The classification of trace fossils depends on the factor considered most significant, leading to several different approaches. For example, a single organism can produce different types of traces, while different organisms can sometimes leave indistinguishable ones. Consequently, unless scientists can identify the original maker of a fossil trace with certainty, they cannot apply phylogenetic taxonomy to trace fossils in most cases. In any case, a system for trace fossil classification will inevitably be artificial (parataxonomic) and based on external morphological characteristics.
On the other hand, organisms with similar behaviors (e.g., feeding, movement, burrowing, etc.) and similar body structures (e.g., worm-like shapes) create very similar structures.
Taxonomic classification is largely formal in nature. Currently, toponomical and ethological classifications of fossil traces are the most widely used. This diversity in approaches reflects the complexity of trace fossil classification and the different interpretive lenses that ichnologists apply.
The Very First Attempt to Classify Traces
One of the first researchers of trace fossils, Edward Hitchcock, proposed two orders in 1844: Apodichnites, which included legless traces left by, for example, worms and mollusks, and Polypodichnites, which included traces of organisms with more than four legs.
Later, J. Salter suggested the name Helminthites for long, curved surface traces and sediment-filled burrows of marine worms without impressions of their lateral appendages.
These early efforts, although crude by modern standards, laid the groundwork for the evolution of trace fossil classification into a more nuanced and structured science.
Taxonomic Trace Fossil Classification
However, the main problem with this type of trace fossil classification is that most trace fossils lack the complex and fine details that would allow for the identification of the specific trace maker, which is crucial for taxonomy. At best, it’s only possible to determine the general group of organisms that left the traces, such as bryozoan borings, trilobite traces like Cruziana, or vertebrate footprints.
Ichnotaxonomy is the formal naming and classification of fossil traces. It classifies the trace fossils of organisms as distinct morphotypes based on their unique forms, regardless of whether they perfectly reflect the body structure of the animal or merely record the behavior of the trace maker. An ichnotaxon refers only to the trace itself, not to its creator. This allows for the possibility that the same animal can leave traces of different forms.
Ichnosystematics (systematic ichnology, systematic paleoichnology) is the study of relationships between ichnotaxa. Since ichnologists rarely deal with ichnotaxa above the ichnogenus level, ichnosystematics remains a weakly developed branch of ichnology.
This taxonomic approach highlights a central principle of trace fossil classification: morphology, not maker, defines the fossil’s identity.
Nomenclature in Trace Fossil Classification
In addition, this is a system for naming and trace fossil classification, which are signs of organism activity preserved in geological layers. The principles behind naming trace fossils differ from those used in the more familiar taxonomy of organisms, which mainly relies on phylogeny.
Ichnologists classify fossil traces based on their morphology. The nomenclature uses the same Latin binomial naming system as in animal and plant systematics. However, trace fossils are included in ichnotaxa, which are separate from Linnaean taxonomy, as they represent sedimentary structures rather than organisms.
Ichnotaxon: A taxon based on the fossilized work of an organism, including fossilized trails, tracks, or burrows (trace fossils) made by an animal.
Ichnogenus. A parataxon1 in ichnology that includes one or more ichnospecies. It is the main classification level for trace fossils, similar to a genus in biological taxonomy. Its definition is wholly morphological (by shape and form), without implying any specific biological relationship. Crucially, an ichnogenus does not necessarily indicate that only one type of animal made it. Indeed, similar activities from different kinds of animals could produce the same ichnogenus. In paleoichnology, the ichnogenus is the most important level of trace fossil classification, even more so than the ichnospecies.
Ichnospecies: The lowest-level category of ichnotaxon typically recognized. The name of an ichnospecies is a binary combination, consisting of the name of the ichnogenus followed by a single specific epithet; this may be in adjectival form and agree with the generic name grammatically, or it may be in genitive form. Because of the variability in external features, it is often difficult to precisely define an ichnospecies. Scientists can assign multiple ichnospecies (or even ichnogenera) to a single trackway with a variable pattern. This is especially true when they find the trackway in separate fragments or observe different behaviors such as walking, running, jumping, or sitting.
Morphofamily (or form-family). A group of ichnogenera with similar morphological traits. These similarities stem primarily from shape and minor features, not biological relationships. So the group may include traces made by unrelated organisms. Because the classification relies purely on form, not lineage, many researchers avoid using this term. They believe ichnotaxa should not have formal groupings above the ichnogenus level.
In the technical terminology of fossil trace classification, the standard suffix is -ichnion (plural: -ichnia).
Ethological Classification
Ethology (or paleoethology) is the study of animal behavior. In ichnology, researchers classify ethological trace fossils according to the behavior of the organisms. However, it’s rarely possible to draw strict boundaries between different categories, as organisms can combine several types of behavior.
The Seilacherian System of Trace Fossil Classification
This system relies on the idea that similar behaviors produce similar trace fossil morphologies. Adolf Seilacher proposed it in 1953. This behavior-based system represents an entirely different axis of trace fossil classification, focused not on form but on inferred life activity.
Cubichnia
Cubichnia (including Volichnia): These are resting traces, signs of an organism’s stillness or rest, left on the surface of soft sediment. These traces form during brief periods of stationary behavior. This behavior could be resting, a potential prey hiding, breathing, rehydration, hibernation, a predator ambushing, and so on. However, it can also involve feeding (marking a transition to Praedichnia), as seen with some predators, like asterozoans (which produce Asteriacites). The shape of these traces reflects, to some extent, the body shape and size of the animal.
Mobile animals that burrow or bore into the substrate and stay there for a while leave behind resting traces. Animals that live within the surface layer of sand—such as many bivalves that create traces like Lockeia—more commonly produce Cubichnia from within the sediment rather than as surface traces. The trace forms when they burrow to settle, disturbing the upper part of the underlying substrate. On hard substrates, limpets and sea urchins can leave resting traces by boring shallow pits.
These behavioral categories, such as Cubichnia, are a core component of trace fossil classification, which allows paleontologists to group trace fossils based on the inferred activity of the organism.
Examples: Asteriacites, Ichnocumulus, Lockeia, and Rusophycus.
Lockeia are bilaterally symmetrical, elongated mounds of almond or ovoid shape (convex hyporelief) or depressions (concave epirelief), which taper at one or both ends. The surface is usually smooth but can be uneven; it may have a median longitudinal ridge (hyporelief) or groove (epirelief). Some mounds are slightly arcuate or sinuous. They typically measure 12–45 mm in length and 9–23 mm in width. The infill is similar to the host rock.
They occur in all types of aquatic environments where bivalves live, including marine, brackish, and freshwater rivers and lakes. Bivalves created these traces, and we know of them from the Cambrian period to the present day. Synonym: Pelecypodichnus.
Domichnia
Domichnia are dwelling traces, created by organisms as permanent or semi-permanent homes. These include burrows, holes, and cavities that provide protection from predators, competitors, and environmental changes. The structures range from simple pits (e.g., Bergaueria) or vertical tubes (e.g., Skolithos) to complex, branched systems (e.g., Thalassinoides). Many of these structures, such as Ophiomorpha, have linings that provide stability or help control water flow. The walls may also feature bioglyphs, reflecting the trace maker’s appendages or method of digging (e.g., Entobia, Spongeliomorpha).
The producers of domichnia can be sessile suspension feeders, predators, or detritivores. They must possess specific adaptations to dig, irrigate, and maintain an open burrow. Dwelling structures often have specialized features, such as multiple openings (e.g., U-shaped tubes) or angled connections to improve the inflow of oxygen and nutrients. Some species actively irrigate their burrows, while others rely on passive flow created by the structural design.
Some burrows can house several individuals, even different species. Domichnia can persist for generations, with their morphology evolving over time to accommodate growth or repair. For example, the burrows of the fiddler crab (Uca pugnax) expand from simple shafts to U-shapes. Spreiten, or layered structures, can reflect deposit feeding, burrow expansion, or adaptation to environmental changes. Domichnia is one of the most common and well-established ethological classes. This category plays a central role in trace fossil classification by demonstrating how behavioral patterns like dwelling can persist across different sedimentary environments and geological periods.
Examples: Arenicolites, Bergaueria, Conichnus, Cylindricum, Diplocraterion, Entobia, Ophiomorpha, Palaeophycus, Skolithos, Spongeliomorpha, and Thalassinoides.
Palaeophycus is straight or slightly curved, smooth or ornamented, cylindrical, predominantly horizontal structures. Branching, if present, is irregular and lacks bulges. The burrow walls are lined. The infill is passive, usually massive, unstructured, and similar to the host rock.
Researchers typically interpret these traces as open dwelling burrows (Domichnia) or feeding structures (Fodinichnia), depending on the specific ichnospecies and context. The creators of these traces were likely filter feeders or predators (possibly polychaetes). In continental settings, they could have been semi-aquatic insects or beetles. They are widespread from the late Precambrian to the present day.
Ophiomorpha consists of simple and complex burrow systems made by organisms (specifically, the dwellings of decapods and shrimp-like animals) living in high-energy marine coastal, sublittoral, and shelf environments. They are most abundant in marine coastal settings.
The burrow walls are bumpy and nodular, primarily composed of dense and evenly distributed disk-shaped, ovoid, or irregularly polygonal pellets that provide support for the burrow. Branching is irregular, often forming Y- and T-shapes. This ichnogenus relies on taxonomic criteria that focus on how the walls are constructed rather than on the burrow’s overall shape. It is a classic example of Domichnia. They appear in the early Permian and become widespread from the Jurassic period onwards, notably in the Nereites ichnofacies.
Bergaueria is a trace fossil representing the dwelling of Cnidarians (specifically, sea anemones). It appears as a mound-shaped lump on the underside of sandstone beds, with hemispherical ends. It’s vertically oriented, ranging from oval to circular in outline. The burrow is short, plug-like, and vertical with an elliptical base and a central depression at the basal end. The burrow margins are smooth, with a very discrete lining. Ornamentation is absent. The burrow’s infill is similar to the host rock.
The diameter of the basal end ranges from 14 to 20 mm, and its height is typically 5 mm. It preserves in positive hyporelief. These burrows often occur in dense populations. For the ichnospecies Bergaueria perata, a nearly ubiquitous presence of concentric ornamentation is noted. It occurs in shallow to deep marine deposits and falls into either the Domichnia or Cubichnia category. This fossil dates from the Ediacaran to the Silurian (Ludlow) periods. Furthermore, the rotational movement of arthropods can leave similar traces (Cheiichnus).
Synonym: Kulindrichnus.
Fodinichnia
Fodinichnia (including Xylichnia): These are feeding traces, specifically subsurface feeding traces. They represent another critical behavioral group in trace fossil classification, helping to distinguish subsurface feeding strategies from surface grazing and other forms of biogenic activity. This category includes burrows and borings created by deposit feeders as they ingest the substrate. The extensive, often three-dimensional, feeding structure can be open or gradually filled (with spreiten). Typically, these traces do not overlap or repeat.
The burrows can be simple, typically with active infill (Planolites, Scoyenia, Taenidium); branched (some Arthrophycus); U-shaped (Rhizocorallium); radial (Dactyloidites, Gyrophyllites); or complex (Treptichnus) with various orientations. These traces usually remain as full-relief endichnia.
Synonym: Fodichnia.
Examples: Arthrophycus, Chondrites, Dactyloidites, Daedalus, Gyrophyllites, Planolites, Phycodes, Rhizocorallium, Scoyenia, Taenidium, and Treptichnus.
Planolites are small, 1–5 mm, unlined, and rarely branched, straight or slightly winding, elongated cylindrical burrows. Their orientation can be horizontal or inclined. There is no lining. Burrows may intersect. They appear throughout the Ediacaran Period and the Phanerozoic Eon.
The composition of the burrow infill differs from the lithology of the surrounding rock. Sometimes, only the burrow’s contents distinguish Planolites from passively filled Palaeophycus burrows. Worm-like deposit feeders and arthropods (e.g., spiders, insects) are examples of the various burrowing invertebrates that form them during their feeding process. The organism consumed sediment as it moved and backfilled the burrow behind it.
Most Planolites fossils are from North America, but you can also find them in South America, Europe, Asia, Africa, and Antarctica. They occur in various environments: shallow and deep marine deposits, continental terrestrial and aquatic deposits, and alluvial, lacustrine, and eolian environments. They belong to the Fodinichnia (deposit-feeding) group.
Rhizocorallium are large, U-shaped burrows, oriented horizontally or inclined within 10° of the sedimentary bedding plane. The burrows are elongated, ribbon-like, straight, or sinuous, with some showing nearly parallel longitudinal scratch marks on the burrow walls. These burrows can be very large, over a meter in length. However, their width is usually only up to 2 centimeters, limited by the size of the organisms that produced them. These traces likely represent fodinichnia—burrows made by animals, probably polychaetes or crustaceans, as they searched through the sediment for food. Rhizocorallium belong to the Glossifungites Ichnofacies and occur in both shallow and deep marine environments, with a geologic range extending from the Cambrian to the present.
Rogerella is a small, unbranched boring found in shells, corals, brachiopods, bivalves, gastropods, echinoids, and other carbonate substrates. It has the shape of a pouch with a slit-like opening perpendicular to the substrate surface. Acrothoracican barnacles create such traces today. These crustaceans extend their legs upward through the opening for filter feeding.
The ichnogenus displays behaviors related to both feeding (Fodinichnia) and dwelling, indicating how these organisms interacted with their substrates. Rogerella belong to the Teredolites Ichnofacies. They have existed from the Devonian Period to the present day.
Teichichnus is a distinctive ichnogenus, characterized by a vertical, arcuate burrow formed by the superimposition of thin “tongues” of sediment, creating a series of tightly packed, upwardly concave laminae. People generally classify Teichichnus as fodinichnia because the organism developed a habit of following the same path through varying sediment heights, which helps it avoid re-traversing the same area. These “tongues” are often quite sinuous, possibly indicating a nutrient-poor environment where animals had to cover a larger area of sediment to find sufficient nourishment. The structure can reach up to 10 cm in height.
It belongs to the Cruziana ichnofacies in shallow marine environments, but it is also rarely present in deep-water deposits. Polychaetes, echiurans, holothurians, trilobites, or crustaceans likely formed these traces. They occur from the early Cambrian period.
Zoophycos is an ichnogenus with a cosmopolitan distribution, formed by the movement and feeding of polychaete worms. Zoophycos occurs in both flat and spiral (corkscrew-like) forms. It consists of a central tunnel (several centimeters long and usually 1 centimeter or less wide) where the invertebrate organism temporarily resided and spiraling, arcuate layers of lateral feeding traces (typically tens of centimeters wide) formed as the animal searched for food around its central home. The overall shape of the structure is conical. Helical burrows can exceed a meter in both vertical and horizontal extent, while the diameter of the passages typically does not exceed 7 mm.
They could simply be feeding traces of deposit-feeding worms. Alternatively, the animal might have used the burrows to store food resources. Zoophycos typically occurs in deep-sea marine silts and sands, often between turbidite layers, but it can also appear in shallow-marine storm deposits. In the Silurian, it was limited to deep waters, but from the Devonian to the end of the Permian, it also appeared in coastal areas. It exist from the early Cambrian to the present day. It belongs to the Fodinichnia group.
Synonym: Taonurus.
Right: Phycosiphon incertum. Photo and drawing of the image. Arrows indicate the places where one trace of Phycosiphon overlaps another. An asterisk indicates the overlap of a fragment of the Chondrites trace on the Phycosiphon trace. Late Triassic (Late Carnian-Early Rhaetian); Southeastern Pamir, Kobrigen locality.
Phycosiphon is a typical fossil trace of deposit feeders. These are small, flat, and curved burrow systems. They can be inclined or bedding-parallel and filled with spreiten (ribs or laminae). They consist of protruding U-shaped lobes with a dark, finer-grained core surrounded by a poorly defined, lighter, coarser-grained, sinuous halo. The spreiten may sometimes not be visible.
Phycosiphon traces developed in sediment rich in organic matter. The worm-like trace maker fed on this substance, creating a lobed structure within the sediment in the area it consumed. The surrounding halo gallery represents the animal’s final, concluding pass before it moved on to the next organically enriched area.
The diameter of the trace is typically 0.2–0.5 cm, with lobes 2–3 mm wide and up to 16 mm long. They are exclusively marine. Their record extends from the Cambrian to modern times. Synonym: Anconichnus.
Pascichnia
Pascichnia are grazing traces, or surface feeding traces. Pascichnia are grazing traces in the form of spiral, winding trails or shallow burrows, left by organisms that use the substrate for feeding. Feeding and movement occur simultaneously, with the traces often touching previous sections to maximize coverage of the area. Preserved in positive hyporelief, these traces commonly appear unbranched, running parallel to the seafloor. They also tend to be more regular than Repichnia. Complex shapes, such as spirals and meanders, rarely overlap, and new parts of the traces only touch old ones.
Examples of meandering traces include Nereites, while spiral ones include Spirorhaphe. Traces can be continuous or discontinuous, depending on whether the trace maker lifts its body or feeding organ.
Typically, Pascichnia are shallow subsurface grazing traces. Surface pascichnia are rare in the fossil record. The notable exceptions are bioerosional examples such as Radulichnus and Gnathichnus. The boundaries between Pascichnia and Fodinichnia (deposit feeding within sedimentary rocks) can sometimes blur, especially when organisms produce traces that are very shallow and barely penetrate the substrate. However, the key point for Pascichnia is the emphasis on surface or subsurface grazing left by a moving organism. Their unique structure and feeding motion distinguish them as a separate class in trace fossil classification. This makes Pascichnia especially important for understanding surface-based feeding behaviors.
Examples: Cosmorhaphe, Desmograpton, Gnathichnus, Helmintoida, Helminthopsis, Helminthoidichnites, Helminthorhaphe, Nereites, Polykampton (Polycampton), Radulihnus, Spirorhaphe.
Gnathichnus is a bioerosion trace fossil appearing as shallow grooves, pits, and scratches on hard substrates (typically shells, rocks, or hard calcium carbonate ground). The form is unbranched and star-shaped. These traces were created by regular echinoids (sea urchins) grazing the surface with their five-toothed feeding structures, known as “Aristotle’s Lantern.”
Desmograpton is a three-dimensional fossil of graphoglyptids. It consists of hypichnial double rows of winding U- or J-shaped strands of moderate width, with raised sections along the curves. The connecting bridges are oriented at an angle to the trail axis and create a zigzag pattern. The burrow is straight or slightly wavy, with smooth, convex hyporelief and lateral extensions. The diameter of the burrow varies from 0.3 to 1.7 mm. Irregularities in the substrate can cause noticeable distortions in the pattern of the tunnels. It occurs in shale deposits from the Silurian to the Miocene, formed in deep-water environments.
Helminthoidichnites are small, simple, relatively thin, and unbranched, irregularly winding trails, or less commonly, circular paths or burrows, preserved as positive hyporeliefs. They lie along horizontal laminae in sandstone. The surface is smooth. The structures are mostly horizontal and curved but can also be straight. They sometimes overlap with other specimens but do not self-intersect.
The diameter is relatively constant, about 1–3 mm, with a maximum length (along the bedding plane) of about 15–95 mm. The infill is identical to the host rock. In Helminthoidichnites, loops are only occasionally present, unlike in Gordia, where loops are the most characteristic feature.
Researchers interpret it as a grazing trace (pascichnia) formed in shallow tiers by mobile organisms, most likely vermiform animals. Found in deep-water marine environments. Helminthoidichnites tenuis is one of the most common components of the ichnofauna from the Ediacaran and early Cambrian periods.
Helminthopsis consists of irregular, wavy, unbranched, or sparsely branched traces that meander across sediment surfaces. The meanders are horizontal, usually broad and low, and composed of straight segments interspersed with short, uneven, irregularly curvilinear segments. Crucially, Helminthopsis traces never intersect each other.
The burrow surface is typically smooth, but in some cases, transverse constrictions are present. The diameter varies from 1.0 to 5 mm, yet it remains constant within each specimen; length ranges from 42 to 80 mm, and the width is about 4 mm.
Traces remain as semi-reliefs in fine-grained turbiditic sandstones. The irregular meander distinguishes specimens from Helminthoidichnites by their tendency to undulate and from Gordia by the absence of self-intersection. Helminthopsis are grazing traces created by a deposit-feeding organism, evidently polychaetes or priapulids. It is common in deep-water marine deposits but occasionally observed in shallow-marine environments. It is a classic example of grazing traces (Fodinichnia). They range from the Ediacaran period to the present.
Helminthoida resembles Helminthopsis but typically has more regular meanders, often forming tightly packed spirals or non-overlapping loops. These parallel, winding, smooth burrows usually appear on a layer’s surface. The burrows are unbranched, do not intersect, and do not penetrate one another. Such a systematic structure, therefore, is characteristic of efficient grazing.
They occur in low-energy marine environments, both shallow and deep-water. The trace makers are burrowing marine organisms, such as worms or other worm-like creatures. They have existed since the Cambrian period.
Helminthorhaphe consists of convex, unbranched hyporelief traces, made up of burrows that form numerous narrow and high-amplitude meanders, which can be regular or irregular, parallel or non-parallel, and closely spaced. The trace width is about 3–4 mm, and the meander length is up to 150 mm. The distance between meanders is 2–3 times greater than the burrow diameter.
The burrow surface is smooth, lacking longitudinal stripes or transverse ripplings. Furthermore, bulges in curved sections are very rare and weakly developed. Marine invertebrate graphoglyptids, which were deposit feeders, left these traces. They occur from the Late Cretaceous to the Miocene in deep-water, medium- and thin-bedded sandy turbidites and flysch deposits. (Fan et al., 2017. From morphology to behavior: Quantitative morphological study of the trace fossil Helminthorhaphe).
Polykampton is a trace fossil produced by a “worm-like” organism inhabiting the deep-sea endobenthic zone. It consists of a horizontal, straight, or sinuous central cylindrical tunnel with lateral, oblique spreiten lobes alternating on either side of the tunnel.
Indeed, scientists hypothesize that the worm-like organism gathered organic-rich, fine-grained material from the sediment surface and stored it within the burrow, particularly in the lobes. This stored material would then be utilized during times of food scarcity on the seafloor.
Polykampton occurs in Upper Albian to Oligocene deposits of the Alps, Apennines, Carpathians, and Betic Cordillera. Consequently, it can be interpreted as a sequestrichnion2, a trace fossil where an organism stores resources. (Uchman et al., 2019).
Models of Nereites Trace Fossils
Nereites is an ichnogenus of trace fossils characterized by an irregularly winding trail. It consists of a central furrow bordered on both sides by regular, closely spaced, oval or circular lobes, which typically show fine striations.
The central furrow is 4–5 mm wide with a rounded bottom. Narrow, more or less evenly spaced ridges, grouped in threes or fours and curved in one direction, cross the central furrow and extend a short distance along its sides. The meanders can be dense and vary in width, shape, and size.
Worm-like organisms, horseshoe crabs, and hermit crabs likely created these traces. Significantly, Nereites fossils range in age from the Cambrian period to the present day.
There are two primary models explaining the formation of Nereites trace fossils:
The “Worm Model” (Convex Hyporelief)
This model suggests that Nereites traces are feeding burrows (fodinichnia) or grazing trails (pascichnia) created by worm-like organisms. These interpretations highlight the complexity of trace fossil classification, where a single ichnogenus may be placed into multiple behavioral categories based on morphology and context.
These organisms fed on sediment in various marine environments, including lagoonal, deltaic, and deep-sea settings. Nereites of this type commonly appear on the upper surfaces of thin turbidites in sandy and silty abyssal plain regions.
The “Arthropod Model” (Concave Epirelief)
This model proposes that Nereites are locomotion traces (repichnia) formed by the legs of arthropods. These traces consist of a central furrow with lobes on both sides, indicative of the arthropod’s movement.
Nereites irregularis is a relatively small Nereites ichnospecies characterized by numerous closely spaced, deeply winding trails that tend to coil. The trails are typically horizontal and can exhibit both regular and irregular directions. The central wide tunnel is usually thicker than the narrow, rarely observed lateral enveloping zones. In dense meanders, the enveloping zones may touch or overlap. The bends of the trails touch or are rarely separated by up to 10 mm. Nereites irregularis is generally considered to be grazing trails (pascichnia) left by worms.
Synonyms: Helminthoida labyrinthica, Helminthoida crassa.
Three specimens of Nereites biserialis; the largest one shows a short transition to uniserial and larger dimples (small arrow) in the central tunnel (ct); the asymmetry and concavity of the ridges in biserial dimples indicate the movement of the producer (large arrow); the enveloping zone (ez) is indicated by diffuse laminae. The rectangle indicates the location of the sketch and corresponding cross-section shown in the upper left; the direction of movement of the producer is indicated by the asymmetry of the margins of the dimples. Seymour Island. © Olivero & Cabrera
Nereites biserialis consists of irregular, winding tracks that form a central groove, flanked on both sides by alternating double rows of small pits. These pits are the result of eroded terminal structures once filled with muddy, globular fecal pellets that had occupied the central tunnel of the burrow. Smooth or lobed sediment layers further border the rows.
Hermit crabs inhabiting elongated shells, such as those of Cerithium, likely produced these distinctive double rows. The presence of alternating double rows within the central tunnel is a key diagnostic feature of Nereites biserialis.
Nereites unserialis is winding trails consisting of a single furrow surrounded by a single row of lobes. Hermit crabs inhabiting trochiform shells3 likely produce Nereites unserialis, unlike the roughly symmetrical Nereites biserialis.
Neonereites multiserialis is straight, winding, or irregularly curving chains of various depressions (when preserved on the upper surfaces of beds) or mounds (when preserved on the lower surfaces of beds, which is a more typical preservation). Traces typically appear as positive hyporelief preserved on sandstone soles.
Neonereites multiserialis is found in Silurian (Wenlockian) and Tertiary (Eocene) strata, indicating deep-water flysch environments. Neonereites represents either an endogenic burrow or a trace left by a benthic organism, most likely an annelid worm. Synonym: Scalarituba.
Repichnia
Repichnia are ichnological traces (tracks or trails) formed by organisms during directed movement, such as crawling or walking, usually along straight or curved paths. These traces reflect the movement of animals across soft sedimentary surfaces and preserved as fossilized impressions. Examples include Diplichnites, Cruziana, and Protichnites, as well as trackways resembling those of early amphibians from Glenisla, Australia.
Repichnia typically appear as simple, shallow, continuous, or discontinuous marks. They primarily indicate directed movement without combining with other behaviors like feeding or resting. These traces align with sedimentary bedding planes and preserved either as positive hypichnia or negative epichnia.
The vast majority of tetrapod traces are repichnia.
Repichnia include subcategories such as Gradichnia, Natichnia, Navichnia, Cursichnia, and Volichnia. Traces of some transitional species, like anguidae lizards with reduced limbs, do not have a separate subcategory.
Some repichnia take the form of trails created by organisms without appendages. These can be simple (Mermia), bilobed (Cruziana), chevron-shaped (Protovirgularia), or, less commonly, meniscate or annulate. In contrast, traces left by animals with appendages are recurring, discontinuous imprints reflecting limb movement. Examples include the archosaur traces Chirotherium, the dinosaur traces Anomoepus, and arthropod traces like Aulichnites, Climactichnites, Crossopoda, Dimorphichnus, Diplichnites, and Scolicia, among others. Repichnia, with its focus on locomotion, rounds out the primary behavioral groups used in trace fossil classification, emphasizing movement as a diagnostic factor in interpreting ancient life.
Chirotherium refers to Triassic-period fossils composed of five-toed footprints and complete trackways. These prints remarkably resemble the hands of apes and bears, with the outermost toe splayed out like a thumb.
The first Chirotherium tracks were discovered in 1834 in Thuringia, Germany (upper Buntsandstein, Solling Formation), and are dated to approximately 243 million years ago (Early Triassic, Pelsonian). However, other research suggests even older Chirotherium traces from the Olenekian (Lucas and Schoch, 2002), and subsequently, by 1838, they found them in England.
The creatures that left these traces likely belonged to archosaur groups such as the Prestosuchidae or Rauisuchidae, which relate to the ancestors of crocodiles. These were large predators with semi-erect gaits.
Synonym: Cheirotherium.
Volichnia: Traces of Aerial or Leaping Organisms
Volichnia are traces left by flying (insects and birds) or leaping organisms during landing and takeoff, or marks made by their wings or other appendages touching a soft substrate. This category applies only in exceptional cases to true landing and takeoff traces (surface disturbances) of swimming or flying tracemakers (e.g., Toganoxichnus).
Very few fossilized Volichnia exist where the tracemaker’s wings left impressions during flight over land, despite being quite common in recent times. Therefore, to be recognized, Volichnia should ideally be associated with a sharply beginning or ending repichnia (locomotion trace). Volichnia is a subcategory within repichnia.
Other Ethological Classes
Since the advent of behavioral categorization, several additional ethological classes of traces have been proposed and accepted, covering other types of behavior. Including Seilacher’s original five ethological trace groups, eleven classes are currently accepted by the ichnological community (Bromley 1996), with a number of others holding provisional status. Ichnotaxonomy and Seilacher’s behavioral framework, along with these categories, highlight the multifaceted nature of trace fossil classification, integrating behavior, structure, and context.
The additional currently accepted groups are as follows:
Aedificichnia: Above-Ground Traces
Aedificichnia refers to the construction of above-ground structures that extend vertically from the substrate surface. These structures can serve various purposes, including skeletal extensions, ventilation, protection, nesting (e.g., mud dauber wasps), or colonial dwellings (e.g., termite mounds).
Some of these structures might also fit into other categories. For example, caddisfly larval tubes and sandy “reefs” built by sabellariid polychaetes could be considered domichnia, as they represent isolated burrow linings. Spiderwebs, on the other hand, might exhibit praedichnia (specifically irretichnia).
An example is Chubutolithes (a wasp nest). Note that researchers reassigned some examples in this category, now classifying them under domichnia and calichnia.
Agrichnia: Farming Traces
Agrichnia, also known as “gardening traces” or cultivation traces, are a systematic network of burrows. Organisms likely built these structures either to trap very small organisms (meiofauna or meiobenthos) or to cultivate bacteria or other food sources.
Invertebrates typically construct most burrows in this category with a highly symmetrical pattern to maximize the internal surface area. They typically show regular branching and are open structures, allowing for repeated passage by the tracemakers. The organism would continuously inspect this burrow system to hunt for any small organisms that wandered in. Agrichnia are commonly produced in deep-water environments, just below the surface of terrigenous4 (hemipelagic5) muds. They usually appear on the lower surface of sandstones preserved as positive hyporeliefs.
The morphology of these burrows varies from branched meanders to spirals or networks; for example, the spirally arranged system of burrows and chambers in Spirodesmos. They are also analogous to galleries in which leafcutter ants (such as Atta colombica) cultivate fungi. A significant group of ichnogenera commonly interpreted as Agrichnia are the graphoglyptids. Their systematic and often symmetrical burrow patterns make Agrichnia is a notable component of trace fossil classification, clearly highlighting the intentional spatial arrangement within trace structures.
Paleodictyon is an ichnotaxon characterized by a distinctive hexagonal network, much like a honeycomb, with cells typically measuring 1–3 cm. It features raised, prominent partitions forming the grid. This ichnotaxon is a representative of the Nereites ichnofacies.
Researchers have found it in both deep-water and shallow-water deposits, ranging from the Cambrian period to the present day. The organism responsible for creating these traces remains unknown.
One hypothesis suggests that Paleodictyon might be the result of microbial farming by an unidentified animal. In this scenario, the animal would line the burrows with an organic substance, thereby allowing microbes to proliferate, and then consume them.
Alternatively, another theory proposes that Paleodictyon is an inorganic structure, possibly formed by natural Rayleigh-Bénard convection cells. (Lilia Popova, 2009. The Problem of Paleodictyon).
Equilibrichnia: Maintaining Position in Sediments
Equilibrichnia, or equilibrium traces, record how organisms adjusted their burrows in response to changing sedimentation rates. As sediment built up or was eroded, the tracemakers altered their burrow vertically to maintain vital functions like ventilation.
These adjustments leave behind characteristic structures called spreiten, which show the organism’s previous positions. Protrusive spreiten form during erosion, while retrusive spreiten develop when sediment accumulates. Unlike fugichnia, which indicate a rapid escape from sudden burial, equilibrichnia reflect a slow, adaptive response to environmental shifts. This behavior highlights long-term interaction with the substrate and plays a distinct role in trace fossil classification.
A classic example is Diplocraterion, though not all species in this genus show this behavior. In contrast, tracemakers like Arthraria lacked spreiten and couldn’t maintain such equilibrium. Thus, in trace fossil classification, terms like equilibrated domichnia or equilibrated fodinichnia (e.g., Rosselia socialis) may more precisely describe these trace types.
Diplocraterion is an ichnogenus that describes distinctive U-shaped burrows. These are single, short, vertical burrows featuring a connecting web-like structure called a spreiten between the two arms of the “U.”
The burrow tubes typically range from 5 to 15 mm in diameter, with the two arms separated by 4–7 cm, and burrow depths usually between 2 and 15 cm. The upper openings, which reached the seafloor surface, sometimes have funnel-shaped enlargements.
This type of trace fossil represents an organism’s gradual adjustment to changing sedimentation and erosion rates. It reflects the animal’s efforts to maintain a specific depth within the substrate. This vertical movement creates two types of spreiten: protrusive spreiten (formed between the paired tubes) and retrusive spreiten (formed below the paired tubes). Some ichnospecies show both types.
Diplocraterion is a classic example of equilibrichnia (equilibrium traces). These traces have existed from the Cambrian to the Cretaceous periods. Synonym: Corophioides.
Fugichnia: Escape Traces
Right: Escape structures that produced by upward escape behavior of mobile bivalves. © Gingras et al.
Fugichnia, or escape traces, are structures formed when an organism attempts to escape burial due to rapid sediment deposition, such as from turbidity or density currents.
These are temporary structures with undefined boundaries and, unlike permanent dwelling burrows, lack reinforced walls. Escape traces are typically oriented vertically or obliquely. However, horizontal escape traces also exist, created when tracemakers tried to flee infaunal predators. The burrows often show chevron patterns, thus indicating the upward direction of the organism’s movement. Fugichnia usually appear as full-relief endichnia.
Examples include Conichnus, Corophioides, and Rosselia.
Previously, researchers defined the Taphichnia category as traces of unsuccessful attempts to escape burial. However, whether an escape attempt was successful or not doesn’t change the fact that the animal still leaves behind escape traces. Therefore, researchers reassigned Taphichnia to Fugichnia.
Rosselia refers to vertical and oblique burrows characterized by downward-pointing, concentric cone structures. These burrows transition downwards into a long, sometimes curved tube. They represent the dwelling traces of a sessile (stationary) organism. When weathered, a Rosselia trace can resemble Asterosoma. Their record extends from the Cambrian to modern times.
Praedichnia: Traces of Predation
Praedichnia are fossilized traces that provide evidence of predatory behavior. This category includes Mordichnia (bite marks) and Irretichnia (trap traces). In the broader framework of trace fossil classification, Praedichnia illustrates how behavioral evidence—not just locomotion or dwelling—can be preserved and systematically categorized.
Common examples of Praedichnia include drill holes left in shells by carnivorous snails (created chemically or mechanically), chips along shell margins, or, most notably, bite marks found on the bones of some vertebrates.
Identifying predation on soft substrates is challenging, with only a few known examples, such as trilobites feeding on “worms” and fish preying on unidentified invertebrates (Osculichnus). It’s also difficult to determine whether gnawing and scratching marks on bones occurred during a predator’s attack or long after the prey’s death. For this reason, Praedichnia also encompasses traces resulting from scavenging activities.
Oichnus describes unbranched, round, or pit-shaped drill holes found in shells. Predatory cephalopods and gastropods create these holes, which reflect predator-prey interactions within mollusk communities.
These small, millimeter-sized holes are typically round or oval. They feature a wider outer opening that narrows inward, possess smooth walls, and are oriented perpendicular to the shell’s surface. Indeed, carnivorous gastropods from the Naticidae and Muricidae families likely produced such holes.
Osculichnus is a trace fossil interpreted as evidence of predation by jawed fishes (gnathostomes). These traces appear as clusters of bipartite, elliptical mounds on the bedding surfaces of gray or brownish sandstones, typically deposited in estuarine or bay environments. The mounds have an approximately elliptical shape, divided into two lobes by a central, undulating groove, with the lobes often differing in width.
The bilobate structure of Osculichnus represents the impressions of both the upper and lower jaws of a predatory fish pressing into the sediment while searching for prey. This morphology strongly supports an origin linked to jawed vertebrates.
The makers of these traces may have been hunting for infaunal organisms, such as the producers of Sinusichnus or other small invertebrates living within the substrate. Based on the overall form and detailed morphology, especially in specimens from the Late Devonian, researchers most likely attribute Osculichnus to dipnoan (lungfish) predators.
The earliest known examples of Osculichnus come from the Upper Devonian Wutong Formation in Hubei Province, South China.
- Parataxon—an artificial taxon used to describe a fossil based on morphology with no relation to genetic reality. ↩︎
- Sequestrichnia – an ethological category of marine trace fossils recording the collection and stowage of nutritional material within burrows. ↩︎
- A trochiform shell is a type of gastropod (snail) shell that resembles a top or a spinning top in shape. ↩︎
- Terrigenous sediments are a result of rock erosion on land, and rivers carry them into the sea. These sediments primarily accumulate on the continental shelf. ↩︎
- Hemipelagic sediments are marine deposits made up of clay and silt particles that come from land. They also contain some biological material from nearby landmasses or from organisms living in the water. ↩︎
