فسیلهای اثری-Trace fossils

Trace fossils

Fossilized evidence of animal behavior, also known as ichnofossils, biogenic sedimentary

structures, bioerosion structures, or lebensspuren. The fossils include burrows, trails, and trackways created by animals in unconsolidated sediment, as well as borings, gnawings, raspings, and scrapings excavated by organisms in harder materials, such as rock, shell, bone, or wood. Some workers also consider coprolites (fossilized feces), regurgitation pellets, burrow excavation pellets, rhizoliths (plant root penetration structures), and algal stromatolites to be trace fossils.  See also: Stromatolite

Trace fossils are important in paleontology and paleoecology, because they are fossils that provide information about the presence of unpreserved soft-bodied members of the original communities, life habits of fossil organisms, evolution of certain behavior patterns through geologic time, and biostratigraphy of otherwise unfossiliferous deposits. Trace fossils also are useful in sedimentology and paleoenvironmental studies, because they are sedimentary structures that are preserved in place and are very rarely reworked and transported, as body fossils of animals and plants commonly are. This fact allows trace fossils to be regarded as reliable indicators of original conditions in the sedimentary environment. The production of trace fossils involves disruption of original stratification and sometimes results in alteration of sediment texture or composition.

Occurrence

Trace fossils occur in sedimentary deposits of all ages from the late Precambrian to the Recent. Host rocks include limestone, sandstone, siltstone, shale, coal, and other sedimentary rocks. These deposits represent sedimentation in a broad spectrum of settings, ranging from subaerial (such as eolian dunes and soil horizons) to subaqueous (such as rivers, lakes, swamps, tidal flats, beaches, continental shelves, and the deep-sea floor).  See also: Depositional systems and environments

Preservation

Organisms may produce fossilizable traces on the sediment surface (epigenic structures) or within the sediment (endogenic structures). Trace fossils may be preserved in full three-dimensional relief (either wholly contained within a rock or weathered out as a separate piece) or in partial relief (either as a depression or as a raised structure on a bedding plane). Simply because a trace fossil is preserved on a bedding plane does not indicate that it originally was an epigenic trace.

Adolf Seilacher, a German paleontologist, proposed the following terminology to describe the preservational mode of trace fossils: full relief (enclosed entirely within the sediment); positive epirelief (ridge, mound, or other raised structure on the upper surface of a bed); negative epirelief (groove, pit, or other indentation on the upper surface of a bed); positive hyporelief (raised structure on the sole of a bed); and negative hyporelief (indentation on the sole of a bed). Anders Martinsson, a Swedish paleontologist, proposed an alternative system to characterize the preservation of trace fossils: endichnia (enclosed entirely within the sediment); epichnia (exposed in relief on the upper surface of a bed); hypichnia (exposed in relief on the sole of a bed); and exichnia (entirely removed from the sediment in which it was produced).

Diagenetic alteration of sediment commonly enhances the preservation of trace fossils by differential cementation or selective mineralization. In some cases, trace fossils have been preferentially replaced by chert, dolomite, pyrite, glauconite, apatite, siderite, or other minerals.  See also: Diagenesis

Classification and nomenclature

The study of trace fossils is known as ichnology. The prefix “ichno-” (as in ichnofossil and ichnotaxonomy) and the suffix “-ichnia” (as in epichnia and hypichnia) commonly are employed to designate subjects relating to trace fossils. The suffix “-ichnus” commonly is attached to the ichnogenus name of many trace fossils (as in Dimorphichnus and Teichichnus).

In the nineteenth century, many trace fossils were mistakenly identified as fossil plants, so they were given the genus and species names of plants in accordance with established principles of Linnean taxonomy. Subsequent recognition that these fossils actually were biogenic sedimentary structures that represented animal activity in the sediment has not stopped the practice of assigning formal taxonomic names to trace fossils. Usually, geologists differentiate between trace fossils and body fossils by the terms “ichnogenus” and “ichnospecies” when speaking of trace fossils. Rules of the International Code of Zoological Nomenclature generally apply to trace fossils at the genus and species level. Although some workers have proposed higher taxonomic levels for trace fossils, such as ichnofamilies or ichnophyla, none of these have gained universal acceptance.  See also: Taxonomy

Fossil behavior

Trace fossils provide tangible information of the activities of ancient organisms, because they represent particular behavior patterns related to dwelling, feeding, locomotion, and resting. Ichnologists have established several behavioral categories of trace fossils, including the following eight groups that are most widely recognized today (Figs. 1,2,3,4,5,6,7,8,9,10): domichnia (dwelling structures, such as permanent burrows or agglutinated tubes); fodinichnia (burrows produced in the process of mining the sediment for food); agrichnia (burrows produced in order to farm or trap food inside the sediment); praedichnia (traces of predation); pascichnia (feeding trails, either within the sediment or on the sediment surface); repichnia (locomotion trails and trackways); fugichnia (escape traces, usually produced by an animal crawling out from beneath a rapidly deposited pile of sediment); and cubichnia (resting or nesting traces).

Fig. 1  Domichnial dwelling burrow with pelleted wall, probably created by a burrowing crustacean. Ophiomorpha, Cretaceous, North Dakota. (From W. Häntzschel, in C. Teichert, ed., Treatise on Invertebrate Paleontology, pt. W, revised, University of Kansas Press, 1975)

Fig. 2  Highly branched, fodinichnial mining burrow, possibly produced by a deposit-feeding crustacean or worm. (a) Chondrites, Tertiary, Austria. (b) Chondrites, Tertiary, Spain. (From W. Häntzschel, in C. Teichert, ed., Treatise on Invertebrate Paleontology, pt. W, revised, University of Kansas Press, 1975)

Fig. 3  Agrichnial farming trace of an unknown organism, composed of a hexagonal, meshlike network of tunnels. Paleodictyon, Tertiary, Poland. (Photograph by W. Häntzschel)

Fig. 4  Agrichnial farming traces of unknown organisms, including a double-spiral tunnel (Spirorhaphe) and a meshlike network of tunnels (Paleodictyon). Tertiary, Austria. (Photograph by W. Häntzschel)

Fig. 5  Loosely meandering, agrichnial farming trace of an unknown organism. Cosmorhaphe, Tertiary, Poland. (From W. Häntzschel, in C. Teichert, ed., Treatise on Invertebrate Paleontology, pt. W, revised, University of Kansas Press, 1975)

Fig. 6  Praedichnial boring drilled in a bivalve shell by a carnivorous gastropod. Oichnus, Recent, Texas. (Photograph by A. A. Ekdale)

Fig. 7  Tightly meandering, pascichnial grazing trail, created by an unknown worm. Helminthoida, Tertiary, Austria. (From W. Häntzschel, in C. Teichert, ed., Treatise on Invertebrate Paleontology, pt. W, revised, University of Kansas Press, 1975)

Fig. 8  Repichnial trail of an arthropod, possibly a trilobite. Cruziana, Cambrian, Alberta. (Photograph by J. P. A. Magwood)

Fig. 9  Unnamed escape structure (fugichnial trace) in sandstone. Cretaceous, Utah. (Photograph by A. A. Ekdale)

Fig. 10  Cubichnial resting trace of an ophiuroid brittle star. Asteriacites, Jurassic, Germany. (Photograph by W. Häntzschel)

Environmental implications

Trace fossils are useful to geologists as indicators of ancient environments of deposition. Recurrent assemblages of trace fossils that represent certain environmental conditions, such as water depth, salinity, or character of the sea floor, are known as ichnofacies. Ichnofacies are named after common ichnogenera that exemplify this association.  See also: Facies (geology)

Adolf Seilacher established a bathymetric zonation of universal ichnofacies representing unconsolidated sediments in marine settings, which can be found throughout the geologic column all over the world. The Skolithos ichnofacies typically represents nearshore, often intertidal, environments characterized by well-sorted clastic sediments that are dominated by primary sedimentary structures. Most of the trace fossils are domichnia, repichnia, and fugichnia. The Cruziana ichnofacies represents offshore settings, generally within wave base (that is, in water shallow enough for waves to move sediment grains on the sea floor). The trace fossils include an abundance of domichnia, repichnia, fugichnia, cubichnia, pascichnia, or fodinichnia. The Zoophycos ichnofacies usually represents quiet-water conditions far from shore, often on a submarine slope. The trace fossils are characterized by a low-diversity assemblage of fodinichnia. The Nereites ichnofacies represents fine-grained, distal turbidites that were deposited in relatively deep water. The trace fossils consist mainly of pascichnia and agrichnia. Another deep-water trace fossil association, simply referred to as the deep-sea ichnofacies, represents pelagic sedimentary environments, and it is characterized by a moderate-diversity assemblage of fodinichnia that were deeply emplaced within the sediment.  See also: Marine sediments; Turbidite

Other marine ichnofacies have been established for substrates that were not unconsolidated sediment. The Glossifungites ichnofacies represents very firm substrates (highly compacted but uncemented sediment). The Trypanites ichnofacies represents fully lithified substrates (cemented sedimentary rock and calcareous shell material). The Teredolites ichnofacies represents wood substrates. In all three cases, the dominant trace fossils are domichnia, namely borings, which were excavated by organisms with very special adaptations for penetrating the harder substrates.

Seilacher grouped all trace fossil associations in continental settings into one ichnofacies, the Scoyenia ichnofacies. Although it is widely recognized that several nonmarine ichnofacies exist in the geologic record, no precise delineation of these varied ichnofacies has achieved universal acceptance. Some workers have established various local ichnofacies that have yet to be accepted as having universal application.

Ichnofabric

The activities of burrowing and boring organisms can profoundly affect many aspects of the texture and internal structure of a sedimentary deposit, as sediment grains are sorted, modified, and redistributed by infaunal (living within the sediment below the sediment surface) animals. Sediment fabrics that result from bioturbation (vertical mixing of sediment by the burrowing activities of animals) and bioerosion activities are called ichnofabrics. In many situations, such as in pelagic carbonate deposits, continuous sedimentation and simultaneous bioturbation allow for the superimposition of different suites of organism traces, thus producing composite ichnofabrics (Fig. 11). Ichnofabric analysis can shed light on the paleoecology of the infaunal community, including trophic relationships and tiering structure. Ichnofabrics also allow the stability and firmness of the original substrate to be interpreted by examining the distinctness and degree of deformation of trace fossils. Interstitial oxygen conditions in the original sediment may be deciphered from the abundance and preservational modes of deep-tier burrows. Early diagenetic processes, including differential cementation and secondary mineralization, can enhance the preservation and visibility of ichnofabrics.

Fig. 11  Complex, composite ichnofabric created by numerous successive phases of burrowing in a fine-grained pelagic carbonate deposit, which has been weakly cemented to become chalk. Upper Cretaceous, Denmark. (Photograph by A. A. Ekdale and R. G. Bromley)

Sedimentologic implications

Trace fossils reflect the interplay among the three important sedimentologic processes of deposition, erosion, and burrowing of the sediment by organisms (causing disturbance or obliteration of primary stratification). Slow, continuous deposition, as occurs in the oxygen-rich water of offshore shelf and deep-sea environments, usually is accompanied by a total burrowing of the sediment. Numerous trace fossils, especially domichnia, fodinichnia, and pascichnia, characterize such situations. In contrast, slow, continuous deposition in eutrophic lakes or restricted marine basins that contain oxygen-depleted bottom water will yield laminated sediment without trace fossils, because the available oxygen is insufficient to support bottom-dwelling animals. Rapid, continuous deposition, as occurs in prograding beaches and laterally accreting point bars, commonly is reflected by sparse trace fossils (mainly fugichnia, cubichnia, and domichnia) superimposed on a sedimentary fabric of primary bed forms and primary sedimentary structures.

Discontinuous deposition usually results from the alternation of slow and rapid sedimentation events, as exemplified by turbidites, or from the alternation of rapid sedimentation and erosion events, as occurs during major storms along a marine coastline. In the former case, the fine-grained units of a turbidite represent a lengthy period of slow deposition, during which numerous kinds of organisms lived on and in the sea floor, creating a wide variety of fossilizable traces (especially pascichnia, fodinichnia, and agrichnia). The coarser-grained unit of the same turbidite represents sudden deposition by a turbidity current, and this unit is characterized by domichnia of organisms that colonized the new sediment immediately after the turbidite event. Thus, turbidite sequences actually contain two separate generations of trace fossils: a predepositional trace fossil association in the fine-grained units and a postdepositional trace fossil association in the coarse-grained units.

In the latter case, laminated-to-burrowed sedimentary sequences represent the effects of storms, which erode the sea bottom and resuspend the sediment. As a storm subsides, sediment is redeposited, and the organism community is reestablished. The vertical transition from laminated sediment layers to burrowed sediment layers reflects a declining sedimentation rate, and the top of the sequence is bounded by an erosional unconformity that marks the next erosive storm event.