Thirteen sedimentary facies were identified within the Serra Grande Group deposits exposed in the study area. These facies are represented by distinct fine- to coarse-grained sandstones, fine-grained heterolithic beds, and siltstones, as described and interpreted in Table 1. Coarse-grained to conglomeratic sandstones dominate the lower and upper portions of the Serra Grande Group sedimentary succession (Ipú and Jaicós formations, respectively) (Figure 2). The middle section consists of amalgamated beds of well-sorted, fine- to medium-grained sandstones interspersed with subordinate heteroliths and siltstones (Tianguá Formation). The deposits are predominantly lenticular in geometry and were grouped into four facies associations, representing braided delta deposits within a shallow marine platform setting.

A set of 36 ichnotaxa were recognized in the Serra Grande Group (Table 2), with most of the structures occupying superficial layers within the substrate (shallow tiers, sensuBromley, 1996). The ichnofauna is both morphologically diverse and abundant, with the greatest concentration of trace fossils occurring in the fine-grained sandstones of the Tianguá Formation (BS 4–5), with subordinate records at the top of the Ipú Formation and at the base of the Jaicós Formation (BS 1–2) (Figure 2). Heimdallia chatwinni and epichnial ridges are the most abundant ichnofossils throughout the sedimentary succession (Figure 5B). Trace fossils are commonly observed on bedding planes and are dominated by long horizontal burrows with straight to slightly curved trajectories preserved in full relief, epirelief, and hyporelief. Preservation in hyporelief is observed primarily in the FA2 deposits (facies Ht and Sla). The pavements are generally exposed in riverbeds and waterfalls and extend over large areas, allowing the observation of evidence of biostabilization, mainly wrinkle structures and leveling ripples (Figures 3N, O). Two distinct ichnocenoses were identified in the deposits of the Serra Grande Group: a glacial ichnocenosis, characterized by a very low-diversity suite of shallow burrows, and a postglacial ichnocenosis, formed by low to moderately diverse suites dominated by horizontal trace fossils.

The Hirnantian glaciation had a significant impact on the marine invertebrate biota at the end of the Ordovician (e.g., Delabroye and Vecoli, 2010). The combination of dramatic cooling and sea-level fall strongly affected the benthic community in shallow seas, triggering the first Phanerozoic mass extinction (e.g., Sheehan, 2001; CocksTorsvik, 2021). The Parnaíba Basin was located nearest the South Pole during the Hirnantian (Caputo, 1984; Caputo and Santos, 2019; CocksTorsvik, 2021), and our data indicate a significant ichnofaunal turnover in the deposits of the Serra Grande Group, which represents the basin infill during the Late Ordovician–Early Silurian.

The benthic biota living in the Parnaíba Basin during the Hirnantian are represented by the glacial paleoichnocenosis, preserved in the upper portion of the Ipú Formation (FA1 deposits). The paucity of trace fossils, their localized occurrence, and the low ichnodiversity in the glacial suite indicate the environmental harshness during the Hirnantian cooling. The main components of the trace fossil suite consist of resting/dwelling traces of large-sized burrowing sea anemones (Conichnus isp.) and burrows of potential commensal or scavenger organisms (Palaeophycus tubularis), suggesting severe restrictions on ecospace occupation by other endobenthic organisms. Beaconites, Circulichnis, Cylindrichnus, Furnaisichnus, Planolites, and Skolithos were also reported in deposits of the Ipú Formation (e.g., Viana et al., 2010; Souza et al., 2015; Rusinelli et al., 2025) but were not observed in the study area. The material described as Conichnus by Viana et al. (2010) was posteriorly recognized as body fossils of sea anemones (Barroso, 2016) and recently named as Arenactinia ipuensis (Barroso et al., 2025). Although the body fossil characterization is well-established, the ichnotaxobases of Conichnus can also be clearly accessed (see Table 2 for description). Thus, we assume it as a coupled body and trace fossil preservation.

The ichnogenus Conichnus primarily occurs in high-energy, shallow marine environments (e.g., Pemberton et al., 1988), which are particularly hostile for benthic invertebrate fauna in polar settings, due to the prevalence of erosional processes that limit food source and substrate stability, the low water temperatures, and the strong salinity fluctuations during melt water input (e.g., Watson et al., 2018). A few species of burrowing sea anemones can survive in these harsh conditions (e.g., Williams, 1981; Fautin et al., 2013; Benedict et al., 2025), colonizing sandy or muddy substrates in sheltered settings (Ansell and Peck, 2000; Watson et al., 2020).

There is currently a gap in knowledge regarding the biology and ecology of invertebrate benthic species in polar waters (Watson et al., 2018). Due to ice cover, benthic species are scarce and have a limited dispersion in the shallow Ross Sea (Antarctica) (Dayton and Oliver, 1977). Thatje et al. (2008) suggest that the thick Antarctic ice cover limited primary production and food sources, hence limiting the shallow marine benthic life to a few habitats. Sessile invertebrates, because of their restricted capacity to move and disperse, were severely affected by the glaciation. The few burrowing sea anemones recorded in shallow marine settings of Antarctica are part of a low-diversity benthic community, along with a few other burrowing species, mostly mollusk bivalves and polychaetes (e.g., Watson et al., 2020). The low ichnodiversity in the glacial suite possibly reflects this low diversity observed in equivalent current settings, indicating that only a few endobenthic organisms can tolerate the extreme conditions of polar shallow benthos since their diversification in the Cambrian–Ordovician. Burrowing sea anemones play an important role as ecosystem engineers, increasing the organic input in the sediment around their burrows through bioirrigation (e.g., Kristensen et al., 2012; Mángano et al., 2024). This process is crucial for increasing the food supply in the substrate in high-energy settings such as polar shallow seas. This might explain the occurrence of Palaeophycus isp. running toward Conichnus isp. in the glacial suite.

The most striking feature of the glacial suite is the large size of Conichnus isp. Although sea anemones show a substantial growth in cold waters (e.g., Chomsky et al., 2004), most extant sea anemone species exhibit diameters ranging from 1.5 to 5 cm (El-Bawab, 2020). The burrowing sea anemones recorded in shallow marine polar settings rarely surpass 10 mm in diameter (e.g., Watson et al., 2018, Watson et al., 2020). Likewise, plug-shaped trace fossils attributed to sea anemones, such as Bergaueria, Conostichus, and Conichnus, show average diameters up to 5 cm (e.g., Alpert, 1973; Pemberton et al., 1988). Curiously, the limited number of reported Hirnantian ichnofauna in Gondwana beds also contain large Conichnus, with diameters ranging from 7 to 23 cm (Le Heron, 2010; Davies et al., 2020; Rusinelli et al., 2025).

A study carried out by Anthony (1977) on the high-latitude sea anemone Metridium senile demonstrated that prey capture by large anemones is more efficient in low-flow regimes. Accordingly, they tend to inhabit low-energy, protected areas within high-energy shallow settings. The lack of spreiten and the absence of stacked Conichnus in FA1 indicate that sedimentation rates were low, not forcing the sea anemones to reposition themselves in the substrate. The presence of sharp-walled Arenicolites isp., which indicates stiff-ground colonization, further supports the low sedimentation rates. Preservation of the glacial suite exclusively along the northeastern border of the Parnaíba Basin (Figures 1, 2) also suggests substrate occupation in a preferred setting, probably in protected zones.

In contrast, the postglacial deposits (Tianguá and Jaicós formations; FA2 and FA3) exhibit a higher ichnodiversity, as evidenced by the ichnofauna of the tidal flat, prodelta, delta input, storm, and delta trace fossil suites. Twenty-seven ichnotaxa and a few unidentified trace fossils occur in the postglacial deposits, compared to three in the glacial beds, indicating a significant ichnofaunal turnover. This turnover appears to have occurred shortly after the end of the Hirnantian glaciation, as 21 distinct ichnotaxa are found in the tidal flat suite at the very base of the Tianguá Formation, which has an early Llandovery age attested by biostratigraphy (e.g., Grahn et al., 2005) and by the presence of Arthrophycus alleghaniensis and Cruziana acacencis (Memória et al., 2023).

We find that Heimdallia chatwinni (Figure 4R) is the main ichnomarker of the initial postglacial transgression episodes in the Parnaíba Basin. It forms moderately bioturbated beds in the uppermost strata of the Ipú Formation. This occurrence indicates that the glacial deposits were still available as softgrounds for the pioneer settlers (FA2, Table 1). As a deeper-tier trace fossil (e.g., Bromley and Ekadale, 1986), H. chatwinni was preserved at the top of the glacial beds, surviving the erosive transgression pulses that potentially destroyed the shallow-tier burrows. H. chatwinni has also been found below the base of transgressive beds in Lower Paleozoic Gondwana deposits and is considered an ichnological indicator of marine transgressions (e.g., Buatois et al., 1998; Bradshaw and Harmsen, 2007; Savage et al., 2013; Sedorko et al., 2017; Bradley et al., 2018; Davies et al., 2020; Memória et al., 2021).

The tidal flat suite, found in tide-influenced delta plain deposits (FA2, Table 1), is the most diverse assemblage of postglacial paleoichnocenosis. It also exhibits the highest ichnodisparity, with 11 distinct architectural patterns of horizontal burrows representing feeding, grazing, and crawling behaviors, two vertical architectures of dwelling or resting burrows, and two distinct complex burrows for dwelling–feeding purposes (Table 2). The significant ichnofaunal turnover observed since the very base of the Tianguá deposits suggests that climate amelioration was rapid, given the early Llandovery age of these beds (e.g., Memória et al., 2023). A global eustatic rise in sea level marked the early Llandovery, following the Hirnantian glacial episode (e.g., Johnson et al., 1991). Sea-level rise and continental input driven by the substantial meltwater discharge provided new benthic ecospace and favorable conditions for substrate colonization in coastal and shallow marine habitats. The occurrence of mica and phytodetritus in FA2 deposits (Spl, Ssar, and Mp facies; Table 1) indicates this continental input (e.g., Bhattacharya and Walker, 1991; Bhattacharya, 2010), which likely increased food availability on these bottoms. The preservation of the ripple crests, double mud drapes, and macroscopic microbially induced sedimentary structures (leveling ripple and wrinkle structures) at the top of sandstone beds in the Ssar and Sla facies suggests substrate biostabilization and the prevalence of a moderate- to low-energy hydrodynamic regime, which was essential for food storage in the benthos. Evidence of substrate biostabilization also implies short periods of stillness (e.g., Noffke et al., 2001; Davies and Shillito, 2018), which are common in tidal cycles (e.g., Walker, 1992; Dalrymple, 1992, Dalrymple, 2010).

The tracemakers preferentially colonized the shallow and middle tiers of the substrate, indicating that resources were concentrated in the upper sediment layers. Feeding and grazing are the primary behaviors reflected by the trace fossils in the tidal flat suite, suggesting an abundance of food and low ecological pressure. This evidence implies that the newly formed ecospace had favorable ecological conditions, which reduced competition and increased the survival and breeding chances of pioneer populations (e.g., Bromley, 1996; Buatois and Mángano, 2011; Gougeon et al., 2018; Buatois et al., 2020). These less stressful conditions, coupled with potentially warmer waters, are assumed to have been the main trigger of the ichnofaunal turnover that followed the end of the Hirnantian Ice Age in the Parnaíba Basin.

Microbial mats probably had an important role in establishing stable ecological conditions in these shallow benthic habitats (e.g., Seilacher and Pflüger, 1994; Seilacher, 1999). The presence of leveling ripple and wrinkle structures on the top of the bioturbated pavements of the Ssar and Sla facies (Figure 3) indicates microbial mat development in the tidal-influenced deposits of FA2. These structures occur together with fully preserved ripple crests and are well exposed on wide pavements extending for hundreds of meters, bearing a significant number of straight to gently curved horizontal shallow burrows (Didymaulyponomos rowei, Didymaulichnus lyelli, and epichnial ridges) and trails (Psammichnites plummeri) that follow long courses (≥ 1 m) (Figures 4N, O, 5F, I). This pattern of bioturbation is frequently seen in present-day intertidal environments with meso- and macrotidal regimes during low tide (e.g., Getty and Hagadorn, 2008), where microbial mats are pervasive (Noffke et al., 2001; Mata and Bottjer, 2009, MataBottjer, 2013; Davies et al., 2016). Microbial mats are ecosystem engineers and play a significant role in tidal flats, being responsible for most primary productivity, maintaining moisture in the substrate of exposed beds during low tides, and providing habitats for several macro- and meiofaunal organisms (e.g., Horne and Goldman, 1994; Seilacher, 2007; Cuadrado et al., 2014; Lv et al., 2016). The trace fossils are preserved in a thin layer that overlies the ripple crests and do not cross them, suggesting that microbial mats were thick and that the tracemakers explored the mat itself, rather than behaving as undermat miners (e.g., Seilacher, 2007; Mángano et al., 2016). Epichnial preservation of shallow burrows, such as D. rowei, D. lyllei, and the epichnial ridges, in full relief was also possible due to biostabilization, which prevented substrate erosion (e.g., Davies and Shillito, 2018; Cuadrado et al., 2021).

Following the Llandovery maximum flooding, the Parnaíba Basin experienced a highstand phase, resulting in the establishment of coastal deltaic systems (e.g., Barrera et al., 2020), represented by the coarsening- and thickening-upward succession that characterizes the upper portion of the Serra Grande Group. These deposits include the prodelta, delta input, and storm trace fossil suites preserved in Ht, Sla, and Shs facies of FA3 (Tianguá Formation), as well as the delta trace fossil suite preserved in facies Spl and Ssg of FA2 (Jaicós Formation) (Table 1; Figure 3). The degree of bioturbation and the ichnodiversity both decreased toward the top of the deltaic succession, most likely due to variations in salinity and oxygen availability in the substrate caused by the discharge of freshwater in prodelta settings (e.g., MacEachern et al., 2005; MacEachern and Bann, 2020). Despite these stressful ecological conditions, ichnodisparity remained moderate to high, suggesting the survival of some strategic ecological niches.

The prodelta suite is the most diverse assemblage of deltaic deposits. It consists of feeding, grazing, dwelling/equilibrium, and crawling traces, which are often reported in shallow marine environments. Burrow diameters/widths, however, are usually half or smaller than those found in the tidal flat suite, and bioturbation intensity drops from high (BS 5–6) to moderate (BS 3) near the top of the hererolithic succession. These features are characteristic of shallow marine prodelta environments (e.g., MacEachern and Bann, 2020) and reflect the response of benthic fauna to stress generated by freshwater input in lower shoreface settings (e.g., Pemberton et al., 2001; Buatois et al., 2005; MacEachern et al., 2007).

The delta input trace fossil suite occurs in the middle portion of the deltaic succession, within thin-bedded, coarsening-upward cycles composed of Ht and Sla facies (FA3, Table 1) that pinch out the prodelta deposits. The reduction in ichnodiversity by half, the prevalence of burrows produced by marine opportunistic organisms, the small size of the burrows, and the dominance of Phycosiphon in this suite are consistent ichnological signatures of rapid sedimentation and stressful physicochemical conditions caused by river discharge in shallow marine settings (e.g., BromleyEkadale, 1986; MacEachern et al., 2005, MacEachern et al., 2007; Dafoe et al., 2010; Hurd et al., 2014; Bayet-Goll and Neto De Carvalho, 2016; Bhattacharya et al., 2020; MacEachern and Bann, 2020). Oxygen depletion apparently was a key factor limiting substrate colonization, as two of the five ichnotaxa present in the suite (Chondrites and Phycosiphon) are produced by chemosymbionts (Ekdale and Mason, 1988; Uchman et al., 2016). The small size of the trace fossils also reflects responses to oxygen and salinity fluctuations (e.g., Wightman et al., 1987; Buatois et al., 2005).

Storm deposits (Shs facies) occur at the top of the deltaic succession (FA3, Table 1), enhancing oxygen conditions and providing a colonization window for upper-tier niches in the substrate (e.g., PembertonMacEachern, 1977). The storm suite represents a postevent colonization and is dominated by vertical burrows inhabited by suspension- and filtering-feeder polychaetes, crustaceans, and brachiopods feeding at the substrate/water interface. Although the ichnodiversity remains low, there is a change in trace fossil composition, probably driven by the reestablishment of more stable physicochemical conditions in shoreface settings, boosted by storm waves (e.g., PembertonMacEachern, 1977). The presence of Rosselia and Lingulichnus in the storm suite indicates that these storm surges occurred frequently or promoted high rates of sedimentation, as their tracemakers can quickly adjust their burrows under such energetic conditions (e.g., Nara, 1997, Nara, 2002; Horodyski et al., 2024; Netto et al., 2014).

The delta trace fossil suite is found locally in thin-bedded, fine-grained micaceous sandstones at the base of the Jaicós Formation (Spl and Ssg facies, Table 1) and represents lateral expressions of FA2 at the base of FA4. Despite the presence of Chondrites and dubious occurrences of Bergaueria, the ichnotaxa present in this suite fairly occur in freshwater/oligohaline settings (e.g., Buatois et al., 2005). The low ichnodiversity and degree of bioturbation (BS 1–2), along with the dominance of simple facies-crossing burrows, such as Palaeophycus, Planolites, and Lockeia, are also indicative of strong salinity fluctuations in these deltaic settings (e.g., Buatois et al., 2005; MacEachern et al., 2007; Lima and Netto, 2012; La Croix et al., 2015).

The trace fossil suites present in the Serra Grande Group represent distinct expressions of shallow marine ichnofaunas commonly reported in the Early Paleozoic (e.g., Ekdale and Bromley, 2003; MacEachern et al., 2005, MacEachern et al., 2007; Buatois and Mángano, 2011; MacEachern et al., 2012). The prodelta suite represents the resident ichnofauna (preevent suite) of the Parnaíba Basin shallow marine environments, while the delta input, storm, and delta suites represent the postevent substrate colonization (Pemberton and MacEachern, 1977; MacEachern et al., 2005, MacEachern et al., 2007). Despite the fall in ichnodiversity, ichnodisparity remains high, indicating that unique niches in the endobenthic community could be maintained even under ecological stress. This suggests the existence of periods of more stable ecological conditions in the shoreface settings, when ecological stressors were less severe, or that stress-tolerant animals (sensu Vermeij, 1978) filled these niches.

The massive evolutionary radiation triggered by the GOBE resulted in an exponential increase in marine biodiversity, as well as the development of trophic networks and new ecological niches (Sepkoski, 1995; Webby et al., 2004; Fortey and Cocks, 2005; Harper, 2006; CocksTorsvik, 2021; Servais et al., 2023; Liu et al., 2025). This led to a rise in ichnodiversity and ichnodisparity in the Early Paleozoic strata (e.g., Mángano and Buatois, 2007; Mángano et al., 2016). During the Late Ordovician, trace fossils were found in marine deposits worldwide (Figure 7). The Katian or Katian–Hirnantian ichnofaunas are relatively diverse (e.g., Kerr and Eyles, 1991; McCann, 1990; Mikuláš, 1990; Alvaro et al., 2007; Hansen et al., 2009; Belaústegui et al., 2016; Bayet-GollNeto De Carvalho, 2016; Knaust and Desrochers, 2019; Toom et al., 2019; Knaust et al., 2022; Moghalu et al., 2025; Singh et al., 2024), suggesting substrate colonization prior to the first pulse of the LOME (Harper et al., 2014). However, their record in the Hirnantian (after LOME) is primarily concentrated in low-paleolatitude deposits (Supplementary Material 1), far north of the ice caps in high-latitude Gondwanan terranes (e.g., Hanken et al., 2016; Knaust and Desrochers, 2019; Knaust et al., 2022). Low-latitude shallow marine ichnofaunas can be found in the Langøyene and Røyse formations of the Baltoscandian Basin, Norway (Hanken et al., 2016), as well as the Ellis Bay Formation of Anticosti Island, NE Canada (Knaust and Desrochers, 2019). Deeper-marine assemblages were reported in the Welsh Basin, Wales, UK (e.g., McCann, 1990; Benton and Hiscock, 1996). Knaust et al. (2022) assumed a Hirnantian age for the ichnofossiliferous deposits that characterize the middle portion of the Kosov Formation (Prague Basin, Czech Republic). However, these deposits occur approximately 70 m below the first appearance of the Hirnantian fauna, characterized by the Hirnantia sagittifera community at the very top of the Kosov Formation (Mikuláš, 1990). Considering that this fauna marks the beginning of the Hirnantian (Cocks and Torsvik, 2021), the described ichnofauna is older, probably Katian.

Currently, the record of trace fossils in Hirnantian glacial deposits appears to be limited to Gondwana (Figure 8B; Supplementary Material 1). Le Heron (2010) described plug-shaped burrows with an approximate diameter of 6 cm in the Ordovician glacial pavements of the Tamadjert Formation in Algeria, which he called “enigmatica” and interpreted as resting burrows of “coelenterates” (Cnidaria). The ichnofauna, similar to that of the Ipú Formation, is extremely limited in diversity and occurs locally, with a prevalence of large cnidarian burrows. This scarcity likely reflects extremely harsh environmental conditions, as Algeria and NE Brazil were located at high latitudes (> 60° S) during the Hirnantian, near the South Pole (Cocks and Torsvik, 2021; Figure 8). A more diverse ichnofauna, composed of eight ichnotaxa, was reported by Davies et al. (2020) in glaciogenic deposits of South Africa (Pakhuis Formation, Table Mountain Group), representing a retreating ice sheet formed in lower latitude (~ 30° S; Figure 8). As in other Gondwana Hirnantian ichnofaunas, large plug-shaped burrows (Metaichna), with diameters of approximately 20 cm, are present in the South African assemblage. The presence/prevalence of plug-shaped burrows, likely produced by soft-bodied cnidarians, in the Gondwana Hirnantian ichnofauna reinforces the hypothesis that these animals could tolerate the marked physicochemical fluctuations caused by glaciation in shallow-marine settings.

In contrast to the rich record of benthic organisms and their traces during the Ordovician, the glacial Hirnantian strata show a general reduction of 45% in ichnodiversity, depicting the sharp decline that followed the LOME, when 85% of global biodiversity vanished (e.g., Harper et al., 2014) (Figure 7; Supplementary Material 1). This reduction reaches 74% in Gondwana ichnofaunas and 87% in deposits from paleolatitudes > 60° (Figure 8), indicating that the severe ecological conditions caused by glaciation impoverished the shallow-marine benthos of Gondwana, resulting in low ichnodiversity during the Hirnantian Ice Age. The abrupt sea-level fall at the beginning of the Hirnantian and the advance of ice caps strongly constrained the occupation of shallow-marine benthos in higher southern paleolatitudes. Despite the reduction in ichnodiversity, ichnodisparity remains high, suggesting that the tracemakers occupying the main ecological niches were tolerant species (e.g., Vermeij, 1978; Davies et al., 2020).

The Llandovery (early Silurian) is marked by a rapid recovery of burrowing benthic faunas in shallow seas (e.g., Kumpulainen et al., 2006), with trace fossil diversity almost tripling worldwide. This pattern is also observed in the Gondwana realm across all paleolatitudes (Figure 8; Supplementary Material 1). Climate amelioration and global sea-level rise following the demise of the Hirnantian Ice Age (e.g., Landing and Johnson, 2003; Caputo and Santos, 2019; CocksTorsvik, 2021) reopened the colonization window for nontolerant/opportunistic tracemakers (e.g., Pollard et al., 1993; Goldring et al., 2004, Goldring et al., 2007). The most frequent trace fossils in Llandovery deposits are Chondrites, Cruziana, Palaeophycus, Phycodes, Planolites, and Rusophycus (Supplementary Material 1). Ichnodisparity also increased in the Llandovery, as evidenced by 37 distinct architectural categories by Buatois et al. (2017) in the reported ichnofaunas, while 38 occur in the preglacial Ordovician ichnoassemblages (Figure 7). This significant rise in ichnodisparity suggests that optimal or near-optimal environmental conditions were reestablished in the marine realm at the beginning of the Silurian (e.g., Buatois and Mángano, 2013), likely triggered by warmer temperatures (Figure 7). The ichnodisparity in the Serra Grande Group follows this same pattern, with the occurrence of 22 distinct architectural categories in the Tianguá Formation, which represents the Llandovery beds in the Parnaíba Basin. Considering that only three architectural designs were represented in the Hirnantian ichnofaunas reported in the Ipú Formation (Figures 7, 8), the end of the Hirnantian Ice Age seems to have been the main driver of the ichnofaunal turnover observed in the Serra Grande Group.