Three‐dimensional reconstruction, taphonomic and petrological data suggest that the oldest record of bioturbation is a body fossil coquina

Fossil material assigned to Nenoxites from the late Ediacaran Khatyspyt Formation of Arctic Siberia (550–544 Ma) has been presented as evidence for bioturbation prior to the basal Cambrian boundary. However, that ichnological interpretation has been challenged, and descriptions of similar material from other global localities support a body fossil origin. Here we combine x‐ray computed tomography, scanning electron microscopy and petrographic methods to evaluate the body or trace fossil nature of Nenoxites from the Khatyspyt Formation. The fossilized structures consist of densely packed chains of three‐dimensionally preserved silicic, bowl‐shaped elements surrounded by distinct sedimentary halos, in a dolomitized matrix. Individual bowl‐shaped elements can exhibit diffuse mineralogical boundaries and bridging connections between elements, both considered here to result from silicification and dolomitization during diagenesis. This new morphological and petrological evidence, in conjunction with recent studies of the late Ediacaran tubular taxa Ordinilunulatus and Shaanxilithes from China, suggest that the Khatyspyt specimens most probably reflect a coquina deposit of Shaanxilithes‐like body fossils. Our data support the possibility of Shaanxilithes‐like organisms representing total group eumetazoans.


P H Y L O G E N E T I C studies predict a Neoproterozoic origin
for Metazoa tens of millions of years prior to the start of the Cambrian, and imply that the stem groups of most bilaterian phyla originated during the Ediacaran Period (635-539 Ma) (e.g.Erwin et al. 2011;dos Reis et al. 2015).Although often contentious (Cunningham et al. 2017), claims for body fossils of animals from the late Ediacaran Period (c.574-539 Ma) have been bolstered by recent discoveries, analyses and developmental data, which together support the presence of total group (e.g.Liu et al. 2014;Evans et al. 2017;Cai et al. 2019;Dunn et al. 2019Dunn et al. , 2021) ) and crown-group (Dunn et al. 2022) eumetazoans by the middle Ediacaran, as well as candidate bilaterians in Ediacaran fossil assemblages younger than c. 558 Ma (e.g.Fedonkin & Waggoner 1997;Gold et al. 2015;Evans et al. 2020;Willman & Slater 2021).
Material described from the c. 550-544 Ma Khatyspyt Formation of Siberia (Sokolov & Fedonkin 1984;Fedonkin 1988;Rogov et al. 2012Rogov et al. , 2013aRogov et al. , 2013b;;Cherry et al. 2022) is of critical importance for understanding the timing and nature of both behavioural innovation and metazoan evolution more broadly.Fossil material assigned to the ichnogenus Nenoxites Fedonkin, 1976, by Rogov et al. (2012) was interpreted by those authors as evidence for burrowing involving meniscate backfilling, on the basis of taphonomic, palaeoecological and sedimentological lines of observation.That evidence includes morphologies consistent with phobotaxis (Rogov et al. 2013a) and vertical escape behaviour, the absence of branching, disturbance of primary lamination, secondary erosion of candidate meniscae by subsequent sedimentary deposits, and the presence of connective structures reminiscent of mucus-wall drains (Rogov et al. 2012).Such an interpretation would extend the ichnofossil record of bilaterians capable of producing complex backfill burrow structures (and thus their associated impact on sediments), by c. 10 myr (M angano & Buatois 2020;Tarhan et al. 2020;Darroch et al. 2021).
The ichnological interpretation of the Khatyspyt material has been challenged by suggestions that it may instead represent the body fossil remains of tubular organisms (Brasier et al. 2013).Other researchers accept that the material may document bioturbation, but by mycetozoan amoebozoans rather than by metazoans (G amez Vintaned & Zhuravlev 2013).Over the past two decades, similar material from Siberia, Ukraine, China, India and Namibia (e.g.Shen et al. 2007;Dong et al. 2008;Zhuravlev et al. 2009;Meyer et al. 2012;Rogov et al. 2013aRogov et al. , 2013b;;Tarhan et al. 2014;Darroch et al. 2016;Wood et al. 2017;Zhu et al. 2017;Ivantsov 2018;Luo & Miao 2020;Liu et al. 2022 Here we reanalyse the Khatyspyt Formation Nenoxites material in an attempt to discriminate between body and trace fossil interpretations.Digital reconstruction of the preserved structures from computed tomography (CT) scans permits visualization of their three-dimensional (3D) morphology.Optical microscopy and scanning electron microscopy (SEM) studies elucidate the taphonomic and diagenetic history of the material, and provide insight into its formation.Our results permit both re-evaluation of the trace fossil interpretation proposed for this material, and direct comparison of the Khatyspyt Formation material with similar, broadly coeval, body fossil material.

GEOLOGICAL SETTING
The fossiliferous Khatyspyt Formation of the late Ediacaran Khorbusuonka Group reaches 245 m in thickness and consists of alternating thin limestones and shales, finely laminated thin-to medium-bedded limestones, planar-to wavy-laminated thick-bedded limestones, and intraclastic dolomitized limestones occurring as laterally persistent channelized deposits over tens to hundreds of metres, all punctuated by thin interbeds of volcanic tuff and packages of shales.Sedimentological observations suggest deposition in a starved intracratonic rift basin developed in a marine inner-ramp setting influenced by wave and current activity, with intraclastic limestones representing debris flows deposited adjacent to inferred syn-sedimentary faults (Knoll et al. 1995;Pelechaty et al. 1996;Nagovitsin et al. 2015;Kolesnikov et al. 2018).
The studied material was collected from finely laminated silicified and dolomitized mudstones in the uppermost part of Member 3 of the Khatyspyt Formation (Nagovitsin et al. 2015, section 0602), cropping out at the mouth of Anabyl Creek, left tributary of the Khorbusuonka River, Olenek Uplift, Republic of Sakha, Siberia, Russia (Fig. 1A).Extensive regional silicification in the uppermost beds of Member 3 is attributed to the presence of thin, devitrified volcanic tuff interbeds.A diatreme dated to 543.9 AE 0.3 Ma (the Tas-Yuryakh volcanic complex; Bowring et al. 1993) penetrates the Khatyspyt Formation as well as the overlying Turkut Formation and lowermost part of the Syhargalakh Formation, providing a minimum age constraint for the material under study (Fig. 1;Rogov et al. 2012;Nagovitsin et al. 2015).A maximum age constraint of 550 Ma is provided by a detrital zircon U-Pb isotopic age from the top of the underlying Maastakh Formation (Cherry et al. 2022).
The studied horizons are part of a 2.64-m-thick finingupward package consisting of planar-and hummockystratified pelsparite, overlain by a succession of alternating finely laminated sparite and micrite interbeds.The finely laminated limestone interbeds are characterized by erosive bases and flat tops.Erosional scours cut down to a depth of up to 17 mm and are filled with finely laminated limestone (Fig. 1C, D).The upper part of limestone interbeds can consist of densely packed meniscate structures that occasionally become selectively silicified (Fig. 1C).An interval of 17.23 m logged in the upper part of Member 3 of the Khatyspyt Formation consists of 17 such packages of different thickness.
The studied sample is a 50-mm-thick bed originating from one of these partly silicified limestone interbeds, and was collected from a horizon 26.77 m from the base of the section (Figs 1C-E, 2).The bed has an erosive base, which cuts down to a depth of up to 11 mm into the underlying fossil-bearing stratum (best seen at left in Fig. 2A, also magnified in Rogov et al. 2012, fig.1A-B).The scour was initially filled by finely laminated limestone that does not contain fossils (Fig. 1C; 'dark layer' in Fig. 2B; see also Rogov et al. 2012, fig. 1A-B).The fossil material is confined to the upper portion of the studied stratum (Fig. 2), and gives the appearance of being inverse graded, with larger elements being located towards the top of the bed.The top of the studied stratum has been scoured (Fig. 2B, also magnified in Rogov et al. 2013a, fig.1A).In the studied sample, fossils are preserved three-dimensionally, although elsewhere in the Khatyspyt Formation similar structures can manifest as low hypo-or epirelief on the sediment surface (Rogov et al. 2012, fig. 3).The figured beds are interpreted to have been deposited in a shallow marine storm-influenced environment on a carbonate ramp (Duda et al. 2020).

MATERIAL AND METHOD
A triangular block of the fossil-bearing horizon (CAMSM X 50382.1;lateral dimensions: 6.30 cm, 5.50 cm and 5.42 cm, height: 3.10 cm) (Figs 1C, 2D) was scanned using x-ray microtomography (lCT) at the Natural History Museum in London with a Nikon Metrology HMX ST 225 instrument.X-rays were generated using a tungsten target, with an accelerating voltage of 180 kV, a current of 140 lA, and no filtration.A total of 3142 projections were collected with a 708 ms exposure, and reconstructed to a create a 33.7 lm voxel size volume.In order to explore the data, the 32-bit floating point volume was loaded in Drishti import v2.5.1 (Limaye 2012), then windowed and the histogram stretched around the grey levels of the sample.Data were exported as an 8-bit PNG stack.This stack was then imported into SPIERSedit v2.2.0 (Sutton et al. 2012).The low attenuation contrast of the scan coupled with the density of the rock resulted in beam hardening artefacts (i.e. the greyscale values for each phase were darker inside the sample).These were minimized by creating broadly circular regions of interest in the sample, and applying different linear threshold values to each.From these thresholds, a surface was exported to SPIERSview v2.2.0, and the smallest islands were removed (object, island removal, tiny).The resulting mesh was then exported and then imported into Blender v2.79 (see Garwood & Dunlop (2014) for an overview), in which mesh islands were separated for individual inspection following the methodology outlined in Parry et al. (2017).This provided an overview of the morphology.However, the low contrast, the beam hardening and the presence of diagenetically introduced crystals of different densities inside the specimens, coupled with differential compaction of individual elements, hampered attempts to identify precise fossil outlines in the lCT slice data.These factors precluded the tracing of individual chains of elements through the slab.As such, the specimens presented here were traced by following the trajectory of the elements in AVIZO v8 and searching for other elements in close proximity.If multiple elements were nearby (a frequent situation), the surrounding series were also traced, enabling us to manually assign elements to the correct series.Thin sections cut perpendicular to bedding through the Siberian material (CAMSM X 50382.2.1 to CAMSM X 50382.4.1) were carbon-coated and analysed using a Hitachi S3500-N scanning electron microscope with a silicon drift detector at the School of Earth Sciences, University of Bristol.Crystal chemistry and element maps were acquired using energy dispersive spectroscopy at 15 kV, and a working distance of either 15 or 20 mm.Weight percent elemental maps were generated using standardless quantification (i.e.all count totals were normalized to 100%).The hand specimen and thin sections are accessioned in the Sedgwick Museum, University of Cambridge, UK (CAMSM).All raw files relating to lCT scans, tomographic reconstruction, and SEM are stored in the Bristol Data Repository (Psarras et al. 2023).
In situ sedimentological context of the studied material from the Khatyspyt Formation.A, general view of the sedimentology at the mouth of Anabyl Creek, Khorbusuonka River, Siberia; the undulating erosive base of the fossil-bearing bed is highlighted in dark brown.B, scouring of the fossil bed by overlying deposits; the fossils themselves are truncated by scours in some places; note the dark layer possessing internal lamination in the lowest millimetres of the fossil-bearing bed.C, sedimentology immediately surrounding the fossil-bearing horizon (which appears to have smaller fossil elements towards the base of the bed and larger elements towards the top).D, close-up view of the fossil-bearing horizon showing 'series' of pale green 'elements' surrounded by dark, olive-green 'halos'; variation in the size of elements through the bed is indicated, and could be interpreted as evidence for inverse grading if the elements were originally particles deposited in the sediment; alternatively, the variation in size may reflect a progressive increase in the amount of diagenetic overgrowth of silicic material towards the top of the bed (discussed in the text).Ruler has mm and cm increments.

Three-dimensional reconstruction
Three-dimensional reconstruction of the Khatyspyt material from lCT data shows multiple bowl-shaped structures, here termed 'elements' (referred to as 'intervening menisci' by Rogov et al. 2012, and comparable to the 'crescentic segments' or 'internal nested tubes' of Wang et al. 2021, and'dishes' of Liu et al. 2022), serially arranged in 'series ' (i.e. the 'strings' in Liu et al. 2022) (Figs 3, 4).Note that the packages of sediment occurring between these bowl-shaped structures were described as 'menisci' by Rogov et al. (2012).Each series of elements is enclosed in a distinct cylindrical-elliptical region of sediment of slightly different colour and mineralogical composition to the surrounding substrate, here termed a 'halo' (Figs 1F,2D,3E).This halo is equivalent to that described by Rogov et al. (2012, fig. 3), who interpreted it as a diagenetic artefact of silicification, and also to halos described in the Chinese taxon Ordinilunulatus by Liu et al. (2022, fig.7), who interpreted it as a biological structure.The width of the surrounding halo remains constant throughout a series (Fig. 3E, see also Rogov et al. 2013b, fig. 1c), but halos can be of variable clarity, with some being almost impossible to distinguish from other halos or the substrate, while others, typically surrounding large, well-defined series, have sharp boundaries (e.g.Fig. 2D).
Digital reconstruction of 55 series shows that the elements constituting individual series are consistent in shape and size (Fig. 3).Individual series have broadly consistent element length and length : width ratios (ratios range from 1 to 2; Fig. 5A), with elements from different series measuring 0.63-2.55mm in length and 0.37-1.85mm in width (n = 151 measured elements).Although the elements are reasonably uniform in shape (mean length : width ratio = 1.52, mode = 1.5, Fig. 5), extreme ratios (min.= 1, max.= 3.48) are observed (Fig. 5A, outlier elements).Series are not observed to branch, and do not cross-cut one another in three dimensions.Adjacent elements in a series can be in direct contact, or separated by distances of up to 0.42 mm (Fig. 4).Most digitally reconstructed specimens have a thin, thread-like connection between adjacent elements (248 of 253 observed elements), here termed a 'bridge' (sensu Brasier et al. 2013; referred to as a 'connection' in Ordinilunulatus by Liu et al. 2022), but in the Khatyspyt material this bridge is not consistently positioned with respect to the centre of the elements, and its length and width are variable even in individual series (Figs 3,4).Importantly, when a bridge is observed, its size is considerably greater than the voxel size for the CT scans, suggesting that the absence of such structures between some elements is a genuine absence rather than an artefact of the resolution of the CT data.Individual series follow a variety of 3D trajectories at all angles to the bedding surface, including straight and sub-vertically aligned series, as well as examples that change direction by up to 180°( Fig. 3F).The maximum observed change in angle between two consecutive elements is 50°.Most individual series eventually collide with other specimens when tracked through the sample, preventing conclusive determination of whether they possess distinct terminal morphologies (see the full sample image mesh in the Data Repository).The longest measured individual series contained 31 elements.Examples of isolated elements, not in obvious association with any neighbouring series, were also observed.No obvious, consistently shaped terminal structure was observed in the studied population.

Petrology and SEM
Optical microscopy and SEM analyses confirm a largely dolomitic composition for both the sedimentary matrix and the halos.Micron-scale angular rhombs of sucrosic dolomite, often with clear compositional zoning (Fig. 6F), comprise c. 70% of the matrix, with the remainder composed of feldspars, quartz and minor pyrite grains (Fig. 6).This mineralogical composition is confirmed by elemental mapping of the sample (Fig. 7), which indicates the presence of dolomite (areas of enrichment in Ca, Mg) (Fig. S1), quartz (Si) and feldspars (Al, K).Notably, the halo shows little compositional difference to the surrounding sediment: there appears to be slightly more dolomite (Ca and Mg in the elemental maps) with respect to silica in the halo (Fig. 7), but this perception may result from differences in grain size rather than composition.In contrast, the fine-grained material comprising the bowl-shaped elements is largely silicic (Fig. 6), with occasional larger crystals of intergrown dolomite and calcite, the latter inferred to have formed during late-stage diagenesis as a void infill (e.g.Fig. 6E).Dolomite rhombs infiltrate the silicic bowl-shaped elements at their margins, producing a boundary between these elements and surrounding sediment that is diffuse rather than sharp (Fig. 6D).Silicic material compositionally identical to that in the bowl-shaped elements is present between dolomite crystals in both the halos and the surrounding sediment (Fig. 7).

Interpretation of the material examined
Petrological analysis of the Khatyspyt Formation material confirms the dolomitic composition of the matrix and the silicification of incorporated elements, as recognized by Rogov et al. (2013b).We also recognize a late-stage void-filling crystallization event that has overprinted some of the silicic elements (Fig. 6E).Clearly the specimens have undergone a complex taphonomic history, and we concur with previous authors that diagenetic silicification is likely to be responsible for the diffuse boundaries and ).An originally biomineralized structure would not be expected to possess an infill of randomly arranged micro-and meso-quartz crystals, and therefore we can discount primary silica biomineralization as an explanation for the elements.Given the propensity for dolomite to form late-stage diagenetic destructive replacement textures (as seemingly evidenced here by its overgrowth of silicic material in the matrix; Fig. 6F), and the observed incursion of dolomite crystals into the silicic elements (Fig. 6D), we infer that silicification of this material occurred early, with dolomitization of the matrix taking place at a later stage in the burial history.This inferred early silicification event is probably the result of devitrification of local tuffs in the Khatyspyt Formation, but is consistent with the association of other global occurrences of Nenoxites-like material with silica (although we note that several of those fossils also have associations with clay minerals; e. Morphologically, the structures are composed of concavo-convex elements arranged in stacked series, with frequent thread-like connections (Fig. 3), all enveloped in a cylindrical halo (Fig. 3E).The precise width and morphology of the halo cannot always be identified due to interaction with neighbouring series and conflation by diagenetic alteration, but the width of elements in individual series is relatively consistent, while different series have a range of widths (Fig. 5A).Elements possess similar length : width ratios of c. 1.5 across the studied population (Fig. 5B).
Importantly, we recognize that the prominent halos surrounding the elements (most clearly observed in the studied hand specimen as a darker green rim around the elements) possess the same mineralogy as the surrounding sedimentary matrix (Fig. 7).If halos reflect a biological or materials, or they could result from decay or diagenetic processes during lithification.Early diagenetic microcrystalline silicification of the elements, removing any available pore space inside them, is our favoured explanation for the observed mineralogical distinction, but it is not exclusive to a trace fossil interpretation.It is also possible that the relative depletion of silica in the halos results from the dissolution and removal of pre-existing siliceous material from those areas during silicification of the elements (promoted by organic matter in the elements), although it is difficult to distinguish between these possibilities from the available data.In all scenarios, silicification of the elements would have been followed by dolomitization of both the sedimentary matrix and the halos (Fig. S2).
The Khatyspyt material can be compared with Upper Cretaceous cherts of the Ekofisk Formation ('Fabric II' of Blinkenberg et al. well as the irregular shapes of the elements (Fig. 5), the inconsistent appearance and placement of bridging structures, and the variable clarity of the halos (Fig. 2D).In sum, our taphonomic data suggest that the bowl-shaped elements and halos had different original compositions, but cannot in isolation distinguish between body and trace fossil explanations for the Khatyspyt material.
From a sedimentological perspective, the most parsimonious explanation for the sedimentological context of the fossil-bearing horizons is that high energy flows created a series of erosive scours, which have first been filled by cross-bedded fine sediment, before being draped by planar laminae (Fig. 2).In individual beds, this is all likely to have happened during one event, with cross-beds preferentially filling the scours as flow velocity decreased, before being draped by planar laminae once the scours were filled.The fossil structures largely lie in and above the planar-laminated part of the beds, although they can also be found (albeit rarely) in the uppermost crosslaminae (e.g.Rogov et al. 2012), implying that they were either present and deposited during the later stages of the flow (if they are particles, i.e. body fossils), or were emplaced after sediment deposition had ceased, and cut down through the sedimentary laminae at a later date (if they are trace fossils).
It is possible that the apparent inverse grading results from a progressive increase in the amount of diagenetic overgrowth of silicic material towards the top of the fossil-bearing unit, but it is difficult to determine whether this was the case from the available data (Fig. S2).
Assuming that the increased size of elements towards the top of the bed is an original signal, increased nutrient or oxygen availability for trace-making organisms offers a reasonable explanation for the presence of larger burrows closer to the sediment-water interface, in which case the assemblage could be interpreted as a post-turbidite or post-tempestite ichnocoenosis (Leszczy nski & Seilacher 1991).However, apparent grading is more readily explained by the sorting of discrete objects in a rapidly deposited sedimentary accumulation.
The observation that the fossil material lies above laminated fine-grained sediment would appear to be at odds with an interpretation of the fossils representing a lag deposit, given that if they were behaving as sedimentary particles settling in a waning flow, they might be expected to exhibit normal grading and to lie at the base of the deposit, following the erosive base as in a traction carpet layer in a granular flow (e.g.Lowe 1982).However, an alternative explanation is that the series and elements behaved as buoyant particles in a clast-rich debris flow.In this scenario, larger clasts would be expected to be entrained towards the top of the flow, resulting in reverse grading of any subsequent deposit.Such an explanation is consistent with the repetition of this sedimentary sequence in the local succession (i.e.successive debrite events), and with the random orientations of individual series in the beds (given that each individual series would have been chaotically transported and then rapidly deposited as flow velocity diminished).Observation of rare isolated elements, both in the Khatyspyt Formation and elsewhere, is explained by the series consisting of transportable objects that could disarticulate into individual elements, and is difficult to explain in a meniscate burrow interpretive framework.A debrite interpretation is also consistent with the general storm-and wave-influenced carbonate ramp depositional environment evidenced by the surrounding beds, and previous interpretation of debris flows in the stratigraphic succession (summarized in the Geological Settings section above).Inverse grading might be expected to involve an upward increase in both clast size and volumetric percentage, which also appears to be the case here (Fig. 1E).
Crucially, if the bed was burrowed or bioturbated, the depth of bioturbation would be expected to be consistent along the length of the bed (i.e.being controlled by the redox gradient in the sediment).Instead, the elements in the Khatyspyt Formation material appear to follow the pre-existing topography of the fine-grained planar laminae at the base of the flow (e.g.Fig. 1C, D).This is inconsistent with expected burrowing behaviour but can be explained by the deposition of particles entrained in a debris flow.Rogov et al. (2012Rogov et al. ( , 2013b) ) suggested that scouring of the upper surface of the bed, including scours that cut across individual elements in the bed (Fig. 2B), is more compatible with a trace fossil origin for the Khatyspyt material.The observation of scoured elements logically rules out the fossils being originally mineralized (because the scouring currents would not be expected to be strong enough to erode mineralized elements in a soft sediment).Indeed, the occurrence of Nenoxites specimens elsewhere as 2D compression fossils (e.g.Kolesnikov et al. 2018) provides strong evidence against them having been robustly mineralized original structures.However, an originally soft, sediment filled, or semi-lithified structure could feasibly have been torn or deformed by currents, and there are multiple examples of late Ediacaran organic-walled tubular body fossils that could potentially have behaved in the way we observe if subjected to debris flows.There is uncertainty surrounding both the taphonomic behaviour of invertebrates in debris flows, and the anatomy of these Ediacaran candidate body fossils, but experimental work with modern polychaetes has demonstrated that those soft-bodied organisms can be transported for tens of kilometres in sedimentary density flows without appreciable damage (Bath Enright et al. 2017).Assemblages of tubular body fossils from other late Ediacaran deposits (e.g.mineralized Cloudina specimens in the Nama Group of Namibia) can exhibit comparable densities and abundances within individual event beds (Mehra & Maloof 2018), demonstrating that low-diversity accumulations of body fossils are common in comparable late Ediacaran palaeoenvironments.On the balance of available evidence, we consider the sedimentological context of the Khatyspyt Formation Nenoxites to be most consistent with a body fossil interpretation for the specimens.Considerable research on those taxa confirms that they reflect body fossils of tubular organisms with an annulated body enclosed in an external tube (Luo & Miao 2020;Wang et al. 2021;Liu et al. 2022;Yi et al. 2022).

Insights from other global fossil assemblages
Comparisons between Nenoxites and tubular body fossils similar to Shaanxilithes have precedent (e.g.Hua et al. 2004;Shen et al. 2007;Cai et al. 2011;Meyer et al. 2012;Tarhan et al. 2014;Ivantsov 2018;Luo & Miao 2020;Wang et al. 2021;Yi et al. 2022), with some of those authors considering Nenoxites to be a tubular body fossil, potentially of protistan grade (Luo & Miao 2020).Such comparisons have been based on both morphological and taphonomic evidence, with some authors noting that multiple taxa may actually reflect taphonomic variants of Nenoxites curvus (e.g.Ivantsov 2018).Taxa such as Helanoichnus, Parahorodyskia, and Longbizuiella from the Liuchapo Formation of China (found in association with Nenoxites and the morphologically similar taxon Ordinilunulatus) can possess similar halos (e.g.Dong et al. 2008;Liu et al. 2022;Yi et al. 2022).Those taxa plus Shaanxilithes (note that Helanoichnus has been considered a taphonomic variant of Shaanxilithes ningqiangensis; Wang et al. 2021) possess millimetric internal segments or annulation, sometimes with bridges of material between them (e.g.Luo & Miao 2020;Liu et al. 2022;Yi et al. 2022).Like Nenoxites, Helanoichnus, Longbizuiella and Shaanxilithes can also be preserved either as 2D compressions or 3D silicifications (Chai et al. 2021;Yi et al. 2022).Some Liuchapo Formation Nenoxites taphomorphs show sudden changes in direction (Luo & Miao 2020) and, like the Khatyspyt material, show no evidence for cross-cutting of specimens.Nenoxites has been described from strata older than 557.3 AE 0.6 Ma in the Lamtsa Formation of Russia (Grazhdankin 2014;Yang et al. 2021) et al. 2021), on the basis of material preserved as compressed 2D structures.Displaced segments in individual series in that material were explained as resulting from compaction or decomposition/partial decay of their surrounding (soft tissue) external tube (Wang et al. 2021, fig. 7, red arrows).The 3D material from the Khatyspyt Formation does not show any original cavities inside the elements.However, a gross morphology consisting of a nested, robust internal tube inside a more flexible, potentially unmineralized outer tube (see Wang et al. 2021, figs 5-7, 9, 11) could be consistent with the material we describe.In such a scenario, the halo might define the position of the outer tube, while individual elements may reflect the remains of internal body structure.Alternatively, our petrological data indicate that the halos could reflect diagenetic artefacts resulting from the transport of silica to the elements.
The Khatyspyt Formation material can be compared with smaller tubular branching fossils from China showing similar internal segmented 3D morphologies (Sun et al.Similarly other proposed diagnostic characteristics of those Nenoxites species (namely size, mode of preservation, degree of curvature, spacing between elements, and the irregular nature of element margins) are not considered here to be taxonomically informative; they all have the potential to result from taphonomic processes in the Khatyspyt Formation, and the prevalence of diagenetic silicification and dolomitization at many Nenoxites localities worldwide raises concerns that such characters are not original.Given that all variants can commonly be found in the same deposits, we consider it more likely that their different appearance results from taphonomic alteration of a single original morphotype.
There are taxonomic complexities among Ediacaran tubular body fossils that remain to be resolved.The type material of the type species of Shaanxilithes (S. ningqiangensis Xing et al., 1984) is represented by straight ribbon-like structures c. 6-8 mm wide with sharp boundaries and no visible meniscate sub-structure.That type material as originally defined is therefore dissimilar from Nenoxites curvus.Conversely, a second species, Shaanxilithes erodus Zhang, 1986, does closely resemble Nenoxites curvus.There is therefore an argument for synonymization of Shaanxilithes erodus with Nenoxites curvus, but not the type species Shaanxilithes ningqiangensis (and thus the genus Shaanxilithes), which seems to represent a distinct taxon.Hua et al. (2004) further complicated the issue by expanding the diagnosis of Shaanxilithes ningqiangensis and including sinusoidally curved fossils with a meniscate sub-structure that resemble Nenoxites curvus.We refrain from formal synonymy at this stage because of the heterogeneous nature of the assemblage currently encompassed by the concept of Shaanxilithes, but we note that Shaanxilithes as a genus is in need of a thorough revision, and the Khatyspyt Nenoxites bear strong similarities to some of the material currently assigned to it.Nevertheless, the Khatyspyt material provides support for the candidacy of the Shaanxilithestype suite of fossils as late Ediacaran (c.558-539 Ma) index fossils (Chai et al. 2021;Liu et al. 2022).

Affinity
The main challenges to phylogenetic placement of late Ediacaran macrofossils include the uncertainty surrounding the mechanisms by which soft body parts of early animals were preserved, and the difficulty in identifying synapomorphies shared with extant taxa (Cunningham et al. 2017).The type species of Shaanxilithes (S. ningqiangensis) was originally described as a trace fossil (Xing et al. 1984), but material assigned to the genus has since been considered as being derived from an alga (Hua et al. 2004), a pogonophoran (Chen et al. 1975), a foraminifera-like protist (Luo & Miao 2020) or a metazoan of uncertain phylogenetic position (Bengtson & Zhao 1992;Shen et al. 2007;Meyer et al. 2012;Tarhan et al. 2014;Zhang et al. 2015).Wang et al. (2021) suggested that Shaanxilithes is closely related to cloudinids, which are conventionally interpreted as eumetazoans (e.g.Vinn & Zato n 2012;Schiffbauer et al. 2020;Yang et al. 2020;Shore & Wood 2021), under the assumption that Shaanxilithes has an internal tubular structure composed of nested funnels inside a flexible external sheath.The Khatyspyt Nenoxites, and Shaanxilithes more generally, may exhibit evidence for histological differentiation into what can be interpreted as distinct tissue layers (Wang et al. 2021, fig. 11).Morphologically similar features are achieved by multicellular cyanobacteria, with a mucus sheath enveloping the component cells.However, taphonomy experiments indicate a preservational pathway for multicellular cyanobacteria that contrasts with that seen in Nenoxites and Shaanxilithes, with the cells degrading rapidly, leaving behind a flaccid sheath (Bartley 1996;Cunningham et al. 2012;Manning-Berg et al. 2022).Furthermore, the geometry of the compartments and their interconnection is incompatible with a cyanobacterial interpretation.Although eukaryotes are diverse, there are a limited number of multicellular lineages and few of these show histological differentiation.Slime mould fruiting bodies have been interpreted to exhibit epithelia (Dickinson et al. 2011(Dickinson et al. , 2012)), but their organization is quite distinct from Nenoxites.Otherwise, red algae can achieve relatively complex histological differentiation (Dixon 1973;Lobban & Wynne 1981;Cole & Sheath 1990), but there is no evidence that they undergo differential taphonomy (e.g.Xiao et al. 2004) and the internal compartments in Nenoxites and Shaanxilithes are again difficult to rationalize with a rhodophyte interpretation.Furthermore, the red and green algal models are not compatible with evidence that these body fossils comprise a coquina.This leaves metazoans as the only really credible interpretative model.Depending on whether sponges or ctenophores are accepted as the sister-lineage to all other animals (the sponge-sister topology would lead to the inference of epithelial differentiation being a derived trait; e.g.Nielsen 2019), there remains the possibility that Shaanxilithes is best interpreted as a total group eumetazoan.

CONCLUSION
Reassessment of fossils from the late Ediacaran Khatyspyt Formation of Siberia suggests that material originally described by Rogov et al. (2012) as trace fossils instead reflects a densely packed body fossil coquina of transported, Shaanxilithes-like flexible, non-mineralized tubular organisms.Our observations from 3D reconstruction, sedimentology and petrological analysis confirm that the Khatyspyt Formation material is more consistent with a body fossil origin for Nenoxites.The presence of isolated elements; the lack of cross-cutting specimens; the erosive truncation of specimens by beds above; a lack of branching; reverse grading; mineralogically distinct halos; variable shape; sharp directional changes; and the variable presence of bridge structures are all, in combination, more consistent with a body fossil interpretation (Luo & Miao 2020;Liu et al. 2022;Yi et al. 2022).These observations supplement previously made arguments against an ichnological origin for Khatyspyt Nenoxites (Brasier et al. 2013;G amez Vintaned & Zhuravlev 2013) including the presence of connections between the proposed 'menisci' and the inconsistent morphology and size of the elements, and emphasize the similarity of Nenoxites material to Shaanxilithes-type body fossil material from South China.The taxonomy of such specimens is in need of considerable revision in light of these and other recent discussions (e.g.Luo & Miao 2020;Chai et al. 2021;Wang et al. 2021;Liu et al. 2022;Yi et al. 2022). The

F
I G . 1 .Stratigraphic, geographical and geological context for the studied material.A, location of the field area in northern Siberia (Russia) (star).B, the Anabyl Creek section along the left tributary of the Khorbusuοnka River (lat.71.200625, long.123.726222 in WGS84 datum), with the fossil-bearing strata indicated towards the top of the section (star).C-D, close-up view of two fossiliferous horizons (f) delineated by erosional surfaces (bold red lines); note that the scour at the base of the upper fossil-bearing bed has initially been filled by cross-bedded and then planar-laminated sediment (coloured in D), which is devoid of fossils.E, polished slab of the studied material (CAMSM X 50382.1).F, close-up view of E, showing the halo boundaries surrounding a series.Scale bar lengths/ divisions represent 1 cm (C-E).Stratigraphy follows Nagovitsin et al. (2015).

4
P A P E R S I N P A L A E O N T O L O G Y

F
I G . 3 .3D digital reconstructions of Nenoxites specimens from the Khatyspyt Formation.A, group of 11 densely packed series of elements.B, one individual series, consisting of 10 elements.C, individual series of 12 bowl-shaped elements.D, sketch showing how the measurements of length 'L' and width 'W' were extracted from individual elements of the boxed area of series C. E, a single specimen, without (left), and with (right) its associated halo of broadly constant width.F, a series presenting an overall 180°change in direction.Scale bars denote the maximum width of individual elements (A-C, E) or halo (E).irregular shape of many individual elements and series (e.g.Rogov et al. 2013b g. Dong et al. 2008; Luo & Miao 2020; Yi et al. 2022).

F
I G . 4 .A-D, views from different angles of a sub-set of elements from a single series (broader context seen at top right), showing their bowl-shaped morphology (e.g.outlined element in A, indicated by the green arrow), the bridge-like connections between elements (black arrows), and examples of non-connected pairs of elements (red arrows).Scale bars indicate the maximum width of the terminal element.Note that individual elements in this single series are of similar size, while connections between elements are not consistent in size, shape or position.F I G . 5 .A, length : width ratio of elements in 13 thirteen reconstructed series; note that the means range between 1.3 and 1.65 (mean = 1.52), indicating that the mean length of each element is c. 1.5-fold larger than its width; whiskers represent the lower and upper limits of the data from individual elements in each series, with outlier points visible in two cases (series 3 and 12).B, frequency distribution of the length : width ratios of 151 individual elements; the distribution is positively skewed (2.92) and exhibits positive kurtosis (20.3); most elements have a length : width ratio of between 1.3 and 1.7.biologically induced feature, they probably had a distinct original composition relative to the elements they encompass, in order to explain their differential preservation (with the elements having been entirely silicified and then largely unaffected by later dolomitization).True burrow meniscae would be assumed to comprise packages of sediment (potentially of the same, or different, composition to the surrounding matrix), and the organic-rich sediment between such true meniscae could feasibly undergo preferential silicification (Butts 2014).Alternatively in a body fossil model, the halos and elements may originally have been constructed of different biological F I G .6 .SEM and light microscopy of the Khatyspyt Formation material in thin section CAMSM X 50382.2.1.A, SEM backscatter; B, optical microscopy plane-polarized; C, optical microscopy cross-polarized views of two series of elements.D, close up SEM backscatter image of the diffuse, irregular margin of an element (left), showing the presence of isolated rhombic dolomite crystals (light grey) in the element.E, cross-polarized optical microscopy image of a series with a large, mineralized vug of calcite and dolomite replacing some of the elements (centre).F, zoning in dolomite crystals in the matrix, which seem to have overgrown or truncated preexisting calcite and silicate minerals.Scale bars represent: 1 mm (B, C); 0.5 mm (E); 0.01 mm (F).
2021), which formed via progressive silica replacement interspersed with calcite recrystallization(Blinkenberg et al. 2021).Such cycles of silica dissolution and recrystallization could have filled voids between original calcite crystals and created voids inside individual elements, which could then be filled by later precipitation of calcite to produce the observed carbonate fabrics.In this model, either silicification or recrystallization could have produced the diffuse element boundaries we observe, as F I G .7 .Elemental maps showing the margin of an element and the surrounding sedimentary matrix.The yellow dashed line in the top left image indicates the boundary between halo (H) and matrix (M); the green dashed line reflects the boundary between element (E) and halo.There is little dolomite or calcite inside the elements (as indicated by the Ca and Mg maps).Silica (Si map) is abundant inside the elements, but also common in patches throughout the halo and matrix.Outside the element, K-feldspars are highlighted by the Al and K maps, and rare probable iron sulfides are shown by the Fe and O maps (i.e. the oxygen-poor regions in the latter).Colour brightness reflects elemental weight percentage per pixel.All scale bars represent 200 lm. 1 0 P A P E R S I N P A L A E O N T O L O G Y 20562802, 2023, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/spp2.1531 by Test, Wiley Online Library on [13/11/2023].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2019), but the studied material here is nonbranching.Yi et al. (2022) discussed the similarity of Nenoxites and Palaeopascichnus specimens from the Liuchapo Formation (see also Luo & Miao 2020), and propose synonymization of P. minimus, P. meniscatus and P. jiumensis within Nenoxites.Those authors also erected the new species Nenoxites irregularis and Nenoxites jishouensis (Yi et al. 2022).Palaeopascichnus jiumenensis and N. jishouensis have since been synonymized in the new taxon Ordinilunulatus (Liu et al. 2022), which those authors distinguish from Nenoxites curvus by their possession of fewer elements, more consistent placement of bridges, and the frequent presence of a terminal spherical structure (Liu et al. 2022).However, in all other respectsthe morphology of Ordinilunulatus is similar to the Khatyspyt material, and ontogenetic or taphonomic factors could be responsible for the stated differences.
T A B L E 1 .Global distribution and stratigraphic occurrence of previously reported Shaanxilithes/Nenoxites-like specimens.et al. (1980) Cherts, siltstones and sandstones Shen et al. (2007) (fig.4:7-8); Wang et al. (2021) (figs 3-9) Pang et al. (2021) (fig.S3) Zhengmuguan *Yang & Zheng (1985) (pl.1, fig.8) Cherts, siltstones and sandstones *Li et al. (1997) (pl.5, fig.2) Shen et al. (2007) (fig.4: 1-6) Yuhucun *Luo et al. (1982) Siltstones *Luo et al. (1988) Luo et al. (1991) (no figured specimen) Zhang et al. (2015) (figs 3-5) Tang et al. (2015) (figs 4-5) Gu et al. (2018) (no figured specimen) Dengying *Chen et al. (1975) (pl. 1, figs 8-9) Limestones, siltstones and mudstones *Xing et al. (1984) (pl.8, figs 19-20) *Zhang (1986) (pl.4, figs 11, 13b) Weber et al. (2007) (fig.6) Cai et al. (2011) (fig.1G-I) Meyer et al. (2012) (figs 2, 4) Zhou et al. (2019) (fig.3C) An et al. (2020) (fig.1D-F) Chai et al. (2021) (figs 3-4) Fang et al. (2021) (figs 4-6) Taozichong Hua et al. (2004) (figs 1-6) Siliceous dolostones at the bottom of Taozichong Formation Tuerkeng Wang et al. (2021) (figs 3-9) Siltstones Liuchapo Dong et al. (2008) (figs 7, 10) Cherts, siltstones and sandstones Wang et al. (2012) (fig.4H) Chen et al. (2015) (no figured specimen) Chen & Feng (2019) (fig.2E-F) Luo & Miao (2020) (figs 3E-G, 4A-K, 6A, 7F-L, 8, 9A-C) Ye et al. (2020) (fig.3A) Yi et al. (2022) (figs 3F, 4, 5A-E, 12G-L) Liu et al. (2022) (figs 7, 11C-D) Laobao Dong et al. (2015) (fig.3C) Cherts Piyuancun Dong et al. (2012) (fig.4A-D) Siliceous siltstones and shales Chen & Feng (2019) (no figured specimen) Russia Khatyspyt Zhuravlev et al. (2009) (fig.2); Rogov et al.Darroch et al. (2016) (figs 2C-D, 6) Slabby siltstones and fine-grained sandstones *Studies that reference material that has not been directly observed by us (either via physical study or observation from photographs).P S A R R A S E T A L .: I N T E R R O G A T I N G T H E O L D E S T B I O T U R B A T I O N 1 320562802, 2023, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/spp2.1531 by Test, Wiley Online Library on [13/11/2023].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License body fossil interpretation of Khatyspyt Nenoxites shifts the origin of sediment-penetrative burrowing and bioturbation (most probably indicative of bilaterian animals) closer to the Ediacaran-Cambrian boundary.The current oldest examples of sediment-penetrative burrows and patches of bioturbation now occur in offshore marine environments in the latest Ediacaran Period (e.g.M angano & Buatois 2020; Darroch et al. 2021), with bioturbation not reaching substantial intensities until Cambrian Age 2(M angano & Buatois 2020).Our results also expand the range of taphonomic variability exhibited by tubular organisms that are candidates for latest Ediacaran index fossils.Finally, Shaanxilithes-type body fossils are likely to reflect an addition to the diversity of candidate metazoans in the late Ediacaran.B O W R I N G , S. A., G R O T Z I N G E R , J. P., I S A C H S E N , C. E., K N O L L , A. H., P E L E C H A T Y , S. M. and K O L O S O V , P. 1993.Calibrating rates of Early Cambrian evolution.Science, 261, 1293-1298.B R A S I E R , M. D., C O W I E , J. and T A Y L O R , M. 1994.Decision on the Precambrian-Cambrian boundary.Episodes, 17, 95-100.B R A S I E R , M. D., M c I L R O Y , D., L I U , A. G., A N T C L I F F E , J. B. and M E N O N , L. R. 2013.The oldest evidence of bioturbation on Earth: Comment.Geology, 41, e289.B U A T O I S , L. A. and M AN G A N O , M. G. 2011.Ichnology: Organism-substrate interactions in space and time.Cambridge University Press, 366 pp.B U A T O I S , L. A. and M AN G A N O , M. G. 2016.Ediacaran ecosystems and the dawn of animals.27-72.In M AN G A N O , M. and B U A T O I S , L. (eds) The trace-fossil record of major evolutionary events.Springer.Topics in Geobiology, 39.B U D D , G. E. and J E N S E N , S. 2000.A critical reappraisal of the fossil record of the bilaterian phyla.Biological Reviews, 75, 253-295.B U T T S , S. H. 2014.Silicification.The Paleontological Society Papers, 20, 15-34.B Y K O V A , N., G I L L , B. C., G R A Z H D A N K I N , D., R O G O V , V. and X I A O , S. 2017.A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: implications for its taphonomy and paleoecology.Geobiology, 15, 572-587.B Y K O V A , N., L O D U C A , S. T., Y E , Q., M A R U S I N , V., G R A Z H D A N K I N , D. and X I A O , S. 2020.Seaweeds through time: morphological and ecological analysis of Proterozoic and early Palaeozoic benthic macroalgae.Precambrian Research, 350, 105875.C A I , Y., H U A , H., Z H U R A V L E V , A. Yu., G AM E Z V I N -T A N E D , J. and I V A N T S O V , A. 2011.Discussion of 'First finds of problematic Ediacaran fossil Gaojiashania in Siberia and its origin'.Geological Magazine, 148, 329-333.C A I , Y., X I A O , S., L I , G. and H U A , H. 2019.Diverse biomineralizing animals in the terminal Ediacaran Period herald the Cambrian explosion.Geology, 47, 380-384.C A R B O N E , C. and N A R B O N N E , G. M. 2014.When life got smart: the evolution of behavioral complexity through the Ediacaran and early Cambrian of NW Canada.Journal of Paleontology, 88, 309-330.C H A I , S., W U , Y. and H U A , H. 2021.Potential index fossils for the Terminal Stage of the Ediacaran System.Journal of Asian Earth Sciences, 218, 104885.C H E N , C. and F E N G , Q. 2019.Carbonate carbon isotope chemostratigraphy and U-Pb zircon geochronology of the Liuchapo Formation in South China: constraints on the Ediacaran-Cambrian boundary in deep-water sequences.Palaeogeography, Palaeoclimatology, Palaeoecology, 535, 109361.C H E N , M. E., C H E N , X. G. and L A O , Q. Y. 1975.An introduction to the Metazoa fossil from the upper Sinian System in southern Shensi and its stratigraphic significance.Scientia Geologica Sinica, 2, 181-192.[in Chinese with English abstract] C H E N , D., Z H O U , X., F U , Y., W A N G , J. and Y A N , D. 2015.New U-Pb zircon ages of the Ediacaran-Cambrian boundary strata in South China.Terra Nova, 27, 62-68.C H E N , Z., C H E N , X., Z H O U , C., Y U A N , X. and X I A O , S. 2018.Late Ediacaran trackways produced by bilaterian animals with paired appendages.Science Advances, 4 (6), eaao6691.C H E N , Z., Z H O U , C., Y U A N , X. and X I A O , S. 2019.Death march of a segmented and trilobate bilaterian elucidates early animal evolution.Nature, 573, 412-415.C H E R R Y , L. B., G I L L E A U D E A U , G. J., G R A Z H D A N -K I N , D. V., R O M A N I E L L O , S. J., M A R T I N , A. J. and K A U F M A N , A. J. 2022.A diverse Ediacara assemblage survived under low-oxygen conditions.Nature Communications, 13, 7306.C O L E , K. M. and S H E A T H , R. G. 1990.Biology of the red algae.Cambridge University Press.C U N N I N G H A M , J. A., T H O M A S , C.-W., B E N G T S O N , S., M A R O N E , F., S T A M P A N O N I , M., T U R N E R , F. R., B A I L E Y , J. V., R A F F , R. A., R A F F , E. C. and D O N O -G H U E , P. C. J. 2012.Experimental taphonomy of giant sulphur bacteria: implications for the interpretation of the embryo-like Ediacaran Doushantuo fossils.Proceedings of the Royal Society B, 279, 1857-1864.C U N N I N G H A M , J. A., L I U , A. G., B E N G T S O N , S. and D O N O G H U E , P. C. J. 2017.The origin of animals: can molecular clocks and the fossil record be reconciled?BioEssays, 39, e201600120.D A R R O C H , S. A. F., B O A G , T. H. and R A C I C O T , R. A. 2016.A mixed Ediacaran-metazoan assemblage from the Zaris Sub-basin, Namibia.Palaeogeography, Palaeoclimatology, Palaeoecology, 459, 198-208.D A R R O C H , S. A. F., C R I B B , A. T., B U A T O I S , L. A., G E R M S , G. J. B., K E N C H I N G T O N , C. G., S M I T H , E. F., M O C K E , H., O ' N E I L , G. R., S C H I F F B A U E R , J. D., M A L O N E Y , K. M., R A C I C O T , R. A., T U R K , K. A., G I B S O N , B. M., A L M O N D , J., K O E S T E R , B., B O A G , T. H., T W E E D T , S. M. and L A F L A M M E , M. 2021.The trace fossil record of the Nama Group, Namibia: exploring the terminal Ediacaran roots of the Cambrian explosion.Earth-Science Reviews, 212, 103435.D I C K I N S O N , D. J., N E L S O N , W. J. and W E I S , W. I. 2011.A polarised epithelium organised by beta and alpha catenin predates cadherin and metazoan origins.Science, 331, 1336-1339.D I C K I N S O N , D. J., N E L S O N , W. J. and W E I S , W. I. 2012.An epithelial tissue in Dictyostelium challenges the traditional origin of metazoan multicellularity.BioEssays, 34, 833-840.D I X O N , P. S. 1973.Biology of the Rhodophyta.Oliver and Boyd.D O N G , L., X I A O , S., S H E N , B. and Z H O U , C. 2008.Silicified Horodyskia and Palaeopaschichnus from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis.Journal of the Geological Society, 165, 367-378.D O N G , L., S O N G , W., X I A O , S., Y U A N , X., C H E N , Z. and Z H O U , C. 2012.Micro-and macrofossils from the Piyuancun Formation and their implications for the Ediacaran-Cambrian boundary in southern Anhui.Journal of Stratigraphy, 36, 600-610.D O N G , L., S H E N , B., L E E , C.-T. A., S H U , X., P E N G , Y., S U N , Y., T A N G , Z., R O N G , H., L A N G , X., M A , H., Y A N G , F. and G U O , W. 2015.Germanium/silicon of the Ediacaran-Cambrian Laobao cherts: implications for the bedded chert formation and paleoenvironment interpretations.Geochemistry, Geophysics, Geosystems, 16, 751-763.d o s R E I S , M., T H A W O R N W A T T A N A , Y., A N G E L I S , K., T E L F O R D , M. J., D O N O G H U E , P. C. J. and Y A N G , Z. 2015.Uncertainty in the timing of origin of animals and the limits of precision in molecular timescales.Current Biology, 25, 2939-2950.D U D A , J.-P., T H I E L , V., R EI T N E R , J. and G R A Z H -D A N K I N , D. 2016.Opening up a window into ecosystems with Ediacara-type organisms: preservation of molecular fossils in the Khatyspyt Lagerst€ atte (Arctic Siberia).PalZ, 90, 659-671.D U D A , J.-P., L O V E , G. D., R O G O V , V. I., M E L N I K , D. S., B L U M E N B E R G , M. and G R A Z H D A N K I N , D. V. 2020.Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerst€ atte (Arctic Siberia, Russia).Geobiology, 18, 643-662.D U N N , F. S., L I U , A. G. and G E H L I N G , J. G. 2019.Anatomical and ontogenetic reassessment of the Ediacaran frond Arborea arborea and its placement within total group Eumetazoa.Palaeontology, 62, 851-865.D U N N , F. S., L I U , A. G., G R A Z H D A N K I N , D., V I X S E -B O X SE , P., FLA NN ER Y-S U TH E RLA ND , J., GR EE N, E., H A R R I S , S. and D O N O G H U E , P. C. J. 2021.The developmental biology of Charnia and the eumetazoan affinity of the Ediacaran rangeomorphs.Science Advances, 7 (30), eabe0291.D U N N , F. S., K E N C H I N G T O N , C. G., P A R R Y , L. A., C L A R K , J. W., K E N D A L , R. S. and W I L B Y , P. R. 2022.A crown-group cnidarian from the Ediacaran of Charnwood Forest, UK.Nature Ecology & Evolution, 6, 1095-1104.E R W I N , D. H., L A F L A M M E , M., T W E E D T , S. M., S P E R L I N G , E. A., P I S A N I , D. and P E T E R S O N , K. J. 2011.The Cambrian conundrum: early divergence and later ecological success in the early history of animals.Science, 334, 1091-1097.E V A N S , S. D., D R O S E R , M. L. and G E H L I N G , J. G. 2017.Highly regulated growth and development of the Ediacara macrofossil Dickinsonia costata.PLoS One, 12, e0176874.E V A N S , S. D., H U G H E S , I. V., G E H L I N G , J. G. and D R O S E R , M. L. 2020.Discovery of the oldest bilaterian from the Ediacaran of South Australia.Proceedings of the National Academy of Sciences, 117, 7845-7850.F A N G , R., L I A N G , Y., H U A , H. and Z H A N G , Z. 2021.First report of the problematic Ediacaran fossil Shaanxilithes from the Jiu-cheng Member of Zhujiaqing Section in Huize, Yunnan Province.Acta Palaeontologica Sinica, 60 (1), 2020035.F E D O N K I N , M. A. 1976.Traces of multicellular animals in the Valdai Group.Izvestiya Akademii Nauk SSSR, Seriya Geologicheskaya, 4, 129-132.F E D O N K I N , M. A. 1988.Paleoichnology of the Precambrian-Cambrian transition in the Russian Platform and Siberia.12.In L A N D I N G , E., N A R B O N N E , G. M. and M Y R O W , ; Yi et al. 2022) has been described, offering multiple points of comparison.Concurrently, meticulous study of extensive collections of the body fossil Shaanxilithes Xing et al., 1984, a potential index fossil for latest Ediacaran strata (e.g.Meyer et al. 2012; Chai et al. 2021), has identified multiple taphonomic modes for Shaanxilithes preservation (Wang et al. 2021), some of which bear strong resemblance to the Siberian Nenoxites material (similarities between some Nenoxites and Shaanxilithes specimens were also recognized by Meyer et al. 2012, Ivantsov 2018 and Rogov et al. 2012, while more recently Nenoxites and certain Palaeopascichnus specimens from South China have been considered to reflect taphonomic variants or closely related taxa; Luo & Miao 2020; Yi et al. 2022; although see also Liu et al. 2022).
20562802, 2023, 6, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/spp2.1531 by Test, Wiley Online Library on [13/11/2023].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Rogov et al. (2012) recognized the morphological similarity between Khatyspyt Formation Nenoxites material and the late Ediacaran taxa Shaanxilithes ningqiangensis, Helanoichnus helanensis and Palaeopascichnus minimus, P. meniscatus, and P. jiumenensis from China (e.g.Shen et al. 2007; Dong et al. 2008; note that the latter taxon, which can be an order of magnitude smaller than the Khatyspyt material, was discussed as a species of Nenoxites by Yi et al. 2022 and then formally synonymized within Ordinilunulatus jiumenensis by Liu et al. 2022).
and in the Liuchapo Formation of South China, which lies close to the Ediacaran-Cambrian boundary (540.7 AE 3.8 (AE6.6)Ma; Chen & Feng 2019).All globally reported material is of late Ediacaran age and occurs predominantly in silicified deposits, including limestones, siltstones and mudstones (Table 1; Yi et al. 2022).This age range corresponds with that of the known range of Helanoichnus, Longbizuiella, Ordinilunulatus and Shaanxilithes (Chai et al. 2021; Liu et al. 2022; Yi et al. 2022), while the preservational style hints at a favourable taphonomic window for preservation of such organisms in rock units that have undergone silicification.Specimens of Shaanxilithes from the late Ediacaran of North China have been interpreted as nested internal tubes encased in flexible, annulated external tubes (Wang