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A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
record of
biological activity In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or ...
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 Mineralization may refer to: * Mineralization (biology), when an inorganic substance precipitates in an organic matrix ** Biomineralization, a form of mineralization ** Mineralization of bone, an example of mineralization ** Mineralized tissues ar ...
. The study of such trace fossils is ichnology and is the work of ichnologists. Trace fossils may consist of impressions made on or in the substrate by an organism. For example,
burrow An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of s ...
s, borings (
bioerosion Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and ...
), 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
coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name is ...
s (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 Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objecti ...
, of their makers.


Occurrence

Traces are better known in their fossilized 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 of taxonomy has been erected. At the highest level of the classification, five behavioral modes are recognized: * 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 An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' and ταξις, ''taxis'' meaning ...
are classified somewhat differently in zoological nomenclature than taxa based on body fossils (see
trace fossil classification Trace fossils are classified in various ways for different purposes. Traces can be classified taxonomically (by morphology), ethologically (by behavior), and toponomically, that is, according to their relationship to the surrounding sedimentary ...
for more information). Examples include: *Late
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
trace fossils from intertidal settings include ''
Protichnites ''Protichnites'' is an ichnogenus of trace fossil consisting of the imprints made by the walking activity of certain arthropods. It consists of two rows of tracks and a medial furrow between the two rows. This furrow, which may be broken, set at ...
'' and '' Climactichnites'', amongst others *
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
dinosaur footprints including ichnogenera such as '' Grallator'', '' Atreipus'' and '' Anomoepus'' *
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
to
Recent The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...
termite mounds, which can encompass several square kilometers of sediment


Information provided by ichnofossils

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.


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 Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
footprints,
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...
trails and the burrows made by clams and
arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...
are all trace fossils. Perhaps the most spectacular trace fossils are the huge, three-toed footprints produced by
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and related
archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...
s. 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
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
s. Some of these
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...
castings are the only fossil record we have of these soft-bodied creatures.


Paleoenvironment

Fossil footprints made by tetrapod
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s 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
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s,
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s,
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s or
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s 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. 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 layer of the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
Muschelkalk The Muschelkalk (German for "shell-bearing limestone"; french: calcaire coquillier) is a sequence of sedimentary rock strata (a lithostratigraphic unit) in the geology of central and western Europe. It has a Middle Triassic (240 to 230 million ...
epoch, throughout wide areas in southern
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
. The base of the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period is defined by the first appearance of the trace fossil '' Treptichnus pedum''. Trace fossils have a further utility as many appear before the organism thought to create them, extending their stratigraphic range.e.g.


Ichnofacies

Ichnofacies An ichnofacies is an assemblage of trace fossils that provides an indication of the conditions that their formative organisms inhabited. Concept Trace fossil assemblages are far from random; the range of fossils recorded in association is constra ...
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 ''Nereites'' is a genus of trace fossil. Modern tracemakers of incipient ''Nereites'' include worm-like organisms,Seilacher, A., 2007. Trace Fossil Analysis. Springer, Berlin, Heidelberg (226 pp.) horseshoe crabsMartin, A.J., Rindsberg, A.K., 20 ...
, Glossifungites,
Scoyenia ''Scoyenia'' is an ichnogenus An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' a ...
, Trypanites,
Teredolites ''Teredolites'' is an ichnogenus of trace fossil, characterized by borings in substrates such as wood or amber. Club-shaped structures rimming mid-Cretaceous Burmese amber were formerly identified as the fungal sporocarps ''Palaeoclavaria burmiti ...
,'' 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 Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
, 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.


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

The earliest complex trace fossils, not including microbial traces such as stromatolites, date to . 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 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. The first evidence of burrowing which is widely accepted dates to the
Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...
(Vendian) period, around . 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
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
s. The traces observed imply simple behaviour, and point to organisms feeding above the surface and burrowing for protection from predators. Contrary to widely circulated opinion that Ediacaran burrows are only horizontal the vertical burrows '' Skolithos'' are also known.M. A. Fedonkin (1985). "Paleoichnology of Vendian Metazoa". In Sokolov, B. S. and Iwanowski, A. B., eds., "Vendian System: Historical–Geological and Paleontological Foundation, Vol. 1: Paleontology". Moscow: Nauka, pp. 112–116. (in Russian) The producers of burrows '' Skolithos declinatus'' from the Vendian (
Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...
) beds in
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
with date 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. Some Ediacaran trace fossils have been found directly associated with body fossils. '' Yorgia'' and ''
Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal that lived during the late Ediacaran period in what is now Australia, China, Russia and Ukraine. The individual ''Dickinsonia'' typically resembles a bilaterally symmetrical ribbed oval. Its a ...
'' are often found at the end of long pathways of trace fossils matching their shape. The feeding was performed in a mechanical way, supposedly the ventral side of body these organisms was covered with
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...
.A. Yu. Ivantsov. (2008)
"Feeding traces of the Ediacaran animals"
HPF-17 Trace fossils ? ichnological concepts and methods. International Geological Congress - Oslo 2008.
The potential
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is est ...
related '' Kimberella'' is associated with scratch marks, perhaps formed by a
radula The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
, further traces from appear to imply active crawling or burrowing activity.According to As the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
got underway, new forms of trace fossil appeared, including vertical burrows (e.g. '' Diplocraterion'') and traces normally attributed to
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s. 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 Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pl ...
organisms were rapidly diversifying during the early
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
. 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. Evidence of the first animals that appear to have been fully terrestrial dates to the Cambro-Ordovician and is in the form of trackways. Trackways from the Ordovician Tumblagooda sandstone allow the behaviour of other terrestrial organisms to be determined. The trackway ''
Protichnites ''Protichnites'' is an ichnogenus of trace fossil consisting of the imprints made by the walking activity of certain arthropods. It consists of two rows of tracks and a medial furrow between the two rows. This furrow, which may be broken, set at ...
'' represents traces from an amphibious or terrestrial arthropod going back to the Cambrian.


Common ichnogenera

*'' Anoigmaichnus'' is a
bioclaustration Bioclaustration is kind of interaction when one organism (usually soft bodied) is embedded in a living substrate (i.e. skeleton of another organism); it means “biologically walled -up”. In case of symbiosis Symbiosis (from Greek , , "liv ...
. It occurs in the Ordovician bryozoans. Apertures of ''Anoigmaichnus'' 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 the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period onwards. * '' Asteriacites'' is the name given to the five-rayed fossils found in rocks and they record the resting place of
starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
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 The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
period of
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
. * '' Burrinjuckia'' is a bioclaustration. ''Burrinjuckia '' includes outgrowths of the brachiopod's secondary shell with a hollow interior in the mantle cavity of a brachiopod. * ''
Chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primi ...
'' (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 a
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
(roundworm). ''Chondrites'' are found in marine sediments from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period of the
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
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 wide) than most of the other trace fossils made by
invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...
. The trails were produced on sandy tidal flats during
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
time. While the identity of the animal is still conjectural, it may have been a large
slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a ...
-like animal – its trails produced as it crawled over and processed the wet sand to obtain food. * '' 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 Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
or allied arthropod. ''Cruziana'' are most common in marine sediments formed during the
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
era, particularly in rocks from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
and Ordovician periods. Over 30 ichnospecies of ''Cruziana'' have been identified. See also Isopodichnus. * '' Entobia'' is a boring produced by endolithic clionaid
sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
consisting of galleries excavated in a carbonate substrate; often has swollen chambers with connecting canals. * ''
Gastrochaenolites ''Gastrochaenolites'' is a trace fossil formed as a clavate (club-shaped) boring in a hard substrate such as a shell, rock or carbonate hardground. The aperture of the boring is narrower than the main chamber and may be circular, oval, or dumb- ...
'' are clavate (club-shaped) borings also produced in calcareous hard substrates, usually by
bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...
s. * '' 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. * ''
Protichnites ''Protichnites'' is an ichnogenus of trace fossil consisting of the imprints made by the walking activity of certain arthropods. It consists of two rows of tracks and a medial furrow between the two rows. This furrow, which may be broken, set at ...
'' 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
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s. 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 The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
seas, but similar ones extend into the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...
. * ''
Rhizocorallium ''Rhizocorallium'' is an ichnogenus 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 ro ...
'' is a type of
burrow An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of s ...
, 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 The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
rocks of the
Yorkshire Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a Historic counties of England, historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other Eng ...
Coast (eastern
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...
), but the width is usually only up to , restricted by the size of the organisms producing it. It is thought that they represent fodinichnia as the animal (probably a
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
) scoured the
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
for food. * ''
Rogerella ''Rogerella'' is a small pouch-shaped boring (a type of trace fossil) with a slit-like aperture currently produced by acrothoracican barnacles. These crustaceans extrude their legs upwards through the opening for filter-feeding (Seilacher, 1969 ...
'' is a small pouch-shaped boring with a slit-like aperture currently produced by acrothoracican
barnacles A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosiv ...
. * ''
Rusophycus ''Rusophycus'' is a trace fossil ichnogenus allied to '' Cruziana''. ''Rusophycus'' is the resting trace, recording the outline of the tracemaker; ''Cruziana'' is made when the organism moved. The sculpture of ''Rusophycus'' may reveal the appr ...
'' 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 ''Skolithos'' (formerly spelled ''Scolithus'' or ''Skolithus'') is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow. It is produced by a variety of organisms in shallow marine environment ...
' of northwest
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
. The 'pipes' that give the rock its name are closely packed straight tubes- which were presumably made by some kind of
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...
-like organism. The name given to this type of tube or burrow is ''Skolithos'', which may be in length and between in diameter. Such traces are known worldwide from sands and
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...
s deposited in shallow water environments, from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period (542–488 Ma) onwards. * '' Thalassinoides'' are burrows which occur parallel to the bedding plane of the rock and are extremely abundant in rocks, worldwide, from the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
period onwards. They are repeatedly branched, with a slight swelling present at the junctions of the tubes. The burrows are cylindrical and vary from in diameter. ''Thalassinoides'' sometimes contain scratch marks, droppings or the bodily remains of the
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...
s which made them. * '' Teichichnus'' has a distinctive form produced by the stacking of thin 'tongues' of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
, 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 Bioclaustration is kind of interaction when one organism (usually soft bodied) is embedded in a living substrate (i.e. skeleton of another organism); it means “biologically walled -up”. In case of symbiosis Symbiosis (from Greek , , "liv ...
) formed by an organism that inhibited growth of the crinoid host stereom. * '' Trypanites'' are elongated cylindrical borings in calcareous substrates such as shells, carbonate hardgrounds and limestones. Usually produced by worms of various types and sipunculids.


Other notable trace fossils

Less ambiguous than the above ichnogenera, are the traces left behind by
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s such as ''
Hibbertopterus ''Hibbertopterus'' is a genus of eurypterid, a group of extinct aquatic arthropods. Fossils of ''Hibbertopterus'' have been discovered in deposits ranging from the Devonian period in Belgium, Scotland and the United States to the Carboniferous p ...
'', a giant "sea scorpion" or eurypterid of the early
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
era. This marine
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
produced a spectacular track preserved in Scotland.
Bioerosion Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and ...
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.Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1–103. For a comprehensive bibliography of the bioerosion literature, please see the External links below. The oldest types of
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
tail-and-footprints date back to the latter
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
period. These
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
impressions have been found in
Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...
,
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
,
Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
, and
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
. A sandstone slab containing the track of tetrapod, dated to 400 million years, is amongst the oldest evidence of a vertebrate walking on land. Important
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
trace fossils are the Laetoli (
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...
) footprints, imprinted in volcanic ash 3.7 Ma (million years ago) – probably by an early
Australopithecus ''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Australo ...
.


Confusion with other types of fossils

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 Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...
as fucoids ( Fucales, a kind of
brown algae Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and p ...
or
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
). 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 Alfred Gabriel Nathorst (7 November 1850 – 20 January 1921) was a Swedish Arctic explorer, geologist, and palaeobotanist. Life He was born in Väderbrunn in Sweden. Nathorst's interest in geology was awoken by Charles Lyell’s ‘’Pr ...
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.


Gallery of trace fossils

File:FaringdonCobble.JPG, Numerous borings in a
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
cobble, Faringdon,
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...
; see Wilson (1986) File:BoredEncrustedShell.JPG, Sponge borings ('' Entobia'') and encrusters on a modern bivalve shell, North Carolina File:Entobia Prairie Bluff Chalk Formation Cretaceous.JPG, ''Entobia'' from the Prairie Bluff Chalk Formation (Upper
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
). Preserved as a cast of the excavations. File:Gyrochorte convex epirelief.jpg, Trace fossil ''Gyrochorte'' from the
Carmel Formation The Carmel Formation is a geologic formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Utah, Colorado, north east Arizona and New Mexico. Part of the Colorado Plateau, this formation was laid down in the Middle ...
(Middle Jurassic) of SW Utah. File:Helminthopsis01.JPG, ''Helminthopsis'' ichnosp., a trace fossil from the Logan Formation (Lower
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
) of
Wooster, Ohio Wooster ( ) is a city in the U.S. state of Ohio and the county seat of Wayne County. Located in northeastern Ohio, the city lies approximately south-southwest of Cleveland, southwest of Akron and west of Canton. The population was 27,232 at ...
File:Gigandipus.JPG, ''Gigandipus'', a
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
footprint in the Lower
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
File:CretaceousLockeia121911.jpg, ''Lockeia'' from the
Dakota Formation The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Wester ...
(Upper Cretaceous) File:DevonianLockeia121911.jpg, ''Lockeia'' from the
Chagrin Shale The Chagrin Shale is a shale geologic formation in the eastern United States that is approximately 365 million years old. The Chagrin Shale is a gray shale that begins thin and deep underground in north-central Ohio. As it proceeds east, the for ...
(Upper Devonian) of northeastern
Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...
. This is an example of the trace fossil ethological group Fugichnia. File:Gnathichnus Cenomanian 020413.JPG, '' Gnathichnus pentax'' echinoid trace fossil on an oyster from the
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
of Hamakhtesh Hagadol, southern Israel File:Naticid borehole Calvert.jpg, Naticid boring in ''Stewartia'' from the Calvert Formation, Zone 10, Calvert County, Maryland (Miocene) File:Trace fossils Bull Fork Ordovician OH.JPG, Trace fossils as convex hyporeliefs on bottom of bed; Bull Fork Formation (Upper Ordovician); Caesar Creek, Ohio File:Tridactyl ornithopod dinosaur track YORYM-1998.335.JPG, Inverted trace fossil of an unidentified
tridactyl In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. It comes from the Greek word δακτυλος (''dáktylos'') = "finger". Sometimes the ending "-dactylia" is use ...
ornithopod Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous w ...


History

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
's ''
The Formation of Vegetable Mould through the Action of Worms #REDIRECT The Formation of Vegetable Mould Through the Action of Worms {{R from other capitalisation ...
'' is an example of a very early work on ichnology, describing
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a ...
and, in particular, the burrowing of
earthworms An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...
.


See also

* * * Brutalichnus * * * - fossilized footprints * * * * * * * *


References


Further reading

* Bromley, R.G., 1970. "Borings as trace fossils and '' Entobia cretacea'' Portlock as an example", pp. 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: 109–11

* Yochelson, E.L. and Fedonkin, M.A., 1993. Paleobiology of ''Climactichnites'', and Enigmatic Late Cambrian Fossil. Smithsonian Contributions to Paleobiology 74:1–74.


External links


Encyclopaedia-style article about trace fossils






{{DEFAULTSORT:Trace Fossil Trace fossils, Paleozoology Bioindicators