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Abstract

Compression-preserved acid-resistant organic-walled microfossils (OWM) entombed in fine-grained siliciclastic rocks provide an essential tool for biostratigraphic correlation, especially when alternative methods are scarce. These fossils offer vibrant insights into the evolution and diversification of the Proterozoic history of eukaryotes. In this study, the authors report the new occurrence of exceptionally well-preserved acanthomorphic acritarch taxa, specifically Trachyhystrichosphaera Timofeev and Hermann, from the Neoproterozoic carbonaceous shale of the Raipur Group in the Chhattisgarh Supergroup. OWM identified here comprise two species of Trachyhystrichosphaera: T. aimika and T. botula. Globally, recovered species of Trachyhystrichosphaera are found in assemblages of Latest Mesoproterozoic and early Neoproterozoic OWM and are considered potential index fossils of Tonian age (1000-720 Ma, early Neoproterozoic). Present data from the Saradih Limestone of the Raipur Group collectively add to the growing diversity of Tonian fossils, which were previously constrained to be older than 1000 Ma, that is, Mesoproterozoic.

Keywords

Tonian organic-walled microfossil crown group eukaryotes Chhattisgarh supergroup India

INTRODUCTION

Compression-preserved, acid-resistant, organic-walled microfossils (OWM), including characteristic and index microfossils entombed in fine-grained siliciclastic rocks, provide a suitable tool for biostratigraphical correlation and deciphering the age of fossil-bearing strata. These OWMs endorse remarkable mile spots in the Proterozoic oceans, especially the origin, evolution, and diversification of early eukaryotic life. Moreover, during the Tonian period (1000-720 Ma), in the Neoproterozoic, there was a pronounced stepwise expansion in global taxonomic diversity and morphological variation, encompassing traits like scales, tests, and biomineralisation—a time often linked to crown group eukaryote diversification (Knoll et al., 2006; Mughal et al., 2025; Parfrey et al., 2011; Shields-Zhou et al., 2016; Xiao & Tang, 2018; Yang et al., 2016). These crown group eukaryotes show comparisons to specific modern eukaryotic clades, including amoebozoans, stramenopiles, fungi, green and red algae (Butterfield, 2005a, 2007, 2009, 2015; Butterfield et al., 1994; Lenton et al., 2014; Loron, Francois, et al., 2019; Peterson & Butterfield, 2005; Porter & Riedman, 2019; Porter et al., 2003; Tang et al., 2020) with their enigmatic affinities. Moreover, eukaryote-derived sterane biomarkers gave the impression initially during this period, ca. 900-800 Ma (Brocks, 2018; Love et al., 2009). Given these evolutionary changes, several OWM (including acritarchs and vase-shaped microfossils [VSM]) have been projected to become potential index taxa intended for this interval.

In contrast, acanthomorphic acritarch Trachyhystrichosphaera aimika—spherical vesicle with few, irregularly distributed, hollow cylindrical processes, considered as a potential late Mesoproterozoic and Tonian index fossil that is documented from more than a dozen localities worldwide in rocks ca. ~1100-720 Ma (Baludikay et al., 2016; Beghin et al., 2017; Butterfield et al., 1994; Li et al., 2019; Loron, Rainbird, et al., 2019; Riedman & Sadler, 2018; Srivastava, 2009; Tang et al., 2017a; Tang et al., 2017b; Tang et al., 2013, 2015) displaying pronounced perspective to endorse the delineation of the Tonian Global Boundary Stratotype Section and Point (GSSP). Other OWM are Cerebrosphaera globosa, ca. ~800-740 Ma and VSM, ca. ~760-730 Ma, which reflect the most promising markers of the latest Tonian time. In the geological account, Trachyhystrichosphaera was instituted by Timofeev and Hermann (1976) as two genera with the type species T. aimika from the Latest Mesoproterozoic Lakhanda Group in the Siberian Platform. After that, several published records have critically reviewed its worldwide distribution and stratigraphic inferences with recently reported radiometric dates.

In Indian records, Trachyhystrichosphaera has been described from the Vindhyan Supergroup (Prasad et al., 2005; Srivastava, 2009) and the lower Madhubani Group of Ganga Supergroup (Tang et al., 2017a) of the peninsular region. The present study documents a new occurrence of the acanthomorphic acritarch Trachyhystrichosphaera in the carbonaceous shale of the Saradih Limestone in the Raipur Group, Chhattisgarh Supergroup, and discusses its biostratigraphic implications. The latest Palaeoproterozoic-Neoproterozoic Chhattisgarh Supergroup in the Indian subcontinent has been the focus of contemporary geobiological investigation because of its thick unmetamorphosed sediments and well-preserved organic-walled fossils (Singh & Babu, 2013; Singh & Sharma, 2016, 2021, 2022; Singh et al., 2019a, 2019b, 2021). The sedimentary succession of the Chhattisgarh basin is one of the most promising Proterozoic basins of Peninsular India, exposed to an area of over 33000 km² in parts of Odisha and Chhattisgarh states. Nearby, ~2,300 metres thick, largely undeformed and unmetamorphosed Chhattisgarh Supergroup unconformably places over the Bastar Craton. It is divided into two sub-basins: (a) the Baradwar Sub-basin to the east and (b) the Hirri Sub-basin to the west (Figure 1). However, the absence of consistent radiometric and biological age limitations for this sedimentary sequence confines our capacity to fully capitalise on the Chhattisgarh Supergroup’s rich geological and palaeontological history. Subsequently, the debate on the position of the different stratigraphic units in the Chhattisgarh basin is well discussed in different publications (Basu et al., 2013). A debate exists regarding the position of the Singhora Group within the Chhattisgarh Supergroup, with some questioning its independent identity and considering it a lateral extension of the Chandarpur Group (Dhang & Patranabis-Deb, 2012). Recently, the independent identity of the Singhora Group has been established based on the occurrence of age-restricted fossil Tappania in the Singhora Group (Singh et al., 2019b) and Jacutianema in the Chandarpur Group (Singh & Sharma, 2016). Additionally, the depositional age of the topmost level of the Chhattisgarh Supergroup is especially controversial. The occurrence of Tonian age-restricted taxa Trachyhystrichosphaera in the Raipur Group would be significant in determining the age of the upper Chhattisgarh succession.

Figure 1.

Generalised geological map of the Baradwar Sub-basin in and around Saradih area showing the location of the study area (modified after Patranibs-Deb & Chaudhuri, 2008).

GEOLOGICAL BACKGROUND AND AGE OF THE RAIPUR GROUP

The Baradwar Sub-basin, covering an area of ~8000 km², lies east of the main Chhattisgarh basin and is well-exposed in parts of Chhattisgarh and Odisha states. It contains a ~2300 m thick sedimentary sequence of mixed siliciclastic-carbonate rocks, unconformably overlying the basement composed of the Sonakhan Greenstone Belt and Sambalpur Granite (Chakraborty et al., 2015; Patranabis-Deb, 2004). Lithostratigraphically, the Chhattisgarh Supergroup is divided into four groups (in ascending order): the Singhora, Chandarpur, Raipur, and Kharsia (Table 1).

Table 1.

Generalised stratigraphic succession of Chhattisgarh Supergroup in Baradwar and Hirri Sub-basins (after Mukherjee et al., 2014).

	BARADWAR SUB-BASIN		HIRRI SUB-BASIN
CHHATTISGARH SUPERGROUP	Kharsia Group	Nandeli ShaleSarnadih Sandstone	Raipur Group	Maniari FormationHirri FormationTarenga FormationChandi FormationGunderdehi FormationCharmuria Formation
	~~~~~~~~~~~~Unconformity~~~~~~~~~~~~~~
	Raipur Group	Churtela ShaleSaradih Limestone*Gunderdehi ShaleSarangarh Limestone
	Chandarpur Group	Kansapathar FormationChaporadih FormationLohardih Formation	Chandarpur Group	Kansapathar FormationChaporadih FormationLohardih Formation
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Unconformity~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Singhora Group	Chhuipali FormationBhalukona FormationSaraipali FormationRehatikhol Formation	Not Exposed
Archean Basement (Sonakhan & Sambalpur Granites)

The Raipur Group, the third unit in this sequence, consists of ~950 m of carbonate-dominated sediments and is well-exposed near Saradih, ~22.4 km NNE of Sarangarh. A broad marine subtidal to intertidal paleoenvironment has been proposed for its stromatolite-rich succession (Moitra, 2003). However, Patranabis-Deb (2004) suggested a shallow-water platform setting for its carbonates in the Baradwar Sub-basin. The Raipur Group is further subdivided into four formations (ascending order): the Sarangarh Limestone, Gunderdehi Shale, Saradih Limestone, and Churtela Shale (Patranabis-Deb & Chaudhuri, 2008) (Table 2). The Sarangarh Limestone, the basal unit, gradationally overlies the Chandarpur Group and comprises black, brown, grey, and mauve limestone, transitioning upward into purple shale. The Gunderdehi Shale is dominated by brown calcareous shale (~90% of its thickness), with minor green shale, stromatolitic limestone, sandstone, and tuff. The Saradih Limestone, overlying the Gunderdehi Shale, consists mainly of limestone and dolomite with minor shale. In the Hirri Sub-basin, it correlates with the Chandi Formation. Its sedimentary architecture features thick interbedded dolomite, green shale, chert, limestone, black shale intercalations, and massive dolomite (Patranabis-Deb & Chaudhuri, 2008), with gently dipping dolomite as the dominant lithology. The Churtela Shale, stratigraphically equivalent to the Tarenga Formation in the Hirri Sub-basin, overlies the Saradih Limestone (Chakraborty et al., 2015; Mukherjee et al., 2014). It consists of red shale, green tuffaceous shale/mudstone, and minor dolomite (Patranabis-Deb & Chaudhuri, 2008).

Table 2.

Generalised lithostratigraphic succession of the Chhattisgarh Supergroup (Das et al., 1992; Patranabis-Deb & Chaudhuri, 2008; Mukherjee & Ray, 2010).

	GROUP	FORMATION	LITHOLOGY	AGE
CHHATTISGARH SUPERGROUP	Kharsia	Nandeli ShaleSarnadih Sandstone	Gypsiferous purple shale and dolomiteSandstone and conglomerate
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Unconformity~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Raipur	Churtela ShaleSaradih Limestone*Gunderdehi ShaleSarangarh Limestone	Purple shale and tuffDolomite/Stromatolitic Limestone, shaleCalcareous shale with stromatolitic limestoneFlaggy limestone and shale	1000 Ma (Tuff)¹
	Chandarpur	Kansapathar FormationChaporadih FormationLohardih Formation	Quartz areniteGlauconitic sandstone/siltstone, black shaleSubarkose with basal conglomerate	1641 ± 120 Ma (Dolerite Intrusive)²
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Unconformity~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Singhora	Chhuipali FormationBhalukona FormationSaraipali FormationRehatikhol Formation	Stromatolitic limestone and Variegated shaleQuartz arenite and minor shaleVariegated shale/siltstone, tuff/porcellaniteSandstone with conglomerate at the base	c.1500 Ma (Tuff)³
Archean Basement (Sonakhan & Sambalpur Granites)

Geochronological Constraints

SHRIMP U–Pb dating of zircons of rhyolitic tuffs (Sukhda and Dhamda tuffs), which occurred at the top of the Raipur Group, yielded an age of ~1000 Ma (Basu et al., 2008; Bickford, Basu, Mukherjee, et al., 2011; Bickford, Basu, Patranabis-Deb, et al., 2011; Patranabis-Deb et al., 2007). Bickford, Basu, Patranabis-Deb, et al. (2011) reported a Concordia upper intercept age of 993 ± 8 Ma from magmatic zircons in the Dhamda Tuff (Tarenga Formation, coeval with the Sukhda Tuff in the Churtela Shale). Carbon isotope stratigraphy further suggests a late Mesoproterozoic age for the Charmuria and Chandi carbonates (George & Ray, 2020; George et al., 2019). These studies support a Mesoproterozoic depositional age (1500-1000 Ma) for the Chhattisgarh basin.

Contradictions with palaeontological evidence

However, biostratigraphic data conflict with these geochronological constraints. Based on the occurrence of OWM and VSM, previous studies estimated a late Riphean (~800-700 Ma) age for the Raipur Group (Gupta, 2004; Moitra, 2003; Schnitzer, 1969). Stromatolite assemblages in the Chandi Formation were also interpreted as mid- to upper Riphean age (Chatterjee et al., 1990). In contrast, permineralised microfossils in the Saradih Limestone (below the Sukhda Tuff) suggest a Cryogenian age (Babu et al., 2014; Singh & Babu, 2013; Singh et al., 2019a). Additionally, Ediacaran marker microfossils (Obruchevella) have been reported from the Saradih Limestone (Singh et al., 2019a), further complicating age interpretations. Thus, while geochronological data predominantly support a Mesoproterozoic age, palaeontological evidence suggests younger (Neoproterozoic) depositional intervals, highlighting unresolved discrepancies in the basin’s chronostratigraphic (Basu & Bickford, 2015).

MATERIALS AND METHODS

For palaeobiological research, samples were obtained from the carbonate unit of the Saradih Limestone, which outcrops along both banks of the Mahanadi River near Saradih Village (21°43″36.75N; 83°08″02.53E) in Janjgir district, Chhattisgarh. The lower section of the sequence consists of light to dark grey limestone, overlain by thin black shale interbeds (~2–3 metres thick), exposed on the left bank near Ghatula Chhote Village (21°42″31.40N; 83°07″36.16E). The upper dolomitic portion contains black chert nodules (up to 15 cm in size). This chert is dense, black, and thinly bedded (0.5–1.0 cm), with nodular forms reaching up to 40 cm in length and 20 cm in thickness, as well as ellipsoidal lenses (up to 15 cm long and 8 cm wide) exhibiting a waxy lustre on fresh conchoidal surfaces. The limestone is fine-grained, ranging from light to dark grey, and typically fractures conchoidally. Black shale appears as thin intercalated layers (~2–3 metres) within the limestone, displaying abundant stromatolitic columns. The specimens discussed in this study were collected from the lower part of this section, as illustrated in Figure 2. Acid digestion was performed using standard and low-manipulation techniques (Butterfield et al., 1994; Grey, 1999; Singh & Sharma, 2021), employing 40% hydrofluoric acid to extract microfossils and organic residues sequentially. These maceration methods minimise fragmentation and preserve delicate morphological features, such as elongated tubular processes and large vesicles. The recovered organic remains were then hand-picked and mounted on glass slides using Canada Balsam (refractive index = 1.5).

Figure 2.

Generalised lithostratigraphic column: (a) Chhattisgarh Supergroup (after Patranibs-Deb & Chaudhuri, 2008) and (b) The Mahanadi River section at Saradih Village shows the sampling locations.

Transmitted Light Microscopy (TLM) was used to analyse fossils extracted from carbonaceous shale. Approximately 100 palynological slides were examined using an Olympus BX53 transmitted light microscope at 40X and 100X (oil immersion) magnifications to capture fine morphological details of the microorganisms. The observed specimens were photographed and measured using an Olympus DP26 digital camera with supporting software. Each specimen is documented with its corresponding England Finder coordinates for precise location reference.

These can be retrieved from the repositroy of Birbal Sahni Institute of Palaeosciences vide statement no. BSIP-1517 & BSIP-1657.

SYSTEMATICS

The microfossil assemblage extracted from the carbonaceous shale of the Saradih Limestone is dominated by sphaeromorphic acritarchs and cellular trichome-bearing cyanobacterial filaments, with a small amount of acanthomorphic acritarchs. Large-size smooth-walled sphaeromorphs Leiosphaeridia, having vesicle diameters ranging from 100 to 350 µm, were recorded in all fossiliferous samples. Due to its simple vesicle morphology, its biological affinity is ambiguous. Along with smooth-walled sphaeromorphs, some morphologically complex acanthomorphs, netromorphs, and other OWM, such as VSM, branched tubular microfossils, have also been recovered from Saradih Limestone. Among all other OWMs, the most significant form of biostratigraphic significance is acanthomorphic acritarch Trachyhystrichopshaera, which has been proposed as a potential index taxon for the Tonian period. Morphologically, Trachyhystrichopsharea is characterised by large spheroidal to sausage-like vesicles with sporadically distributed hollow, cylindrical and conical processes. The processes″ morphology, density, and communication with the vesicle play a significant role in acanthomorphic acritarch taxonomy. Regarding the size parameters, vesicles vary in size (129 µm–312 m), and their process is (0.2 m–3.0 µm) in basal width. Based on morphological characteristics, two species of Trachyhystrichopshaera, namely T. aimika and Trachyhystrichosphaera botula are described in the present study. Their systematic description and geographic distribution, listed in alphabetical order, are discussed below.

Group – ACRITARCHA Evitt 1963

Subgroup – ACANTHOMORPHITAE Downie, Evitt, and Sarjient, 1963

Genus – TRACHYHYSTRICHOSPHAERA Hermann, 1976 in Timofeev et al., 1976, emend. Hermann and Jankauskas, 1989 in Jankauskas et al., 1989, emend. Butterfield, 1994 in Butterfield et al., 1994 emend. Tang in Tang et al., 2013

Type species – T. aimika Hermann, 1976 (in Timofeev et al., 1976)

T. aimika Hermann in Timofeev et al., 1976, emend. Butterfield et al., 1994

Plate 1, Figures 1–5

PLATE 1. (1–4):

TLM micrographs of Trachyhystrichosphaera aimika from the Saradih Limestone (arrow indicates the position of spiny processes). 1. Type specimen slide no BSIP 17594, England Finder No. V49/2, 1.1. The magnified view of the box area in Figure 1 shows the nature of spiny processes emerging from the vesicle.; 2. Specimen slide no. BSIP 17595, England Finder No. Q56/3; 3. Specimen slide no. BSIP 17596, England Finder No. S46/4; 4. Specimen slide no. BSIP 16414, England Finder No. W20/4; 4.1 magnified view of box area in Figure 4 showing broken processes with outer ?membrane; 5. Specimen slide no. BSIP 17598, England Finder No. Q44/2; 5.1. magnified view of Figure 5 showing spiny processes. Scale bar for each specimen = 25 µm.

Synonymy

1969 Nucellosphaeridium bellum sp. n. Timofeev, pp. 23–24, Pl. 6: 6

1976 T. aimika sp. n. Hermann; Timofeev et al., p. 48, PIs. 19: 6, 8; 20: 1–3.

1984 Trachyhystrichosphaera vidalii Hermann; Knoll, pp. 154–156, Figure 8A–8K.

1994 T. aimika Hermann; Butterfield et al., 1994, Figures 18A–18K, 19G, and synonyms therein.

1995 T. vidalii Knoll; Zang, p. 170, Figure 23A–23E.

1998 Trachyhystrichosphaera sp.; Butterfield and Rainbird, Figure 3H.

2000 Trachyhystrichosphaera stricta Hermann; Gnilovskaya et al., pl. 1, Figure 19.

2001 T. aimika Hermann; Samuelsson and Butterfield, Figure 10E.

2004 T. aimika Hermann; Veis et al., pl. 1, Figure 4.

2004 T. stricta Hermann; Veis et al., pl. 1, Figure 5.

2004 T. vidalii Hermann; Veis et al., pl. 7, Figure 7.

2005 T. aimika Hermann; Butterfield, p. 178, Figure 9.

2007 Prolatoforma aculeata Mikhailova; Vorob″eva et al., pl. 1, Figure C.

2007 T. aimika Hermann; Vorob″eva et al., pl. 1, Figure B.

2008 Trachyhystrichosphaera aimica [sic] Hermann; Nagovitsin, et al., Figure 2e–2f.

2009a T. aimika Hermann; Vorob″eva et al., Figure 4t.

2009b T. aimika Hermann; Vorob″eva et al., p. 183, Figures 8–11.

2009c Trachyhystrichosphaera sp., Srivastava, Figure 4.

2013 T. aimika Hermann; Tang et al., p. 175, Figures 8–10.

2014 T. aimika Hermann; Xiao et al., 2014a, p. 217, Figure 6C.

2015 T. aimika Hermann; Tang et al., p. 315, Figure 17.

2016 T. aimika Hermann; Baludikay et al., Figure 6D–6L.

2017 T. aimika Hermann; Beghin et al., Figures 3v and 4a–4e.

2017 Trachyhystrrichosphaera aimika Hermann, Tang et al., Figure 4A–4F.

2019 T. aimika Hermann, Loron et al., Figure 8G–8H.

Stratum typicum – Holotype specimen and slide No. 49/2. Laboratoriia biostratigrafi i IGGD AN SSSR, Turukhansk collection. Turukhansk Region, Krasnoyarsk District, R. Miroedikha, Miroedikha Formation (Riphean) (Timofeev 1969, Pl. 6: 6).

Locality – Mahanadi River section, Saradih Village, Janjgir District, Chhattisgarh.

Type material – The type specimen of T. aimika from Saradih Limestone is illustrated in Pl. I, Figure 1, slide no. BSIP 17594.

Description – Originally spheroidal to sub-spheroidal; unlayered vesicle; with heteromorphic, sparsely distributed short, hollow, cylindrical and conical processes. Some specimens may find an inner membrane. Processes penetrate the outer membrane. Processes are projected in the group in some specimens (Plate 1, Figure 5.1). Vesicle diameter ranges between 70 and 284 µm (average = 167 µm); lengths of the processes range between 1.0 and 7.0 (average = 4 µm) µm and 0.2–3.0 µm basal width. A total of 08 well-preserved specimens were measured.

Remarks – The acanthomorphic acritarch, Trachyhystrichoisphaera aimika, has been considered a potential Tonian index fossil (Tang et al., 2013). It was initially reported from the 1109 ± 37 Ma (Re-Os isochron age) Atar/EI Mreïti Group of Democratic Republic of Congo (Beghin et al., 2017; Rooney et al., 2010). On the size parameters, the maximum diameter of this species is documented as up to 2700 µm and the process length is up to 58 µm (Pang et al., 2020). Additionally, based on this species, the Tonian age has been suggested for the Sirbu Shale, Bhander Group (Srivastava, 2009); the latest Mesoproterozoic—early Neoproterozoic age for lower Madhubabi Group of the Ganga Supergroup (Tang et al., 2017b). Identified specimens in the present material are morphologically similar to those described as T. aimika from the Svanbergfjellet and Draken Conglomerate Formations of Spitsbergen (Butterfield et al., 1994) and early Neoproterozoic Liulaobei Formation of North China (Tang et al., 2013). Processes of the described specimens range from 1.0 to 7.0 µm long (Pl 1.2). Whereas in the Liulaobei specimen, processes are up to 36 µm long (Tang et al., 2013, p. 175). By the size parameter, they are smaller than the specimens of the Saradih Limestone (157–312 µm).

Age and Geographic distribution – Globally, T. aimika is distributed in the uppermost Mesoproterozoic to Tonian successions.

T. botula Tang, 2013 in Tang et al., 2013

Plate 2, Figures 1–6

PLATE 2.

Transmitted light microscopic micrograph of Trachyhystrichosphaera botula from the Saradih Limestone, Raipur Group (arrow indicates the position of spiny processes in each micrograph). (1) Specimen slide no. BSIP 16416, England Finder No. F29/1; 2. Specimen slide no. BSIP 17595, England Finder No. S25/3; 3. Specimen slide no. BSIP 16412, England Finder No. K65/4; 4. Specimen slide no. BSIP 17595, England Finder No. V30/4; 5. Specimen slide no. BSIP 17595, England Finder No. Q44/2; 6. Specimen slide no. BSIP 17597, England Finder No. H22. Scale bar for each specimen = 25 µm.

Synonymy

2013 T. botula Tang; Tang et al., p. 175, Figures 11 and 12A.

2016 T. botula Tang; Baludikay et al., Figure 6M–6O.

2017 T. botula Tang; Beghin et al., pl. 4, Figure f–i.

2017 T. botula Tang; Tang et al., Figure 4G–4H.

2019 T. botula Tang; Li et al., Figure 9J–9O.

S. typicum – Holotype specimen and slide 11-LLB-A-50-13, Liulaobei Formation in the Huainan Group, North China (early Neoproterozoic) (Tang et al., 2013, Figure 11D)

Locality – Mahanadi river section, Saradih Village, Janjgir District, Chhattisgarh.

Type material – The type specimen of T. botula from Saradih Limestone is illustrated in Pl. 2, Figure 1, slide no. BSIP 16416.

Description – Originally elongated, sausage-shaped/tomaculate vesicles, with a variable number of randomly distributed processes (1–3 around the margin) emerge from the vesicle, and 2–8 processes on the vesicle surface. The vesicle wall is moderately thick and often irregularly folded. Processes are hollow, cylindrical, relatively short (3–5 µm long), widened at the base and blunt or slightly rounded/sharp at the distal end. Vesicle 38–311 µm long (average = 230 µm) and 16–200 µm (average = 150 µm) wide. The absence of the outer membrane may be due to preservation bias. A total of 06 well-preserved specimens were measured.

Remarks – The acanthomorphic acritarch, T. botula, was initially reported from the 900-750 Ma Liulaobei Formation of North China (Tang et al., 2013) as a tomaculate or sausage-like vesicle with bluntly rounded spines. T. botula differs from the T. aimika and Trachyhystrichosphaeria polaris by its elongated and tomaculate vesicles. Available records show that T. botula mostly co-occurs with T. aimika in Tonian successions. Similarly, the Saradih microfossils population also shows co-occurrence of T. botula with T. aimika, similar to other occurrences from Africa, China, Congo and India (Baludikay et al., 2016; Beghin et al., 2017; Li et al., 2019; Tang et al., 2013; Tang et al., 2017b). In addition, this species, recorded from the Atar/El Mreïti Group in West Africa, has up to 400 µm long and 190 µm wide vesicles (Beghin et al., 2017). In comparison, specimens from the Tonian Tongjiazhuang Formation in China have flattened vesicles ranging from 65 to 281 µm long and 32–136 µm wide, with cylindrical processes ranging from 1.5 to 5.2 µm long (Li et al., 2019). The type specimen of T. botula in the present material has a vesicle dimension of 38 × 16 µm with eight short cylindrical and conical processes (Plate 1, Figure 6), and the outer membrane is absent. It closely resembles the Liulaobei and Tongjiazhuang specimens (Li et al., 2019; Tang et al., 2013). In addition, the co-occurrence of sausage-shaped T. botula with T. aimika from the half a dozen late Mesoproterozoic to Tonian sedimentary succession suggests a possible linkage of the ontogenetic stage of T. aimika (Tang et al., 2013, 2020) may be a transitional form between monads and diads of T. aimika (Li et al., 2019).

Age and Geographic distribution – Globally, T. botula is found in Tonian successions alongside T. aimika.

DISCUSSION

Fossil records of Trachyhystrichosphaera and its biostratigraphic potential

Refinement in the geological time scale is a continuous process. To achieve this goal, inputs like the recording of global geological events, large-scale atmospheric changes, celestial impacts and significant biological evolutionary changes are studied to help demarcate the time boundaries. Distinct biostratigraphic events are the key foundation for characterising the entire Phanerozoic era. Nonetheless, the Proterozoic Era is arbitrarily divided using carbon isotope anomalies and lithostratigraphic marker units without distinguishing biostratigraphic events (Dehler, 2014). The time interval between ~1100 Ma and ~720 Ma represents a significant period in Proterozoic history. This time is considered as time to the end of the ‘Boring Billion″ period in relation to the global carbon cycle (Lyons et al., 2014), and also witnessed a critical evolutionary transition in the diversification of complex eukaryotes (Knoll et al., 2006; Xiao, 2013a, 2013b; Xiao & Tang, 2018). Recently, ample OWM and macroscopic carbonaceous compression fossil taxa have been documented worldwide from the late Mesoproterozoic to Tonian sedimentary successions. They have shown a great potential as biostratigraphic index fossils and have continued to provide new insights into the rise of crown group complex eukaryotic life (Anderson et al., 2024; Mughal et al., 2025). Smooth-walled sphaeromorphs, namely Leiosphaeridia, Synsphaeridium, Simplassosphaeridium and Pterosphaeromorpha, are the most common constituents and long-ranging taxa of the Proterozoic time (Javaux & Knoll, 2017; Loron, Rainbird, et al., 2019; Sergeev, 2009; Xiao, 2013b). At the same time, a few taxa such as Tappania plana Yin, Valeria lophostriata Jankauskas, Dictyosphaera macroreticulata Yin, Jacutianema solubila Jankauskas, T. aimika Hermann, C. globosa Ogurtsova & Sergeev, Bangiomorpha pubescessse Butterfield; Ourasphaera giraldae Loron and VSM show restricted stratigraphic distribution several Mesoproterozoic–early Neoproterozoic (Tonian) successions worldwide (Adam et al., 2017; Butterfield, 2015; Hill et al., 2000; Javaux, 2011; Javaux et al., 2013; Mughal et al., 2025; Porter, 2020; Porter & Riedman, 2023; Riedman et al., 2023; Singh & Sharma, 2014; Strauss et al., 2014; Xiao, 2013b). However, many of these fossiliferous horizons are poorly age-constrained without consistent radiometric dates.

Among others, OWM, similar to Trachyhystrichosphaera, are regarded as a potential index taxon, frequently documented as a standard component in fossil assemblages from late Mesoproterozoic to early Neoproterozoic (Tonian) deposits worldwide. (Pang et al., 2020). Timofeev and Hermann in Timofeev et al. (1976) originally described the genus Trachyhystrichosphaera with its type species T. aimika from the latest Mesoproterozoic (>1005 ± 4 Ma, baddeleyite U–Pb age) Lakhanda Group in Siberia (Rainbird et al., 1998; Timofeev et al., 1976). Diagnostically, these were large spheroidal, ovoid, or elongated/sausage-shaped vesicles (ranging up to and exceeding 500 µm in diameter), featuring a sparse distribution of heteromorphic, hollow processes (10–30 µm) that were irregularly arranged and freely connected to the vesicle cavity (Timofeev and Hermann in Timofeev et al., 1976). The processes may or may not have an outer membrane nearby, and the vesicle might contain an inner body (Tang et al., 2013) since Trachyhystrichosphaera has been described from the geochronologically established silicified chert and shale horizons globally and is playing a key role in establishing the GSSP for the Tonian (Pang et al., 2020).

Besides the type species T. aimika, 16 taxonomically distinct species of the three-dimensionally preserved OWM Trachyhystrichosphaera have been documented from late Mesoproterozoic to Neoproterozoic silicified cherts and shale successions worldwide. These species are differentiated based on their variable vesicle morphology and spiny process arrangements. These species include: T. vidalli (Knoll, 1984; Knoll & Calder, 1983; Knoll et al., 1991); T. bothnica, Trachyhystrichosphaera sp. (Tynni & Donner, 1980); T. membranacea, T. megalia (Pjatiletov, 1988); T. magna (Allison & Awramik, 1989); T. cyathophora, T. stricta, and T. cf. magna (Sergeev, 1991); T. parva, T. truncata, T.? anelpa, T. hystricosa, T. triangula and T. tribulosa (Samuelsson et al., 1999; Zang & Walter, 1992). In a critical review, Butterfield et al. (1994) have further emended the generic diagnosis of the genus Trachyhystrichosphaera to describe large spheroidal vesicles of variable size (113 µm and 702 µm) with one to many irregularly distributed hollow processes, 3–8 µm in diameter, and the presence/absence of an outer membrane. Based on emendation in morphology, eight species (T. vidalli; T. bothnica,? Trachyhystrichosphaera sp.; T. membranacea, T. megalia; T. magna; T. cyathophora, T. stricta, and T. cf. magna) were retained as a junior synonym of T. aimika; however, the remaining seven were rejected from the genus Trachyhystrichosphaera by Butterfield (Butterfield et al., 1994). Butterfield et al. (1994) also proposed a new species—T. polaris from the ~ Tonian age Svanberfjellet Formation (~802-783Ma) of Svalbard (Zhang et al., 2023). T. polaris is characterised by an echinate, spheroidal carbonaceous vesicle (95–235 µm in diameter) with a variable number of irregularly distributed processes (Butterfield et al., 1994). In recent years, T. aimika has been widely documented from the late Mesoproterozoic to early Neoproterozoic successions of Africa, Australia, Canada, China, Europe, Siberia and Svalbard (Tang et al., 2020). These occurrences includes the ~1109 Ma-1105 Ma Atar/El Mreïti Group, Taoudeni Basin (Beghin et al., 2017) and the ~1065 Ma-948 Ma Mbuji-Mayi Supergroup (Baludikay et al., 2016) in Africa; the ~811.5 Ma-716.5 Ma Alinya Formation (Zang, 1995) in Australia; the Tonian (~807 ± 8Ma) Leshukon, Nyafta and lower Vychegda (or Okos Formation) formations in Baltica (Veis et al., 2004; Vorob″eva et al., 2009a, 2009b); the ~900 Ma-750 Ma Liulaobei Formation (Dong et al., 2008; Tang et al., 2013), the ~820 Ma-720 Ma Gouhou Formation (Tang et al., 2015; Xiao et al., 2014), and the ~1070 Ma-910 Ma Tongjiazhuang Formation, Tumen Group (Li et al., 2019) in China; the ~1150-716 Ma Escape Rapids, Nelson Head, Grassy Bay and Wynniatt formations of Lower Shaler Group (Butterfield & Rainbird, 1998; Loron, Rainbird, et al., 2019) in Canada; the ~1025 ± 40Ma Lakhanda Group (Nagovitsin et al., 2008), the ~820 Ma-720 MaTukulan,Kulady, and Khastakh formations (Nagovitsin et al., 2015) in Siberia; the ~110 Ma-720 Ma lower Madhubani Group of the Ganga Supergroup (Tang et al., 2017a; Xiao et al., 2016). These reports somehow predate the antiquity of Trachyhystrichosphara aimika at ~1150 Ma and are constrained to ~720 Ma (Nagovitsin et al., 2015; Pang et al., 2020; Tang et al., 2017a; Tang et al., 2013, 2015).

Recently, Tang et al. (2013) emended the diagnosis of the OWM Trachyhystrichosphaera by adding a character: an outer membrane surrounding the processes. They also discarded the bifurcating nature of the processes to accommodate an additional species, Trachyhystrochosphaera botula, documented from the Tonian (~900 Ma-750 Ma) Liulaobei Formation of North China. Diagnostically, T. botula is described as a tomaculate or sausage-like vesicle (195–365 µm in length and 81–162 µm in width) with rounded ends, with heteromorphic, cylindrical/conical processes (up to 39 µm long) (Tang et al., 2013). Additionally, due to similar morphology, Tang et al. (2013) further proposed the P. aculeata as a synonym of T. aimika that was earlier described by Mikhailova and Podkovyrov (1992) from the Ural. Recently published reports revealed that the T. botula always co-exists with T. aimika, which has been reported from the late Mesoproterzoic to Tonian succession such as Mbuji-Mayi Supergroup (Baludikay et al., 2016) and the Atar/El Mreïti Group (Beghin et al., 2017) of Africa, Tonian lower Madhubani Group of Ganga Supergroup of India (Tang et al., 2017a; Xiao et al., 2016) and the Tongjiazhuang Formation of Tumen Group of China (Li et al., 2019) and the Tonian Liulaobei Formation of China (Tang et al., 2013). More importantly, several species of Trachyhystrichosphaera were documented from the Ediacaran successions, such as the Amadeus Basin, Australia (Zang & Walter, 1992); the Doushantuo Formation of China (Awramik et al., 1985; Yin et al., 2007). These reports have been critically scrutinised (Butterfield et al., 1994) and further transferred into other genera and species such as Tianzhushania spinosa (Zhang et al., 1998), Appendisphaera, Gyalosphaeridium and Knollisphaeridium (Grey, 2005). Given the limited distribution and dominance of T. aimika between approximately 1150 Ma and 720 Ma, we support its consideration as a potential index fossil in biostratigraphic studies and as a key tool for establishing the GSSP for the Tonian within the Proterozoic timeframe as proposed by Pang et al. (2020).

Biostratigraphic connotation of Trachyhystrichosphaera in Chhattisgarh Supergroup

Traditionally, the microfossil assemblage studied from the Saradih Formation is characterised by abundant large-sized sphaeromorphs and filaments with a relatively low amount of acanthomorphs, followed by the occurrence of helically coiled microfossil Obruchevella and Rugosoopsis (Singh et al., 2019a). Most precisely, in the taxonomic composition of the Saradih microfossil assemblage, the co-existence of two species of the Tonian potential index taxon Trachyhystrichosphaera has been unambiguously identified, namely T. aimika Hermann in Timofeev et al., 1976, emend. Butterfield et al., 1994 and T. botula Tang in Tang et al., 2013 (Plates 1 and 2). Their vesicle morphology and process distribution differentiate them. The other associated OWMs, unambiguously considered as crown group eukaryotes, such as Valkyria borealis, Proterocladus major, Proterocladus minor, and Comasphaeridium tonium and VSM Cyclocyrillium simplex (not included in this study), also restricted their occurrence in Tonian assemblages (Butterfield, 2015; Butterfield et al., 1994; Mughal et al., 2025; Sergeev, 2009). On a lighter note, this assemblage is distinct from distinct Ediacaran Complex Acanthomorphic Palynoflora (ECAP), which are characterised by diverse taxa of large acanthomorphs (Grey, 2005; Liu et al., 2013, 2014; Moczydłowska, 2008; Morais et al., 2021; Sergeev et al., 2011; Vorob″eva et al., 2009a, 2009b; Wu et al., 2024; Xiao et al., 2024) in not having many characteristic forms of ECAP assemblage, nonetheless similar to the Tonian microfossil assemblages from the Tonian Svanbegfjellet Formation and Dracken Conglomerate of Svalbard (Butterfield et al., 1994; Knoll, 1984); the Tonian Liulaobei Formation (Tang et al., 2013) and the Tongjiazhuang Formation of the Tumen Group of China (Li et al., 2019) and other Tonian strata worldwide summarised in Pang et al. (2020). In association of other age-diagnostic microfossils comprehensively the Raipur assemblage has drawn a parallel linkage with well-dated Tonian assemblages of Wynnniat Formation, Spitsbergen (Butterfield, 2005b); the Alinya Formation (~811.5-716.5 Ma) in Australia (Riedman & Porter, 2016; Zang, 1995); the lower Vychegda Formation (807 ± 8 Ma) in Baltic (Vorob″eva et al., 2009b); the Lone Land Formation (~1050-720 Ma) in Canada (Samuelsson & Butterfield, 2001); the Gouhou Formation (~820-720 Ma) in North China (Tang et al., 2015); the Liulaobei Formation (~900-750 Ma) in North China (Tang et al., 2013); the Khastakh Formation (~820-720 Ma) in Siberia (Nagovitsin et al., 2015); the Svanbergfjellet Formation (~810-788 Ma) in Svalbard (Butterfield et al., 1994); the Chichkan Formation (~800-750 Ma) in Kazakhstan (Sergeev & Schopf, 2010); the Chuar Group (~780-740 Ma) in Arizona, United States (Porter & Riedman, 2016). Similarly, their co-occurrences in the well age-constrained Sirbu Shale Formation (~900-866 Ma) of the Bhander Group, Vindhyan Supergroup (Srivastava, 2009) and in the late Mesoproterozoic to Tonian (~1100-720 Ma) Lower Madhubani Group of Ganga Supergroup (Tang et al., 2017a; Xiao et al., 2016) also advocate a Tonian age for the Saradih Formation (~800-720 Ma) of Raipur Group.

The available geochronological data for the Chhattisgarh basin remains inconsistent. Based on various parameters, several studies have proposed an early Mesoproterozoic to Neoproterozoic age for the rocks of the Chhattisgarh Supergroup. However, recent improvements have been made in dating the Chhattisgarh sediments, particularly the Raipur Group of rocks. From paleomagnetic studies, Murti (1987) suggested that the Gundredehi Shale of the Raipur Group dates to approximately 1250-1300 Ma. A comparative stable isotope (C and O) study of the Raipur Limestone with the δ13C curve for the late Proterozoic (Kaufman et al., 1992) led Chakraborty et al. (2002) to propose an Upper Riphean age for the Raipur Limestone. Distinct rhyolitic tuff bands in the Churtela Shale (=Tarenga Formation)— stratigraphic unit above the Trachyhystrichosphaera-bearing Saradih Limestone—have been dated using SHRIMP U-Pb zircon analysis, yielding ages of 1007 ± 20 Ma (Patranabis-Deb et al., 2007) and 993 ± 8 Ma (Bickford, Basu, Mukherjee, et al., 2011). Similarly, using δ13C isotope stratigraphy and available radiometric age data from the Charmuria Limestone (also known as Sarangarh Limestone) and Chandi Limestone (also known as Saradih Limestone), George et al. (2019) suggested a depositional age for the Raipur Group between 1.0 and 1.2 Ga.

Additionally, the Saradih Limestone revealed the prolific occurrence of early Ediacaran marker microfossils, such as Obruchevella spp. (Singh et al., 2019a) However, it also lacks a distinct ECAP. The appearance of T. aimika and T. botula, along with other age-diagnostic microfossils, holds significant biostratigraphic value, helping to refine the depositional timeline of the basin. The comprehensive analysis of these two species of Trachyhystrichosphaera and other Neoproterozoic age-restricted microbiota, along with the absence of typical ECAP and glacial evidence in the Saradih assemblage, suggests that the Raipur Group likely dates to the Neoproterozoic, most likely the Tonian to early Cryogenian. If the Saradih Limestone is indeed of late Tonian age, this finding raises questions about the geochronologically proposed stratigraphic position of the Sukhda and Sapos tuffs (Basu et al., 2008).

CONCLUDING OBSERVATIONS

The black carbonaceous shale of the Saradih Limestone shows moderate occurrence of acanthomorphic acritarch Trachyhystrichosphaera, namely T. aimika; and T. botula are associated with another Neoproterozoic (900-720 Ma) microbiota. Their co-occurrence in the Saraidh assemblage suggests a late Tonian age for the Saradih Formation.

Globally, T. aimika is considered a potential index taxon with nearly all of its occurrences related to the Tonian period and a key tool in establishing the GSSP for the Tonian period in geological time scale.

The present finding adds a new occurrence of Trachyhystrichosphaera acritarch in the early Neoproterozoic biosphere of the world. It is being documented for the first time, possibly from the late Tonian carbonaceous shale of the Saradih Limestone of the Raipur Group in the Chhattisgarh Supergroup.

The present documentation of the Trachyhystrichosphaera from the Raipur Group suggests Neoproterozoic age, coeval to Liulaobei Formation, Tongjiazhuang Formation of China and Svanbegfjellet Formation of Svalbard (900-720 Ma) and adds a new fossiliferous horizon in the geographic distribution.

Footnotes

Acknowledgements

We express our sincere gratitude to the Director, Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow, for granting access to the laboratory facilities essential for this research and permitting the publication (Ref. No.: BSIP/RDCC/105/2024-2025). MS is thankful to the CSIR for awarding the Emeritus Fellowship (21/1161/24). We are grateful to the anonymous reviewers for their insightful and constructive suggestions, which significantly improved the quality of this manuscript.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: DST-Birbal Sahni Institute of Palaeosciences, Lucknow.

ORCID iD

Veeru Kant Singh

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Tonian age marker acritarch taxa Trachyhystrichosphaera aimika Hermann from the Mesoproterozoic Chhattisgarh Supergroup,Baradwar sub-basin,central India and its stratigraphic significance

Abstract

Keywords

INTRODUCTION

Generalised geological map of the Baradwar Sub-basin in and around Saradih area showing the location of the study area (modified after Patranibs-Deb & Chaudhuri, 2008).

GEOLOGICAL BACKGROUND AND AGE OF THE RAIPUR GROUP

Generalised stratigraphic succession of Chhattisgarh Supergroup in Baradwar and Hirri Sub-basins (after Mukherjee et al., 2014).

Generalised lithostratigraphic succession of the Chhattisgarh Supergroup (Das et al., 1992; Patranabis-Deb & Chaudhuri, 2008; Mukherjee & Ray, 2010).

Geochronological Constraints

Contradictions with palaeontological evidence

MATERIALS AND METHODS

Generalised lithostratigraphic column: (a) Chhattisgarh Supergroup (after Patranibs-Deb & Chaudhuri, 2008) and (b) The Mahanadi River section at Saradih Village shows the sampling locations.

SYSTEMATICS

DISCUSSION

Fossil records of Trachyhystrichosphaera and its biostratigraphic potential

Biostratigraphic connotation of Trachyhystrichosphaera in Chhattisgarh Supergroup

CONCLUDING OBSERVATIONS

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

ORCID iD

References