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Trace fossil

Trace fossils, also called ichnofossils (ιχνος or ikhnos meaning "trace" or "track"), are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings (bioerosion), footprints and feeding marks, and root cavities. The term in its broadest sense also includes the remains of other organic material produced by an organism - for example coprolites (fossilized droppings) or chemical markers - or sedimentological structures produced by biological means - for example, stromatolites. Trace fossils contrast with body fossils, which are the fossilised remains of parts of organisms' bodies, usually altered by later chemical activity or mineralisation.

Sedimentary structures, for example those produced by empty shells rolling along the sea floor, are not produced through the behaviour of an organism and not considered trace fossils.

The study of traces is called ichnology, which is divided into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern traces. This science is challenging, as most traces reflect the behaviour--not the biological affinity--of their makers. As such, trace fossils are categorised into form genera, based upon their appearance and the implied behaviour of their makers.

Occurrence

Traces are better known in their fossilised form than in modern sediments. This makes it difficult to interpret some fossils by comparing them with modern traces, even though they may be extant or even common. The main difficulties in accessing extant burrows stem from finding them in consolidated sediment, and being able to access those formed in deeper water.

Trace fossils are best preserved in sandstones; the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.

Classification

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 taxonomy has been erected. At the highest level of the classification, five bahavioural modes are recognised:

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.

Information provided by ichnofossils

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 the depositional environment. Attempts to deduce such traits as whether a deposit is marine or non-marine have been made, but shown to be unreliable.

Paleoecology

Trace fossils provide us with indirect evidence of life 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.

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 by clams and arthropods are all trace fossils.

Perhaps the most spectacular trace fossils are the huge, three-toed footprints produced by dinosaurs and related archosaurs. These imprints give scientists clues as to how these animals lived. Although the skeletons of dinosaurs can be reconstructed, only their fossilized 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 or not the front limbs touched the ground.

However, most trace fossils are rather less conspicuous, such as the trails made by segmented worms or nematodes. Some of these worm castings are the only fossil record we have of these soft-bodied creatures.

Palæoenvironment

Fossil footprints made by tetrapod vertebrates are difficult to identify to a particular species of animal, but they can provide us with valuable information such as the speed, weight, and behavior of the organism that made them. Such trace fossils are formed when amphibians, reptiles, mammals or birds 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 palæo-wind directions.

Assemblages of trace fossils occur at certain water depths, and can also reflect the salinity and turbidity of the water column.

Stratigraphic correlation

Some trace fossils can be used as local index fossils, to date the rocks in which they are found, such as the burrow Arenicolites franconicus which occurs only in a 4 cm (1.6") layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.

The base of the Cambrian period is defined by the first appearance of the trace fossil Trichophycus pedum.

Trace fossils have a further utility as many appear before the organism thought to create them, extending their stratigraphic range.

Ichnofacies

Trace fossil assemblages are far from random; the range of fossils recorded in association is constrained by the environment in which the trace-making organisms dwelt. Palaeontologist Adolf Seilacher pioneered the concept of ichnofacies, whereby the state of a sedimentary system at its time of deposition could be implied by noting the fossils in association with one another.

Inherent bias

Most trace fossils are known from marine deposits. 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, including tidal flats.

Evolution

Putative "burrows" dating as far back as may have been made by animals which fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean; however their uneven width and tapering ends make a biological origin difficult to defend. The first evidence of burrowing which is widely accepted dates to the Ediacaran period, around . During this period, burrows are horizontal, or just below the surface. Such burrows must have been made by motile organisms with heads, which would probably have been bilateran animals. The burrows observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators. The complex, efficient feeding traces common from the start of the ensuing Cambrian period are absent. Some Ediacaran fossils, especially discs, have been interpreted tentatively as trace fossils, but this hypothesis has not gained widespread acceptance. As well as burrows, some trace fossils have been found directly associated with an Ediacaran fossil. Yorgia and Dickinsonia are often found at the end of long pathways of trace fossils matching their shape; the method of formation of these disconnected and overlapping fossils largely remains a mystery. The potential mollusc Kimberella is associated with scratch marks thought to have been formed by its radula, further traces from appear to imply active crawling or burrowing activity.

As the Cambrian got underway, new forms of trace fossil appeared, including vertical burrows and traces normally attributed to arthropods. These represent a “widening of the behavioural repertoire”, both in terms of abundance and complexity.

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 early Cambrian.

Further, less rapid diversification occurred since, and many traces have been converged upon independently by unrelated groups of organisms.

Trace fossils also provide our earliest evidence of animal life on land. The earliest arthropod trackways date to the Cambro-Ordovician, and trackways from the Ordovician Tumblagooda sandstone allow the behaviour of these organisms to be determined. The enigmatic trace fossil Climactichnites may represent an earlier still terrestrial trace, perhaps made by a slug-like organism.

Common ichnogenera

Skolithos: One well-known occurrence of Cambrian trace fossils from this period is the famous 'Pipe Rock' of northwest Scotland. The 'pipes' that give the rock its name are closely packed straight tubes- which were presumably made by some kind of worm-like organism. The name given to this type of tube or burrow is Skolithos, which may be 30 cm (12") in length and between 2 to 4 cm (0.8 to 1.6") in diameter. Such traces are known worldwide from sands and sandstones deposited in shallow water environments, from the Cambrian period (542 to 488 m.y.a) onwards.
Chondrites 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 a nematode (roundworm). Chondrites are found in marine sediments from the Cambrian period of the Paleozoic onwards. They are especially common in sediments which were deposited in reduced-oxygen environments.

Rusophycus are bilobed "resting traces" associated with trilobites and other arthropods such as horseshoe crabs.
Trypanites are elongated cylindrical borings in calcareous substrates such as shells, carbonate hardgrounds and limestones.
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 a trilobite or allied arthropod and, in fact, several different types of trilobite have been discovered at the end of Cruziana trails. Cruziana are most common in marine sediments formed during the Paleozoic era, particularly in rocks from the Cambrian and Ordovician periods. Over 30 species of Cruziana have been identified.
Thalassinoides are burrows which occur parallel to the bedding plane of the rock and are extremely abundant in rocks, worldwide, from the Jurassic period onwards. They are repeatedly branched, with a slight swelling present at the junctions of the tubes. The burrows are cylindrical and vary from 2 to 5 cm (0.8" to 2") in diameter. Thalassinoides sometimes contain scratch marks, droppings or the bodily remains of the crustaceans which made them.
Asteriacites is the name given to the five-rayed fossils found in rocks and they record the resting place of starfish on the sea floor. Asteriacites are found in European and American rocks, from the Ordovician period onwards; and are numerous in rocks from the Jurassic period of Germany.
Rhizocorallium is a type of burrow, 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 the Yorkshire Coast (eastern United Kingdom), but the width is usually only up to 2 cm, restricted by the size of the organisms producing it. It is thought that they represent fodinichnia as the animal (probably a nematode) scoured the sediment for food.
Teichichnus has a distinctive form produced by the stacking of thin 'tongues' of sediment, 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.
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 of arthropods. 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 of Paleozoic seas, but similar ones extend into the Cenozoic.
Climactichnites is the name given to trackways that usually consist of two parallel ridges separated by chevron-shaped raised cross bars. They somewhat resemble tire tracks, and are larger (typically about four inches wide) than most of the other trace fossils made by invertebrates. The tracks were produced on sandy tidal flats during late Cambrian time. While the identity of the animal is still conjectural, it may have been a large slug-like animal - its trackways produced as it crawled over and processed the wet sand to obtain food.

Other notable trace fossils

Less ambiguous than the above ichnogenera, are the traces left behind by invertebrates such as Hibbertopterus, a giant "sea scorpion" or eurypterid of the early Paleozoic era. This marine arthropod produced a spectacular hibbertopteroid track preserved in Scotland.

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 and microborings. 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. For a comprehensive bibliography of the bioerosion literature, please see the External links below.

The oldest types of tetrapod tail-and-foot prints date back to the latter Devonian period. These vertebrate impressions have been found in Ireland, Scotland, Pennsylvania, and Australia.

Important human trace fossils are the Laetoli (Tanzania) footprints, imprinted in volcanic ash 3.7 million years ago (mya) -- probably by an early Australopithecus.

Confusion with other types of fossils

Trace fossils should not be confused with body casts. The Ediacara biota, for instance, primarily comprises the casts of organisms in sediment.

Early geologists gave the name 'fucoid' to a wide variety of markings they found on the bedding planes of sedimentary rocks. The earth scientists frequently misinterpreted these 'fucoid' marks as being the fossilized remains of seaweed. However, in more recent years, these markings have been studied with greater thoroughness. It is now apparent that the 'fucoids' and other markings have in fact been caused by a variety of plants and animals. As a result, these 'fucoid' markings are now termed trace fossils.

Pseudofossils, which are not true fossils, should also not be confused with ichnofossils, which are true indications of prehistoric life.

References

Bromley, R.G., 1970. Borings as trace fossils and Entobia cretacea Portlock as an example, p. 49-90. In: Crimes, T.P. and Harper, J.C. (eds.), Trace Fossils. Geological Journal Special Issue 3.
Bromley, R.G., 2004. A stratigraphy of marine bioerosion. In: The application of ichnology to palaeoenvironmental and stratigraphic analysis. (Ed. D. McIlroy), Geological Society of London Special Publications 228:455-481.
Palmer, T.J., 1982. Cambrian to Cretaceous changes in hardground communities. Lethaia 15:309-323.
Wilson, M.A., 1986. Coelobites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground fauna. Palaeontology 29:691-703.
Wilson, M.A. and Palmer, T.J., 2006. Patterns and processes in the Ordovician Bioerosion Revolution. Ichnos 13: 109-112.
Yochelson, E.L. and Fedonkin, M.A., 1993. Paleobiology of Climactichnites, and Enigmatic Late Cambrian Fossil. Smithsonian Contributions to Paleobiology 74:1-74.