Palaeopascichnus ‌Palij, 1976 is a worldwide distributed Ediacaran genus and is among the most emblematic fossils of this period before the Cambrian Explosion (see Boag et al. 2016; Kolesnikov & Desiatkin 2022). Many studies have been devoted particularly to its description, taxonomy, taphonomy and paleoenvironment (Haines 2000; Dong et al. 2008; Antcliffe et al. 2011; Lan & Chen 2012; O’Donnell 2013; Hawco et al. 2017, 2019; Kenchington et al. 2017; Jensen et al. 2018; Kolesnikov 2018, 2019; Kolesnikov et al. 2018b; Hawco 2020; Liu & Tindal 2020; Desiatkin et al. 2021; Kolesnikov & Desiatkin 2022). Palaeopascichnus is also considered as an Ediacaran biostratigraphic marker (Kolesnikov 2019), although this opinion is not entirely shared (Liu & Tindal 2020).

Traces of organic activity possibly related to the Ediacaran biota were formerly reported in the Neoproterozoic terranes of Central Dobrogea (Eastern Romania) (Oaie 1992, 1993, 1998, 2010). The distinction between ichnofossils and body fossils and their paleontological affinity, however, remained to be precisely determined considering the more recent data on the Ediacaran biota. In the last 15 years, numerous field investigations led to new discoveries of Ediacaran remains, and allowed the revision of some previous determinations (Saint Martin et al. 2012, 2013; Saint Martin & Saint Martin 2018). Among the specimens assigned to Romanian Ediacaran biota, one trace first reported and described by Oaie (1992, 1993) has drawn particular attention. According to this author, it consists of parallel, meandering, segmented “half-moon shaped” imprints, which can be considered as movement trails but their exact ichnological affinity was uncertain. However, a detailed re-examination of this “imprint” leads us to consider it as a body fossil, herein attributed to Palaeopascichnus. The recent discovery of another specimen shows a distribution probably wider than what is observable in the field because of fracturing, schistosity and coverage by lichens. Although they are embedded in metamorphosed schistose sediments, these specimens have the advantage of being sufficiently well preserved to provide additional data on this classic Ediacaran fossil. Details of the structure of this fossil are provided and considerations on the stratigraphic and palaeoenvironmental consequences are discussed.

The study focuses on imprints observed on bed surfaces of a set of sediments outcropping in Central Dobrogea (Fig. 1). This region is located between the Peceneaga-Camena Fault at the north and the Capidava-Ovidiu Fault at the south (Fig. 1B). It belonged to the Moesian platform with a Precambrian basement represented by Middle and Upper Proterozoic terranes. Neoproterozoic deposits were formerly described as “greenschist formation” in a flyschoid series (Kräutner 1988). However, the sedimentological characteristics indicate weakly metamorphosed mainly fine- and medium-grained sediments exhibiting numerous original sedimentary structures (Jipa 1967, 1968). These deposits belong to the 5000 m thick Histria Formation (Seghedi & Oaie 1994, 1995) composed of two lower and upper coarse members (Beidaud and Sibioara members) of sandstone separated by the thinner Haidar member with pelites and siltites (Seghedi & Oaie 1995; Oaie 1999). Seghedi & Oaie (1995), Oaie (1999) and Oaie et al. (2005) argued that the sedimentological, structural and mineralogical features of the Histria Formation point to foreland basin accumulations, in accordance with results of geochemical and detrital zircon distribution data (Żelaźniewicz et al. 2009). The low-grade metamorphic and weakly deformed clastic rocks of the Histria Formation correspond to median to distal turbiditic sequences (Oaie 1998; Oaie et al. 2005; Seghedi et al. 2005; Balintoni et al. 2011; Balintoni & Balica 2016; Melinte-Dobrinescu et al. 2020). The basin may have been filled by sediments issued from a continental margin dominated by an active volcanic arc (Oaie et al. 2005; Seghedi et al. 2005). Nevertheless, the frequent surfaces with various wrinkled structures suggest the occurrence of microbial mats resulting in the formation of Microbially Induced Sedimentary Structures (MISS; Saint Martin et al. 2011, 2012, 2013). Thus these microbial mats clearly challenge the distal turbidite paradigm.

A Late Proterozoic-Early Cambrian age was firstly estimated for the Histria Formation, based on geochemical K/Ar datation (about –572 million years; Giuşcă et al. 1967) and palynological assemblages (Kräutner et al. 1988). Detrital zircon U/Pb ages later suggested a maximum Late Ediacaran depositional age (Żelaźniewicz et al. 2009; Balintoni et al. 2011; Balintoni & Balica 2016). The discovery of various imprints attributable to Ediacaran biota confirms the age of the median Haidar member of the Histria Formation (Oaie 1992, 1993; Saint Martin et al. 2011, 2013; Saint Martin & Saint Martin 2018; Melinte-Dobrinescu et al. 2020).

The sampling of the surface bearing the studied imprints was impossible because of the outcrop conditions, the nature of the rocks, the schistosity and the fracturing. Thus, a faithful replica of the surfaces was made using silicone molding during fieldwork. Then, a back molding in resin was performed (see Hawco 2020). The technique of photogrammetry finally allowed to highlight details of the reproduced surface. Measurements were performed using ImageJ software (Schneider et al. 2012).

Although widespread, Palaeopascichnus is an enigmatic fossil within the Ediacaran biota. It was originally described as a trace fossil by Palij (1976) and thus referred as an ichnogenus. Palaeopascichnus and other similar forms known by numerous Ediacaran occurrences were considered a fossil displacement trace (e.g., Glaessner 1969; Palij et al. 1979; Fedonkin 1981; Cope 1982; Palij et al. 1983; Glaessner 1984; Fedonkin 1985; Crimes 1987; Hofmann 1987; Narbonne et al. 1987; Pacześna 1989; Fedonkin 1990a; Crimes 1992; Fedonkin 1992; Cope & Bevins 1993; Crimes 1994; Jenkins 1995; Waggoner 1998; Gehling et al. 2001; McCall 2006; McIlroy & Horák 2006; Parcha & Pandey 2011; Tiwari et al. 2013; Ivantsov et al. 2015; Levashova et al. 2015; McIlroy & Brasier 2016). The more complete Palaeopascichnus specimen from Casimcea, well visible on a bed surface, was also first interpreted as traces of movement on the background, corresponding to an undetermined ichnogenus (Oaie 1992, 1993). Later, Oaie et al. (2005) related this meandering trace to the Nereites ichnofacies suggesting deep-water environments. Oaie (2010) and Saint Martin et al. (2012) only evoked traces of movement or grazing without further details. More recently, Melinte-Dobrinescu et al. (2020) discussed the similarity of the Casimcea specimen with the ichnogenus Scalarituba ‌Weller, 1899, known only from the Middle Paleozoic to the Recent. Several characters of Palaeopascichnus specimens, also observed in the Romanian samples, rule out the hypothesis of a trace of displacement or grazing: the superposition of the prints, the subdivision of the branches, among others. For these reasons, the interpretation of Palaeopascichnus as a trace of organic activity was generally abandoned in favor of a body fossil. Several possible similarities or affinities were then proposed: undetermined body fossil (Jensen 2003), algae (Haines 2000; Jensen et al. 2006), agglutinated uniseriate foraminifers (Dong et al. 2008), agglutinated xenophyophorian rhizopods (Seilacher et al. 2003; Seilacher & Mrinjek 2011; Seilacher & Gishlick 2015; Kolesnikov 2019; Kolesnikov & Desiatkin 2022), undetermined protozoan (Antcliffe et al. 2011), agglutinated foraminifera (Kenchington et al. 2017), possible protistan (Hawco et al. 2019). The putative affinities of Palaeopascichnus as body fossils are actually closely dependent on how the interpretation of the elements of the chains, often described as chambers. The recent studies were naturally oriented towards an affinity with organisms possessing chambers. The protist hypothesis is then logical and seems legitimate. Thus, morphometry analyses (Kenchington et al. 2017; Hawco et al. 2019; Kolesnikov & Desiatkin 2022) indicated compatibility with a protist nature with an agglutinated body or test. The agglutinated contour of the chambers may point to organisms close to tested protozoa whose phylogeny with fossil or current groups remains to be established (Kolesnikov & Desiatkin 2022). The Romanian specimens lack evidence for agglutination of chamber walls and thus cannot provide definitive arguments in favor of a well-argued affinity. The S1 Casimcea specimen clearly shows a tubular outline containing the succession of segments. On the other hand, surfaces S1 and S2 in Casimcea also display parallel meandering traces of the same width without visible segments, which could correspond to “empty” chains of Palaeopascichnus. Buatois & Mangano (2016) described similar specimens from the Ediacaran of Newfoundland (Canada) and interpreted them as poorly preserved structure mimicking a fossil trace. This fact raises the possibility of hypothetic tube-dwelling segmented organism. Considering the segments structuring the Palaeopascichnus body fossils as chambers does however not resolve the question of communications between chambers and the growth of the supposedly unicellular individual from one chamber to the other.

According to recent studies (Kolesnikov et al. 2018b; Kolesnikov 2019; Kolesnikov & Desiatkin 2022), only P. ‌delicatus, P. ‌linearis and P. ‌gracilis should be retained within the corpus of Palaeopascichnus. Their paleogeographic distribution is worldwide with occurrences for instance in Europe, Asia, Australia, North America. The distribution of P. ‌linearis detailed by Kolesnikov et al. (2018b) and Kolesnikov & Desiatkin (2022) is large: China, India, Newfoundland (Canada), Australia, Norway, Wales (U.K.), Ukraine, Russia (White Sea, Urals, Siberia...). The discovery of P. ‌linearis in Romania enlarges its paleogeographic distribution (westward domain) in line with the previous occurrences reported in Ukraine (Grytsenko 2016; Golubkova et al. 2017; Nesterovsky et al. 2018; Soldatenko 2018).

The age range of all Neoproterozoic terranes of Dobrogea is not clearly established. The Haidar member of the Histria Formation containing the Palaeopascichnus specimens yields other members of the Ediacaran biota (Saint Martin et al. 2013; Melinte-Dobrinescu et al. 2020), such as Beltanelliformis ‌brunsae ‌Menner, 1974 (Saint Martin & Saint Martin 2018). Although the Ediacaran-Cambrian boundary cannot be formally identified on the field, the presence of Palaeopascichnus confirms the Ediacaran age of the Haidar member (Fedonkin 1990b; Fedonkin et al. 2007; Gehling & Droser 2009; Grytsenko 2016; Högström et al. 2013; Brasier et al. 1994; Carbone & Narbonne 2014; Liu & Conliffe 2015; Ebbestad et al. 2016; Ivantsov 2017; Golubkova et al. 2017; Høyberget et al. 2017; Ivantsov 2018; Ivantsov et al. 2018; Jensen et al. 2018; Nesterovsky et al. 2018; Bobrovskiy et al. 2019; Hawco et al. 2019; Kolesnikov 2019; Soldatenko et al. 2019; Liu & Tindal 2020; Desiatkin et al. 2021; Moczydłowska et al. 2021; Kolesnikov & Desiatkin 2022; Bowyer et al. 2023; Clarke et al. 2023; Kolesnikov et al. 2023a, b). It should be noted that the Neoproterozoic sediments of Podolia (Ukraine) dated around -557 Ma (Soldatenko et al. 2019) contain an Ediacaran biota close to that observed in Romania in the Haidar member of the Histria Formation (Saint Martin et al. 2013; Melinte-Dobrinescu et al. 2020; unpublished personal data), which is compatible with the youngest occurrence of Palaeopascichnus ‌linearis according to Kolesnikov & Desiatkin (2022) and Kolesnikov et al. (2023a).

Since all the Histria Formation has been attributed to deposits of turbidite system, the levels containing Palaeopascichnus in Casimcea have been naturally described as belonging to a system of distal fine-grained turbidites (Oaie 1992; Melinte-Dobrinescu et al. 2020). In this case, the comparison with the shallow water ichnogenus Scalarituba suggested by Melinte-Dobrinescu et al. (2020) is irrelevant (Conkin & Conkin 1968; Archer 1984).

The occurrences of Palaeopascichnus are generally related to relatively shallow variable marine settings, from very shallow littoral, to upper slope shelf or deltaic front environments (Narbonne et al. 1987; Haines 1987, 1990; Cope & Bevins 1993; Gehling et al. 2000; Haines 2000; Martin et al. 2000; Grazhdankin 2004; Fedonkin & Vickers-Rich 2007a, b, c, d; Gehling & Droser 2009; Grazhdankin et al. 2009; Corrick 2012; Fedonkin et al. 2012; Gehling & Droser 2013; Menon et al. 2013; Grazhdankin 2014; Pacześna 2014; Dong et al. 2015; Liu et al. 2015; Liu & McIllroy 2015; Buatois & Mangano 2016; Grytsenko 2016; McIlroy & Brasier 2016; Ivantsov et al. 2018; Nesterovsky et al. 2018; Reid et al. 2018; Hawco et al. 2019; Becker-Kerber et al. 2020, 2021, 2024; Bobrovskiy et al. 2020; Xiao et al. 2021; Bowyer et al. 2023; Clarke et al. 2023; Kolesnikov et al. 2023a). The spatial distribution of fossil body assemblages from different Ediacaran deposits led Boag et al. (2016) to integrate Palaeopascichnus into White Sea-type assemblages, in an inner shelf position. The occurrences of Palaeopascichnuslinearis point to the following possible deposit environments: shallow inner shelf (Kolesnikov et al. 2015, 2019), internal ramp affected by wave and current activity (Kolesnikov et al., 2018b), extremely shallow and intertidal setting (Desiatkin et al. 2021; Kolesnikov et al. 2023a, b), prodelta system (Bobkov et al. 2019) or nearshore marine setting (Liu & Tindal 2020). A probable link with subaqueous shrinkage cracks and MISS has also been reported (Gehling & Droser 2009; Hawco et al. 2019; Liu & Tindal 2020). Moreover, the nearest known occurrences of Palaeopascichnus are issued from the Mogilev Formation cropping out Podolia (Ukraine), which points to shallow water conditions with the frequent implication of microbial mats (Fedonkin & Vickers-Rich 2007b; Ivantsov et al. 2015; Grytsenko 2016; Nesterovsky et al. 2018; Soldatenko 2018; Martyshyn & Uchman 2021). The depositional environment of the specimens studied here thus likely corresponds to depth much shallower than that of deep turbidites, in line with our sedimentological observations. The HCS-type structures observed locally at Casimcea (Fig. 3E) and widely distributed at Rahman (Fig. 5B,C) indeed illustrate the influence of storm waves. The co-occurrence of palaeopascichnid remains and HCS-like structures has also been reported in numerous other Ediacaran sedimentary series (e.g., Narbonne et al. 1987; Haines 1987, 1990; Jenkins et al. 1993; Gehling 2000; Gehling et al. 2000; Grazhdankin 2004; McIlroy & Horák 2006; Grazhdankin et al. 2009; Corrick 2012; Kolesnikov et al. 2015; Nagovitsin et al. 2015; McIlroy & Brasier 2016; Bobkov et al. 2019; Becker-Kerber et al. 2020; Shahkarami et al. 2020; Xiao et al. 2021; Bowyer et al. 2023). The Romanian specimens of Palaeopascichnus occur in sediments with oscillating ridges (Casimcea, Fig. 2C) or lingoid ridges (Rahman; Figs 4F, 5A) resulting from currents and/or oscillating waves. The Casimcea and Rahman surfaces with MISS-type textured structures also testify the presence of microbial mats (Figs 3F, 5D-E). According to several studies (Hawco et al. 2019; Becker-Kerber et al. 2020, 2021), microbial mats promoted the preservation of Palaeopascichnus, as also demonstrated by actualistic experiments (Bobrovskiy et al. 2019). More generally, the widespread presence of MISS in other Ediacaran deposits of Central Dobrogea led to the same conclusions (Saint Martin et al. 2011, 2013; Oaie et al. 2012).

Fossils of Palaeopascichnus from Neoproterozoic terranes of Dobrogea confirm the widespread palaeogeographic distribution of this genus as well as its occurrence in shallow marine environments. Because these fossils are not easy to distinguish on the field, like other Ediacaran fossil bodies in the region, we can assume that new field prospecting could lead to the discovery of additional specimens. The Ediacaran outcrops of Romania should be carefully resampled and reinterpreted in terms of palaeoenvironments, owing that the present observations are not compatible with distal turbidic system. The Ediacaran outcrops of Romania thus constitute a potential for understanding a new bioprovince of the Ediacaran biota.

This work was carried out thanks to the ATM program of the Muséum national d’Histoire naturelle, Paris and the research program CNRS-MNHN, France/GeEcoMar, Romania. All the steps of the molding process on the field and counter-molding in the laboratory were made by Philippe Richir. We deeply thank George Oaie† and Antoneta Seghedi (GeoEcoMar) for guiding us in the field of the Ediacaran Romanian terranes and particularly for bringing us to the Casimcea outcrop. We especially thank Johnathan Antcliffe (University of Lausanne) who kindly shared his expertise with us and guided us towards the identification of our specimens. The manuscript has been significantly improved thanks to the reviewers, Dr Volodymyr Grytsenko (National museum of Natural history of Ukraine) and Dr Abederrazak El Albani (Université de Poitiers, France).