Trace fossil

Chirotherium footprints in a Triassic sandstone.
Protichnites trackway from the Cambrian, Blackberry Hill, central Wisconsin.

Trace fossils, also called ichnofossils (sg. /ˈɪknoʊfɒsᵻl/; Greek: ιχνος ikhnos "trace, track"), are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings (bioerosion), urolites (erosion caused by evacuation of liquid wastes), footprints and feeding marks, and root cavities. The term in its broadest sense also includes the remains of other organic material produced by an organism — for example coprolites (fossilized droppings) or chemical markers — or sedimentological structures produced by biological means - for example, stromatolites. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization.

Sedimentary structures, for example those produced by empty shells rolling along the sea floor, are not produced through the behaviour of an organism and not considered trace fossils.

The study of traces is called ichnology, which is divided into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern traces. This science is challenging, as most traces reflect the behaviour — not the biological affinity — of their makers. As such, trace fossils are categorised into form genera, based upon their appearance and the implied behaviour of their makers.

Occurrence

Cross-section of mammoth footprints at The Mammoth Site, Hot Springs, South Dakota.

Traces are better known in their fossilised form than in modern sediments.[1] This makes it difficult to interpret some fossils by comparing them with modern traces, even though they may be extant or even common.[1] The main difficulties in accessing extant burrows stem from finding them in consolidated sediment, and being able to access those formed in deeper water.

Trace fossils are best preserved in sandstones;[1] the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.[1]

Classification

Trace fossils are generally difficult or impossible to assign to a specific maker. Only in very rare occasions are the makers found in association with their tracks. Further, entirely different organisms may produce identical tracks. Therefore, conventional taxonomy is not applicable, and a comprehensive form of taxonomy has been erected. At the highest level of the classification, five behavioral modes are recognized:[1]

Fossils are further classified into form genera, a few of which are even subdivided to a "species" level. Classification is based on shape, form, and implied behavioural mode.

Information provided by ichnofossils

Mesolimulus walchi fossil and track, a rare example of tracks and the creature that made them fossilized together

Because identical fossils can be created by a range of different organisms, trace fossils can only reliably inform us of two things: the consistency of the sediment at the time of its deposition, and the energy level of the depositional environment.[2] Attempts to deduce such traits as whether a deposit is marine or non-marine have been made, but shown to be unreliable.[2]

Paleoecology

Trace fossils provide us with indirect evidence of life in the past, such as the footprints, tracks, burrows, borings, and feces left behind by animals, rather than the preserved remains of the body of the actual animal itself. Unlike most other fossils, which are produced only after the death of the organism concerned, trace fossils provide us with a record of the activity of an organism during its lifetime.

Trace fossils are formed by organisms performing the functions of their everyday life, such as walking, crawling, burrowing, boring, or feeding. Tetrapod footprints, worm trails and the burrows made by clams and arthropods are all trace fossils.

Perhaps the most spectacular trace fossils are the huge, three-toed footprints produced by dinosaurs and related archosaurs. These imprints give scientists clues as to how these animals lived. Although the skeletons of dinosaurs can be reconstructed, only their fossilized footprints can determine exactly how they stood and walked. Such tracks can tell much about the gait of the animal which made them, what its stride was, and whether or not the front limbs touched the ground.

However, most trace fossils are rather less conspicuous, such as the trails made by segmented worms or nematodes. Some of these worm castings are the only fossil record we have of these soft-bodied creatures.

Paleoenvironment

Eubrontes, a dinosaur footprint in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah.

Fossil footprints made by tetrapod vertebrates are difficult to identify to a particular species of animal, but they can provide valuable information such as the speed, weight, and behavior of the organism that made them. Such trace fossils are formed when amphibians, reptiles, mammals or birds walked across soft (probably wet) mud or sand which later hardened sufficiently to retain the impressions before the next layer of sediment was deposited. Some fossils can even provide details of how wet the sand was when they were being produced, and hence allow estimation of paleo-wind directions.[3]

Assemblages of trace fossils occur at certain water depths,[1] and can also reflect the salinity and turbidity of the water column.

Stratigraphic correlation

Some trace fossils can be used as local index fossils, to date the rocks in which they are found, such as the burrow Arenicolites franconicus which occurs only in a 4 cm (1.6") layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.[4]

The base of the Cambrian period is defined by the first appearance of the trace fossil Treptichnus pedum.[5]

Trace fossils have a further utility as many appear before the organism thought to create them, extending their stratigraphic range.[6]

Ichnofacies

Main article: Ichnofacies

Trace fossil assemblages are far from random; the range of fossils recorded in association is constrained by the environment in which the trace-making organisms dwelt.[1] Palaeontologist Adolf Seilacher pioneered the concept of ichnofacies, whereby the state of a sedimentary system at its time of deposition could be implied by noting the fossils in association with one another.[1]

Inherent bias

Diagram showing how dinosaur footprints are preserved in different deposits

Most trace fossils are known from marine deposits.[7] Essentially, there are two types of traces, either exogenic ones, which are made on the surface of the sediment (such as tracks) or endogenic ones, which are made within the layers of sediment (such as burrows).

Surface trails on sediment in shallow marine environments stand less chance of fossilization because they are subjected to wave and current action. Conditions in quiet, deep-water environments tend to be more favorable for preserving fine trace structures.

Most trace fossils are usually readily identified by reference to similar phenomena in modern environments. However, the structures made by organisms in recent sediment have only been studied in a limited range of environments, mostly in coastal areas, including tidal flats.

Evolution

Climactichnites, probably trackways from a slug-like animal, from the Cambrian, Blackberry Hill, central Wisconsin. Ruler in background is 45cm (18") long.

The earliest complex trace fossils, not including microbial traces such as stromatolites, date to 2,000 to 1,800 million years ago. This is far too early for them to have an animal origin, and they are thought to have been formed by amoedae.[8] Putative "burrows" dating as far back as 1,100 million years may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean;[9] however their uneven width and tapering ends make a biological origin so difficult to defend[10] that even the original author no longer believes they are authentic.[11]

The first evidence of burrowing which is widely accepted dates to the Ediacaran (Vendian) period, around 560 million years ago. During this period the traces and burrows basically are horizontal on or just below the seafloor surface. Such traces must have been made by motile organisms with heads, which would probably have been bilateran animals.[12] The trace observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators.[13] Contrary to widely circulated opinion that Ediacaran burrows are only horizontal the vertical burrows Skolithos are also known.[14] The producers of burrows Skolithos declinatus from the Vendian (Ediacaran) beds in Russia with date 555.3 million years ago have not been identified; they might have been filter feeders subsisting on the nutrients from the suspension. The density of these burrows is up to 245 burrows/dm2.[15] Some Ediacaran trace fossils have been found directly associated with body fossils. Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape.[16] The feeding was performed in a mechanical way, supposedly the ventral side of body these organisms was covered with cilia.[17] The potential mollusc related Kimberella is associated with scratch marks, perhaps formed by a radula,[18] further traces from 555 million years ago appear to imply active crawling or burrowing activity.[19]

As the Cambrian got underway, new forms of trace fossil appeared, including vertical burrows (e.g. Diplocraterion) and traces normally attributed to arthropods.[20] These represent a “widening of the behavioural repertoire”,[21] both in terms of abundance and complexity.[22]

Trace fossils are a particularly significant source of data from this period because they represent a data source that is not directly connected to the presence of easily fossilized hard parts, which are rare during the Cambrian. Whilst exact assignment of trace fossils to their makers is difficult, the trace fossil record seems to indicate that at the very least, large, bottom-dwelling, bilaterally symmetrical organisms were rapidly diversifying during the early Cambrian.[23]

Further, less rapid diversification occurred since, and many traces have been converged upon independently by unrelated groups of organisms.[1]

Trace fossils also provide our earliest evidence of animal life on land. The earliest arthropod trackways date to the Cambro-Ordovician,[24] and trackways from the Ordovician Tumblagooda sandstone allow the behaviour of these organisms to be determined.[3] The enigmatic trace fossil Climactichnites may represent an earlier still terrestrial trace, perhaps made by a slug-like organism.

Common ichnogenera

Petroxestes borings in a hardground from the Upper Ordovician of southern Ohio.
Rusophycus trace fossil from the Ordovician of southern Ohio. Scale bar is 10 mm.
Skolithos trace fossil. Scale bar is 10 mm.
Thalassinoides, burrows produced by crustaceans, from the Middle Jurassic, Makhtesh Qatan, southern Israel.
Trypanites borings in an Upper Ordovician hardground from northern Kentucky. The borings are filled with diagenetic dolomite (yellowish). Note that the boring on the far right cuts through a shell in the matrix.
Ophiomorpha and Thalassinoides trace fossils produced by crustaceans found at Camacho formation from the Late Miocene in Colonia Department, Uruguay.

Other notable trace fossils

Less ambiguous than the above ichnogenera, are the traces left behind by invertebrates such as Hibbertopterus, a giant "sea scorpion" or eurypterid of the early Paleozoic era. This marine arthropod produced a spectacular hibbertopteroid track preserved in Scotland.[28]

Bioerosion through time has produced a magnificent record of borings, gnawings, scratchings and scrapings on hard substrates. These trace fossils are usually divided into macroborings[29] and microborings.[30] Bioerosion intensity and diversity is punctuated by two events. One is called the Ordovician Bioerosion Revolution (see Wilson & Palmer, 2006) and the other was in the Jurassic.[31] For a comprehensive bibliography of the bioerosion literature, please see the External links below.

The oldest types of tetrapod tail-and-foot prints date back to the latter Devonian period. These vertebrate impressions have been found in Ireland, Scotland, Pennsylvania, and Australia.

Important human trace fossils are the Laetoli (Tanzania) footprints, imprinted in volcanic ash 3.7 Ma (million years ago) -- probably by an early Australopithecus.[32]

Confusion with other types of fossils

Asteriacites (sea star trace fossil) from the Devonian of northeastern Ohio. It appears at first to be an external mold of the body, but the sediment piled between the rays shows that it is a burrow.

Trace fossils are not body casts. The Ediacara biota, for instance, primarily comprises the casts of organisms in sediment. Similarly, a footprint is not a simple replica of the sole of the foot, and the resting trace of a seastar has different details than an impression of a seastar.

Early paleobotanists misidentified a wide variety of structures they found on the bedding planes of sedimentary rocks as fucoids (Fucales, a kind of brown algae or seaweed). However, even during the earliest decades of the study of ichnology, some fossils were recognized as animal footprints and burrows. Studies in the 1880s by A. G. Nathorst and Joseph F. James comparing 'fucoids' to modern traces made it increasingly clear that most of the specimens identified as fossil fucoids were animal trails and burrows. True fossil fucoids are quite rare.

Pseudofossils, which are not true fossils, should also not be confused with ichnofossils, which are true indications of prehistoric life.

See also

Wikimedia Commons has media related to Trace fossils.

References

  1. 1 2 3 4 5 6 7 8 9 Seilacher, D. (1967). "Bathymetry of trace fossils". Marine Geology 5 (5–6): 413–428. doi:10.1016/0025-3227(67)90051-5.
  2. 1 2 Woolfe, K.J. (1990). "Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica". Palaeogeography, Palaeoclimatology, Palaeoecology 80 (3–4): 301–310. doi:10.1016/0031-0182(90)90139-X.
  3. 1 2 Trewin, N.H.; McNamara, K.J. (1995). "Arthropods invade the land: trace fossils and palaeoenvironments of the Tumblagooda Sandstone (? late Silurian) of Kalbarri, Western Australia". Transactions of the Royal Society of Edinburgh: Earth Sciences 85: 177–210. doi:10.1017/s026359330000359x.
  4. ↑ Schlirf, M. (2006). "Trusheimichnus New Ichnogenus From the Middle Triassic of the Germanic Basin, Southern Germany". Ichnos 13 (4): 249–254. doi:10.1080/10420940600843690. Retrieved 2008-04-21.
  5. ↑ Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne, Guy (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138 (2): 213–218. doi:10.1017/S001675680100509X.
  6. ↑ e.g. Seilacher, A. (1994). "How valid is Cruziana Stratigraphy?" (PDF). International Journal of Earth Sciences 83 (4): 752–758. Bibcode:1994GeoRu..83..752S. doi:10.1007/BF00251073. Retrieved 2007-09-09.
  7. ↑ Saether, Kristian; Christopher Clowes. "Trace Fossils". Retrieved 2009-06-19.
  8. ↑ Bengtson, S; Rasmussen, B (January 2009). "Paleontology. New and ancient trace makers". Science 323 (5912): 346–7. doi:10.1126/science.1168794. PMID 19150833.
  9. ↑ Seilacher, A.; Bose, P.K.; Pflüger, F. (1998-10-02). "Triploblastic Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India". Science 282 (5386): 80–83. Bibcode:1998Sci...282...80S. doi:10.1126/science.282.5386.80. PMID 9756480. Retrieved 2007-05-21.
  10. ↑ Budd, G.E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla" (abstract). Biological Reviews 75 (02): 253–295. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389.
  11. ↑ Jensen, S. (2008). "PALEONTOLOGY: Reading Behavior from the Rocks". Science 322 (5904): 1051–1052. doi:10.1126/science.1166220.
  12. ↑ Fedonkin, M.A. (1992). "Vendian faunas and the early evolution of Metazoa". In Lipps, J., and Signor, P. W., eds., Origin and early evolution of the Metazoa: New York, Plenum Press. (Springer): 87–129. ISBN 0-306-44067-9. Retrieved 2007-03-08.
  13. ↑ Dzik, J (2007), "The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications 286, London: Geological Society, pp. 405–414, doi:10.1144/SP286.30, ISBN 9781862392335, OCLC 156823511 191881597
  14. ↑ M. A. Fedonkin (1985). "Paleoichnology of Vendian Metazoa". In Sokolov, B. S. and Iwanowski, A. B., eds., "Vendian System: Historical–Geological and Paleontological Foundation, Vol. 1: Paleontology". Moscow: Nauka, pp. 112–116. (in Russian)
  15. ↑ Grazhdankin, D. V.; A. Yu. Ivantsov (1996). "Reconstruction of biotopes of ancient Metazoa of the Late Vendian White Sea Biota". Paleontological Journal 30: 676–680.
  16. ↑ Ivantsov, A.Y.; Malakhovskaya, Y.E. (2002). "Giant Traces of Vendian Animals" (PDF). Doklady Earth Sciences (Doklady Akademii Nauk) 385 (6): 618–622. ISSN 1028-334X. Retrieved 2007-05-10.
  17. ↑ A. Yu. Ivantsov. (2008). "Feeding traces of the Ediacaran animals". HPF-17 Trace fossils ? ichnological concepts and methods. International Geological Congress - Oslo 2008.
  18. ↑ New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications 286, London: Geological Society, pp. 157–179, doi:10.1144/SP286.12, ISBN 9781862392335, OCLC 156823511 191881597
  19. ↑ According to Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L. (2000-05-05). "Age of Neoproterozoic Bilatarian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution" (abstract). Science 288 (5467): 841–5. Bibcode:2000Sci...288..841M. doi:10.1126/science.288.5467.841. PMID 10797002.
  20. ↑ Such as Cruziana and Rusophycus. Details of Cruziana’s formation are reported by Goldring, R. (January 1, 1985). "The formation of the trace fossil Cruziana". Geological Magazine 122 (1): 65–72. doi:10.1017/S0016756800034099. Retrieved 2007-09-09.
  21. ↑ Conway Morris, S. (1989). "Burgess Shale Faunas and the Cambrian Explosion". Science 246 (4928): 339–46. Bibcode:1989Sci...246..339C. doi:10.1126/science.246.4928.339. PMID 17747916.
  22. ↑ Jensen, S. (2003). "The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives". Integrative and Comparative Biology (The Society for Integrative and Comparative Biology) 43 (1): 219–228. doi:10.1093/icb/43.1.219. PMID 21680425.
  23. ↑ Although some cnidarians are effective burrowers, e.g. Weightman, J.O.; Arsenault, D.J. (2002). "Predator classification by the sea pen Ptilosarcus gurneyi (Cnidaria): role of waterborne chemical cues and physical contact with predatory sea stars" (PDF). Canadian Journal of Zoology 80 (1): 185–190. doi:10.1139/z01-211. Retrieved 2007-04-21. most Cambrian trace fossils have been assigned to bilaterian animals.
  24. ↑ MacNaughton, R.B.; Cole, J.M.; Dalrymple, R.W.; Braddy, S.J.; Briggs, D.E.G.; Lukie, T.D. (2002). "First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada". Geology 30 (5): 391–394. Bibcode:2002Geo....30..391M. doi:10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2. ISSN 0091-7613.
  25. ↑ Vinn, O.; Wilson, M.A.; Zatoń, M.; Toom, U. (2014). "The trace fossil Arachnostega in the Ordovician of Estonia (Baltica)". Palaeontologia Electronica. 17.3.40A: 1–9. Retrieved 2014-06-10.
  26. ↑ Getty, Patrick; James Hagadorn (2009). "Palaeobiology of the Climactichnites trailmaker". Palaeontology 52 (4): 758–778. doi:10.1111/j.1475-4983.2009.00875.x.
  27. ↑ Getty, Patrick; James Hagadorn (2008). "Reinterpretation of Climactichnites Logan 1860 to Include Subsurface Burrows, and Erection of Musculopodus for Resting Traces of the Trailmaker". Journal of Paleontology 82 (6): 1161–1172. doi:10.1666/08-004.1.
  28. ↑ Whyte, MA (2005). "Palaeoecology: A gigantic fossil arthropod trackway". Nature 438 (7068): 576. Bibcode:2005Natur.438..576W. doi:10.1038/438576a. PMID 16319874.
  29. ↑ Wilson, M.A., 2007. Macroborings and the evolution of bioerosion, p. 356-367. In: Miller, W. III (ed.), Trace Fossils: Concepts, Problems, Prospects. Elsevier, Amsterdam, 611 pages.
  30. ↑
    • Glaub, I., Golubic, S., Gektidis, M., Radtke, G. and Vogel, K., 2007. Microborings and microbial endoliths: geological implications. In: Miller III, W (ed) Trace fossils: concepts, problems, prospects. Elsevier, Amsterdam: pp. 368-381.
    • Glaub, I. and Vogel, K., 2004. The stratigraphic record of microborings. Fossils & Strata 51:126-135.
  31. ↑
    • Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.
  32. ↑ David A. Raichlen, Adam D. Gordon, William E. H. Harcourt-Smith, Adam D. Foster, Wm. Randall Haas, Jr (2010). Rosenberg, Karen, ed. "Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics". PLoS ONE 5 (3): e9769. doi:10.1371/journal.pone.0009769. PMC 2842428. PMID 20339543.

Further reading

External links

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