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AEDIFICICHNIA (BOWN & RATCLIFFE, 1988)
ABOVE-GROUND TRACES
Aedificichnia, introduced by BOWN & RATCLIFFE (1988) for above-ground structures, was regarded as an unnecessary category by VALLON et al. (2016). Such structures may serve diverse purposes such as extension of skeletons, ventilation, protection or, in the case of muddauber wasps, nesting. These structures are better grouped in other categories according to the dominant purpose for which these structures were built. Other traces that were subsequently classed as aedificichnia, such as caddisfly larval tubes (DONOVAN, 1994) and sand ʽreefsʼ constructed by sabellariid polychaetes (EKDALE & LEWIS, 1993) fit well into domichnia because they can be considered as isolated burrow linings. In spider webs, VALLON et al. (2016) saw, contrary to DONOVAN (1994), praedichnia (irretichnia; cf. LEHANE & EKDALE, 2013).
AESTIVICHNIA (VERDE et al., 2007)
AESTIVATION TRACES
Spherical chambers having a lined wall of imbricated pellets and a filling of rounded to meniscate pellets arranged in winding strings were convincingly interpreted as aestivation chambers of earthworms by VERDE et al. (2007). The authors grouped these chambers with other aestivation burrows of amphibians (HEMBREE et al., 2005) and lungfish (VOORHIES, 1975) and recommended either the establishment of a new ethological category (aestivichnia) or incorporation within the domichnia. VALLON et al. (2016) preferred to maintain the aestivichnia as a subcategory of domichnia because the tracemakers construct these structures for protection against temporary worsening of local environmental conditions (cf. RINDSBERG, 2012, p. 55).
AGRICHNIA (EKDALE et al. 1984)
FARMING TRACES
Most of the burrows of this category are built in a highly symmetrical layout to maximise the inner surface area. They usually show true branching and therefore must have been open structures for repeated transit by the tracemakers. The tracemakers presumably irrigated these structures with sulphide-rich pore water to encourage the growth of sulphide-metabolising bacteria. Agrichnia are typically produced in deep-sea environments, just below the sediment surface of hemipelagic mud. They are usually preserved on the lower surface of sandstones (as positive hyporelief) that have been deposited by turbidity currents, which excavate and cast the lower parts of the traces. Burrow morphologies range from branched meanders to spirals or nets. Boundaries between chemichnia, pascichnia and fodinichnia are indistinct, especially in the fossil record. Until LEHANE & EKDALE (2013), trapping traces (irretichnia) were included as agrichnia. SEILACHER (1977), therefore, suggested two functions for agrichnia and regarded the simpler structures (commonly having only few apertures) as traps for migrating meiofauna (for discussion see LEHANE & EKDALE, 2013) and those having numerous apertures at the seafloor as gardening systems similar to the galleries in which leafcutter ants (e.g. Atta colombica) cultivate fungi (BROMLEY, 1996). VALLON et al. (2016) agreed with LEHANE & EKDALE (2013) that trapping prey is a different behaviour than farming. But instead of keeping LEHANE & EKDALE’s irretichnia as a distinct category, they chose to lower it’s rank and put it as a subcategory under praedichnia. However, modern analogues are poorly understood, making the recognition of trapping traces very difficult at present. As BROMLEY (1996) noted, ʽAfter all, there is a natural sequence from the trapping of microbes for food, via the culturing of microbes for food, to the culturing of microbes as symbionts.ʼ
CALCIHNIA (GENISE & BOWN, 1994a)
BROODING TRACES
Originally proposed for insect breeding structures by GENISE & BOWN (1994a), all traces produced for raising and caring of the young by adults of the same species should be incorporated into this category (cf. BROMLEY, 1996; BUATOIS & MÁNGANO, 2011). Observations on Recent examples show that these traces may be constructed to achieve several purposes, ranging from pure protection of the offspring to establishing stable microclimates, e.g. constant humidity and temperature (GENISE & BOWN, 1994a, 1994b; GENISE et al., 2000). In contrast to BROMLEY (1996), fossil leaf-mining of insect larvae or bark-mining traces by xylophagous beetle larvae (e.g. Scolytinae) should mainly be placed in the fodinichnia (MÜLLER, 1982, 1989). In these cases, only the central gallery needs be regarded as a calichnion, the numerous radiating galleries being fodinichnia because they are produced by the feeding larvae rather than their parents.
Burrows may be simple or complex, commonly consisting of one or more chambers that may be connected to adult-sized tunnels for access, but much smaller exit apertures or tunnels for juveniles may exist. Vertebrate nests may range from shallow pits (e.g. MUELLER-TÖWE et al., 2011) to small mounds or tunnels (MARTIN, 2014) or typical ʽbirdsʼ nestsʼ built of twigs (cf. LEHMANN, 2005). Calichnia are usually preserved in full relief.
Notably, structures built by social insects also serve other purposes such as dwelling (domichnia), ventilation or farming (agrichnia), and transitions to the respective ethological categories exist. Additional transitions may occur to ecdysichnia (pupichnia, see VALLON et al., 2015).
CECIDOICHNIA (MIKULÁŠ, 1999)
PLANT REACTION TISSUE
Cecidoichnia were introduced by MIKULÁŠ, 1999 for plant reaction tissue. VALLON et al. (2016) recommend that the term should be replaced by cecidotaxa, because reaction tissues of the substrate to any infestation was excluded from trace fossils by BERTLING et al. (2006).
CHEMICHNIA (BROMLEY, 1996)
CHEMOSYMBIOTIC TRACES
BROMLEY (1996) proposed the category chemichnia for traces left by tracemakers that live in symbiosis with chemoautotrophic bacteria. The symbionts make use of redox differences between oxic and reducing tiers within the sediment. Many different tracemakers are living like this today, so there also is a huge variety of burrow morphology. However, a few reoccurring features exist in order to fulfil both, the tracemaker’s and its internal symbionts’ needs: In general, the makers of chemichnia stay connected to oxygenated water through apertures at the sediment surface, but also penetrate anoxic sediments to mine sulphide- or ammonium-rich porewaters that are required for microbial respiration. Some tracemakers simply commute between the oxic surface sediment and the anoxic tier below (OTT, 1993), whereas others construct deep mines to pump up H2S-rich pore water (SEILACHER, 1990; KĘDZIERSKI et al., 2015). The sulphide oxidation by various symbiotic sulphur bacteria may produce framboidal aggregates of pyrite preserved within the trace fossils (JØRGENSEN & GALLARDO, 1999; JØRGENSEN & NELSON, 2004), e.g. in Trichichnus (KĘDZIERSKI et al., 2015). A halo of lighter coloured surrounding sediment may also be present. Its formation is being linked to oxidation of pyrite or iron monoxides during diagenesis (KĘDZIERSKI et al., 2015).
Many bivalves living in symbiosis with sulphur-oxidising bacteria produce essentially Y-shaped burrows (e.g. Lucinidae, Thyasiridae and Solemyidae; SEILACHER, 1990, 2007). The bivalves live in the upper, more or less U- to V-shaped part of the burrow. The vertical shaft usually reaches far down to the anoxic layers of the sediment from which H2S-rich pore water is pumped up and made available for the bivalveʼs symbionts. These deep shafts may have multiple branches and are highly patterned. Here, many suspected chemichnia show pronounced distancing between branches, the diametre of each burrow is fairly constant and the burrows are phobotactic (FU, 1991; SEILACHER, 1990, 2007). This means that in principle, no newly produced burrow element should crosscut an older part or a neighbouring burrow of the same ichnotaxon. The regular branching pattern optimises both surface area and influx of H2S-rich porewaters (FU, 1991). Some chemichnia are backfilled with sediment differing from the surrounding sediment (mainly in colour owing to different content of organic matter).
Chemosymbiosis may well be active in other types of burrows. Therefore, transitions to other behavioural groups exist, especially to agrichnia, but also to fodinichnia (especially in the fossil record where the tracemaker usually cannot be named and symbiosis with chemoautotrophs remains speculative).
CIBICHNIA (MÜLLER, 1962)
FEEDING TRACES
Introduced by MÜLLER (1962) as a supercategory for all traces connected with feeding. They are comprising SEILACHER’s categories fodinichnia and pascichnia and another category introduced by MÜLLER in the same publication (1962) mordichnia. The supercategory never really was accepted, because it does not yield any ethological information and thus ichnologists prefer to use Seilacher’s ethological groups instead.
CORROSICHNIA (MIKULÁŠ, 1999)
PLANT CORROSION TRACES
In his overview of plant traces, MIKULÁŠ (1999) defined corrosichnia as mostly biochemical, surface-dissolution of hard substrates. Examples are mainly known from the Recent and comprise lichen pits and corrosive root structures. The purpose of these traces probably lies in obtaining minerals directly from a rock source, to augment a soil forming process and also to anchor the plant’s body (MIKULÁŠ, 1999).
CUBICHNIA (SEILACHER, 1953a); incl. Volichnia (sensu WALTER, 1978)
RESTING TRACES
One of the original five categories defined by SEILACHER (1953a) for traces that are created during short-term stationary behaviour. This may be resting, hiding, respiration, rehydration, hibernation, etc. but also feeding (marking the transition to praedichnia) as done by some predators, e.g. asterozoans (producing Asteriacites). More often than as surface traces, cubichnia are produced endogenically by animals that live within a superficial sand layer, e.g. many bivalves (producing e.g. Lockeia). The trace is created when they dig down to establish themselves by disturbing the top of the underlying substrate (e.g. BROMLEY, 1996; SEILACHER, 1953b). In hardgrounds, limpets and sea urchins may create resting traces by boring shallow pits (BROMLEY, 1970; MIKULÁŠ, 1992).
LESSERTISSEUR (1956) distinguished informally between resting and hiding traces. In the second group, the tracemakers would usually be concealed by a thin layer of sediment. Distinction between these, however, is difficult in the fossil record.
WALTER (1978) redefined the volichnia as landing and take-off traces of flying or leaping organisms. Although intended as locomotion traces by MÜLLER (1962, 1989; see also repichnia), WALTERʼs category fits best within the cubichnia (see also BUATOIS & MÁNGANO, 2011). Volichnia (sensu WALTER, 1978) should only be used exceptionally for genuine touching-down and lifting-off traces (surface disturbances) of swimming or flying tracemakers (e.g. Tonganoxichnus). To be recognisable as such, volichnia should ideally be combined with an abruptly starting or ending repichnion (MARTIN, 2013).
Cubichnia (s. l.) are trough-like depressions, shallower than broad, typically preserved as positive hypichnia, but with negative epichnia occurring as well. No lining or other reinforcement is usually present because the surrounding sediment is supported by the tracemakerʼs body. A possible exception to this rule, could be around the respiratory organs where reinforcement elements might be present to maintain an unimpeded water flow (RINDSBERG, 2012). Resting traces are produced by vagile animals digging or boring into the substrate and lingering there temporarily. When the structures are abandoned, the maker may create an exit trace. Resting traces reflect to some extent the outline and ventral morphology of their makers. Together with the retained impressions of digging by appendages such as feet, fins, claws and podia, this fact provides clues to the identity of the tracemaker (e.g. bioprint). Thus, they are the trace fossil group in which a tracemaker assignment has the highest probability of success. Vertical and horizontal repetition or overlapping is common in this ethological category. Transitions exist to repichnia, fugichnia, equilibrichnia, ecdysichnia and praedichnia.
CURSICHNIA (MÜLLER, 1962)
WALKING TRACES
MÜLLER (1962) introduced this category to distinguish moving traces produced by tracemakers with appendages (tracks or trackways) from traces left by crawling or creeping tracemakers (repichnia s. str.). Trackways are made up of repeated sets of discontinuous impressions reflecting the tracemakersʼ appendages and their motion within the substrate (e.g. vertebrate trackway Chirotherium, arthropod trackway Diplichnites). Cursichnia as well as gradichnia (GEYER, 1973) are usually included in repichnia (e.g. BROMLEY, 1996).
DIGESTICHNIA (VALLON, 2012)
DIGESTION TRACES
The category was originally defined by VIALOV (1972) but was little used before its revival by VALLON (2012). Digestichnia include all traces that are made by digestive processes. These are behavioural modifications made to material that has been acted on within the digestive tract of the tracemaker, leaving orally or anally or even being retained as gut contents. In contrast to the term bromalite (fossilised remains of digested material; HUNT, 1992), digestichnion is an ethological rather than descriptive term.
Coprolites are the most common examples, but regurgitalites, cololites and gastroliths also belong to the digestion traces. Identifiable coprolites usually have a distinct shape. Examples and methodology were presented by HÄNTZSCHEL et al. (1968). Coprolites commonly consist of a faecal groundmass that may be phosphatic, calcitic, purely organic or a mix of these types. Remains of body fossils may be present (e.g. VALLON, 2012).
Regurgitalites are mainly produced by vertebrates (especially reptiles, birds and some fish); some molluscs (e.g. cephalopods, suspension-feeding bivalves and gastropods) also regurgitate indigestible matter. These resemble coprolites to some extent, but lack a faecal groundmass and therefore only consist of remains of hard body parts of the tracemakerʼs prey or incidentally ingested sediment particles. Some avian regurgitalites (e.g. of hawks and owls) are moulded into a form that may itself be recognised as a trace of activity (ELBROCH & MARKS, 2001). Hard parts may show fractures from biting and chewing and/or etching, and dissolution features caused by stomach acids and enzymes (e.g. VALLON, 2012). Regurgitalites and coprolites were combined as faecichnia by GEYER, 1973.
Fossilised faecal matter that has not been discarded from the intestinal tract, yet has been named cololite (or more correct, but rarely used cololith; AGASSIZ, 1833). The term is not only used for faeces-filled intestines that are preserved within a body cavity, but also for faecal matter in the shape of intestines isolated from a body fossil (in contrast to coprolite). At least, one example of a cololite has been formally named (Ambergrisichnus MONACO et al., 2014).
WINGS (2004, 2007) divided gastroliths into ʽbiogastrolithsʼ, ʽpatho-gastrolithsʼ and ʽgeo-gastrolithsʼ. Biogastroliths are concretions produced prior to moulting in some crustacean species to store calcium (VALLON, 2012) and so are not regarded as trace fossils. Pathogastroliths (ʽbezoarsʼ) are mainly generated in the stomachs of herbivorous mammals and are agglomerations of swallowed and felted hair or plant fibres. Geogastroliths are deliberately or accidentally swallowed pebbles and sand. These rocks are not traces, but the modifications of their surfaces by stomach processes are (BERTLING et al., 2006; VALLON, 2012). Because gastroliths stay within the stomach for a relatively long time, they typically show abrasion and etching of the surfaces in the form of subparallel grooves, resulting from muscle contractions and other stones contained in the stomach (SCHMEISSER & FLOOD, 2008). Polishing, however, does not take place (WINGS, 2004).
Defaecation and regurgitation may happen in conjunction with all kinds of behaviours, so transitions to each of the other ethological categories may occur. Digestichnia (except on geogastroliths) are preserved as full reliefs and often contain remains of body fossils (hard parts of the prey).
DOMICHNIA (SEILACHER, 1953a)
DWELLING TRACES
Open burrows, borings, simple hollows or cavities occupied by the tracemaker and created as permanent or semi-permanent domiciles are called domichnia (BROMLEY, 1996; SEILACHER, 1953a). The structures protect not only from competitors and predators, but also from temporary changes in the local environment (RINDSBERG, 2012). Forms range from simple, shallow pits (e.g. Bergaueria) or vertical tubes (e.g. Skolithos) via J-, U- or Y-shaped burrows (e.g. Arenicolites, Diplocraterion) to complexly branched traces (e.g. Thalassinoides). A spreite may be present. Most domichnia are vertical to oblique, but the more complex structures are dominated by horizontal elements having only a few shafts connecting several tiers of the burrow with one another or the sediment surface. Open burrows in firmgrounds or borings in hard substrates typically need little reinforcement, but the walls may be sculpted with bioglyphs reflecting the tracemakerʼs appendages or its method of excavating (e.g. Entobia, Spongeliomorpha). Other interior surfaces may be smoothly finished or perhaps lined to limit influx of pore water from the surrounding substrate (e.g. some Gastrochaenolites in porous corals, VALLON et al., 2016; ALLER, 1983). Walls of burrows in substrates that collapse easily are commonly lined (e.g. Ophiomorpha) to ensure stability, to fend off other burrowers or to control pore water flow. After abandonment, domichnia within soft substrates typically collapse or are passively filled with sediment; alternatively, unused elements of burrow systems may be backfilled.
The tracemakers need to possess special adaptations in order to excavate, irrigate and maintain an open burrow (for further reading see, e.g. BROMLEY, 1996; BUATOIS & MÁNGANO, 2011). The producers of domichnia may be sessile suspension feeders (marking the transition to fixichnia), active carnivores waiting in ambush for their prey or detritivores. The trace fossil, however, emphasises the stationary dwelling function and not the trophic group (in contrast to Recent ethological studies). Specialisation exists in different burrows. Burrows with more than one aperture (e.g. U-tubes) often improve the oxygen and nutrient flow for the inhabitant. If the tracemaker does not actively irrigate its burrow by movement of its gills or other appendages, then other burrow modifications are often present (BROMLEY, 1996). In such cases, one aperture may be raised higher above the sediment surface than the other, which causes a difference in current velocity generating a constant passive flow through the burrow (VOGEL, 1978). Different angles at junctions help to irrigate complex burrows. Currents are also produced by the tracemaker moving within the burrow, pushing air or water in front of it. This pressure wave is easier divided at junctions having 120° angles resulting in an even distribution of oxygen-rich fluids within the burrow (cf. VALLON & KJELDAHL-VALLON, 2011).
Another specialisation, preserved especially in palaeosols, was described by VERDE et al. (2007). Spherical chambers having a lined wall of imbricated pellets and a filling of rounded to meniscate pellets arranged in winding strings were convincinglyinterpreted as aestivation chambers of earthworms. VERDE et al. (2007) grouped these chambers with other aestivation burrows of amphibians (HEMBREE et al., 2005) and lungfish (VOORHIES, 1975) and recommended either the establishment of a new ethological category (aestivichnia) or incorporation within the domichnia. VALLON et al. (2016) maintained the aestivichnia as a subcategory of domichnia because the tracemakers construct these structures for protection against temporary worsening of local environmental conditions (cf. RINDSBERG, 2012).
Some domichnial burrows may be inhabited by several individuals, not necessarily belonging to the same species (e.g. callianassid shrimps and innkeeper worms Urechis caupo; BROMLEY, 1996; FISHER & MACGINITIE, 1928). Because they are subsurface structures created sometimes at considerable depths below the sediment-water or sediment-air interface, they have a high preservational potential. But they also may be inhabited for very long periods, sometimes even several generations. Owing to this fact, the morphology of domichnia may vary during ontogeny of the tracemaker (FREY & SEILACHER, 1980). Burrows of the fiddler crab Uca pugnax are initially constructed as a simple shaft and only later expanded to a U-shaped burrow (BASAN & FREY, 1977). Other examples of burrow morphologies changing with time include increasing size of bioglyphs and burrow or boring diameter (BROMLEY, 1970, 1996), adding segments to a burrow system, and filling or closing off segments (SCHÄFER, 1972). Further modifications of domichnial morphology include reinforcement of the burrowʼs lining that also might be opened for expansion, repair, etc.
Several possibilities obtain where a spreite is present. Either sediment feeding is involved (marking a transition to fodinichnia), the tracemaker has adjusted its relative position to the sediment surface during erosion or sedimentation events (equilibrated domichnia) or enlargement of the burrow was needed with the growth of the tracemaker (e.g. in U-tubes). Similarities exist with fixichnia, especially in bioerosional domichnia.
ECDYSICHNIA (VALLON et al., 2015)
MOULTING TRACES
ABEL (1935) had already defined a category for all kinds of moulting. He saw traces of moulting behaviour (ecdysis) in the exuviae of arthropods, shed feathers, hatched eggs or cocoons, etc. As in the case of digestichnia, it must be emphasised that ecdysichnia consist of evidence of behaviour, not merely the exuviae or other body remains. Especially in arthropods, which apart from ʽwormsʼ are the most common tracemakers, ecdysis must be done on a regular basis to allow for growth. VALLON et al. (2015) revived ABELʼs idea, redefining the category of moulting traces and giving it the formal name ecdysichnia. Included are all traces left in or on any substrate by animals that are connected with moulting. This may include pupation (see GENISE et al., 2007 for the recognition of these traces), ecdysis in arthropods or shedding of the skin (e.g. a deer rubbing his newly grown antlers on a tree trunk creating scratches in the bark). However, in the fossil record, probably only insect pupation and arthropod ecdysis will be commonly preserved or recognisable (VALLON et al., 2015). A few detailed observations of fossil moulting traces exist (e.g. BISHOP, 1986; BRANDT, 2002; SEILACHER, 2007; TETLIE et al., 2008; VALLON et al., 2015). Trilobites pressed themselves into sticky mud to fix their old cuticula in one position, thus easing their exit (SEILACHER, 2007). Modern decapods tend to toss and turn to rid themselves of old cuticulae during ecdysis (VALLON et al., 2015). In contrast to trilobites, moulting traces of modern arthropods tend to be more complex and exhibit a greater variety of movements, making it difficult to erect ichnotaxa for these structures.
Pupation chambers within a substrate were summarised as pupichnia by GENISE et al. (2007), who showed how these structures could be distinguished from similar calichnia. This behaviour only applies to a small number of insects, because pupation in cocoons, chrysalides or puparia is much more common. Since pupation is a special case of ecdysis (ABEL, 1935), the pupichnia are best included as a subcategory of the ecdysichnia.
Moulting traces can either be superficial disturbances of the substrate, transitional to cubichnia, or burrow-like subsurface structures (e.g. pupichnia). When produced by arthropods, transitions may exist to other ethological categories. A fugichnion connected to an exuvia was reported by SCHWEIGERT & FRATTIGIANI (2004), but transitions to repichnia and cubichnia will probably turn out to be more common.
EQUILIBRICHNIA (FREY & PEMBERTON, 1985)
EQUILIBRIUM TRACES
Equilibrichna are burrows of all kinds that have to be vertically moved by the tracemakers (FREY & PEMBERTON, 1985) in order to re-establish the necessary functions of the burrow (e.g. ventilation) despite sediment accumulation or erosion. Shifting of the burrow results in a spreite. In case of sediment erosion, a protrusive spreite is produced and when sediment is accumulated a retrusive one. As in fugichnia, the tracemaker may have to adjust because of burial, but in this case has more time to shift the depth of its burrow; and equilibrichnia also include adjustments to the more or less continuous changes in sediment surface level. The resulting traces therefore have clear boundaries, unlike fugichnia. When sediment accumulation is fast, however, the transition to fugichnia is smooth and the traces become less distinct.
Many animals were evidently unable to equilibrate and produced no spreite, e.g. the tracemakers of Arthraria in contrast to Diplocraterion. Thus, perhaps other ichnospecies of Diplocraterion than D. yoyo could be considered as equilibration traces.
BROMLEY (1996) agreed that equilibrichnia constitutes a distinct category of behaviour. The relatively high number of publications using the term probably reflects the ease of recognition of such traces, and their utility in deducing ancient conditions of sedimentation. However, VALLON et al. (2016) recommended to discontinue th usage of this category. The reason why these traces are made is clearly not adjustment to persistent erosion or deposition, but dwelling, or in some cases feeding. Equilibrated domichnia or equilibrated fodinichnia (e.g. Rosselia socialis; NARA, 2002), respectively, would serve as better terms for these structures. If a bivalve resting in its Lockeia is buried, then it might create an indistinct fugichnion atop its cubichnion. But if it responds in a more deliberate fashion, then it creates an extended Lockeia (equilibrated cubichnion). The equilibration trace would be created using the same muscular actions that made the original Lockeia.
FAECICHNIA (GEYER, 1973)
DIGESTION TRACES
Faecichnia almost fits the definition of digestichnia (VALLON, 2012), but GEYER (1973) only included real faeces and regurgitalites, not other traces of digestion such as scratches on gastroliths or intestinally preserved faecal material. Digestichnia in the sense of VIALOV (1972) and VALLON (2012) therefore is preferred.
FIXICHNIA (DE GIBERT et al., 2004)
ATTACHMENT TRACES
Fixichnia (DE GIBERT et al., 2004) represent a special behaviour on hard substrates. These attachment traces bind the epilithic tracemaker at an early ontogenetic stage to a fixed anchor point where they live for the rest of their lives. Therefore, the produced structures are closely related behaviourally to the domichnia (BROMLEY, 1992; EKDALE et al., 1984; MARTINELL, 1989). Many makers of domichnia apart from bioeroders, however, can leave their dwelling structures and produce a new structure somewhere else. Thus, fixichnial morphology is completely different from domichnial morphology, and very special adaptations are necessary to produce these traces. In most attachment traces, the substrate is mechanically or chemically roughened so the tracemaker has a better grip on the surface. Anchoring is achieved by soft or hard body parts (exoskeleton). They are usually surface structures, preserved as shallow negative epichnia. Morphologies range from concentric furrows (Centrichnus) via radially arranged tiny pits (Podichnus) to kidney-shaped (Renichnus), star-shaped (Stellichnus) or serpentiform furrows (made by Recent polychaetes on pebble surfaces, see VALLON et al. (2016).
FODINICHNIA (SEILACHER, 1953a); incl. Xylichnia (GENISE, 1995)
FEEDING TRACES
SEILACHER (1953a) incorporated within this category mainly burrows (and subsequently borings) that are constructed by deposit-feeders while ingesting the substrate. Usually, a dwelling structure that is left open during the lifetime of the tracemaker is combined with a far more extensive feeding structure that may be open or progressively filled (spreite). Burrows may be simple (Planolites, Scoyenia, Taenidium), branched (some Arthrophycus), U-shaped (Rhizocorallium), radial (Dactyloidites, Gyrophyllites) or complex (Treptichnus) with various orientations. A spreite originates by shifting the open part of the burrow (lumen) to one side while depositing the excavated material on the other after extracting its nutrition. The traces are most commonly preserved as full-relief endichnia.
Close relations exist with domichnia and pascichnia as well as agrichnia. Depending on preservation and interpretation, these structures are sometimes difficult to place into only one of the mentioned categories.
KELLY & BROMLEY (1984), GENISE (1995) and BERTLING et al. (2006) amongst others regarded substrate selection as an important ichnotaxobase. Thus, GENISE (1995) introduced xylichnia for wood borings (e.g. Teredolites) as a subcategory of fodinichnia. VALLON et al. (2016) agreed with this placement because substrate feeding has to be regarded as a more important behaviour than the selection of any particular substrate.
FUGICHNIA (FREY, 1973)
ESCAPE TRACES
Escape structures are produced by tracemakers that have been buried by sudden sediment accumulations (FREY, 1973). They are temporary structures, producing only disturbed sedimentary lamination and therefore are not originally open or reinforced, but instead show undefined boundaries. Owing to the panic reactions of the tracemakers during sudden burial, the traces usually are vertical to oblique and show vertical repetition. In case of originally simple domichnia like Skolithos or Diplocraterion, cone-in-cone or U-in-U structures are produced, respectively (BUATOIS & MÁNGANO, 2011). In addition to these vertical escape traces, a few horizontal examples have been documented from Recent studies where the tracemakers were trying to escape infaunal predators (BEHRENDS & MICHAELIS, 1977; BROMLEY, 1996). In the fossil record, these will not be recognisable, so VALLON et al. (2016) restricted fugichnia to traces produced during escape from burial, as FREY (1973) originally intended. Fugichnia are generally preserved as full-relief endichnia.
Transitions may exist with ecdysichnia (VALLON et al., 2016) and repichnia. The category ʽtaphichniaʼ (PEMBERTON et al., 1992) was defined as traces of unsuccessful attempts to escape burial. VALLON et al. (2016) regarded this category as an unnecessary subset of fugichnia, because the reactive behaviour reflected by taphichnia is the same as in fugichnia. Whether the attempted escape from being buried alive is successful is irrelevant, as escape traces are produced in either case. Additionally, the recognition of taphichnia depends in practice on the tracemaker having hard parts. Soft-bodied tracemakers are not ordinarily preserved, and in such cases, the trace will look like a ʽlucky-escapeʼ fugichnion, when in fact it was a tragedy.
GRADICHNIA (GEYER, 1973)
ʽWALKINGʼ TRACES
This category equals MÜLLER’s (1962) cursichnia and is defined as tracks and trackways. As a special form of locomotion, this category fits into repichnia and is therefore redundant.
IMPEDICHNIA (TAPANILA, 2005)
EMBEDMENT TRACES
Impedichnia (TAPANILA, 2005) was introduced as category for ʽsymbioticʼ bioclaustration, with symbiosis being used in its broad sense to include antagonistic as well as mutually beneficial interactions. This category is at first glance very closely related to domichnia, especially resembling the ones produced by bioeroders. However, neither the host nor the infesting organism actively manipulates the substrate, in contrast to bioeroders. Cases where the embedded organism uses chemicals, appendages etc. locally to prevent tissue growth of the surrounding host organism could be viewed as a special type of bioerosion. Embedment structures, whether produced by a symbiont, commensal or parasite, or to accommodate one, were excluded by BERTLING et al. (2006) from trace fossils along with plant reaction tissues (e.g. plant galls induced by wasps) and skin infections or rashes caused by micro-organisms. VALLON et al. (2016) therefore did not recommend the use of impedichnia, though they recognised the need for nonichnologic names to accommodate these structures. VALLON et al. (2016) recommend that the term impedichnia be replaced by impeditaxa, a neutral term specifically for bioclaustrating taxa that would encourage research while not including –ichnia.
INSOLIDICHNIA (GEYER, 1973)
BIOEROSION TRACES
This category was never recognised or accepted by the ichnological community. GEYER (1973) defined it as any traces in hard substrates, but also qualified that these very often are domichnia and should then be named domichnia. This category, together with the xylichnia and equilibrichnia therefore share a common faith: The most important reason is why the trace was made, not where. This category therefore should not be used!
IRRETICHNIA (LEHANE & EKDALE, 2013)
TRAPPING TRACES
Trapping prey for food is a special case of feeding. VALLON et al. (2016) therefore recommended to place the irretichnia as a subcategory into the group praedichnia. For the trapping of prey external resources need to be employed. These can be pitfalls in a substrate, sticky materials such as spider webs or any other activity where the predator waits and ensnares its prey (LEHANE & EKDALE, 2013). Activity where the predator is actively searching, hunting and subduing its prey will generate traces that are praedichnia in the strict sense. Irretichnia are usually preserved as positive hypichnia, negative epichnia or as full-relief endichnia.
MORDICHNIA (MÜLLER, 1962)
BITING and GNAWING TRACES
MÜLLER (1962) combined feeding traces produced by biting, rasping or gnawing under his category mordichnia. As producers he mainly saw vertebrate, but also acknowledged that some invertebrates may be responsible for these traces, e.g. arthropods cutting into gastropod shells, gastropods rasping off algae from hardgrounds etc. The category was regarded unnecessary by VALLON et al. (2016) as feeding is more important as the way how it is done. Traces are better grouped in the categories praedichnia and pascichnia.
MORTICHNIA (SEILACHER, 2007)
DEATH STRUGGLE TRACES
Mortichnia was proposed as category for traces left by death struggles in the lithographic limestones of Solnhofen (Upper Jurassic, southern Germany) by SEILACHER (2007) (previously called taphoglyphs; SARJEANT, 1975; not to be confused with taphichnia). Any tracemaker can be stricken by a sudden death threat, e.g. predation by other animals, exposure to hostile living conditions, disease, etc., although the fossil record will probably yield few traces that are recognisable as mortichnia. This category is to some extent more interpretive than others whose ethology can be directly read from morphological evidence. Seilacher based this new category on holistic interpretation rather than on trace fossil morphology. VALLON et al. (2015) showed that many Solnhofen mortichnia are not ʽdeath marchesʼ, because the body fossils at the end of the trackways commonly are exuviae rather than corpses and reinterpreted them as ecdysichnia. Genuine mortichnia are rare (e.g. Telsonichnus, the spiral or looped trails produced by Solemya from the Lithographic Limestones of Solnhofen and some further examples recently reported by SCHWEIGERT et al., 2016 and NETO DE CARVALHO et al., 2016). Without the terminal corpses (owing to incomplete preservation), mortichnia, like taphichnia, are not recognisable. Out of the above reasons, VALLON et al. (2016) did not recommend the use of this category.
MOVICHNIA (MÜLLER, 1962)
MOVING TRACES
Supercategory proposed by MÜLLER (1962) for traces including natichnia, cursichnia and repichnia as well as volichnia (sensu MÜLLER, 1962). VALLON et al. (2016) reconstructed MÜLLER’s movichnia but called it by the more familiar term repichnia.
NATICHNIA (MÜLLER, 1962)
SWIMMING TRACES
Traces produced by swimming animals touching the sediment surface with their fins, tails or other extremities. These traces are usually preserved as positive hyporelief or rarer as negative epirelief. Natichnia (MÜLLER, 1962) were reintroduced by VALLON et al. (2016) as a subcategory for repichnia whith which they were combined by e.g. BROMLEY (1996). Best know example is Undichna, a fish-produced swimming trace fossil.
NAVICHNIA (GINGRAS et al., 2007)
WITHIN-SUBSTRATE SWIMMING TRACES
Animals moving through soupgrounds do not produce burrows. Instead of shafts and tunnels, the consistency of the substrate only allows for a preservation of disrupted lamination. This type of locomotion traces was named navichnia by GINGRAS et al. (2007). VALLON et al. (2016) recommended placing these traces as a subcategory of repichnia.
PASCICHNIA (SEILACHER, 1953a)
GRAZING TRACES
When a particular area of substrate has to be efficiently exploited for food, spiral or meandering trails or shallow burrows are produced. Feeding and locomotion happen at the same time. Certain sections often touch previously made parts of the same trace, so the largest possible area is covered. Crossings or reworking of previous trace-sections by the same or even different individuals of the same tracemaker-species are almost always avoided. Defined by SEILACHER (1953a) as surface to near-surface traces, most of his original and subsequent examples, have since been proved to be subsurface structures produced by epifaunal detritus- or infaunal deposit-feeders, respectively (RINDSBERG, 2012); accordingly, the term is redefined here to fit usage during the past sixty years. Modern ecologists distinguish surface and subsurface feeding (e.g. LOPEZ & LEVINTON, 1987) because specific life strategies and adaptions are required for each feeding method. In the fossil record, this differentiation is hardly achievable because surface traces have such a low preservation potential compared to subsurface traces. Additionally, we must ask according to TINBERGEN (1963) “What was the function of the traces?” and not “How was it produced?”. Because the overall morphology of the traces is similar despite their different position with regard to the substrate surface, and all were made in pursuit of surficial or near-surficial detritus, distinction between surface and shallow subsurface traces seems unnecessary and artificial. If the distinction would be meaningless to the organisms that made the traces, then why should ichnologists insist on it?
Pascichnia are usually preserved in positive hyporelief, less commonly in negative epirelief. The usually meandering (Nereites) or spiral (Spirorhaphe) course of many such traces points to the exploitation of a food source. The traces are generally constructed parallel to the sea floor. They are unbranched and more regular than repichnia. Simpler forms (curved to looped structures) may cross each other, but traces with a strict pattern and more complex morphologies (spirals and meanders) do usually not. The latter pascichnia include guided meanders in which the boundary of the newer part of the constructed trace touches the previously produced part (thigmotaxis). In most cases, pascichnia are continuous trails, though tracemakers that can lift their feeding organ or entire body off the substrate may produce discontinuous feeding traces. The above-mentioned examples are subsurface grazing traces. Genuinely surficial pascichnia are rare in the fossil record, and few have been named. The clearest examples are bioerosional grazing traces such as Radulichnus and Gnathichnus.
MÜLLER (1982, 1989) classified and characterised several insect and insect larval feeding traces in leaves, below the bark or within the wood and placed them within the fodinichnia. VALLON et al. (2016) agreed with this placement, but this was a lapsus. These insect traces belong within the pascichnia because their food source is exploited systematically in highly patterned forms and the resulting traces are parallel to the substrate surface.
Boundaries between repichnia (in genuine surface traces), fodinichnia (in subsurface traces) and agrichnia (in very regular examples) may be hard to draw in specific cases.
POLYCHRESICHNIA (HASIOTIS, 2003)
MULTIFUNCTIONAL TRACES
Polychresichnia (HASIOTIS, 2003) encompass structures made by social insects. These complex and frequently large structures are multifunctional, representing different kinds of behaviour simultaneously. VALLON et al. (2016) agreed with BUTOIS & MÁNGANO (2011) that a discrete ethological category for such multipurpose traces is redundant. Most trace fossils reflect more than one activity or behaviour, and transitions between or overlapping of categories are rather the rule than the exception. Probably most specimens ascribed to polychresichnia could be accommodated at least in part within the calichnia.
PRAEDICHNIA (EKDALE, 1985); incl. Mordichnia (MÜLLER, 1962) and Irretichnia (LEHANE & EKDALE, 2013)
PREDATION TRACES
Traces of predation (EKDALE, 1985) show interactions between a predator and its prey. Often the hard body parts of the prey carry the traces of predation, such as round drillholes (Oichnus) and chipped margins in shells or gnawing and biting traces on bones (named ʽmordichniaʼ by MÜLLER, 1962). Most praedichnia are therefore bioerosional structures (BROMLEY, 1981, 1993). Soft-substrate predation causing only indistinct sedimentary disturbances, might not be recognised as praedichnia, but rather be interpreted as endogenic repichnia or fugichnia. A few examples from soft substrates have been documented, e.g. trilobites preying on ʽwormsʼ (e.g. BERGSTRÖM, 1973; JENSEN, 1990) and fish preying on unidentified invertebrates (Osculichnus; DEMIRCAN & UCHMAN, 2010).
It can be problematic to distinguish whether gnawing and scratching traces on bones were produced during a predatorʼs attack or long after the death of the ʽpreyʼ. VALLON et al. (2016) have therefore included traces resulting from scavenging within praedichnia despite the fact that ecologists draw a distinction between scavenging and predatory behaviours.
Most predation traces are preserved in negative epirelief, but positive hyporelief may occur. Transitions exist towards repichnia for both predators and prey. Prey can be attacked by predators while occupied in all kinds of behaviours, so all traces of other categories may abruptly end in a praedichnion. As for traces left by the predators, they might be mistaken as repichnia or cubichnia.
Concerning irretichnia, VALLON et al. (2016) agreed with LEHANE & EKDALE (2013) that trapping prey and farming are distinct behaviours that should not in principle be combined under the category agrichnia. In contrast to regular preying, trapping involves the use of external resources, such as pitfalls or sticky substrates. However, only a tiny fraction of trapping traces can be recognised in the fossil record, and, more to the point, we should ask why the organism made the trace. Because ultimately the function of the trace was predation, VALLON et al. (2016) recognised irretichnia as a subcategory of praedichnia. Morphologies therefore comprise conical depressions in loose sediment, open pits and physical snares produced of a sticky substrate such as silk in spider webs. Single traces usually are very regularly spaced with no overlapping. While regarding irretichnia as a subcategory of praedichnia, VALLON et al. (2016) recognised that the complex irretichnia seen in deep-sea environments may be extraordinarily difficult to interpret as such and may easily be confused with agrichnia.
PUPICHNIA (GENISE et al., 2007)
PUPATION TRACES
GENISE et al. (2007) defined and established the pupichnia. They are characterized by their passive fillings or in the case of (sub-)recent pupation chambers the lack of it. Chambers are usually constructed from the inside, resulting in walls that are completely smooth on the inside, but rough or pelleted on the outside (GENISE et al., 2007). In pupichnia, the structures are produced by the same animal in different ontogenetic stages (in contrast to calichnia where one or more adults are producing the structure for their offspring). Pupichnia were regarded as subcategory of ecdysichnia by VALLON et al. (2016).
QUIETICHNIA (MÜLLER, 1962)
RESTING TRACES
Supercategory proposed by MÜLLER (1962) for traces including domichnia and cubichnia. Like cibichnia, this supercategory never really was accepted, because it does not yield any ethological information and thus VALLON et al. (2016) recommended not to use this category.
SPHENOICHNIA (MIKULÁŠ, 1999)
PLANTROOT PENETRATION TRACES
As another trace fossil category attributed to plants, the sphenoichnia MIKULÁŠ (1999) are the most common of all plant trace fossils both in recent and in fossil examples. The traces are generated by the usually slow growth of roots and rootlets and their penetration and therefore manipulation of the substrate which results in soil clasts of various sizes between the roots. Because they are produced by the roots, these traces are constructed in order to provide water and nutritions for the plant while simultaneously anchoring it within the ground (MIKULÁŠ, 1999).
REPCIHNIA (SEILACHER, 1953a); incl. Gradichnia (GEYER, 1973), Natichnia (MÜLLER, 1962, 1989), Navichnia (GINGRAS et al., 2007), Cursichnia (MÜLLER, 1962), Volichnia (sensu MÜLLER, 1962)
LOCOMOTION TRACES
SEILACHER (1953a) introduced this category for ʽcrawlingʼ traces. However, repichnia are understood in a broader sense today and comprise all traces produced during locomotion (BROMLEY, 1996). They are generally simple and shallow and can be either continuous or interrupted. However, they mainly reflect directed locomotion and are not visibly combined with other behaviour (in contrast e.g. to pascichnia). Locomotion traces follow bedding planes and are mainly preserved as positive hypichnia or negative epichnia.
Continuous disturbances of sediment with more or less parallel sides are called trails (=ʽrepichniaʼ sensu SEILACHER, 1953a; sensu MÜLLER, 1962) and are made by tracemakers without appendages. They may be simple (e.g. Mermia), bilobate in cross section (e.g. some Cruziana) or show a chevron-like morphology (e.g. some Protovirgularia); rarely, they may be meniscate or annulate. Trackways are made up of repeated sets of discontinuous impressions reflecting the tracemakersʼ appendages and their motion within the substrate (= the unfortunately named, at least for the English-speaking audience, ʽcursichniaʼ MÜLLER, 1962; e.g. vertebrate trackway Chirotherium, arthropod trackway Diplichnites). Repichnia produced by evolutionarily transitional animals with reduced limbs (e.g. some anguid lizards) have not been named separately.
Natichnia (MÜLLER, 1962, 1989) are produced by tracemakers swimming close to the sediment surface (e.g. Undichna) or hovering just above it. Swimming traces were included in repichnia by BROMLEY (1990, 1996). Diffuse bioturbation of tracemakers swimming through a soupground (ʽnavichniaʼ of GINGRAS et al., 2007) should therefore be included in the repichnia as well (cf. WETZEL & UCHMAN, 1998).
MÜLLER (1962) included natichnia, cursichnia and repichnia as well as volichnia (sensu MÜLLER, 1962) within his movichnia or movement traces. Only very few volichnia (sensu MÜLLER, 1962) where the tracemakersʼ wings made impressions while flying just above the ground are documented, despite being fairly common in the Recent (MARTIN, 2013). In essence, VALLON et al. (2016) have reconstructed MÜLLERʼs movichnia but called it by the more familiar term repichnia. Although his system received wide attention as a figure reprinted by OSGOOD (1970) and HÄNTZSCHEL (1975), MÜLLERʼs original work, published in the German Democratic Republic, remained unread by most ichnologists outside Germany.
Most transitions from repichnia are to cubichnia and when feeding is involved also to pascichnia. Rarely, they can end in praedichnia or ecdysichnia.
VOLICHNIA (sensu MÜLLER, 1962)
FLYING TRACES
Animals, especially insects and birds, flying on very low height above a very plastic substrate may create flying traces. Then, their wings or other appendages may touch the ground creating a trace similar to a repichnion, but all in all more irregular. This irregularity is caused by subtle differences in flying height and perhaps the changing of the closeness of the appendages to the body core, so not all appendages or wings are touching the substrate at the same time. These traces usually are preserved as positive hypichnia or negative epichnia. VALLON et al. (2016) agreed with BROMLEY (1996) to incorporate them into repichnia.
VOLICHNIA (sensu WALTER, 1978)
LANDING- and TAKE-OFF TRACES
In addition to the original definition of volichnia by MÜLLER (1962), WALTER (1978) proposed that volichnia also should include landing and take-off traces of flying animals. This view was later extended and take-off and landing traces of swimming organisms were added (e.g. VALLON et al., 2016), mainly because there is no morphological difference between take-off and landing traces of flying or swimming tracemakers.
XYLICHNIA (GENISE, 1995)
TRACES IN WOOD
KELLY & BROMLEY (1984), GENISE (1995) and BERTLING et al. (2006) amongst others regarded substrate selection as an important ichnotaxobase. Thus, GENISE (1995) introduced xylichnia for wood borings (e.g. Teredolites) as a subcategory of fodinichnia. VALLON et al. (2016) agreed with this placement because substrate feeding has to be regarded as a more important behaviour than the selection of any particular substrate.
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