Variably oriented linear burrow with a rope-like or wrinkled texture. Unbranched but may cross-cut. Interpreted as the feeding trace of terrestrial larvae. |
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Skolithos Vertical, typically unlined burrow that is generally filled with material identical to the surrounding substrate. Roughly the diameter of a straw. Similar to Diplocraterion but not demonstrably paired. Interpreted as the dwelling trace of a variety of invertebrates. Image from James St. John via Flickr ; CC BY 2.0 DEED . |
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Spirorhaphe A roughly bedding-plane parallel burrow that spirals inward and then reverses in the center to form a matched pair of coils. Interpreted as the farming trace of a polychaete worm. Image from Falconaumanni via Wikimedia Commons ; CC BY-SA 3.0 |
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Taenidium Unlined sinuous burrow with pronounced meniscate backfill. Interpreted as the feeding trace of a worm-like organism. Image by Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Tetrapod trackway Any number of named trackways that have footprints from front and back feet. Marks from toes commonly preserved. Morphology highly dependent upon substrate. Tetrapod locomotion trace. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
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Thallasinoides A relatively large horizontal to vertical unlined burrow; may branch to form a Y- or a T-shape. Planolites and Thallasinoides are similar, the main difference is size - the former is more the size of a noodle and the latter is more the size of a hot dog or sausage. Note: if burrow has a knobby texture via fecal pellet lining, it is called Ophiomorpha . Interpreted as a feeding or dwelling burrow of a crustacean. Image from Mark Wilson via Wikimedia Commons ; public domain . |
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Undichnia Assemblage of sinuous grooves formed by dragging fish fins. Tail trace is commonly sinusoidal; pectoral fin traces are parallel to each other and more gently curved. Interpreted as a fish locomotion trace. Image from Figure 7 in Ronchi, A.; Marchetti, L.; Klein, H.; Groenewald, G.H. A Middle Permian Oasis for Vertebrate and Invertebrate Life in a High-Energy Fluvial Palaeoecosystem of Southern Gondwana (Karoo, Republic of South Africa). Geosciences 2023, 13 , 325. https://doi.org/10.3390/geosciences13110325 ; CC BY 4.0 DEED |
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Zoophycos A central vertical burrow that has corkscrew-shaped tiers of sheet-like subhorizontal burrows that make a "rooster tail” or "pinwheel" pattern on bedding planes. Three dimensional form rarely preserved; flattened preservation roughly along bedding planes is much more common. Most are intereted as the feeding burrow of a worm-like organism. Image from Michael C. Rygel via Wikimedia Commons ; CC BY-SA 3.0 . |
Seilacherian (Behavioral) Ichnofacies
Trace fossils record behaviors and those behaviors are dictated by environmental conditions; consequently certain types and assemblages of trace fossils tend to occur together in time and space because the behaviors they record work in a given environment. These assemblages are called ichnofacies and are named for a distinctive trace fossil that is typical of that environment. It is worth noting that a given trace fossil can be present in a variety of environments and the focus is on the assemblage of traces, not a single diagnostic trace fossil. Although nearly two dozen ichnofacies have been described, the diagrams provided below highlight the four most important/common marine ichnofacies ( Skolithos , Cruziana , Zoophycos , and Nereites ), two continental ichnofacies ( Scoyenia and Mermia ), and three substrate-dependent ichnofacies ( Glossofungites , Teredolites , and Trypanites ).
Marine Ichnofacies
Terrestrial Ichnofacies
Substrate-Dependent Ichnofacies
Figure \(\PageIndex{16}\): Sand-filled trace fossils in black shales of the lower Pebbly Beach Formation (Permian). These palimpsest trace fossils were probably excavated in firm substrate and could be interpreted as part of the
Glossofungites
Ichnofacies (
Michael C.
Rygel
via
Wikimedia Commons
;
CC BY-SA 4.0
).
Readings and Resources
- PBS Eons, Something Has Been Making This Mark For 500 Million Years (Video on Paleodictyon ) - https://www.youtube.com/watch?v=Pz1fccY3S84
- KU Ichnology webpage - https://ichnology.ku.edu/
- Ichnofossils at FossilID.info - https://fossiilid.info/112?mode=in_baltoscandia
- Digital Atlas of Ancient Life's trace fossil webpage - https://www.digitalatlasofancientlife.org/vc/trace-fossils/
- Bromley, R.G., 1996, Trace fossils—Biology, taphonomy, and applications [2d ed.]: London, Chapman and Hall, 384 p. - https://www.taylorfrancis.com/books/mono/10.4324/9780203059890/trace-fossils-richard-bromley
- Ekdale, A.A., Bromley, R.G., and Pemberton, S.G., 1984, Ichnology; the use of trace fossils in sedimentology and stratigraphy: Society of Economic Paleontologists and Mineralogists, Short Course No. 15, 317 p.
- Frey, R.W., 1973, Concepts in the study of biogenic sedimentary structures: Journal of Sedimentary Research, v. 43, no. 1, p. 6–19. - https://chooser.crossref.org/?doi=10.1306%2F74D726C1-2B21-11D7-8648000102C1865D
- MacEachern, J.A., Bann, K.L., Gingras, M.K., and Pemberton, S.G., 2007, Applied Ichnology, SEPM Short Course Notes, No. 52, 145 p. - https://sedimentary-geology-store.com/catalog/book/applied-ichnology
- Knaust,D., and Bromley, R.G., 2012, Trace Fossils as Indicators of Sedimentary Environments; Developments in Sedimentology v. 64, 924 p. - https://www.sciencedirect.com/bookseries/developments-in-sedimentology/vol/64/suppl/C
- Seilacher, A., 1953, Studien zur Palichnologie I. Uber die methoden der Palichnologie: Neues Jahrbuch Geologie Palaontologie, Abh. 96, p. 421–452.
- Seilacher, A., 2007, Trace Fossil Analysis, Springer-Verlag, Berlin, 226 p. - https://link.springer.com/book/10.1007/978-3-540-47226-1