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Geological record of biological activity From Wikipedia, the free encyclopedia
A trace fossil, also known as an ichnofossil ( /ˈɪknoʊfɒsɪl/; from Greek: ἴχνος ikhnos "trace, track"), is a fossil record of biological activity by lifeforms but not the preserved remains of the organism 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 by mineralization. The study of such trace fossils is ichnology - the work of ichnologists.[2]
Trace fossils may consist of physical impressions made on or in the substrate 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 example coprolites (fossilized droppings) or chemical markers (sedimentological structures produced by biological means; for example, the formation of stromatolites). However, most sedimentary 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 into paleoichnology, or the study of trace fossils, and neoichnology, 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 into form genera based on their appearance and on the implied behaviour, or ethology, 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.
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]
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 in zoological nomenclature than taxa based on body fossils (see trace fossil classification for more information). Examples include:
Trace fossils are important paleoecological and paleoenvironmental indicators, because they are preserved in 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 the depositional 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 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. 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 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 the front limbs touched the ground or not.
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.[citation needed]
Fossil footprints made by tetrapod vertebrates 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 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 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 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+1⁄2 in) layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.[9]
The base of the Cambrian period is defined by the first appearance of the trace fossil Treptichnus pedum.[10]
Trace fossils have a further utility, as many appear before the organism thought to create them, extending their stratigraphic range.[11]
Ichnofacies are assemblages of individual trace fossils that occur repeatedly in time and space.[12] Palaeontologist Adolf 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 are Skolithos, Cruziana, Zoophycos, Nereites, Glossifungites, Scoyenia, Trypanites, Teredolites, and Psilonichus.[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 the Cretaceous–Paleogene mass extinction, to aid in understanding environmental factors involved in mass extinction events.[14][15]
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, including tidal flats.[citation needed]
The earliest complex trace fossils, not including microbial traces such as stromatolites, date to 2,000 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 by amoebae.[17] Putative "burrows" dating as far back as 1,100 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 the Ediacaran (Vendian) period, around 560 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 been bilateran animals.[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 burrows Skolithos are also known.[24] The producers of burrows Skolithos declinatus from the Vendian (Ediacaran) beds in Russia with date 555.3 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 and Dickinsonia 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 with cilia.[27] The potential mollusc related Kimberella is associated with scratch marks, perhaps formed by a radula,[28] further traces from 555 million years ago appear to imply active crawling or burrowing activity.[29]
As the Cambrian got underway, new forms of trace fossil appeared, including vertical burrows (e.g. Diplocraterion) and traces normally attributed to arthropods.[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 early Cambrian.[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 Ordovician Tumblagooda sandstone allow the behaviour of other terrestrial organisms to be determined.[8] The trackway Protichnites represents traces from an amphibious or terrestrial arthropod going back to the Cambrian.[36]
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 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 of tetrapod tail-and-footprints date back to the latter Devonian period. These vertebrate impressions have been found in Ireland, Scotland, Pennsylvania, and Australia. 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]
Important human trace fossils are the Laetoli (Tanzania) footprints, imprinted in volcanic ash 3.7 Ma (million years ago) – probably by an early Australopithecus.[46]
Trace fossils are not body casts. The Ediacara 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 of sedimentary rocks as fucoids (Fucales, a kind of brown algae or seaweed). However, even during the earliest decades of the study of ichnology, some fossils were recognized as animal footprints and burrows. Studies in the 1880s by A. G. Nathorst and Joseph 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 are not true fossils, should also not be confused with ichnofossils, which are true indications of prehistoric life.
Charles Darwin's The Formation of Vegetable Mould through the Action of Worms[a] is an example of a very early work on ichnology, describing bioturbation and, in particular, the burrowing of earthworms.[47]
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