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,
Fig. 2 Highly branched, fodinichnial mining
burrow, possibly produced by a deposit-feeding crustacean or worm. (a)
Chondrites,
Fig. 3 Agrichnial farming trace of an unknown
organism, composed of a hexagonal, meshlike network of tunnels. Paleodictyon,
Fig. 4 Agrichnial farming traces of unknown
organisms, including a double-spiral tunnel (Spirorhaphe) and a meshlike network
of tunnels (Paleodictyon).
Fig. 5 Loosely meandering, agrichnial farming
trace of an unknown organism. Cosmorhaphe,
Fig. 6 Praedichnial boring drilled in a bivalve
shell by a carnivorous gastropod. Oichnus, Recent,
Fig. 7 Tightly meandering, pascichnial grazing
trail, created by an unknown worm. Helminthoida,
Fig. 8 Repichnial trail of an arthropod,
possibly a trilobite. Cruziana, Cambrian,
Fig. 9 Unnamed escape structure (fugichnial
trace) in sandstone. Cretaceous,
Fig. 10 Cubichnial resting trace of an ophiuroid
brittle star. Asteriacites, Jurassic,
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,
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.
- R. G. Bromley, Trace Fossils: Biology, Taphonomy and Applications., 2d ed., Chapman and Hall, 1996
- A. A. Ekdale, Palaeogeog. Palaeoclimatol. Palaeoecol., 50:63–81, 1985
- A. A. Ekdale, Palaios, 3:464–472, 1988
- A. A. Ekdale, R. G. Bromley, and S. G. Pemberton, Ichnology: Trace Fossils in Sedimentology and Stratigraphy, 1984
- W. Häntzschel, in C. Teichert (ed.), Treatise on Invertebrate Paleontology, pt. W, revised, University of Kansas Press, 1975
فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils
فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils
فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils
فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils-فسیلهای اثری-Trace fossils
این وبلاگ تمامی موضوعات و مقالات و اطالاعات تخصصی زمین شناسی را که از سایتهای علمی جهان برگرفته شده در اختیار بازدیدکنندگان محترم قرار می دهد.گفتنی است که مطالب موجود در این وبلاگ در نوع خود بی نظیر بوده و از هیچ وبلاگ ایرانی ای کپی برداری نشده است و اگر هم شده منبع آن به طور کامل ذکر شده است.