Atrace fossil, also known as anichnofossil (/ˈɪknoʊfɒsɪl/; fromGreek:ἴχνοςikhnos "trace, track"), is afossil record ofbiological activity bylifeforms but not the preserved remains of theorganism itself.[1] Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or bymineralization. The study of such trace fossils isichnology - the work ofichnologists.[2]
Trace fossils may consist of physical impressions made on or in thesubstrate by an organism.[3] For example,burrows, borings (bioerosion),urolites (erosion caused by evacuation of liquid wastes),footprints, feeding marks, and root cavities may all be trace fossils.
The term in its broadest sense also includes the remains of other organic material produced by an organism; for examplecoprolites (fossilized droppings) or chemical markers (sedimentological structures produced by biological means; for example, the formation ofstromatolites). However, mostsedimentary structures (for example those produced by empty shells rolling along the sea floor) are not produced through the behaviour of an organism and thus are not considered trace fossils.
The study of traces – ichnology – divides intopaleoichnology, or the study of trace fossils, andneoichnology, the study of modern traces. Ichnological science offers many challenges, as most traces reflect the behaviour – not the biological affinity – of their makers. Accordingly, researchers classify trace fossils intoform genera based on theirappearance and on the implied behaviour, orethology, of their makers.
Traces are better known in their fossilized form than in modern sediments.[4] This makes it difficult to interpret some fossils by comparing them with modern traces, even though they may be extant or even common.[4] The main difficulties in accessing extant burrows stem from finding them in consolidated sediment, and being able to access those formed in deeper water.
Thiscoprolite shows distinct top and bottom jaw bite marks, possibly from a prehistoricgar fish. Discovery location:South Carolina,US; age:Miocene; dimensions: 144.6 mm × 63.41 mm (5.693 in × 2.496 in); weight: 558 g (1 lb 3.7 oz)
Trace fossils are best preserved in sandstones;[4] the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.[4]
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:[4]
Domichnia, dwelling structures reflecting the life position of the organism that created it.
Fodinichnia, three-dimensional structures left by animals which eat their way through sediment, such as deposit feeders;
Pascichnia, feeding traces left by grazers on the surface of a soft sediment or a mineral substrate;
Cubichnia, resting traces, in the form of an impression left by an organism on a soft sediment;
Repichnia, surface traces of creeping and crawling.
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.
To keep body and trace fossils nomenclatorially separate,ichnospecies are erected for trace fossils.Ichnotaxa are classified somewhat differently inzoological nomenclature than taxa based on body fossils (seetrace fossil classification for more information). Examples include:
Mesolimulus walchi fossil and track, a rare example of tracks and the creature that made them fossilized together
Trace fossils are important paleoecological and paleoenvironmental indicators, because they are preservedin situ, or in the life position of the organism that made them.[5] 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 thedepositional environment.[6] Attempts to deduce such traits as whether a deposit is marine or non-marine have been made, but shown to be unreliable.[6]
Trace fossils provide us with indirect evidence oflife 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. Unlike body fossils, which can be transported far away from where an individual organism lived, trace fossils record the type of environment an animal actually inhabited and thus can provide a more accurate palaeoecological sample than body fossils.[7]
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 byclams andarthropods are all trace fossils.
Perhaps the most spectacular trace fossils are the huge, three-toed footprints produced bydinosaurs and relatedarchosaurs. These imprints give scientists clues as to how these animals lived. Although the skeletons of dinosaurs can be reconstructed, only theirfossilized 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 the front limbs touched the ground or not.
However, most trace fossils are rather less conspicuous, such as the trails made bysegmented worms ornematodes. Some of theseworm castings are the only fossil record we have of these soft-bodied creatures.[citation needed]
Fossil footprints made by tetrapodvertebrates 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 whenamphibians,reptiles,mammals, orbirds 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.[8]
Assemblages of trace fossils occur at certain water depths,[4] and can also reflect the salinity and turbidity of the water column.
Some trace fossils can be used as localindex fossils, to date the rocks in which they are found, such as the burrowArenicolites franconicus which occurs only in a 4 cm (1+1⁄2 in) layer of theTriassicMuschelkalk epoch, throughout wide areas in southernGermany.[9]
Ichnofacies are assemblages of individual trace fossils that occur repeatedly in time and space.[12] PalaeontologistAdolf Seilacher pioneered the concept of ichnofacies, whereby geologists infer the state of a sedimentary system at its time of deposition by noting the fossils in association with one another.[4] The principal ichnofacies recognized in the literature areSkolithos,Cruziana,Zoophycos,Nereites, Glossifungites,Scoyenia,Trypanites,Teredolites, andPsilonichus.[12][13] These assemblages are not random. In fact, the assortment of fossils preserved are primarily constrained by the environmental conditions in which the trace-making organisms dwelt.[13] Water depth,salinity, hardness of the substrate, dissolved oxygen, and many other environmental conditions control which organisms can inhabit particular areas.[12] Therefore, by documenting and researching changes in ichnofacies, scientists can interpret changes in environment.[13] For example, ichnological studies have been utilized across mass extinction boundaries, such as theCretaceous–Paleogene mass extinction, to aid in understanding environmental factors involved in mass extinction events.[14][15]
Diagram showing how dinosaur footprints are preserved in different deposits
Most trace fossils are known from marine deposits.[16] 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, includingtidal flats.[citation needed]
The earliest complex trace fossils, not including microbial traces such asstromatolites, date to2,000 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 byamoebae.[17]Putative "burrows" dating as far back as1,100 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;[18] however their uneven width and tapering ends make a biological origin so difficult to defend[19] that even the original author no longer believes they are authentic.[20]
The first evidence of burrowing which is widely accepted dates to theEdiacaran (Vendian) period, around560 million years ago.[21] 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 beenbilaterananimals.[22] The traces observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators.[23] Contrary to widely circulated opinion that Ediacaran burrows are only horizontal the vertical burrowsSkolithos are also known.[24] The producers of burrowsSkolithos declinatus from the Vendian (Ediacaran) beds inRussia with date555.3 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.[25] Some Ediacaran trace fossils have been found directly associated with body fossils.Yorgia andDickinsonia are often found at the end of long pathways of trace fossils matching their shape.[26] The feeding was performed in a mechanical way, supposedly the ventral side of body these organisms was covered withcilia.[27] The potentialmollusc relatedKimberella is associated with scratch marks, perhaps formed by aradula,[28] further traces from555 million years ago appear to imply active crawling or burrowing activity.[29]
As theCambrian got underway, new forms of trace fossil appeared, including vertical burrows (e.g.Diplocraterion) and traces normally attributed toarthropods.[30] These represent a "widening of the behavioural repertoire",[31] both in terms of abundance and complexity.[32]
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 earlyCambrian.[33]
Further, less rapid[verification needed] diversification occurred since,[verification needed] and many traces have been converged upon independently by unrelated groups of organisms.[4]
Trace fossils also provide our earliest evidence of animal life on land.[34] Evidence of the first animals that appear to have been fully terrestrial dates to the Cambro-Ordovician and is in the form of trackways.[35] Trackways from the OrdovicianTumblagooda sandstone allow the behaviour of other terrestrial organisms to be determined.[8] The trackwayProtichnites represents traces from an amphibious or terrestrial arthropod going back to the Cambrian.[36]
Anoigmaichnus is abioclaustration. It occurs in the Ordovician bryozoans. Apertures ofAnoigmaichnus are elevated above their hosts' growth surfaces, forming short chimney-like structures.
Arachnostega is the name given to the irregular, branching burrows in the sediment fill of shells. They are visible on the surface of steinkerns. Their traces are known from theCambrian period onwards.[37]
Asteriacites is the name given to the five-rayed fossils found in rocks and they record the resting place ofstarfish on the sea floor.Asteriacites are found in European and American rocks, from theOrdovician period onwards, and are numerous in rocks from theJurassic period ofGermany.
Burrinjuckia is a bioclaustration.Burrinjuckiaincludes outgrowths of thebrachiopod's secondary shell with a hollow interior in the mantle cavity of a brachiopod.
Chondrites (not to be confused with stony meteorites of the same name) are small branching burrows of the same diameter, which superficially resemble the roots of a plant. The most likely candidate for having constructed these burrows is anematode (roundworm).Chondrites are found in marine sediments from theCambrian period of thePaleozoic onwards. They are especially common in sediments which were deposited in reduced-oxygen environments.
Climactichnites is the name given to surface trails and burrows that consist of a series of chevron-shaped raised cross bars that are usually flanked on either side by a parallel ridge. They somewhat resemble tire tracks, and are larger (typically about 10 cm or 4 in wide) than most of the other trace fossils made byinvertebrates. The trails were produced on sandy tidal flats duringCambrian time. While the identity of the animal is still conjectural, it may have been a largeslug-like animal – its trails produced as it crawled over and processed the wet sand to obtain food.[38][39]
Cruziana are excavation trace marks made on the sea floor which have a two-lobed structure with a central groove. The lobes are covered with scratch marks made by the legs of the excavating organism, usually atrilobite or allied arthropod.Cruziana are most common in marine sediments formed during thePaleozoic era, particularly in rocks from theCambrian andOrdovician periods. Over 30 ichnospecies ofCruziana have been identified. See alsoIsopodichnus.
Entobia is a boring produced by endolithic clionaidsponges consisting of galleries excavated in a carbonate substrate; often has swollen chambers with connecting canals.
Gastrochaenolites are clavate (club-shaped) borings also produced in calcareous hard substrates, usually bybivalves.
Oikobesalon is an unbranched, elongate burrow with single-entrance and circular cross-section produced by terebellid polychaetes. They are covered with thin lining which has a transverse ornamentation in the form of fusiform annulation.
Petroxestes is a shallow groove boring produced by mytilacean bivalves in carbonate hard substrates.
Planolites is a small, 1-5mm (0.039–0.197 in), unlined and rarely branched, elongate burrow with fill that differs from the host rock, and is found throughout theEdiacaran and thePhanerozoic.
Protichnites consists of two rows of tracks and a linear depression between the two rows. The tracks are believed to have been made by the walking appendages ofarthropods. The linear depression is thought to be the result of a dragging tail. The structures bearing this name were typically made on the tidal flats ofPaleozoic seas, but similar ones extend into theCenozoic.
Rhizocorallium is a type ofburrow, the inclination of which is typically within 10° of the bedding planes of the sediment. These burrows can be very large, over a meter long in sediments that show good preservation, e.g.Jurassic rocks of theYorkshire Coast (easternUnited Kingdom), but the width is usually only up to2 centimetres (3⁄4 in), restricted by the size of the organisms producing it. It is thought that they represent fodinichnia as the animal (probably anematode) scoured thesediment for food.
Rusophycus are bilobed "resting traces" associated with trilobites and other arthropods such as horseshoe crabs.
Skolithos: One well-known occurrence of Cambrian trace fossils from this period is the famous 'Pipe Rock' of northwestScotland. The 'pipes' that give the rock its name are closely packed straight tubes- which were presumably made by some kind ofworm-like organism. The name given to this type of tube or burrow isSkolithos, which may be 30 cm (12 in) in length and between2 and 4 cm (3⁄4 and1+1⁄2 in) in diameter. Such traces are known worldwide from sands andsandstones deposited in shallow water environments, from theCambrian period (542–488Ma) onwards.
Thalassinoides are burrows which occur parallel to the bedding plane of the rock and are extremely abundant in rocks, worldwide, from theJurassic period onwards. They are repeatedly branched, with a slight swelling present at the junctions of the tubes. The burrows are cylindrical and vary from2 to 5 cm (3⁄4 to 2 in) in diameter.Thalassinoides sometimes contain scratch marks, droppings or the bodily remains of thecrustaceans which made them.
Teichichnus has a distinctive form produced by the stacking of thin 'tongues' ofsediment, atop one another. They are again believed to be fodinichnia, with the organism adopting the habit of retracing the same route through varying heights of the sediment, which would allow it to avoid going over the same area. These 'tongues' are often quite sinuous, reflecting perhaps a more nutrient-poor environment in which the feeding animals had to cover a greater area of sediment, in order to acquire sufficient nourishment.
Tremichnus is an embedment structure (i.e.bioclaustration) formed by an organism that inhibited growth of the crinoid host stereom.
Less ambiguous than the above ichnogenera, are the traces left behind byinvertebrates such asHibbertopterus, a giant "sea scorpion" oreurypterid of the earlyPaleozoic era. This marinearthropod produced a spectacular track preserved in Scotland.[40]
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[41] and microborings.[42][43] 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.[44] For a comprehensive bibliography of the bioerosion literature, please see the External links below.
The oldest types oftetrapod tail-and-footprints date back to the latterDevonian period. Thesevertebrate impressions have been found inIreland,Scotland,Pennsylvania, andAustralia. A sandstone slab containing the track of tetrapod, dated to 400 million years, is amongst the oldest evidence of a vertebrate walking on land.[45]
Asteriacites (sea star trace fossil) from theDevonian 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. TheEdiacara 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 ofsedimentary rocks as fucoids (Fucales, a kind ofbrown algae orseaweed). However, even during the earliest decades of the study of ichnology, some fossils were recognized as animal footprints and burrows. Studies in the 1880s byA. G. Nathorst andJoseph 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 arenot true fossils, should also not be confused with ichnofossils, which are true indications of prehistoric life.
Neoichnology – the study of modern/contemporary traces resultant from the behavior of biological organismsPages displaying wikidata descriptions as a fallback
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