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Definition: Bioturbation / Trace Fossils / Ichnotaxa


Bioturbation

Any animal that lives in or on sediments – that digs or burrows or simply moves across the surface – will create some kind of disturbance in those sediments.

In the broadest sense, any kind of sediment disturbance is bioturbation. Often, however, the term is employed more restrictively to refer to the disruption of fine sedimentary layering by digging organisms.

Archean and most Proterozoic sedimentary rocks are notable for their distinct lack of any bioturbated textures. Very fine layers in these rocks are perfectly preserved, suggesting an almost complete lack of any burrowing organisms. Trace fossils and bioturbation first appear commonly in sedimentary rocks after the end of the last major Proterozoic glaciation, the Varanger-Marinoan ice age, approximately 600 to 590 Ma.

Trace Fossils, Ichnofossils

Trace fossils, or ichnofossils, are the evidence of bioturbation preserved in sediments, produced in soft sediments and hard substrates as a result of the living activities of organisms. They include surface tracks and trails, subsurface burrows and borings, as well as fecal material and the marks produced by dying animals (Häntzschel 1975). To a large extent, the factors that control the distribution of different kinds of trace fossils are environmental rather than temporal (Seilacher 1964a, b; Osgood 1970; Häntzschel 1975; Fedonkin 1977; Savrda & Bottjer 1986), so most ichnofossil taxa have long stratigraphic ranges.

Development Through Time

The first trace fossils are very simple trails: The earliest clear indications of animals are trace fossils from about 570 Ma – structures, such as trails, that record animal activity. Before Cambrian time these traces are rare and minute, 1 mm or so wide for the most part, and were probably formed by creeping worms. Younger trace fossils progressively increase in complexity through time.

Traces are mostly found in marine sediments, but some of the most famous are terrestrial. Among larger and more recent animals, dinosaur trackways are relatively common, intensively studied, and have engendered an entire literature of their own. Perhaps the most famous, however, is the homonid trackway discovered by Mary Leakey's team at Laetoli.

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Ichnotaxa

Trace fossils are given Linnaean-style (binomial) taxonomic names, just as if they were actual organisms. The assignment of names to actual animals (or parts of animals) is usually dictated by the rules set out in the ICZN (International Code of Zoological Nomenclature; there is also an ICBN for botanical names). However, trace fossils are not actual organisms or parts of organisms, so they cannot be given Linnaean names recognised by the International Committee on Zoological Nomenclature.

Nevertheless, morphologically distinctive trace fossils are given genus and species names by ichnologists for the sake of international communication. For example, a simple, unbranched, unlined, horizontal burrow might be given the name Planolites, and varieties of that same basic morphology can be identified as "species" under that same "genus" name (e.g. Planolites montanus). To avoid confusion with binomial nomenclature used in naming body fossils, trace fossils are named as ichnogenera (plural of ichnogenus) and ichnospecies.

Very occasionally, it may be possible (e.g. by finding a fossilised body fossil at the end of a trail or burrow) to learn which particular organism makes a particular trace fossil – assuming the trace is distinctive enough to be sure that apparently identical versions are not created by several different organisms – but mostly the "owner" is unknown. Part of the confusion regarding this nomenclature arises from the common lack of connection between the trace fossil name and the name of its original trace maker. Some trace fossils were made by unknown, extinct, or poorly preserved organisms, hence the ichnogenus name can not be expected to reflect a specific trace maker.
Consequently, a trilobite trackway may have been made by a species of the trilobite Isotelus, but the trilobite trackway itself might be called Cruziana. (It must be remembered that the same trace fossil could have been made by many organisms.)
Even dinosaur tracks can rarely be matched with a specific dinosaur; most ichnologists are satisfied enough to say that certain trackways were made by sauropods or theropods.

References

Fedonkin, M.A. 1977: Precambrian–Cambrian ichnocoenoses of the east European platform. In Crimes, T.P.; Harper, J.C. (eds.) 1977: Trace fossils 2. Geological Journal, Special Issue no. 9. Liverpool: Seel House Press, pp. 183–94.

Häntzschel, W. 1975: Trace fossils and problematica. In Teichert, C (ed.) 1975: Treatise of Invertebrate Paleontology (2nd Edition), part W, Miscellanea, supp 1. University of Kansas and Geological Society of America, Boulder, Colorado, and Lawrence, Kansas, 269 pp.

Osgood, R.G. Jr. 1970: Trace fossils of the Cincinnati area. Palaeontographica Americana 6(41): 281–444.

Savrda, C.E.; Bottjer, D.J. 1986: Trace-fossil model for reconstruction of paleo-oxygenation in bottom waters. Geology 14: 3–6.

Seilacher, A. 1964a: Biogenic sedimentary structures. In Imbrie, J.; Newell, N.D. (eds.) 1964: Approaches to paleoecology . John Wiley & Sons, New York, pp. 296-316.

Seilacher, A. 1964b: Sedimentological classification and nomenclature of trace fossils. Sedimentology 3: 253-256.


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