Trace Makers Research Articles

Cicada nymph trace fossils from South American Maastrichtian paleosols

The Cretaceous was a pivotal Period in the evolution of major insect groups, including cicadas. Insect diversification led to an increased complexity in the paleosol ichnofabrics due to the emergence of chambers associated with reproductive and feeding behaviors. Therefore, any new record of trace fossils attributed to cicadas from the Cretaceous period represent a significant advancement in understanding the diversification of this group and the emergence of insect feeding chambers, throughout the geological record. This work describes the record of the ichnogenus Feoichnus attributed to feeding chambers of cicada nymphs (Hemiptera: Cicadidae) from Maastrichtian paleosols developed in floodplain deposits of The Marília Formation (Bauru Basin, Brazil). The described Feoichnus chambers are elongated, hemispherical, and upwardly concave, with a lining covering the inner surface of the wall. The inner surface exhibits thin, irregular ridges and grooves, likely associated with fine roots. Feoichnus occurs in association with Beaconites antarcticus and root traces found within the feeding chamber, it could be indicating the movement of the trace maker and the feeding chamber from one root to another. These characteristics indicate the xylem-feeding behavior of Feoichnus trace maker in accordance with extant cicada nymph ecology. The presence of Beaconites antarcticus associated with Feoichnus and rhizoliths probably represents the foraging behavior of cicada nymphs and the displacement of feeding chambers in well-drained, rooted soils. Therefore, the cicada nymph could be the trace maker of both traces representing feeding and locomotion behavior. This contribution expands the knowledge about plant–insect interactions and insect groups that compose the Cretaceous paleosols ichnofabrics from South America.

Small topographical variations controlling trace maker community: Combining palaeo- and neoichnological data at the Porcupine Abyssal Plain

Ichnological research has generally assumed that abyssal plains are dominated by quiescent, homogenous environmental conditions. Thus, deep-sea trace fossil assemblage changes have been usually linked to significant spatial and temporal environmental variations. Here, we conducted a comparative ichnological analysis between a small abyssal hill (50 m elevation) and the surrounding abyssal plain; this modest bathymetric variation is known to generate substantial environmental heterogeneity for the benthic fauna community of the Porcupine Abyssal Plain (c. 4850 m depth), Northeast Atlantic. Based on X-ray data from a 5 × 5 core grid emplaced in two box cores, we compared hill and plain bioturbational sedimentary structures, including trace fossil assemblages (e.g., ichnotaxonomy) and biodeformational structures (e.g., mixed-layer depth). We observed that topographically-enhanced near-bottom currents over the hill likely produce significant changes in depositional dynamics and sediment properties (e.g., grain size, organic matter content and degradation), and control specificities of bioturbational sedimentary structures (e.g., trace fossils, mixed layer attributes such as thickness, mottled background, discrete traces). Palaeoichnological data suggested that the abyssal plain had experienced consistent conditions during the last thousands of years while the abyssal hill recorded improving environmental conditions for the trace maker community. Our results highlight the complexity of the deep-sea environment, demonstrating that small changes in bioturbated sedimentary assemblages appear even within the same box core (m-scale), and that substantial changes can occur due to environmental heterogeneity (e.g., subtle topographic variations) at the local scale (km-scale). Considering the vast global extent of abyssal hill terrain, we suggest that their influence on the bioturbational sedimentary record may be significantly under-appreciated and require more attention in palaeoenvironmental reconstructions.

Coleopteran burrows from foreshore settings in Florianópolis, Brazil

While insects are versatile animals capable of occupying harsh environmental conditions, not many are present in the marine environment, and has generally been discounted as trace makers in such settings. During the intra-congress field trip of ICHNIA 2024, however, we observed multiple surficial burrows on foreshore sediments that were constructed by beetle larvae. While there are several reports of insect burrows from backshores, eolian dunes, salt marshes, tidal creeks, washover fans, and distributary channels, we are not aware of ichnological studies of these burrows from the foreshore. Moreover, some of these studies failed to identify or mention the entity of the trace makers, and have not stressed enough the significance of burrowing marine insects on palichnology and paleoecology. Some entomologists on the other hand have recognized insects in marine environments, and some insect groups (such as Coleoptera and Diptera) have burrowing larvae in brackish to marine environments. Understanding the burrow morphologies of marine insects compared to freshwater insects or other marine invertebrates may help us understand the ecological significance and evolutionary history of insects in marine habitats.

Enigmatic vertebrate swimming trace fossils from the Wapiti Formation, Alberta, Canada, and their implications for paleoenvironmental reconstruction

Vertebrate ichnofossil research, when compared with invertebrate ichnological analyses, tends to put greater emphasis on identifying the trace makers rather than associating the trace fossils with animal behaviors to interpret local paleoenvironmental conditions. In 2011, several sandstone blocks bearing unusual deformational structures were collected from the Wapiti Formation exposed near Red Willow Falls, Alberta, Canada. The sandstone blocks were analyzed for their sedimentary characteristics and the morphology of the enigmatic structures in order to identify their origins and their significance for paleoenvironmental interpretation. The deformational features are interpreted to be the swim tracks of small vertebrates attempting to escape a flash flood event. The sedimentology of the sandstone suggests a rapid increase in water flow in an overbank setting, and the orientation of the traces indicate that the trace maker was initially moving against, and then moving perpendicular to, the current. This is likely the first case of a trace fossil exhibiting clear association between vertebrates and flash flood events. As such, these traces provide an excellent example of the utility of vertebrate trace fossils for assessing organism response to changing environmental conditions, thus facilitating interpretation of local geological events.

Thalassinoides ichnofabrics from the Zhangxia Formation (Cambiran Miaolingian series) in Henan province: Bioturbators as Ecosystem engineers

Bioturbation, a crucial ecosystem engineering behavior in marine environments, significantly influences fluid flow and the physical environment. Trace fossil records serve as valuable tools for understanding the impact of bioturbation by ecosystem engineers on sedimentary substrates. This study utilizes the abundant trace fossil Thalassinoides found in the Zhangxia Formation of the Middle Cambrian (Miaolingian, Wuliuan) in the Dengfeng section of western Henan Province, China, as research materials to analyze the evolution of trace makers' ecosystem engineering behavior and its influence on sediment mixing. Investigations reveal that the producers of Thalassinoides adopt diverse survival strategies in response to environmental changes, leading to varying degrees of sediment mixing. Despite the potential impact of environmental fluctuations on benthic organisms, constraining their excavation into deeper sediment layers, the complex engineering behaviors of the ecosystem continued to evolve during the mid-Cambrian period. These ecosystem engineers not only modify the sedimentary substrates, expanding ecological space, but also potentially accelerate the rate of nutrient cycling in the seafloor, thereby serving as a driving force for further prosperity within the early Paleozoic benthic communities.

Late Miocene mammalian burrows in the Camacho Formation of Uruguay reveal a complex community of ecosystem engineers

We report fossil mammal burrows from backshore beach facies in the Camacho Formation of southern Uruguay, of Late Miocene (Huayquerian SALMA) age. The presence of desiccation cracks and rhizoliths indicate the occurrence of relatively extended periods of subaerial exposure and the incipient development of vegetation. The analysis of the burrows' spatial extent, size, and structure reveals the existence of exceptionally well-preserved and intercrossing tunnel systems. We show the existence of different size classes of burrows, which indicate that at least four different taxa were responsible for their construction. Considering the inferred body masses of the trace makers obtained from allometric relationships and the body masses of taxa recovered for the Camacho Formation, the burrows may have been produced by a combination of the following mammals: one of several rodents, notoungulates, cingulates, folivorans, and a carnivoran. The fossil association represents an exceptional case of a community of ecosystem engineers in the Late Miocene of southeastern South America.

Quantitative ichnology: a novel framework to determine the producers of locomotory trace fossils with the ichnogenus Gordia as a case study

AbstractTrace fossils record the interactions between organisms and their surroundings, and can therefore provide unique insights into the coevolution of trace makers and the environment. However, identifying the producers of trace fossils is challenging because different animals can create very similar traces and many ichnotaxa can therefore only be attributed to broad morphological grades. For example, simple horizontal traces like Gordia are generally suggested to have been produced by vermiform organisms, potentially encompassing a range of animal phyla. This uncertainty makes it difficult to decipher their palaeobiological significance through major evolutionary events and episodes of environmental change. We have developed new mathematical approaches for identifying previously unrecognized signatures left by the trace makers of simple marine locomotory traces. We calculated the deviation angle series of self‐crossing traces made by extant isopods, polychaetes, gastropods and nematodes, computing the frequency spectrum and autocorrelation function in each case. The results reveal that each of these taxa left unique markers during the trace‐making process, reflecting differences in their anatomy and locomotory behaviour. We were able to identify the possible trace makers of several early Palaeozoic Gordia specimens, demonstrating that ichnospecies within the same ichnogenus can be created by distantly related animals with very different morphologies and/or behaviours. This novel mathematical framework has great potential for identifying the possible producers of diverse trace fossils through deep time, helping to uncover the earliest evidence of certain animals or behaviours. It also has great potential for quantifying ichnotaxonomy, consolidating the link between ichnology and palaeobiology.

The First Artropod Trackways in the Lower Triassic of Eastern Europe from the New Locality Mansurovo, Orenburg Region

A description is given of trace fossils from the Lower Triassic Gostevskaya Formation of the Mansurovo locality, Orenburg Region, represented by counterimpressions on the lower contact of sandstone with wave ripples. The ichnological characteristics of the trackways and their individual imprints are similar to the ichnospecies Diplichnites triassicus (Linck, 1943), which is widespread in fluvial and lake sediments of the Carboniferous-Triassic of Europe and North America. For D. triassicus most likely the trace maker from the group of branchiopod arthropods Notostraca, which moved in the aquatic environment. Most likely, the microbial mat developed on the surface of the sand substrate acted as a trace carrier, and the subsequent preservation and burial of arthropod trackways took place with the direct participation of the microbial mat.

Macaronichnus ‘co-occurrence’ in offshore transition settings: Discussing the role of tidal versus fluid muds influence

Macaronichnus is a key trace fossil in palaeoceanographic, palaeoclimatic, and petroleum exploration research. Small ichnosubspecies such as Macaronichnus segregatis segregatis, M. s. lineiformins, M. s. maeandriformis and M. s. spiriformis, typically occur in wave-dominated foreshore sands where large M. s. degiberti was never found. The latter shows a wide environmental distribution, occurring in sandy deposits of tidal channels, tidal bar sand sandridges, tidal-flat sand sheets, shorefaces, bioturbated sandy shelf, shelf storm-sheets, shelf sand ridges, and upper slopes. Small M. segregatis and large M. s. degiberti have not been observed to date due the ecological segregation of the tracemakers. An abundant record of large M. s. degiberti in a Tortonian (Late Miocene) mixed carbonate-siliciclastic unit from the Betic Cordillera (southern Spain) has been studied. Occurrence of M. s. degiberti is the result of the interaction of tidal and waves, storm influenced environment determining high-energy conditions and associated palaeoenvironmental parameters as shifting substrates, organic matter availability, and oxygenated pore and bottom-waters. Locally, associated to M. s. degiberti appear small, sinuous traces infilled by light material that were originally assigned to M. s. maeandriformis, and very rare M. s. spiriformis. However, the absence of the typical rim of Macaronichnus avoid a conclusive assignment. The coexistence of both small traces (?M. s. maeandriformis) and large M. s. degiberti is identified in the deposits underlying mudstone layers, revealing the importance of mud deposition during tidal slack water intervals or linked to fluid mud events favouring the co-occurrence of the trace makers of both Macaronichnus ichnosubespecies. This fact would have significant palaeobiological and palaeoecological implications, and could be the first record of both ichnosubspecies in the same intervals.

Lower Jurassic trace fossil Zoophycos from the Ksour Mountains of northwestern Algeria and their palaeoecological and palaeoenvironmental significance

The Zoophycos ichnofacies is reported for the first time from the Lower Jurassic (Toarcian) deposits of Algeria. The trace fossil-bearing strata in the Ksour Mountains, northwestern Algeria, are an alternation of fine-grained limestones and marls. The assemblage is characterised by its low diversity, consisting of eight ichnospecies (Chondrites intricatus, C. targionii, Chondrites isp., Palaeophycus isp., Thalassinoides isp., Zoophycos brianteus, Z. cauda-galli, and Zoophycos isp.), representing the Zoophycos ichnofacies ethologically dominated by deposit-feeding behaviours of different producers. The abundance of Zoophycos and Chondrites within a low diversity assemblage suggest stressful conditions most likely indicating oxygen depletion within the sediment. Moreover, the presence of reworked burrows by Chondrites could indicate ecological succession. This is probably due to environmental conditions getting more stressful, and causing the need for exploiting the nutrient stored in the abandoned burrows after the death of the trace maker. This process is primarily associated with an oxygen-depleted environment, compelling the animal to engage in it as a means of obtaining energy, rather than actively searching for food within the sediment. On the other hand, the occurrence of Thalassinoides may indicate an improvement of palaeoecological conditions and short periods with partial increase of oxygenation. The ichnological, palaeontological and palaeoecological data of the studied succession indicate a relatively deep-water environment located well below the storm wave base at a deeper offshore setting.

A ROWELLELLA (LINGULATA, BRACHIOPODA) NESTLER IN A TRYPANITES BORING FROM THE MIDDLE ORDOVICIAN OF ESTONIA: AN EARLY COLONIZER OF HARD SUBSTRATE BORINGS

ABSTRACT A steinkern of an endoceratid nautiloid siphuncle contains a Trypanites sozialis boring with a lingulate brachiopod Rowellella sp. shell inside. The steinkern of this endoceratid formed during early lithification of the sediment on the seafloor. The lithified steinkern of this siphuncle was either initially partially exposed to the seawater or was exhumed and stayed exposed on the seafloor, where it was colonized by boring organisms. This bioerosion resulted in numerous Trypanites borings in the siphuncle. After the death or exit of the Trypanites trace maker, a vacant boring was colonized by a small lingulate nestler Rowellella sp. This lingulate was likely preadapted to life in hard substrate borings when it first found its way into borings in living substrates in the Late Ordovician. The increased availability of hard substrate borings, combined with the increased predation pressure due to the GOBE, enhanced the colonization of hard substrate borings by lingulate brachiopods.

Northernmost (Subarctic) and deepest record of Paleodictyon: paleoecological and biological implications

Paleodictyon is one of the most iconic and widespread of trace fossils in the geological record. However, modern examples are less well known and restricted to deep-sea settings at relatively low latitudes. Here, we report the distribution of Paleodictyon at six abyssal sites near the Aleutian Trench. This study reveals for the first time the presence of Paleodictyon at Subarctic latitudes (51°–53°N) and at depths over 4500 m, although the traces were not observed at stations deeper than 5000 m suggesting that there is some bathymetric constraint for the trace maker. Two small Paleodictyon morphotypes were recognized (average mesh size of 1.81 cm), one having a central hexagonal pattern, the other being characterized by a non-hexagonal pattern. Within the study area, Paleodictyon shows no apparent correlation with local environmental parameters. Finally, based on a worldwide morphological comparison, we conclude that the new Paleodictyon specimens represent distinct ichnospecies that are associated with the relatively eutrophic conditions in this region. Their smaller size may reflect this more eutrophic setting in which sufficient food can be obtained from a smaller area in order to satisfy the energetic requirements of the tracemakers. If so, then Paleodictyon size may provide some assistance when interpreting paleoenvironmental conditions.

Luzon predators: Clues from a fossil with bite marks

Luzon Island remained isolated throughout its emergent history with its paleofauna often considered depauperate. However, the terrestrial vertebrate fossil record of the island is still poorly known. A new fossil-bearing paleochannel deposit was discovered in La Union Province, northwestern Luzon, Philippines. This paleochannel is inferred to be of Pleistocene age and has yielded fossils tentatively classified as a bovid (Bubalus sp.), a cervid (Rusa sp.), a suid (Sus sp.), and a large crocodile (Crocodylidae). Interestingly, a proximal fragment of the left tibia of a cervid shows multiple bite marks. This study analyzes the bite marks and modifications on the specimen to infer its possible trace maker and explore its implications in understanding the ancient predatory guild of Luzon. Three sets of modifications were recognized on the specimen, namely the bite marks, the broken proximal end and the dry bone fractures. A crocodilian agent for the bite marks is supported by the bisections, their occurrence in the paleofaunal assemblage and the fluvial setting of the deposit. However, an unknown carnivoran agent interpretation cannot be fully disregarded given the ambiguity of bisected marks and the possibly gnawed proximal end of the specimen. Unfortunately, given the lack of in situ context and apparent overprinting, the extent of the peri- and post-mortem modification remains uncertain which hinders the identification of the predatory agent. Verification of the observations reported in this study would require the systematic collection and detailed taphonomic analysis of additional fossil specimens from the deposit.

Deep-Sea Echinoid Trails and Seafloor Nutrient Distribution: Present and Past Implications

The movement patterns of deep-sea bioturbational fauna are believed to be the result of the organism’s interactive response to the perceived spatial distribution of nutritional resources on the seafloor. To address this hypothesis, we examined the movement paths of Echinocrepis rostrata -a common epibenthic bioturbator echinoid in the northeast Pacific Ocean- through fractal analysis in order to characterize how they cover the seafloor during foraging. We used an 18-yr time series photographic record from 4100-m depth at an abyssal site in the eastern North Pacific (Sta. M; 34°50′N, 123°00′W; 4100 m depth). Echinocrepis rostrata paths showed low fractal values (1.09 to 1.39). No positive correlation between particulate organic carbon (POC) flux measured from sediment traps at 600 m and 50 m above bottom and fractal dimension (FD) values was observed. The movement of echinoids was characterized by high-speed periods, followed by slower speed periods and higher turning rates. These slow-speed periods were correlated with higher sinuosity values, slightly wider turning angles, and numerous cross-cuts. Based on visual estimation of seafloor phytodetritus coverage, we hypothesize that its small-scale distribution may be the primary determinant of echinoid feeding movement patterns rather than the bulk amount of nutrients. Finally, this finding reveals new insights into the morphological studies of trace fossils, indicating that trails of past echinoid trace makers could help to evaluate nutrient availability/distribution in the ancient deep-sea and help to decipher past climate-induced changes.

Trace fossil characterization during Termination V and MIS 11 at the western Mediterranean: Connection between surface conditions and deep environment

Trace fossil assemblages are studied at Ocean Discovery Program (ODP) Site 977 to characterize the response of the macrobenthic trace maker community to deep paleoenvironmental conditions during the Termination V (TV) and interglacial Marine Isotope Stage (MIS) 11 at the western Mediterranean Alboran Sea. An assemblage composed of Chondrites, Planolites, Scolicia, Thalassinoides and Zoophycos is identified, showing notable variations in ichnodiversity, abundance and Bioturbation Index, that were analyzed in detail. The integration of ichnological information with sediment color and high-resolution coccolithophore records from Site 977, evidenced that variations in macrobenthic trace maker community are primarily controlled by oxygen availability and surface organic productivity patterns. During TV, high surface organic productivity by intense Alboran Upwelling System enhanced the deep organic accumulation that, together with reduced deep-water removal, resulted in a decrease of bioturbation and the formation of an Organic Rich Layer. Moderate and stable surface production through MIS 11c reduced deep food availability, resulting in an oligotrophic and stable deep environment. This is reflected by relatively abundant trace fossils in lighter sediments. Intra-interglacial increase in surface organic production at ~405 ka is evidenced by increased organic matter preservation. Minor impact of western Mediterranean circulation on deep-water removal, but a plausible stronger control by Bernoulli aspiration intensities in the region, is, in overall, observed during these intervals. During the Heinrich-type (Ht) events 3 and 2, increased trace fossil diversity and ameliorated oxygenation is driven by limited surface organic production, but intense western Mediterranean deep-water circulation and enhanced regional deep-water removal.

Thomas Webster's Tulip Alcyonium (Lamellaecylindrica, trace fossil) in the Upper Greensand Formation (Albian) of the Isle of Wight

Large, ramified burrow systems with a thick lamination and bulbous enlargements are common in the Upper Greensand Formation (Albian) of Ventnor, Isle of Wight. From there, these trace fossils were first described by Thomas Webster more than two centuries ago. Although also characteristic outside their area of original description, these structures and Webster's fabulous documentation have fallen into oblivion and only recently were assigned to the ichnogenus Lamellaecylindrica. The two included ichnospecies, L. paradoxica and L. ludwigae reflect different behaviours of their trace maker (possibly a holothurian) in adaption to variations in the sedimentary conditions and current energy. A comparison of the associated ichnofauna with that of the Upper Greensand Formation at Lyme Regis, about 100 km to the west, reveals subtle but significant differences in the trace-fossil distribution, ichnodiversity and ichnoabundance, indicating a proximal (lower shoreface to offshore transition) to distal (inner shelf) depositional trend from west to east. Given the total bioturbation of the greater part of this shelf succession, these subtle variations in the ichnofauna provide valuable information for interpretation of the depositional environment and differences in the energy level where primary sedimentary structures are absent.

X-ray microtomography analysis to approach bioturbation's influence on minor-scale porosity distribution: A novel approach in contourite deposits

Micro-CT analysis is employed for the first time to evaluate the effects of bioturbation on porosity distribution in contourite facies, namely: i) sandy clastic contourites from the Late Miocene Moroccan Rifian Corridor; and ii) dominant-calcareous contourites from Eocene- Middle Miocene Cyprus paleoslope. Porosity distribution is affected by bioturbation due to trace maker behavior and burrow infilling, but there is no clear, single relationship. Macaronichnus, especially M. segregatis, located in sandy clastic contourite facies would have the greatest impact, increasing porosity due to the grain-selective deposit-feeding behavior of the trace maker. Porosity values are up to three times higher in the burrow rim than in the infilling material, which favors a dual-porosity flow medium. Samples with M. s. degiberti record higher porosity values in the host sediment than in the burrow fill, but any increase of porosity in the burrow rim is not certain. This variable influence on porosity distribution (M. segregatis vs. M. s. degiberti) might be associated with different trace maker behaviors during feeding activities, or even with variable producers. Chondrites located in muddy chalk contourite facies has a neutral effect on porosity distribution, porosity data of the host sediment and the filling material being similar, in agreement with a comparable grain size. Thalassinoides located inside calcarenitic contourite facies has a minor effect on porosity distribution, showing similarities between passively infilled material and the host sediment. Our results also reveal the importance of secondary diagenetic overprints when homogenizing primary differences. Accordingly, understanding the impact of bioturbation on porosity distribution in contourite facies appears to be a key factor for evaluating the real potential of these unconventional reservoirs. This study represents a first step forward in discerning the relationship between contourite facies, ichnological features, petrophysical properties and reservoir geology.

A unique notostracan trace fossil assemblage from the Upper Triassic Chinle Formation (northeastern Utah, USA) and its paleoecological and paleoenvironmental implications

The Upper Triassic (Norian) Chinle Formation of the southwest USA is world-renowned for its diverse fossil assemblages. Here we describe a new trace fossil assemblage from the formation in Dinosaur National Monument (northeastern Utah), and discuss its paleoenvironmental implications. The trace fossils comprise freshwater bilobate resting and feeding traces (Isopodichnus-Rusophycus-morphotype and Isopodichnus-Cruziana-morphotype respectively) as well as walking traces (Acripes), probably of notostracan crustaceans, in a previously unknown subparallel alignment. Extant notostracans are known to live in ephemeral bodies of freshwater with a low water level and temporary desiccation. However, associated sedimentological features such as prod marks and groove casts indicate that the traces were produced under the influence of a low velocity, unidirectional current which is typical for sheet-flows – an interpretation independently corroborated by further sedimentological data. Thus, the trace fossil assemblage supports the presence of rheotaxis in notostracans, a thus far little-known behavior in these animals. Based on the inferred ecology of the trace maker, the sedimentological features associated with the traces, and the sedimentology of the surrounding rocks, the traces were most likely formed in an ephemeral part of a fluvial system dominated by sheet-flow events. This interpretation supports the results of previous studies suggesting that the upper Chinle Formation at Dinosaur National Monument area is characterized by ephemeral low flow regime sheet-flows deposited on mudflats.

Response of macrobenthic trace maker community to the end-Permian mass extinction in Central Spitsbergen, Svalbard

The End Permian Mass Extinction (EPME, ca. 252 Ma) is considered as the most severe biodiversity crisis in the Phanerozoic, affecting more than 90% of marine species and 75% of terrestrial species. In recent years, ichnological analysis has emerged as a proxy to better understand the paleoenvironmental changes associated with this event, as well as their effect(s) on the biota and its recovery. This study focuses on the ichnological analysis of an extended upper Permian – Lower Triassic succession from the DD-1 core, drilled in Deltadalen, Central Spitsbergen. Variations in degree of bioturbation and ichnodiversity across the Permian-Triassic transition allow for the characterization of pre-, syn-, and post-EPME phases. Pre-EPME is characterized by a high degree of bioturbation (BI = 5), and a diverse trace fossil assemblage (13 ichnotaxa) with abundant Nereites, frequent Planolites and Phycosiphon, and the more or less local record of Asterosoma, Bergaueria, Chondrites, Macaronichnus, Palaeophycus, Taenidium, Teichichnus, Thalassinoides, Zoophycos, meniscate horizontal burrows and undifferentiated vertical traces, indicative of oxygenated bottom and pore-waters, as well as high nutrient availability in the benthic habitat. The EPME was followed by an interval characterized by the scarcity (BI = 1) or even absence (BI = 0) of trace fossils. The decrease in ichnodiversity was abrupt at first and more progressive subsequently, as the conditions were inhospitable (i.e., very low oxygenation) in the benthic habitat. In this context, short-lasting episodes of improved paleoenvironmental conditions (from anoxia to dysoxia) allowed for intermittent colonization of the substrate. The post-EPME interval is characterized by a gradual recovery of the trace maker community in terms of abundance (BI from 1 to 5) and ichnodiversity (total of 10 ichnotaxa), revealing a relatively rapid reestablishment of favourable paleoenvironmental conditions for the benthic habitat, with better‑oxygenated bottom and pore-waters, as well as improved nutrient availability at or just below the seafloor. This new ichnological information highlights (i) the absence of a total extinction of the trace-maker community, despite a marked reduction in abundance and ichnodiversity during the EPME, and (ii) the rapid recovery of the benthic fauna during the Induan Age, ca. 150 kyr after the EPME occurred.

Sedimentology and neoichnology of a muddy translating point bar in the fluvio-tidal transition, Serpentine River, BC, Canada

Point bars are common in the fluvio-tidal transition of rivers, yet detailed characterization of their sedimentology and neoichnology is rarely reported. Herein, we present sedimentological and ichnological descriptions for a seaward translating point bar located in the fluvio-tidal transition of the Serpentine River, BC, Canada. This bar accumulates in mainly polyhaline water, and channel flow is tide-dominated. The landward bar has steep channel banks and grain size fines from medium sand in the thalweg to mud in the upper intertidal zone. The seaward side of the bar forms a bench-like morphology that is steeply inclined in the intertidal zone, has a wide, gently dipping platform in the lower intertidal/uppermost subtidal zone, and a steep lower subtidal channel bank. Sediment sizes fine from coarse- to medium-grained sand in the thalweg, to mud in the lower intertidal zone. Medium-grained sand is deposited on the middle- and upper-intertidal zone of the seaward-side of the bar as a result of flow reversal and impingement of the flood tidal current against the seaward side of the translating point bar.The neoichnological assemblage of the intertidal part of the translating point bar is characteristic of brackish-water environments, and comprises low diversities of diminutive structures that occur locally in high densities (Bioturbation Indices 3–4). The neoichnological distribution correlates closely with the macrobenthos distribution, which in turn indicates marine-nutrient distribution. Marine nutrient supply is likely controlled by the bar's morphology. Biogenic structures attributable to Arenicolites, Skolithos, Polykladichnus and Siphonichnus mark the middle and upper intertidal zones, as well as the lower intertidal zone along the landward side of the bar. The trace makers that produce this trace assemblage are resilient to prolonged subaerial exposure and employ diverse feeding strategies. The lower intertidal zone along the seaward side of the bar is affected by decreasing current velocities across the wide bench-like morphology, which permits regular deposition of marine nutrients. Here, population counts are high and trace makers specialize in surface deposit- and filter-feeding. Traces include Gyrolithes-like structures and Siphonichnus. The Gyrolithes and Gyrolithes-like burrows that are produced by S. kowalevskii at the study site are hypothesized to potentially reflect the trace maker's anatomical predisposition, and not represent some spatial or anchoring advantage.This work represents the first published study on the neoichnology of a tide-dominated, translating point bar in the fluvio-tidal transition. The model we present is intended as an analog to show the variability in fluvio-tidal point bars and how they might be manifested in the sedimentary record.