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Definition: Bioturbation / Trace Fossils / Ichnotaxa
BioturbationAny 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 a complete lack of any burrowing organisms. Trace fossils and bioturbation first appear in sedimentary rocks after the end of the last major Proterozoic glaciation, the Varanger-Marinoan ice age, approximately 600 to 590 Ma. |
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Trace Fossils, IchnofossilsTrace fossils, or ichnofossils, are the evidence of bioturbation preserved in sediments. 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. |
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| The beginning of the Cambrian Period at 543 Ma is marked by the first records of larger, vertical traces, and from then until the beginning of the explosion the traces become an order of magnitude wider, more diverse, and more complex, suggesting a gradual diversification of larger animals (Crimes and Droser, 1992). Indeed, the lower boundary of the Cambrian is now defined by the occurrence of a distinctive horizontal burrow trace fossil, Trichophycus (formerly Phycodes) pedum in the reference section at Fortune Head, southeastern Newfoundland. |
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IchnotaxaTrace 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 recognized 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. |
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| Very occasionally, it may be possible (e.g. by finding a fossilized 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. When you recall that one of the disadvantages of trace fossils is that the same trace fossil could have been made by many organisms, think of the difficulty of trying to match every Cruziana with each of the hundreds of trilobite genera. | |||||||||||
| 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. |
ReferencesCowie, J.W. and Brasier, M.D. eds. (1989): The Precambrian-Cambrian Boundary. Clarendon Press. Crimes and Droser (1992): |
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