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Trace fossil

A trace fossil, also ichnofossil ( /ˈɪknoʊfɒsɪl/; from Greek: ἴχνος ikhnos 'trace, track'), is a geological record of biological activity. Ichnology is the study of such traces, and is the work of ichnologists. Trace fossils may consist of impressions made on or in the substrate by an organism: for example, burrows, borings (bioerosion), urolites (erosion caused by evacuation of liquid wastes), 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 fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization.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 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: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. 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.Ichnofacies are assemblages of individual trace fossils that occur repeatedly in time and space. 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. The principal ichnofacies recognized in the literature are Skolithos, Cruziana, Zoophycos, Nereites, Glossifungites, Scoyenia, Trypanites, Teredolites, and Psilonichus. 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. Water depth, salinity, hardness of the substrate, dissolved oxygen, and many other environmental conditions control which organisms can inhabit particular areas. Therefore, by documenting and researching changes in ichnofacies, scientists can interpret changes in environment. 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.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).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.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; however their uneven width and tapering ends make a biological origin so difficult to defend that even the original author no longer believes they are authentic.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.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.Numerous borings in a Cretaceous cobble, Faringdon, England; see Wilson (1986).Sponge borings (Entobia) and encrusters on a modern bivalve shell, North Carolina.Entobia from the Prairie Bluff Chalk Formation (Upper Cretaceous). Preserved as a cast of the excavations.Helminthopsis ichnosp.; a trace fossil from the Logan Formation (Lower Carboniferous) of Wooster, Ohio.Gigandipus, a dinosaur footprint in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah.Lockeia from the Dakota Formation (Upper Cretaceous).Lockeia from the Chagrin Shale (Upper Devonian) of northeastern Ohio. This is an example of the trace fossil ethological group Fugichnia.Gnathichnus pentax echinoid trace fossil on an oyster from the Cenomanian of Hamakhtesh Hagadol, southern Israel.Naticid boring in Stewartia from the Calvert Formation, Zone 10, Calvert Co., MD (Miocene).Trace fossils as convex hyporeliefs on bottom of bed; Bull Fork Formation (Upper Ordovician); Caesar Creek, Ohio.Inverted trace fossil of an unidentified tridactyl ornithopodCharles Darwin's The Formation of Vegetable Mould through the Action of Worms is an example of a very early work on ichnology, describing bioturbation and, in particular, the burrowing of earthworms.

[ "Sediment", "Paleontology", "Skolithos", "Ediacaran biota", "Anabarites", "Undichna", "Swartpuntia" ]
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