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A trace fossil, also ichnofossil ( /ˈɪknfɒsɪl/; from Greek: ἴχνος ikhnos "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization. Ichnology is the study of such trace fossils 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 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.

Occurrence

Cross-section of mammoth footprints at The Mammoth Site, Hot Springs, South Dakota

Traces are better known in their fossilized form than in modern sediments.[1] This makes it difficult to interpret some fossils by comparing them with modern traces, even though they may be extant or even common.[1] The main difficulties in accessing extant burrows stem from finding them in consolidated sediment, and being able to access those formed in deeper water.

This coprolite shows distinct top and bottom jaw bite marks, possibly from a prehistoric gar fish. Discovery location: South Carolina, US; age: Miocene; dimensions: 144.6mm X 63.41mm or 5.7” X 2.5”; weight: 558g (1lbs 4oz)

Trace fossils are best preserved in sandstones;[1] the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.[1]

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 of taxonomy has been erected. At the highest level of the classification, five behavioral modes are recognized:[1]

  • 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 in zoological nomenclature than taxa based on body fossils (see trace fossil classification for more information). Examples include:

Information provided by ichnofossils

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 preserved in situ, or in the life position of the organism that made them.[2] 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.[3] Attempts to deduce such traits as whether a deposit is marine or non-marine have been made, but shown to be unreliable.[3]

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.

Paleoenvironment

Eubrontes, a dinosaur footprint in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah

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.[4]

Assemblages of trace fossils occur at certain water depths,[1] 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 12 in) layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.[5]

The base of the Cambrian period is defined by the first appearance of the trace fossil Treptichnus pedum.[6]

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

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 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.[1] This makes it difficult to interpret some fossils by comparing them with modern traces, even though they may be extant or even common.[1] The main difficulties in accessing extant burrows stem from finding them in consolidated sediment, and being able to access those formed in deeper water.

This coprolite shows distinct top and bottom jaw bite marks, possibly from a prehistoric gar fish. Discovery location: South Carolina, US; age: Miocene; dimensions: 144.6mm X 63.41mm or 5.7” X 2.5”; weight: 558g (1lbs 4oz)

Trace fossils are best preserved in sandstones;[1] the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.[1]

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 of taxonomy has been erected. At the highest level of the classification, five behavioral modes are recognized:[1]

  • 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 in zoological nomenclature than taxa based on body fossils (see trace fossil classification for more information). Examples include:

  • Late Cambrian trace fossils from intertidal settings include Trace fossils are best preserved in sandstones;[1] the grain size and depositional facies both contributing to the better preservation. They may also be found in shales and limestones.[1]

    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 of taxonomy has been erected. At the highest level of the classification, five behavioral modes are recognized:[1]

    • 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;
    • C

      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:[1]

      • 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 tr

        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:

        • L

          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.[2] 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.[3] Attempts to deduce such traits as whether a deposit is marine or non-marine have been made, but shown to be unreliable.[3]

          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-b

          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 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.

          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.[4]

          Assemblages of trace fossils occur at certain water depths,[1] 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 12 in) layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.[5]

          The base of the Cambrian period is defined by the first appearance of the trace fossil Assemblages of trace fossils occur at certain water depths,[1] 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 12 in) layer of the Triassic Muschelkalk epoch, throughout wide areas in southern Germany.[5]

          The base of the Cambrian period is defined by the first appearance of the trace fossil Treptichnus pedum.[6]

          Trace fossils have a further utility a

          The base of the Cambrian period is defined by the first appearance of the trace fossil Treptichnus pedum.[6]

          Trace fossils have a further utility as many appear before the organism thought to create them, extending their stratigraphic range.[7]

          Ichnofacies are assemblages of individual trace fossils that occur repeatedly in time and space.[8] 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.[1] The principal ichnofacies recognized in the literature are Skolithos, Cruziana, Zoophycos, Nereites, Glossifungites, Scoyenia, Trypanites, Teredolites, and Psilonichus.[8][9] 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.[9] Water depth, salinity, hardness of the substrate, dissolved oxygen, and many other environmental conditions control which organisms can inhabit particular areas.[8] Therefore, by documenting and researching changes in ichnofacies, scientists can interpret changes in environment.[9] 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.[10][11]

          Inherent bias

          Most trace fossils are known from marine deposits.[12] 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]

          Evolution

          Asteriacites (sea star trace fossil) from the Devonian 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. 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.