Marine and non-marine palynomorphs from the Ordovician sequence of Takche Formation,Spiti,India

Abstract

The palynological investigation of a sequence of the Takche Formation (Middle Ordovician–Early Silurian), Spiti region in the Tethyan Himalaya, reveals new perspectives into Ordovician micro-faunal and floral diversity, palaeoenvironments, biostratigraphy and the likely signatures about early land plants within Gondwana. The recovered palynomorph assemblage is rich in marine forms such as chitinozoans, acritarchs, melanosclerites, and scolecodonts, along with non-marine palynomorphs, which comprises possible cryptospores and phytodebris. The chitinozoan assemblage comprises genera- Belonechitina, Baltochitina, Eisenackitina and Euconochitina. Acritarch assemblage is characterised by Baltisphaeridium, Orthosphaeridium, Stelliferidium, Dactylofusa, Leiosphaeridia, Lophosphaeridium and Focusphaera. Scolecodonts’ assemblage is dominated by simple jaw elements, associated with ctenognath- and placognath-type apparatuses, of which placognath are common. The melanosclerites assemblage includes Mirachitina, Melanoporella, Eichbaumia, Melanorhachis, and Melanosteus. The non-marine palynomorphs of the putative cryptospore assemblage show semblance with the taxa Chelinospora, Didymospora, Laevolancis, Rugosphaera, Dyadospora and Segestrespora. Although preservation is generally poor and thermal maturity is high, the presence of diagnostic taxa across multiple palynological groups permits robust palaeobiological and palaeoenvironmental interpretations. The abundance of marine forms clearly reflects deposition in a distal shallow marine setting, removed from significant terrestrial influence. These findings are consistent with similar studies from comparable sequences in the region and elsewhere along the northern Gondwana. The present study not only enhances our understanding of the Ordovician biodiversity in India but also paves the way to further explore and enrich the global narrative of early plant evolution and palaeobiogeographic connectivity during the early phases of life on land from the Indian subcontinent.

INTRODUCTION

The Ordovician Period represents a pivotal phase in Earth’s history, characterised by the rapid diversification of marine life and the emergence of the earliest terrestrial plants. While much of the research has been focused on the Great Ordovician Biodiversification Event (GOBE) through the study of macrofossil records; organic-walled microfossils, especially palynomorphs such as acritarchs, chitinozoans and possible cryptospores, are proving equally vital for understanding the environmental and evolutionary changes of this era (e.g., Servais et al., 2010). These microfossils serve as sensitive indicators of both marine ecosystem structures and the early colonisation of land by plants, especially in areas situated along the Gondwanan margin during the early Palaeozoic.

The Tethyan Himalaya, particularly of the Spiti region of Himachal Pradesh, preserves one of the few relatively continuous Palaeozoic successions in India and was deposited on a part of the Gondwana palaeocontinent situated at relatively low palaeolatitudes (Figure 1A; Torsvik & Cocks, 2009). As located along the northern margin of the Gondwanan margin, this region occupies a key palaeogeographic position for investigating the biogeographic linkages between the Indian subcontinent and other peri-Gondwanan and palaeoequatorial regions such as Baltica, Avalonia, and North Africa. Although the early Palaeozoic sequences in the Spiti Valley are lithologically well preserved and of considerable geological significance, systematic palynological investigations remain scarce. Previous studies of the Takche Formation (Middle Ordovician–Early Silurian), stratigraphically positioned between the underlying Thango and overlying Muth Formation (Table 1) have focused primarily on macrofaunal elements or sedimentological aspects, leaving the organic microfossil record largely underexplored (Chaubey et al., 2019; Pandey & Parcha, 2018; Shabbar et al., 2020, 2022, 2023; Srikantia & Bhargava, 2021). However, recent analyses demonstrate that despite thermal alteration associated with the Himalayan orogeny, these successions have yielded well-preserved palynomorphs of biostratigraphic and palaeoecological significance (e.g., Tonarová et al., 2024; Wang et al., 2021).

Table 1.

Ordovician–Silurian stratigraphy of Spiti Basin (after Srikantia & Bhargava, 2021; Shabbar et al., 2023).

Group	Formation	Lithology	Age
Kanawar group (Muth, Lipak, Po and Ganmachidam formations)			Devonian– Carboniferous
Disconformity
Sanugba	Takche	Calcareous sandstone, shale, reefoid carbonate	Middle Ordovician–Early Silurian
	Thango	Purple sandstone, maroon shale	Early Ordovician
Angular Unconformity
Haimanta group (Batal and Kunzum La formations)			Terminal Proterozoic–Cambrian

Figure 1.

A. Palaeogeography of the northeastern Gondwana and adjacent peri-Gondwanan areas in early to mid-Ordovician (Arenig) times at 480 Ma. Ab, Alborz terrane; Afg, Afghan terrane; Lut, Lut terrane; M, Madagascar; Qiang, Qiang terrane; San, Sanand terrane; Tau, Taurides terrane (after Torsvik and Cocks, 2009). The star indicates the position of our fossil locality. B. General geological map of northern India and surrounding countries, showing the major structural units of the Himalaya from Nanga Parbat to Namcha Barwa and south of the Indus–Tsangpo suture zone. The inset shows the Indian part of the Tethyan Belt, with the locations of four sub-basins that contain Palaeozoic sedimentary sequences belonging to the Tethyan Himalaya (after Gansser, 1964). C. Geological map of the study area, showing different Palaeozoic and Mesozoic formations; red star indicates the studied locality (after Bagati et al., 1991).

The present study documents a diverse assemblage of marine palynomorphs, including acritarchs, chitinozoans, melanosclerites, and scolecodonts, along with a suite of terrestrial elements such as putative cryptospores and phytodebris. Notably, some of the cryptospore-like forms bear morphological resemblances to established cryptospore taxa, such as Chelinohilates, Didymospora, Dyadospora, Gneudnaspora, Laevolancis, Rugosphaera, Sphaerasacus, and Segestrespora, as well as trilete spore taxa, including Chelinospora. This is of particular significance as it suggests the presence of proximal landmasses supporting the early terrestrial vegetation during the Middle Ordovician. The occurrence of these terrestrial palynomorphs within marine sediments suggests transportation by aeolian or fluvial processes from a Gondwanan hinterland, reflecting similar dispersal patterns observed in coeval deposits of South America, North Africa, and other parts of peri-Gondwana (e.g., Donoghue et al., 2021; Rubinstein & Vajda, 2019; Steemans et al., 2009). Moreover, the recovered chitinozoan taxa such as Belonechitina capitata, Eisenackitina, Conochitina, and Lagenochitina permit a preliminary biostratigraphic assignment of the studied strata within the Darriwilian–Sandbian interval. This interpretation aligns with the established Ordovician zonation schemes for Gondwanan regions (Paris, 1990; Paris et al., 2004). The integrated marine and terrestrial palynological assemblage provides valuable insights into palaeoenvironmental gradients during this time, highlighting the dynamic interface between early land ecosystems and marine environments.

The primary objective of this study is to document and classify the marine and terrestrial palynomorphs from the Takche Formation, evaluate their taxonomic diversity and preservation quality, interpret the depositional environments they represent, and evaluate their significance in both biostratigraphic and palaeogeographic contexts. This not only enhances our understanding of the Ordovician biosphere within the Indian subcontinent but also contributes to the broader global narrative of early plant evolution and palaeobiogeographic connectivity during the initial phases of life on land.

STRUCTURAL AND STRATIGRAPHIC FRAMEWORK

The Spiti region, often referred to as the fossil museum, is one of the most significant and archetypal regions in India for the study of the lower Palaeozoic, particularly for palaeontological and stratigraphical research. The Spiti region is positioned parallel to the general NW-SE trend of the Himalayas (Bagati et al., 1991). It is a part of the Tethyan Himalaya (Figure 1B), which is situated along the hanging wall of the South Tibetan Detachment System (Wiesmayr & Grasemann, 2002). It hosts one of the well-preserved sedimentary sequences, approximately 32808 feet thick, which unconformably overlies the Precambrian basement and spans a stratigraphic range from Cambrian to Cretaceous (Bagati et al., 1991; Bhargava, 2008; Hayden, 1904; Parcha, 2021). The Ordovician–Silurian strata in the Spiti region unconformably overlie Kunzam La Formation (Cambrian) and underlie Muth Formation with an unconformable contact (Bhargava & Bassi, 1998). This sedimentary succession is further subdivided into the Thango and Takche formations (Table 1). It is generally exposed along the snow-covered mountain peaks in the Spiti, Parahio and Pin valleys, reaching elevations of more than 5,600 m above mean sea level.

The studied section of the Ordovician–Silurian sequence, located at Takche (GPS coordinates: 32°27.024ʹN, 77°41.767ʹE), is exposed on the right side of the Spiti River (Figure 1C). It forms the northeastern limb of a large anticline with a NW-SE trend and is also intersected by a major fault with a NE-SW trend (Bagati et al., 1991). In the Takche area (Figure 2), the Takche Formation is composed of sandstone, grey shale, calcareous siltstone, dolomitic limestone/dolomite, and limestone, many of which exhibit an earthy brown on their weathered surfaces (Bhargava, 2008). The shale horizons are well–developed in the middle part of the formation, where they are intercalated with other lithologies.

Figure 2.

A & B. Closer view of the palynomorphs bearing units of Takche Formation C. Panoramic view of Takche Formation, Takche locality. Man in the photograph (white arrow) for scale (Height of man = 152 cm).

MATERIALS AND METHODS

Samples of shale, siltstone, and limestone were collected for the investigation of palynomorphs from the Takche Formation exposed along the right side of the Spiti River, at the Takche locality. The sample location is indicated on the geological map (Figure 1C) with their stratigraphic positions shown in the lithostratigraphic column (Figure 3). A total of 13 samples (T1 to T13, in stratigraphical order) from the shale/siltstone unit of the lowermost part (Farka Muth Member) of the Takche Formation were processed for palynological studies using the standard maceration procedure (HCl-HF-HCl) for the extraction of palynomorphs and briefly treated with HNO₃ and KClO₃. All the samples were thoroughly washed with distilled water using a 10 µm mesh sieve. Permanent slides were prepared for the investigation under light microscopy (Olympus BX5), and selected palynomorphs were photographed using confocal laser scanning microscopy (CLSM)—Leica TCS-SP8 at Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow. The illustrated specimens are housed in the repository of Birbal Sahni Institute of Palaeosciences, Lucknow, under the slide numbers 17231–17249 vide statement number 1608.

Figure 3.

Simplified litholog of Takche Formation, Takche, showing palynomorphs bearing units and distribution of chitinozoan taxa.

RESULTS

Palynomorph recovery and preservation

The detailed analysis of samples collected from the Takche Formation reveals several important insights about the thermal preservation conditions and post-depositional processes. Out of the 13 processed samples from the Takche Formation, the uppermost three samples were more productive and yielded a significant amount of palynological content. The remaining samples also yielded palynomorphs, albeit at a low frequency, with highly degraded organic matter. Approximately 50 g of each sample was macerated; the most productive sample yielded a significant palynological assemblage (280 palynomorphs), while the least productive sample yielded only 10 specimens. The recovered palynomorphs exhibit moderate to poor preservation. Most specimens appear opaque and dark black in colour, indicative of high thermal maturity, consistent with Thermal Alteration Index (TAI) values of 4–5. This advanced maturity is attributed to diagenetic alteration and low-grade metamorphism associated with deep burial and subsequent Himalayan orogeny. Chitinozoans predominantly display flattened vesicles, often partially broken. Acritarchs commonly show broken spines and corroded vesicle walls, while scolecodonts are fragmented, and melanosclerites exhibit breakage and surface degradation.

Marine palynomorphs

Chitinozoans: The chitinozoans, recovered from all 13 samples, are abundant and morphologically diverse within the assemblage. The recovered assemblage includes well-documented genera and species, such as: Euconochitina ?brevis, Belonechitina capitata, B. micracantha, Belonechitina spp., Eisenackitina sp., Eremochitina sp., Baltochitina ?delicata, Conochitina spp.,?Euconochitina sp., Lagenochitina sp., and several fragmented specimens (Lagenochitina?) (Plate 1). These taxa are consistent with Upper Ordovician to Lower Silurian marine settings, suggesting a warm, shallow marine depositional environment for this part of the Takche Formation.

PLATE 1.

Photomicrographs of Chitinozoa recorded from the studied section of Takche Formation near Takche locality, Spiti Valley. Scale bar 20 micron. 1) Euconochitina ?brevis, BSIP Slide No., 17231B, Y34; 2) Belonechitina micracantha, BSIP Slide No., 17232A, T52; 3) Eisenackitina sp., BSIP Slide No., 17233, Q48; 4) Eremochitina sp., BSIP Slide No., 17243, U42; 5) Baltochitina ?delicata, BSIP Slide No., 17243, N12/3; 6) Conochitina sp., BSIP Slide No., 17243, J27/2; 7) Conochitina sp., BSIP Slide No., 17243, E37/3; 8) Belonechitina capitata, BSIP Slide No., 17245, L35/3; 9) Belonechitina capitata, BSIP Slide No., 17245, L35/3; 10) ?Euconochitina sp., BSIP Slide No., 17246A, N56/2; 11) Belonechitina sp., BSIP Slide No., 17248, J61; 12) Belonechitina sp., BSIP Slide No., 17248A, Q33/2; 13) ?Lagenochitina sp., BSIP Slide No., 17249A, F21/2.

Acritarchs: The acritarchs, recovered from all 13 samples, are both taxonomically diverse and ecologically informative. The following genera and species are identified Baltisphaeridium ternatum, B. citrinum, B. longispinosum, B. multipilosum?, Baltisphaeridium sp., Orthosphaeridium bispinosum, O. ternatum?, Orthosphaeridium sp., Stelliferidium stelligerum, Dactylofusa sp., Leiosphaeridia sp., Lophosphaeridium parvum, ?Lophosphaeridium sp., Stelliferidium redonense, ?Aremoricanium sp., Focusphaera sp., ?Pirea sp., Veryhachium sp., Dictyotidium sp., and Neoveryhachium sp. (Plate 2). These taxa are characteristic of shallow to open marine environments, particularly in warm-water regimes, as also noted from the Shiala Formation (Sinha et al., 1998).

PLATE 2.

Photomicrographs of acritarchs recorded from the studied section of Takche Formation near Takche locality, Spiti Valley. Scale bar 20 micron for all. 1) Dactylofusa sp., BSIP Slide No., 17231A, G23; 2) Dactylofusa sp., BSIP Slide No., 17231, N53/2; 3) Orthosphaeridium bispinosum, BSIP Slide No., 17232A, P49/1; 4) Stelliferdium stelligerum, BSIP Slide No., 17232B, N42; 5) Baltisphaeridium longispinosum, BSIP Slide No., 17232, V53; 6) Baltisphaeridium longispinosum, BSIP Slide No., 17232, N33; 7) Leiosphaeridia sp., BSIP Slide No., 17233, L33/2; 8) Leiosphaeridia sp., BSIP Slide No., 17233A, N43/1; 9) Stelliferdium stelligerum, BSIP Slide No., 17234A, L45/2; 10) Aremoricanium sp., BSIP Slide No., 17234, V38; 11) Focusphaera sp., BSIP Slide No., 17234B, Q37/1; 12) Dactylofusa sp., BSIP Slide No., 17234B, O31/3; 13) Leiosphaeridia sp., BSIP Slide No., 17234C, O38; 14) Lophosphaeridium sp., BSIP Slide No., 17243, N39; 15) Orthosphaeridium ternatum, BSIP Slide No., 17243, O23/2; 16) Orthosphaeridium bispinosum, BSIP Slide No., 17241, G29/2; 17) ?Pirea sp., BSIP Slide No., 17242, Q43/3; 18) Orthosphaeridium sp., BSIP Slide No., 17244A, T38; 19) Lophosphaeridium sp., BSIP Slide No., 17246D, W40/2; 20) Orthosphaeridium ternatum, BSIP Slide No., 17245A, G33/4.

Scolecodonts: The scolecodonts, recovered from eight samples (T2, T3, T5, T8, T9, T10, T11, and T13) of the Takche Formation, are dominated by simple jaw elements, with a notable prevalence of the ‘saw-plate type’ morphologies typically associated with ctenognath- and placognath-type apparatuses. The most frequent specimens belong to placognath elements, likely attributable to xanioprionids and mochtyellids, which are characteristic of Ordovician to Silurian deposits (Eriksson et al., 2013; Kielan-Jaworowska, 1968). In addition, several specimens exhibiting smooth, conical morphologies with broad bases and denticulated apices were also identified. These forms are consistent with scolecodonts commonly found in shallow, warm marine environments (e.g., Eriksson et al. 2004, 2016).

Melanosclerites: The melanosclerites recovered from two samples (T10 and T11) of the Takche Formation are of particular significance, primarily due to their overall rarity globally and their rarity in Indian sequences. Prior to the present study, the occurrence of these microfossils in Indian Gondwana was limited to the Garhwal region of Tethys Himalaya (Sinha & Trampisch, 2013). The presence of these microfossils in the Takche Formation, thus, marks a notable biogeographical extension. The identified taxa include Mirachitina sp., Melanoporella polonica, M. clava. M. clava?, Eichbaumia sp., Melanorhachis regularis?, and Melanosteus indica (Plate 3). These taxa are known to be facies-controlled and commonly associated with shallow, open marine depositional environments (Cashman, 1992).

PLATE 3.

Photomicrographs of phytodebris and melanosclerites recorded from the studied section of Takche Formation near Takche locality, Spiti Valley. Scale bar 20 micron for all except 2 and 3 are 10 microns. 1) Fungal hypha, BSIP Slide No., 17239, Q49/3; 2) Cuticle-like sheet, BSIP Slide No., 17243, K35/1; 3) Laevigate tubes, BSIP Slide No., 17244, X32/1; 4) Laevigate tubes, BSIP Slide No., 17233, O45; 5) Tracheid?, BSIP Slide No., 17231, O50/2; 6) Cuticle-like sheet, BSIP Slide No., 17236, S40/1; 7) Cuticle-like sheet, BSIP Slide No., 17237, V42/3; 8) Mirachitina sp., BSIP Slide No., 17244A, G53; 9) Melanoporella polonica, BSIP Slide No., 17244A, E50/4; 10) Eichbaumia sp., BSIP Slide No., 17244A, E45/1; 11) Melanoporella clava?, BSIP Slide No., 17245, K36; 12) Melanoporella clava, BSIP Slide No., 17246C, G51/4; 13) Melanorhachis regularis?, BSIP Slide No., 17246A, S41/1; 14) Melanosteus indica, BSIP Slide No., 17246B, R46/3; 15) Mirachitina sp., BSIP Slide No., 17246C, F54/4.

Non-marine Palynomorphs

?Cryptospores and Phytodebris: The non-marine component of the palynological assemblage, recovered from 11 samples (T1, T3, T4, T5, T6, T7, T8, T9, T10, T11, and T13), is represented by possible cryptospores and phytodebris, though they occur in very low abundance (~1%). This suggests an actual marine depositional setting, situated at a considerable distance from the Gondwanan mainland. The limited terrestrial input support has resulted from aeolian or fluvial transport of spores and plant debris from nearby coastal vegetated areas.

The microfloral assemblage includes: putative cryptospore - range of forms such as monads and dyads, and reveals resemblance with cryptospore taxa such as Didymospora luna, Didymospora sp., Gneudnaspora sp., Laevolancis divellomedium, Sphaerasacus glabellus, Chelinohilates sp., Rugosphaera cerebra, Dyadospora murusdensa, Segestrespora rugosa? as well as trilete spore - Chelinospora sp. and various laevigate tubes, probable tracheid fragments, and cuticle-like sheets (Plates 3 and 4). Despite their low occurrence, these palynomorphs provide probable early evidence of land plant colonisation in India, suggesting marine transport from a Gondwanan source during the Middle to Late Ordovician.

PLATE 4.

Photomicrographs of possible? Cryptospores recorded from the studied section of Takche Formation near Takche locality, Spiti Valley. Scale bar 20 micron for all except 14 is 10 microns. 1) Chelinospora sp., BSIP Slide No., 17232, N26/1; 2) Chelinospora? sp., BSIP Slide No., 17232, G29/4; 3) Didymospora sp., BSIP Slide No., 17234, Q55/2; 4) Gneudnaspora sp., BSIP Slide No., 17235, K44; 5) Laevolancis divellomedium (Chibrikova), BSIP Slide No., 17237, P53/2; 6) Chelinospora sp., BSIP Slide No., 17238, S25; 7) ?Sphaerasacus glabellus, BSIP Slide No., 17240, S24/3; 8) Chelinohilates sp., BSIP Slide No., 17240, R25/4; 9) Sphaerasacus glabellus, BSIP Slide No., 17241, P56/2, CLSM microphotograph; 10) Didymospora luna? , BSIP Slide No., 17242, G30/2; 11) Rugosphaera cerebra, BSIP Slide No., 17245, Q27/3; 12) Rugosphaera cerebra, BSIP Slide No., 17245, L61/3, CLSM microphotograph; 13) Didymospora luna, BSIP Slide No., 17246, E54/4; 14) Rugosphaera sp. cf R. cerebra, BSIP Slide No., 17247, T56/2; 15) Dyadospora murusdensa, BSIP Slide No., 17248, N36/2; 16) Didymospora fucosogranulata? , BSIP Slide No., 17248, L57/3; 17) Segestrespora rugosa?, BSIP Slide No., 17249, S22

DISCUSSION

The palynological assemblage recovered from the Takche Formation reveals a moderately diverse suite of marine and non-marine palynomorphs, including acritarchs, chitinozoans, scolecodonts, melanosclerites, ?cryptospores, trilete spores, and phytodebris. Despite poor preservation and high thermal maturity, the presence of diagnostic taxa across multiple palynological groups enables robust palaeobiological and palaeoenvironmental interpretations.

Thermal maturity and taphonomic implications

The distinctively dark, opaque coloration and the conspicuous structural deformation observed in many of the recovered palynomorphs, such as the flattened vesicles of chitinozoans, corroded margins of acritarchs, and the fragmented remains of scolecodonts, are consistent with the high degree of thermal alteration. This is most plausibly the outcome of prolonged diagenesis and low-grade regional metamorphism, all of which point to deep burial and tectonic overprinting during the Himalayan orogenic events. The general loss of three-dimensionality in vesiculate forms, combined with the near-complete degradation of surface ornamentation, further supports this interpretation. Comparable preservation patterns have been documented in other Ordovician–Silurian successions across the Tethyan Himalaya, where palynomorphs exhibit similar darkened coloration, brittle textures, and morphological collapse, indicative of elevated thermal maturity (e.g., Sinha 2022; Sinha et al., 1998, 2005, 2011; Sinha & Verniers, 2016; Tonarová et al., 2024; Wang et al., 2021). Such characteristics representing thermal maturity are also observed in the palynoassemblages recovered from younger formations of Devonian and Carboniferous ages (Gupta et al., 2022, 2023). This taphonomic signature aligns well with regional metamorphic gradients mapped in the Tethyan belt, where organic matter reflectance values and alteration indices suggest a transition from anchizonal to epizonal metamorphism (Le Fort, 1996; Yin, 2006). In this context, the palynomorph assemblage of the Takche Formation provides a valuable window into a complex interplay of how palaeobiological signals are preserved or, in some cases, lost, under Himalayan tectono-thermal regimes. Moreover, the impact of such overprinting extends beyond mere preservation: it directly influences the recoverability and identifiability of palynomorph taxa with delicate morphotypes (e.g., ornamented acritarchs or thin-walled cryptospores) often disproportionately affected. This may account for the comparatively lower taxonomic richness observed in the more thermally mature samples, underscoring the need for cautious interpretation when reconstructing palaeoecological patterns from such thermally altered assemblages. At the same time, in spite of limitations, it offers a more rigorous investigation of these strata for the want of unequivocal terrestrial palynomorphs.

Palaeoecological and palaeoenvironmental affinities

The palynological assemblage recovered from the Takche Formation is overwhelmingly dominated by marine palynomorphs, particularly acritarchs and chitinozoans, with a moderate representation of melanosclerites and scolecodonts. In contrast, terrestrial components, namely cryptospores and phytodebris, constitute less than 1% of the total palynomorph content. This stark marine-terrestrial disparity suggests sedimentation of these units within a distal, shallow marine shelf environment, sufficiently removed from significant fluvial or deltaic input. Such interpretation is consistent with palaeoenvironmental models proposed for other lower Palaeozoic formations in the Tethyan Himalaya, such as the Shiala Formation (Sinha et al., 2005) and the Ordovician chitinozoan-bearing successions of the Garhwal region (Sinha et al., 2011), both of which record similar marine dominance in their microfossil content.

Acritarch diversity within the assemblage is notable, with genera such as Baltisphaeridium, Leiosphaeridia, Orthosphaeridium, and Stelliferidium being particularly abundant. These taxa are commonly reported from low-latitude, warm-water Gondwanan margins and are typically interpreted as phytoplanktonic forms thriving in well-illuminated, euphotic marine conditions (Servais et al., 2003; Vecoli & Le Hérissé, 2004). Their occurrence in the Takche Formation suggests productive marine waters with sufficient nutrient influx, possibly influenced by upwelling or periodic water column stratification.

Scolecodonts, especially simple saw-blade elements attributable to xanioprionid and mochtyellid polychaetes, also occur in modest numbers. These microfossils are widely regarded as indicative of shallow marine depositional regimes, often associated with stable, warm, epicontinental seas (Hints, 2000; Hints & Eriksson, 2007, 2009; references therein). Although, the facies specificity of scolecodont distribution remains an area of active inquiry, their presence here complements the broader marine signal inferred from other palynomorph groups.

The occurrence of melanosclerites, including the forms tentatively identified as Melanoporella, Melanosteus, and Mirachitina, provides additional support for a shallow, open marine shelf setting. These enigmatic, thermally resistant organic microfossils have been interpreted as possible remains of marine microplankton or gelatinous organisms, though their biological affinities remain debated (Cashman, 1992; Sinha et al., 1996; Sinha & Trampisch, 2013). Nonetheless, their facies-dependent distribution, often restricted to low-energy, well-oxygenated marine environments, makes them useful proxies for palaeoenvironmental reconstruction.

Taken together, the dominance of marine palynomorphs, the specific acritarch and chitinozoan taxa identified, and the presence of melanosclerites and scolecodonts collectively support the deposition of Takche Formation in a relatively stable, offshore marine shelf setting during the Middle Ordovician. This palaeoenvironmental interpretation aligns with broader models of early Palaeozoic marine transgressions along the Gondwanan margin (e.g., Cocks & Torsvik, 2002; Ghienne et al., 2014; Rubinstein & Vaccari, 2004) and contributes to our understanding of marine-terrestrial interactions during the early stages of land plant colonisation (e.g., Edwards et al., 1998; Steemans et al., 2009; Wellman & Gray, 2000).

Implications for terrestrialisation

Although quantitatively minor, the occurrence of ?cryptospores, trilete spores, and phytodebris within the palynological assemblage of the Takche Formation is of substantial palaeobiological and biogeographic importance. The identification of genera such as Chelinohilates, Didymospora, Dyadospora, Gneudnaspora, Laevolancis, Rugosphaera, Sphaerasacus and Segestrespora, widely interpreted as cryptospore taxa derived from early land plants, offers valuable evidence for the presence of terrestrial vegetation on the Gondwanan landmass during the Middle Ordovician. These dispersed spores are generally considered among the earliest records of embryophyte evolution, signalling the advent of terrestrial colonisation by bryophyte-grade plants or their precursors (Edwards & Wellman, 2001; Rubinstein & Vajda, 2019; Wellman et al., 2003). In the context of the Tethyan Himalaya, these microfossils likely represent the oldest known cryptospore-bearing sediments from the Indian subcontinent. Their presence not only fills a geographic gap in the known distribution of early land plant microfossils but also contributes to a growing body of evidence indicating that parts of Gondwana were vegetated much earlier than previously assumed (e.g., Servais et al., 2010; Steemans et al., 2009). These dispersed palynomorphs, though preserved within marine shale and calcareous siltstone, are interpreted as having been transported offshore, likely by aeolian or fluvial processes, from nearby emergent landmasses. This pattern of occurrence is consistent with that observed in other low-latitude Gondwanan and peri-Gondwanan regions, where cryptospore and phytodebris signals in marine sediments have been linked to proximal terrestrial sources (Muro et al., 2018; Rubinstein et al., 2023; Vecoli & Le Hérissé, 2004). The recovery of such taxa in thermally mature but stratigraphically constrained Ordovician successions underscores the need to reevaluate the timing and geography of early land plant colonisation. Importantly, these findings lend further support to the hypothesis that land plant evolution, and by extension, terrestrial ecosystem development, was underway on Gondwana long before the widespread appearance of trilete spores in the Silurian (Gray et al., 1985; Wellman & Gray, 2000). In this light, these preliminary putative findings of cryptospores from the Takche Formation provide a valuable data point for reconstructing the early stages of terrestrialisation and for understanding how emerging terrestrial biotas may have interacted with adjacent shallow marine ecosystems during the Ordovician.

Biostratigraphic and palaeogeographic significance

The chitinozoan assemblage identified from the Takche Formation provides critical biostratigraphic insight into the Ordovician succession of the Tethyan Himalaya. Among the most significant taxa is the Belonechitina capitata that was first described from the Baltic domain by Eisenack (1962). This taxon is well established biostratigraphically from upper Darriwilian to lower Sandbian (Achab & Asselin, 1995; Grahn et al., 1996; Nõlvak & Bauert, 2006; Nõlvak & Grahn, 1993; Paris, 1981; Vandenbroucke, 2004). B. capitata is known to occur from Lasnamägi Stage (463–462.6 Ma) to Rakvere Stage (453.2–452 Ma) of Baltoscandia (Nõlvak & Grahn, 1993). In Baltic domain, B. capitata has been reported from Baltoscandia – Scandinavia, East Baltic, Northeast Poland (Modliński et al., 2002; Samuelsson, Vecoli et al., 2002), and Podolia (Nõlvak & Grahn, 1993), Upper Sandbian of Lithuania (Hints et al., 2016), Late Darriwillin to Early Sandbian of Sweden (Grahn et al., 1996; Grahn & Nõlvak, 2007a; Pouille et al., 2013; Sturkell et al., 2000; Vandenbroucke, 2004), Estonia (Grahn et al., 1996; Nõlvak & Bauert, 2006), Late Abereiden to Late Caradoc (Middle Darriwillian – Early Katian) of NW Germany (Samuelsson, Gerdes, et al., 2002), Caradoc of England (Vandenbroucke, 2008a, 2008b; Vandenbroucke et al., 2005, 2009), Upper Darriwillian to Lower Sandbian of Oslo region, Southern Norway (Grahn & Nõlvak, 2007b). This species of Belonechitina is reported in Northern Gondwana domain from Libya (Molyneux & Paris, 1985), Early Caradoc (Early Sandbian) of Zagros Basin, Southern Iran (Ghavidel-Syooki, 2001), Middle Sandbian of the Algerian Sahara (Oulebsir, 1992; Oulebsir & Paris, 1995) Costonian to Early Harnagian (Late Darriwillian to Early Sandbian) of Saudi Arabia (Al-Hajri, 1995), Late Llanvirn (Late Darriwillian) to Early Caradoc (Early Sandbian) of Portugal (Henry et al., 1974; Paris, 1981). In the Laurentia region, B. capitata has been reported from Late Llanvirn to Early Caradoc (Late Darriwillian to Early Sandbian) of the Arctic Platform, Canada (Achab & Asselin, 1995). This species of Belonechitina has also been reported from Garhwal Tethys Himalaya along with conochitina chydaea, B. micracantha, transitional forms B. capitata and B. micracantha, Angochitina spp., Conochitina spp., Belonechitina spp. and Conochitinidea (Sinha et al., 2011). It has also been reported from Kumaon Tethys Himalaya along with B. micracantha and Desmochitina spp. (Sinha & Verneirs, 2016). Belonechitina micracantha is a widely distributed chitinozoan species, with documented occurrences across various regions, including Europe, North Africa and India. In Europe, it has been extensively reported from the Middle to Late Ordovician strata in Estonia and Sweden. In Estonia, occurrences are noted from the Väo Formation at Uuga Cliff on the Pakri Peninsula, where it is part of a diverse chitinozoan assemblage indicative of Darriwilian age (Tammekänd et al., 2010). In Sweden, the species is identified in the Grötlingbo-1 core section in southern Gotland, contributing to high-resolution biostratigraphic frameworks and regional correlations across the Baltoscandian Basin (Männik et al., 2015). Additionally, B. micracantha has been recorded in the Skogerholmen Formation at Hovedøya Island in the Oslo Fjord, Norway, further supporting its widespread distribution in the Baltoscandian region (Amberg et al., 2017). In North Africa, B. micracantha is reported from Upper Ordovician deposits in both Morocco and Algeria. In Morocco, it occurs in the Bou Ingarf section of the Central Anti-Atlas, within a rich chitinozoan assemblage reflecting shallow marine conditions on the northern Gondwanan margin, aiding in regional correlations with the Armorican Trough (Bourahrouh et al., 2004). In Algeria, its presence in shelfal marine facies confirms its role in establishing regional biozones within the Northern Gondwana framework (Paris, 1990).​ In India, B. micracantha was identified for the first time from the Shiala Formation of the Tethyan Himalaya in Uttarakhand, where it co-occurs with Belonechitina capitata in Darriwilian–Sandbian strata, marking a significant extension of its paleogeographic range into the Indian Gondwanan margin and linking Indian chitinozoan assemblages with those of Baltica and peri-Gondwanan regions (Sinha et al., 2011).

Notably, the occurrence of Belonechitina capitata and B. micracantha, both of which are widely regarded as biostratigraphically significant taxa within the Middle to early Late Ordovician interval (Paris, 1990; Paris et al., 2004), provides strong support for assigning Darriwilian to early Sandbian age for the sampled strata. Fossil invertebrates (brachiopods and tentaculitioids) and warm-water marine macroalgae belonging to Darriwilian to Sandbian times are documented from the units just above the palynomorphs-yielding units (Shabbar, 2021; Shabbar et al., 2020, 2022, 2023). These taxa have well-documented stratigraphic ranges in regions such as Baltica, North Africa, and Avalonia, and their presence in the Indian Himalaya further strengthens correlations across these palaeocontinents. The stratigraphic utility of chitinozoans has been firmly established through decades of global work, owing to their rapid evolutionary turnover and wide palaeogeographic distribution (Achab & Paris, 2007; Grahn, 2006). In the context of the Takche Formation, the recognition of these chitinozoan markers not only corroborates earlier palynological findings from the region (Sinha et al., 2011; Sinha & Verniers, 2016; Wang et al., 2021), but also significantly refines the regional biostratigraphic framework by offering precise temporal constraints for the sequence. From a palaeogeographic standpoint, the chitinozoan evidence provides further support for the biogeographic connectivity of the Indian margin with other Gondwanan and peri-Gondwanan terranes during the Middle Ordovician. The assemblage composition reflects strong faunal similarities with contemporaneous palynological records from Morocco, Argentina, Libya, and Sardinia, suggesting that the Indian plate occupied a similar low- to mid-latitude southern hemisphere position during this period (Achab & Paris, 2007; Ghienne et al., 2014; Paris et al., 1996). The recovery of such cosmopolitan chitinozoan genera in the Takche Formation affirms the region’s inclusion within the broader Gondwanan palaeobiogeographic realm. It hints at possible migration pathways for marine planktonic microfauna along shallow shelf seas that rimmed the supercontinent. In addition, the presence of these taxa in thermally mature sediments reinforces their potential resilience to low-grade metamorphic conditions, a quality that enhances their value in chronostratigraphic reconstruction, especially in tectonically active and metamorphosed terrains such as the Himalayas (Khadka et al., 2025; Sah, 1976; Srivastava & Bhattacharya, 2000). As such, the Takche assemblage contributes not only to refining Ordovician biostratigraphy in India but also to global compilations of chitinozoan distribution and evolution during the GOBE, during which chitinozoans played a key role as index fossils and paleoenvironmental indicators (Servais et al., 2010).

CONCLUSIONS

The palynological investigation of the Takche Formation in the Tethyan Himalayas provides a valuable new perspective on Ordovician paleoenvironments, biostratigraphy, and the early evolution of land plants within the Gondwana realm. Despite the overprints of thermal alteration affecting preservation, the recovered assemblage is dominated by marine forms, including acritarchs, chitinozoans, melanosclerites, and scolecodonts. Clearly, it reflects deposition in a distal shallow marine setting, removed from significant terrestrial influence.

These findings align closely with the palaeoenvironmental interpretations from comparable sequences in the region and elsewhere along the northern Gondwanan margin, including the Shaila Formation of the Garhwal region. Of particular significance is the presence of ?cryptospores and dispersed phytodebris, albeit in low abundance. Recorded ?cryptospore taxa having close resemblance with Chelinospora, Didymospora and Rugosphaera represent possibly some of the earliest land plant spores recorded from India and suggest a vegetated hinterland not far from the depositional site. This subtle terrestrial signal, preserved within a dominantly marine matrix, supports the emerging consensus that the colonisation of land by early plants was already underway by the Middle Ordovician. However, further substantial studies are needed to confirm the unequivocal presence of cryptospores from this part of the globe.

These microfossils thus represent a significant addition to the sparse yet growing record of early terrestrial life from Gondwana. Biostratigraphically, the occurrence of diagnostic chitinozoans, including Belonechitina capitata and B. micracantha, permits a preliminary correlation of the Takche Formation with the Darriwilian–Sandbian interval. These taxa extend the known distribution of chitinozoans within India and highlight the significance of the Tethyan Himalaya in refining Ordovician chronostratigraphy in this part of Gondwana. Furthermore, the close taxonomic affinities of the palynomorph assemblage with those from Baltica, Avalonia and other peri-Gondwanan regions lend further weight to global palaeogeographic reconstructions of the Ordovician world. In summary, the Takche Formation emerges as a key archive for reconstructing Middle to early Late Ordovician palaeoecosystems and offers a valuable window into early marine-terrestrial interactions. The combined marine and terrestrial palynomorph signals, preserved in a geologically complex and tectonically active region, underscore the significance of integrated micropalaeontological approaches for understanding the evolutionary and environmental dynamics of early Earth systems. Ongoing and future research in this region promises to refine our understanding of the timing, nature, and extent of early land plant expansion, as well as its broader implications for Ordovician Earth history.