A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. 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.
Trace fossils may consist of physical impressions made on or in the substrate by an organism. 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.
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 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 Protichnites and Climactichnites, amongst others Mesozoic dinosaur footprints including ichnogenera such as Grallator, Atreipus, and Anomoepus Triassic to Recent 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.
Paleobiology Although one-to-one correspondence between trace fossils and their tracemakers is rarely preserved, trajectory-type trace fossils provide valuable information about the biology and behaviour of the organisms that produced them. Such information includes inferred behaviours (e.g., grazing, foraging, sediment probing, and escape responses, or stereotyped behaviour such as taxis or kinesis), the feeding habits of tracemakers, and the likely functions of particular burrow systems.
Quantitative analyses have further expanded the interpretive potential of trajectory-type trace fossils. Spectral characteristics derived from turning-angle series, such as dominant frequencies Wang, Z. and Rahman, I.A. (2023), Quantitative ichnology: a novel framework to determine the producers of locomotory trace fossils with the ichnogenus Gordia as a case study. Palaeontology, 66: e12686. https://doi.org/10.1111/pala.12686 and bandwidths Zekun Wang, Zhengyuan Chen, (2026) Quantifying navigational capacity in trace fossils using spectral bandwidths: Applications for animal movement ecology through the Ediacaran–Cambrian transition. Palaeogeography, Palaeoclimatology, Palaeoecology, 699: 113973. obtained through Fourier transform, as well as autocorrelation, have been shown to reflect organismal locomotory modes and may help constrain the taxonomic affinity of tracemakers to particular animal groups. In addition, trajectory smoothness analyses based on the continuity of trajectory derivatives have been proposed as proxies for body slenderness or aspect ratio, thereby providing insights into the body plan of the tracemaking organism, including the earliest bilaterians.
Additionally, biophysical modelling and dimensional analysis can be used to estimate the locomotory speeds of tracemakers. For example, Froude number scaling has been employed to relate locomotion speed to trackway dimensions and gait. Alexander (1976) demonstrated that the locomotory speeds of dinosaurs and other tetrapods could be estimated from track size through allometry relationships involving hip height.Alexander, R. M. (1976). Estimates of speeds of dinosaurs. Nature, 261, 129–130.; Similar approaches have subsequently been applied to invertebrate trace fossils. Wang et al. (2024) used Froude number scaling to establish a relationship between the irregularity of furrow levees and locomotory speed, enabling quantitative estimates of the movement dynamics of the invertebrate tracemaking organisms.Wang, Z., Davies, N. S., Liu, A. G., Minter, N. J. & Rahman, I. A. (2024). Identifying signatures of the earliest benthic bulldozers in emergent subaerial conditions during the colonization of land by animals. Proceedings of the Royal Society B, 291, 20241629. Such approaches demonstrate that trace fossils can preserve quantitative information on the biomechanics and locomotory performance of organisms through deep time, providing insights into palaeobiological traits that are otherwise difficult to infer from the body-fossil record alone.
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. 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.
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. Due to a lack of hard parts, body fossils of these soft-bodied animals are rare.
Palaeopathology Ichnofossils can preserve evidence of pathologies in extinct organisms, particularly in cases where there is a large sample size of a given track type and abnormalities can easily be diagnosed in individual trackways.
Paleoenvironment
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.
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 Muschelkalk epoch, throughout wide areas in southern Germany.
The base of the Cambrian 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 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.
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).