High Ichnodiversity Research Articles

Trace fossil zonation in interdune Lakes: example from the neogene of Eastern Patagonia, Argentina

The Upper Miocene-Lower Pliocene Río Negro Formation (Río Negro Province, Argentina) contains diverse and abundant continental trace fossils. The purpose of this contribution is to recognize distinctive trace fossil assemblages in interdune lakes as response to lake expansion and contraction. The interdune deposits host 23 invertebrate, vertebrate, and plant ichnotaxa that are linked to the hydrological dynamics of the lake. During lake expansion a highly diverse subaqueous trace fossil assemblage reflects an infaunal bivalve population living in a littoral zone with swampy vegetation and semiaquatic mammals. The trace fossil assemblage recorded during Lake Highstand is less diverse and mostly linked to bivalve-generated trace fossils reflecting resting, locomotion, and escape/equilibrium behaviour (comparable to the Mermia ichnofacies). The assemblage during lake contraction exhibits high ichnodiversity and is dominated by vertebrate trace fossils reflecting foraging activities of shorebirds, cursorial birds, and herding semiaquatic mammals and ground sloths frequenting water holes. Flat sandy areas with moist substrates marginal to the interdune lake displayed a moderate ichnodiversity and its distinctive components were armadillo meniscate burrows and footprints in cross-section, as well as subvertical calcareous root casts. Neogene interdune lacustrine deposits reflect a particular zonation of trace fossils in response to changing physical conditions.

New mammal and bird tracks from the Lower Oligocene of the Ebro Basin (NE Spain): implications for the Palaeogene ichnological record

ABSTRACT The Ebro Basin (Spain) is one of the few worldwide areas where Palaeogene avian and mammal tracksites have been found. A new unpublished tracksite known as La Sagarreta is here described. The tracksite is located in a sandstone-dominated outcrop from the Early Oligocene Peralta Formation in the northern-central sector of the basin. Six different ichnotaxa have been identified. Four belong to mammals, including perissodactyl (cf. Plagiolophustipus isp), artiodactyl (Megapecoripeda isp. and cf. Pecoripeda isp.) and carnivoramorph tracks (cf. Canipeda isp.), and the other two to avian tracks (Aviadactyla vialovi and Gruipeda dominguensis). La Sagarreta presents a high ichnodiversity with the presence of a medium-sized member likely of the genus Plagiolophus (or relative), one medium- and one small-sized artiodactyl that probably belong to the family Entelodontidae, one medium-size unidentified carnivorous mammal and two types of birds. La Sagarreta tracksite is the most diverse Early Oligocene tracksite of the Ebro Basin and one of the few palaeontologic vertebrate records at the central area of the basin during this age.

Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran

Grazing trails found in the rhythmite beds attributed to the Ediacaran Tacuarí Formation (northern Uruguay) had been identified as the oldest evidence of bilaterians in the world. However, a diverse and abundant invertebrate, mostly arthropod, trace fossil assemblage is found associated with Upper Paleozoic palynomorphs in these same beds at the original localities from where the oldest bilaterian burrows were reported. These ichnoassemblages include Arborichnus repetita, Crescentichnus tesiltus, Cruziana problematica, Diplichnites aff. gouldi, Diplopodichnus biformis, Gluckstadtella cooperi, Helminthoidichnites tenuis, Kingella aff. natalensis, Maculichna varia, Rusophycus isp., Treptichnus pollardi, and Umfolozia sinuosa. The palynological assemblages yielded 89 species composed of 33 spores, 49 pollen grains, four chlorophycean algae, a fungal spore, an acritarch, and an indeterminate species. These occurrences point to a Phanerozoic record. The dominant botanical groups recorded are Lycophyta, Cordaitean, Coniferalean, Glossopteridalean, Corystospermaceae/ Peltaspermaceae, Pteridophyta, algal (Botryococcus, Tetraporina, Brazilea, Quadrisporites), and other groups (Deusilites tenuistriatus, Portalites gondwanensis). The high ichnodiversity, the prevalence of unquestionable arthropod biogenic structures, along with the abundance of ichnotaxa suggests a Carboniferous–Permian age. Therefore, the ichnoassemblages indicate that these strata are not Ediacaran in age and belong to the San Gregorio Formation, which represents the Upper Paleozoic Gondwanan glacial deposits in Uruguay. The ichnofauna is equivalent to others that are well known in deposits of the Late Paleozoic Ice Age from Brazil, Argentina, and South Africa. Furthermore, the new palynologic data from the trace-fossil bearing strata show that they can be correlated to the latest Carboniferous–early Cisuralian Cristatisporites inconstans-Vittatina saccata (IS) Zone of Paraná Basin in Uruguay.

Ichnology and Sedimentology of the Lower Cretaceous Wabiskaw Member (Clearwater Formation) Alberta, Canada

Identifying facies and paleoenvironmental interpretation of the rock record is complicated due to the interplay of independent variables (i.e. wave-, river- and tidal-input) in marginal- and shallow-marine environments. This study integrates sedimentology and ichnology to characterize the depositional environments of the Lower Cretaceous Wabiskaw Member. The deltaic and strand plain shoreface-offshore deposits of Wabiskaw Member of Clearwater Formation of northeast Alberta provide an excellent case study to evaluate the ichnological and sedimentological characteristics associated with laterally and vertically variable stresses in wave-influenced nearshore settings. The Wabiskaw Member is studied using four sedimentary facies associations (Ten sedimentary facies). Facies association 1(FA-1) consists of massive, sharp-based mudstones and lenticular sandstone with small-scale soft-sediment deformation features, syneresis cracks and small-scale oscillation ripples. The ichnological suite mainly includes small grazing and deposit-feeding traces, representing the Phycosiphon Ichnofacies. Facies association 1(FA-1) is interpreted to record deposition in a prodeltaic setting. Facies association 2 (FA-2) is fine-to-medium-grained sandstone and thin mudstone with current, oscillation and combined flow ripples. The low angle and trough cross-beds, soft-sediment deformation structure, mud chips and coal debris are common. The trace fossil suite includes sporadically distributed deposit-feeding traces belonging to the Rosselia Ichnofacies. This facies association is interpreted to be deposited in the delta front to a mouth bar setting. Facies association 3 (FA-3) is gray to bluish-gray mudstone with minor sandstone interbeds (massive, planar parallel to hummocky cross-stratified). FA-3 is moderate to highly bioturbated. The ichnodiversity and abundance is high with dominant horizontal and subordinate vertical traces, including the dominance of deposit and detritus feeding traces representing the archetypal Cruziana Ichnofacies ichnological suite. Facies association 3 (FA-3) is interpreted as being deposited in a storm-influenced offshore environment. Facies association 4 (FA-4) is upper fine to fine-grained glauconitic sandstone with minor mudstone interbeds at the base. Physical sedimentary structures including massive and planar (horizontal and low angle), hummocky, and swaley crossing-stratification. FA-4 is heavily burrowed with a diverse suite of horizontal and vertical traces of deposit and detritus-feeding structures, representing a proximal expression of Cruziana Ichnofacies. Facies association 4 (FA-4) represents distal lower to middle shoreface deposition. The Wabiskaw Member deltaic (prodelta-delta front) deposits, producing characteristic sedimentological-ichnological signatures, distinguishing these deposits from related strand plains. In addition to characteristic deltaic traits (e.g. soft-sediment deformation structures, fluid mud and syneresis cracks), these deposits commonly contain comparably stressed low-diversity and diminutive trace-fossil suites. On the other hand, fully marine strand plain shoreface-offshore complexes are characterized by comparably robust trace fossils with high ichno-diversity and bioturbation intensities.

Secular variations of ichnofossils from the terrestrial Late Permian–Middle Triassic succession at the Shichuanhe section in Shaanxi Province, North China

Abstract Ichnological studies are exceptionally important for reconstruction of ecosystems in the aftermath of mass extinctions, but they have only rarely been undertaken from strata deposited in continental settings during the Permian–Triassic transition. This paper systematically documents seventeen ichnogenera and three ichno-types coupled with plant fossils (mainly equisetaceous stems and roots) from the fluvial–lacustrine Late Permian to Middle Triassic Shichuanhe section in North China. These ichnofossils constitute several ichnoassemblages, including resting traces ( Lockeia ), locomotion traces ( Diplichnites , Kouphichnium , Protovirgularia ), dwelling traces ( Arenicolites , Camborygma , Macanopsis , Palaeophycus , Skolithos ), grazing traces ( Gordia , Helminthoidichnites , Monomorphichnus ) and feeding traces ( Planolites , Scoyenia , Spongeliomorpha , Taenidium ) and some of them were scarcely reported in the coeval sediments of North China. The ichnological criteria, such as ichnodiversity, intensity of bioturbation, and burrow size and depth, exhibit a significant reduction in the latest Permian rocks, accompanied by simplification of trace maker ethology. These signals initially rebound in the early Early Triassic, when the behavioral generalists opportunistically occupied the empty niches and constructed simple but deep Skolithos . A full reconstruction of continental ecosystems should take place in the late Early Triassic to Middle Triassic, with high ichnodiversity, intensive bioturbation and behavior complexity of trace producers. The small, simple but deep burrows occurring in the early Early Triassic would be the ecological responses of the animals to the lethal heat in the terrestrial facies.

Tiering Style and Its Interpretation: Ichnofabric Study in Balikpapan Formation, Kutai Basin, Indonesia

The determination of tiering style is an important task in ichnology. Tiering styles can be modeled by using ichnodiversity, the number of behaviors and number of tiers and their relationships. In this study, this approach was applied in the area of Samarinda, Kutai Basin, Indonesia. The values of ichnodiversity, number of behaviors, and number of tiers were identified. Ichnodiversity was strongly correlated with number of behaviors but weakly correlated with number of tiers. Accordingly, three tiering styles were identified. Typically, tiering styles A, B, and C demonstrated high ichnodiversity, number of behaviors and number of tiers. In general, the colonization window that coincides with the environmental conditions characteristic for deltaic settings is the factor that mostly regulates these tiering styles.

A diverse trackway-dominated marine ichnoassemblage from the Lower Triassic in the northern Paleotethys: Ichnology and implications for biotic recovery

We document a diverse ichnoassemblage from marine interbeds of the Lower Triassic terrestrial succession in the Houzhougongmiao (HZGM) section of Shaanxi Province, northwestern China. Integrated biostratigraphic data (bivalve, palynology and conchostracan) reveals that the ichnofossil-bearing marine beds are Griesbachian (Induan, Early Triassic) in age. The marine interbeds are interpreted to be the result of the earliest Triassic transgression of the Paleotethys Ocean northward onto the southern margin of the North China Craton. The HZGM ichnoassemblage includes 17 ichnospecies in 16 ichnogenera and is dominated by shallow-tier Asteriacites and Biformites produced by ophiuroids, the scratch marks or trackways Dimorphichnus, Diplichnites, and Monomorphichnus produced by arthropods, and a rare occurrence of the fish swimming trace Undichna. Of these, the hook-shaped imprints Biformites, representing the moving arm impressions of ophiuroids, are reported for the first time from the Lower Triassic. These trace-makers are interpreted to have lived in a low energy, semi-restricted, shallow embayment environment. Although possessing relatively high ichnodiversity, the HZGM ichnoassemblage differs clearly from other coeval diverse ichnocoenoses in the lack of complex burrow forms (i.e. Thalassinoides, Diplocraterion, or Rhizocorallium) and a mixed layer, and in having abundant shallow tiers. In addition, high ichnodiversity may occur at both high and low latitudes during the Griesbachian (or Induan). Accordingly, the resting trace or trackway-dominated ichnoassemblage from the HZGM section may represent the initial recovery of the trace-maker ecosystem after the Permian-Triassic mass extinction. Trace-makers such as arthropods and ophiuroids were perhaps opportunistic organisms that proliferated in marginal marine settings when other biota were still suffering post-extinction biotic depletion and environmental stresses.

Early Middle Triassic trace fossils from the Luoping Biota, southwestern China: Evidence of recovery from mass extinction

Trace fossils have proven useful for studying the timing and process of biotic recovery after the Permian–Triassic Mass Extinction (PTME). Recovery stages are defined by comparing successive ichnoassemblages from the latest Permian to the early Middle Triassic. Lower Triassic trace fossils have been explored in some detail, but those of the lower Middle Triassic are less well known. Here, well-preserved fossil materials from the Luoping Biota from Yunnan Province, South China suggest that a fully recovered shallow-marine ecosystem was re-established by the early Middle Triassic. Trace fossil assemblages of the Luoping Biota are characterized by high ichnodiversity, with 14 ichnogenera in the shallow-marine environment of an intra-carbonate platform basin, and nine ichnogenera in the subtidal environment. Such moderate to high ichnodiversity, together with a marked increase in burrow sizes and the common occurrence of key ichnotaxa (e.g. Rhizocorallium and Thalassinoides), suggests that the ichnofauna had reached recovery stage four. In contrast, non-turbiditic strata of the offshore setting record only three ichnogenera, with bioturbation indices never exceeding one. Periodic anoxia in bottom waters was presumably the main control for such a protracted trace fossil recovery in an offshore setting, which otherwise aided the fine preservation of body fossils of the Luoping Biota. Furthermore, event sedimentation (turbidite deposits) in the offshore setting incorporated moderate ichnodiversity and moderate to high bioturbation indices, both interpreted as a result of short-term colonization by transported infaunal animals from proximal settings. The occurrence of variable crustacean traces (e.g. Sinusichnus, Spongeliomorpha, and Thalassinoides) at Luoping and the locomotion traces of marine reptiles, together with abundant fishes and fossil decapods, highlights the value of trace fossils in ecosystem reconstruction after the PTME.

A reappraisal of the Middle Triassic chirotheriid Chirotherium ibericus Navás, 1906 (Iberian Range NE Spain), with comments on the Triassic tetrapod track biochronology of the Iberian Peninsula.

Triassic vertebrate tracks are known from the beginning of the 19th century and have a worldwide distribution. Several Triassic track ichnoassemblages and ichnotaxa have a restricted stratigraphic range and are useful in biochronology and biostratigraphy. The record of Triassic tracks in the Iberian Peninsula has gone almost unnoticed although more than 25 localities have been described since 1897. In one of these localities, the naturalist Longinos Navás described the ichnotaxon Chirotherium ibericus in 1906.The vertebrate tracks are in two sandy slabs from the Anisian (Middle Triassic) of the Moncayo massif (Zaragoza, Spain). In a recent revision, new, previously undescribed vertebrate tracks have been identified. The tracks considered to be C. ibericus as well as other tracks with the same morphology from both slabs have been classified as Chirotherium barthii. The rest of the tracks have been assigned to Chirotheriidae indet., Rhynchosauroides isp. and undetermined material. This new identification of C. barthii at the Navás site adds new data to the Iberian record of this ichnotaxon, which is characterized by the small size of the tracks when compared with the main occurrences of this ichnotaxon elsewhere. As at the Navás tracksite, the Anisian C. barthii-Rhynchosauroides ichnoassemblage has been found in other coeval localities in Iberia and worldwide. This ichnoassemblage belongs to the upper Olenekian-lower Anisian interval according to previous biochronological proposals. Analysis of the Triassic Iberian record of tetrapod tracks is uneven in terms of abundance over time. From the earliest Triassic to the latest Lower Triassic the record is very scarce, with Rhynchosauroides being the only known ichnotaxon. Rhynchosauroides covers a wide temporal range and gives poor information for biochronology. The record from the uppermost Lower Triassic to the Middle Triassic is abundant. The highest ichnodiversity has been reported for the Anisian with an assemblage composed of Dicynodontipus, Procolophonichnium, Rhynchosauroides, Rotodactylus, Chirotherium, Isochirotherium, Coelurosaurichnus and Paratrisauropus. The Iberian track record from the Anisian is coherent with the global biochronology proposed for Triassic tetrapod tracks. Nevertheless, the scarcity of track occurrences during the late Olenekian and Ladinian prevents analysis of the corresponding biochrons. Finally, although the Iberian record for the Upper Triassic is not abundant, the presence of Eubrontes, Anchisauripus and probably Brachychirotherium is coherent with the global track biochronology as well. Thus, the Triassic track record in the Iberian Peninsula matches the expected record for this age on the basis of a global biochronological approach, supporting the idea that vertebrate Triassic tracks are a useful tool in biochronology.

Ichnofabric analysis of the Tithonian shallow marine sediments (Bhadasar Formation) Jaisalmer Basin, India

The shallow marine sedimentary sequence of the Jaisalmer Basin exhibits one of the important and well-developed Tithonian sedimentary outcrops for western India. The ichnology and ichnofabric of the lower part of Bhadasar Formation (i.e., Kolar Dongar Member) belonging to Tithonian age are presented and discussed. The Kolar Dongar Member represents a shallow marine succession that contains 16 ichnotaxa: Ancorichnus ancorichnus, Conichnus conicus, Gyrochorte comosa, cf. Jamesonichnites heinbergi, Imponoglyphus kevadiensis, Laevicyclus mongraensis, Monocraterion tentaculatum, Ophiomorpha nodosa, Palaeophycus tubularis, P. bolbiterminus, Phycodes palmatus, Planolites beverleyensis, Rhizocorallium isp., Rosselia rotatus, R. socialis, and Teichichnus rectus. The ichnofabric analysis divulges five distinct ichnofabrics, each typifying distinct depositional environment within shallow marine conditions. The ichnofabric Ophiomorpha 1 with syn-sedimentary faulting exemplifies high energy conditions typical of lower shoreface environment, whereas the Ophiomorpha 2 ichnofabric typifies upper shoreface environment. The Ancorichnus ichnofabric reflects lower offshore condition of deposition. The high ichnodiversity Ancorichnus–Rosselia ichnofabric is indicative of inner shelf conditions, while low ichno-diversity Teichichnus ichnofabric indicates prevalence of low energy brackish bay environment. Thus, Tithonian Kolar Dongar Member indicates depositional environment ranging from shoreface to offshore to inner shelf and finally to brackish bay environment.

Insights on the Behavior of Late Paleozoic Aquatic Crustaceans (Pygocephalomorpha?): Compound Trace Fossils from Western Argentina

We describe intergradations between the arthropod repichnial trace fossils Diplichnites gouldi (Gevers et al., 1971), Dendroidichnites Demathieu et al., 1992, and Monomorphichnus Crimes, 1970 with two distinct cubichnial traces, Gluckstadtella cooperi Savage, 1971 and Huilmuichnus santracruzensis new ichnogenus and ichnospecies. These compound ichnofossils are part of high ichnodiversity arthropod-dominated trace fossil associations from the Late Carboniferous Agua Escondida Formation of the San Rafael Basin (Argentina). The complex ichnotaxonomy of Diplichnites and the ichnospecies D. gouldi are discussed and some possible working solutions are proposed. The most likely producer of the compound trace fossils are the Pygocephalomorpha, a group of Late Paleozoic crustaceans that are recorded in nearby basins from Uruguay and Brazil. The compound trace fossils are used to infer a number of complex behaviors of pygocephalomorphs in a subaqueous setting, including caridoid (tail flip) or more primitive escape reactions, landing followed by resting and transient resting during normal walking. In addition, different gaits can be inferred from well-preserved D. gouldi, which can be correlated with contrasting trackway morphology.

Bioerosion versus colonisation on Bivalvia: A case study from the Upper Miocene of Cacela (southeast Portugal)

A study of the bioerosion structures and the skeletobionts associated with the most common bivalves (infauna and epifauna) from the classic Upper Tortonian site of Cacela, Algarve region, SE Portugal, revealed 24 different ichnotaxa and five systematic groups of encrusters (Foraminifera, Annelida, Bryozoa, Balanomorpha and Bivalvia). Despite a relatively high ichnodiversity, the percentage of bioerosion in the specimens analysed is quite low (10–12%). This is explained by rapid sedimentation with only short periods of exposure on the sea-floor. The dominant bioerosion structures were linked to the boring activity of nonpredatory organisms. Algal microborings are the most common, followed by annelid borings ( Caulostrepsis-Maeandropolydora), sponge borings ( Entobia) and ctenostome bryozoans ( Pinaceocladichnus). Spatial distribution of bioerosion structures and encrusters allow the reconstruction of three successive stages. The first was restricted to the biosubstrate lifetime, with structures showing a preferred orientation and situated exclusively on the outside of the shells. The second comprises the period immediately after death, with structures that extend outwards and start with the colonization of the interior of the valves, losing their initial orientation. The third stage relates to later postmortem colonisation, with structures on both sides of the valves and without a preferential orientation.

A HIGHLY DIVERSE ICHNOFAUNA IN LATE TRIASSIC DEEP-SEA FAN DEPOSITS OF OMAN

Abstract We encountered a highly diverse ichnofauna within the deep-sea fan deposits of the Upper Triassic Al Ayn Formation in Oman. It comprises 32 ichnogenera: 18 ichnogenera represent predepositional graphoglyptids and other trace fossils that are preserved as casts on turbidite soles, and 14 ichnogenera represent postdepositional trace fossils that penetrate turbidite beds. The relatively large size of the area studied certainly favors encountering a high number of ichnogenera. The diversity we found approximately doubles the value that has often been stated in the literature and contradicts the paradigm that the Triassic represents a time of low ichnodiversity in the deep sea. Although the data are limited, in general the recovery of deep-sea tracemakers has been very slow owing to environmental disturbances that resulted from cold-bottom-water circulation after the Carboniferous–Permian glaciation. The high ichnodiversity in the Al Ayn Formation is explained by its paleogeographic position and local...

Lithofacies distribution of invertebrate and vertebrate trace-fossil assemblages in an Early Mesozoic ephemeral fluvio-lacustrine system from Argentina: Implications for the Scoyenia ichnofacies

This paper discusses the lithofacies distribution and ichnotaxonomic assignment of invertebrate, vertebrate and plant trace fossils in an Early Mesozoic ephemeral fluvio-lacustrine system from northwest Argentina. The overall palaeoenvironment is envisaged as a distal, low-gradient fluvio-lacustrine setting under semi-arid climate, similar to a terminal fan. Depositional lobes (facies association A) are composed of distributary ribbon channels (facies A1), and associated pond and sheetflood deposits (facies A2). Lowlands between depositional lobes (facies association B) contained poorly drained floodplain zones similar to wetlands (facies B1) and shallow closed lakes (facies B2). Depositional lobes were fringed by playa-lake settings (facies association C), including mudflats (facies C1) that are partially eroded by ephemeral fluvial channels (facies C2). The ichnofauna of the Santo Domingo Formation displays high ichnodiversity (30 trace fossil types) and includes 5 trace fossil assemblages that are linked to sedimentary facies. These trace fossil assemblages are highly distinctive as half of the recognised trace fossil types are assigned to different assemblages. The ephemeral fluvial channel assemblage (facies A1 and C2) displays the lower ichnodiversity ( n = 6) and includes Skolithos linearis, Diplichnites sp., Palaeophycus tubularis, Taenidium barretti, Dicynodontipus sp. and footprints in cross-section. The floodplain pond assemblage (facies A2) shows the higher ichnodiversity ( n = 15) and is composed of Diplichnites sp., Helminthoidichnites tenuis, P. tubularis, Rusophycus carbonarius, S. linearis, Spongeliomorpha carlsbergi, T. barretti, appendage marks, burrows with brush-like features, epichnial bilobed traces, hypichnial bilobed ridges, bird-like footprint type A, tridactyl footprints, footprints preserved in cross-section, and small epichnial rounded pits. The poorly drained floodplain assemblage (facies B1) is of moderate ichnodiversity ( n = 10) and consists of H. tenuis, S. carlsbergi, ? Spongeliomorpha sp., imbricated backfilled burrow fillings, root trace type A, Tetrasauropus sp., bird-like footprint type A, large pentadactyl footprints, tridactyl footprints, and footprints preserved in cross-section. The high ichnodiversity ( n = 13) nearshore lacustrine assemblage (facies B2) is the only assemblage that lacks vertebrate traces, and is composed of Cochlichnus anguineus, Cruziana problematica, H. tenuis, Palaeophycus heberti, P. striatus, P. tubularis, Scoyenia gracilis, S. carlsbergi, T. barretti, epichnial bilobed traces, hypichnial bilobed ridges, root trace type B, and scratch marks. The intermediate ichnodiversity ( n = 9) mudflat assemblage (facies C1) is typified by the largest ichnodiversity of vertebrate tracks and includes H. tenuis, P. tubularis, S. carlsbergi, Dicynodontipus sp., Tetrasauropus sp., bird-like footprints of types B and C, large pentadactyl footprints, and tridactyl footprints. The trace fossil assemblages from Santo Domingo Formation are typical representatives of the Scoyenia ichnofacies. The distinctiveness of the studied trace fossil assemblages suggests that a future division of the Scoyenia ichnofacies would be possible, provided that occurrences of invertebrate, vertebrate and plant trace fossils in definite sedimentary facies are considered. The subdivision of the Scoyenia ichnofacies into recurrent groups of trace fossils (ichnosubfacies) with a palaeoecological and palaeoenvironmental meaning can be based on two approaches: distinction of associations of vertebrate tracks and recognition of ichnocoenoses related to substrates with different degrees of water saturation and firmness. The first approach can be based on an evaluation of the recurrent vertebrate ichnoceonoses used to recognise vertebrate ichnofacies. In this way, some of the candidate vertebrate ichnofacies are proposed as subdivisions of the Scoyenia ichnofacies. The second approach is based on the identification of crosscutting relationships between trace fossils and between trace fossils and sedimentary structures within some trace fossil assemblages. Using this methodology, it is possible to define distinct assemblages of trace fossils: a “pre-desiccation suite” with structures lacking ornamentation and developed in a soft substrate and a “desiccation suite” typified by striated traces that crosscut the former. These trace-fossil assemblages help to characterise the changes in organism–sediment interaction during progressive desiccation of a water-lain substrate. Additional neoichnological studies are necessary to assess the exact meaning of different trace fossil features in terms of substrate consistency.

Ichnology of Carboniferous tide-influenced environments and tidal flat variability in the North American Midcontinent

Abstract Trace fossils are sensitive indicators of environmental fluctuations and, accordingly, ichnological studies have the potential to improve facies characterization of marginal-marine systems. Carboniferous intertidal deposits in eastern Kansas and western Missouri accumulated under contrasting palaeoenvironmental conditions, ranging from the open shoreline to fluvio-estuarine transitions. Comparative analysis of these exposures illustrates lateral variations in trace-fossil content and allows characterization of the intertidal ichnofaunas developed in three sub-environments: open marine, restricted bays and fluvio-estuarine transitions. Openmarine tidal flat ichnofaunas are characterized by (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) the presence of both infaunal and epifaunal traces, (4) the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively, (5) dominance of horizontal trace fossils of the Cruziana ichnofacies, (6) presence of multispecific associations, (7) high density, and (8) wide size range. This ichnofauna is present in heterolithic deposits and reflects the activity of a biota that inhabited tidal flats dominated by normal-marine salinities and connected directly to the open sea. Restricted-bay ichnofaunas display (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, (6) the presence of monospecific associations, (7) variable density, and (8) small size. This assemblage occurs in heterolithic facies and records the activity of a brackish-water benthic fauna inhabiting intertidal areas of estuarine basins and embayments. Fluvio-estuarine ichnofaunas are characterized by (1) moderate to relatively high diversity, (2) forms typically present in continental environments, (3) the dominance of surface trails and absence of burrows, (4) temporary structures produced by a mobile deposit-feeding fauna, (5) a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies, (6) moderate density of individual ichnotaxa, (7) absence of monospecific suites, and (8) small size. This assemblage occurs in tidal rhythmites and records the activity of a typical freshwater/terrestrial benthos inhabiting tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions. Through integration of ichnological and sedimentological data, conventional sedimentological interpretations of marginal-marine depositional systems can be refined and enhanced.

The Paradox of Nonmarine Ichnofaunas in Tidal Rhythmites: Integrating Sedimentologic and Ichnologic Data from the Late Carboniferous of Eastern Kansas, USA

The occurrence of trace fossil assemblages dominated by arthropod trackways and surface grazing trails within Carboniferous tidal rhythmites has puzzled sedimentologists and ichnologists, who interpreted them either as marine or nonmarine. The Virgilian (Stephanian) Tonganoxie Sandstone Member (Stranger Formation) at Buildex Quarry (eastern Kansas) consists, for the most part, of planar-laminated coarse grained siltstones deposited on an upper tidal flat, close to or at the fluvial-estuarine transition of a macrotidal estuarine paleovalley. Recurrent thickness fluctuations demonstrate the strong influence of tidal process and provide evidence that these deposits are tidal rhythmites, with thicker strata representing spring tides and thinner ones recording neap tides. The Buildex sequence hosts a moderately diverse ichnofauna composed of arthropod trackways (Dendroidichnites irregulare, Diplichnites gouldi, Diplopodichnus bifurcus, Kouphichnium isp., Mirandaichnium famatinense, Stiallia pilosa, Stiaria intermedia), grazing traces (Gordia indianaensis, Helminthoidichnites tenuis, Helminthopsis hieroglyphica), subsurface feeding traces (Treptichnus bifurcus, T. pollardi, irregular networks), apterygote insect resting and feeding traces (Tonganoxichnus buildexensis, T. ottawensis), fish traces (Undichna britannica, U. simplicitas), and tetrapod trackways. In contrast to trace fossil assemblages from brackish-water estuarine settings, the Buildex ichnofauna is characterized by moderate to relatively high ichnodiversity, ichnotaxa commonly present in terrestrial/freshwater environments, dominance of surface trails and absence of burrows, dominance of temporary structures produced by a mobile deposit-feeder fauna, a mixture of traces belonging to the Scoyenia and Mermia ichnofacies, moderate density of individual ichnotaxa, and absence of monospecific suites. This ichnofauna is thought to record the activity of a typical freshwater/terrestrial benthos. The presence of this mixed freshwater/terrestrial ichnofauna in tidal rhythmites is regarded as indicative of tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions, more precisely in a zone between the maximum limit of landward tidal currents and the salinity limit further towards the sea. Although lithofacies distribution in estuarine valleys is mainly salinity-independent, the distribution of benthos is not. Accordingly, ichnologic studies have the potential to provide a high-resolution delineation of fluvio-estuarine transitions.