1. Introduction
The end-Triassic mass extinction (ETME) is one of Earth’s five major extinction events (Sepkoski Jr, Reference Sepkoski1996) and is causally linked to the eruption of the Central Atlantic Magmatic Province (CAMP), a large igneous province (Davies et al., Reference Davies, Marzoli, Bertrand, Youbi, Ernesto and Schaltegger2017). Its vast volcanic emissions (CO2, SO2 and other volatiles) were probably responsible for the biotic crisis, triggering extreme global warming, disrupting all levels of the marine food web and leading to ecosystem collapse (McElwain et al., Reference McElwain, Beerling and Woodward1999; Lindström et al., Reference Lindström, Sanei, Van de Schootbrugge, Pedersen, Lesher, Tegner, Heunisch, Dybkjær and Outridge2019; Lindström, Reference Lindström2021).
The Early Jurassic, more specifically the Hettangian, is characterized by significant environmental and climatic changes following the extinction event, including sea-level change, ocean acidification and episodes of anoxia (Van de Schootbrugge et al., Reference Van de Schootbrugge, Tremolada, Rosenthal, Bailey, Feist-Burkhardt, Brinkhuis, Pross, Kent and Falkowski2007; Kasprak et al., Reference Kasprak, Sepúlveda, Price-waldman, Williford, Schoepfer, Haggart, Ward, Summons and Whiteside2015; Jost et al., Reference Jost, Bachan, Van de Schootbrugge, Lau, Weaver, Maher and Payne2017; Schöllhorn et al., Reference Schöllhorn, Adatte, Van de Schootbrugge, Houben, Charbonnier, Janssen and Föllmi2020; Trudgill et al., Reference Trudgill, Rae, Whiteford, Adloff, Crumpton-banks, Van Mourik, Cuperus, Corsetti, Doherty, Gray and Greene2025). Although much research has been carried out on macrofossil marine organisms (e.g., bivalves, belemnites, ammonites and ichnofossils) (Hallam, Reference Hallam1996; Wignall & Bond, Reference Wignall and Bond2008; Atkinson & Wignall, Reference Atkinson and Wignall2019) and miospores (Lund, Reference Lund1977; Schuurman, Reference Schuurman1977; Pedersen & Lund, Reference Pedersen and Lund1980; Küerschner et al., Reference Kuerschner, Bonis and Krystyn2007; Bonis et al., Reference Bonis, Kürschner and Krystyn2009; Bonis et al., Reference Bonis, Ruhl and Kürschner2010; Gravendyck et al., Reference Gravendyck, Schobben, Bachelier and Kürschner2020; Lindström et al., Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017, Reference Lindström, Pedersen, Vosgerau, Hovikoski, Dybkjær and Nielsen2023), much less is known about the response of phytoplankton groups to this severe environmental disruption. Palynological research focusing on non-pollen components, such as acritarchs, remains limited to a few pioneer studies (Wall, Reference Wall1965; Morbey, Reference Morbey1975; Sarjeant, Reference Sarjeant1976; Van de Schootbrugge et al., Reference Van de Schootbrugge, Tremolada, Rosenthal, Bailey, Feist-Burkhardt, Brinkhuis, Pross, Kent and Falkowski2007).
Acritarchs are a catch-all group for organic-walled microfossils that cannot readily be classified (Evitt, Reference Evitt1963). Despite their uncertain biological affinity (incertae sedis), discrete groups are generally recognized as resting cysts of microplanktonic organisms (Kroeck et al., Reference Kroeck, Mullins, Zacaï, Monnet and Servais2022). Some acritarchs share similarities with dinoflagellate cysts, including morphological features, biogeochemical properties and their occurrence patterns in marine sediments. Many studies link variations in acritarch morphology, such as process length and overall size, to specific depositional settings (Tyson, Reference Tyson1995; van Soelen & Kürschner, Reference van Soelen and Kürschner2018; Lei et al., Reference Lei, Shen, Algeo, Servais, Feng and Yu2019). Consequently, acritarchs can be used to help interpret past marine environments. Given that the group likely includes many primary producers, acritarchs play a vital role in the marine food chain and act as sensitive indicators of ocean chemistry. Also, they serve as tools for refining palaeoecological models and shedding light on major environmental perturbation events, such as the end-Permian and Hirnantian (end-Ordovician) mass-extinctions (Delabroye et al., Reference Delabroye, Munnecke, Servais, Vandenbroucke and Vecoli2012; van Soelen et al., Reference van Soelen, Twitchett and Kürschner2018), if not yet the end-Triassic event.
Addressing this lacuna, this paper details the combined record of miospore, acritarch, and dinoflagellate cyst distribution from the Prees 2 borehole in the Cheshire Basin, the United Kingdom (Figure 1). It is important to acknowledge that this approach differs from single-group specialized studies, which typically rely on higher counts for specific groups to capture subtle community changes. Although our approach results in lower total counts for individual aquatic groups, it offers the advantage of linking vegetation changes with marine ecosystem responses within the same sample set.
Figure 1. Palaeogeographic map illustrating: (a) A detailed view of southern UK, indicating the location of Prees 2 in the Cheshire Basin during the Early Hettangian. The modern UK outline and the extent of Jurassic–Cretaceous sedimentary basins follow Hesselbo et al. (Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023), which is adapted from the BGS 1:1,500,000 series tectonic map. The palaeogeographic features are adapted from Bradshaw et al. (Reference Bradshaw, Cope, Cripps, Donovan, Howarth, Rawson, West, Wimbledon, Cope, Ingham and Rawson1992), with inferred boundaries shown as dashed lines. (b) Palaeogeographic map of NW Europe during the Triassic–Jurassic transition, and (c) global palaeogeographic reconstruction showing continental configuration and the positions of the Panthalassa and Tethys Oceans after Bos et al. (Reference Bos, Lindström, Van Konijnenburg-van Cittert, Hilgen, Hollaar, Aalpoel, Van der Weijst, Sanei, Rudra and Sluijs2023) and references therein. Rectangles in (b) and (c) are the areas of panels (a) and (b) respectively.
A total of 92 samples, from the uppermost Triassic through to the lower Sinemurian, were analyzed, providing new palynological data for the northwestern Tethys region. Prees 2 was drilled in the Cheshire Basin in 2020–2021 as part of the Jurassic Earth System and Timescale (JET) project, under the auspices of the International Continental Scientific Drilling Program (ICDP). The retrieved core, ∼600 m long, provides a valuable Lower Jurassic record, spanning the upper Norian to lower Pliensbachian (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023).
1. a. Geological setting
1.a.1. Cheshire basin
Located in northwest England (Figure 1), the Cheshire Basin encompasses an area of 3,500 km2 and contains the most extensive Permian–Triassic sedimentary fill in the UK, reaching ∼4 km in thickness (Evans et al., Reference Evans, Rees and Holloway1993; Newell, Reference Newell2018). This southeast-dipping half-graben basin was controlled by the Wem-Red Rock Fault System (Newell, Reference Newell2018). The wider rift system developed across England and adjacent regions and was related to the early stages of the break-up of the supercontinent Pangaea (e.g., Coward, Reference Coward1995).
During the late Permian to Triassic periods, these extensional basins were filled with clastic sediments and evaporites deposited in aeolian, fluvial and playa-lacustrine depositional systems that traversed the region (Chadwick & Evans, Reference Chadwick and Evans1995; Newell, Reference Newell2018). As rifting in the North Atlantic progressed, these continental basins transitioned to fully marine conditions during the latest Triassic (Mayall, Reference Mayall1981), with marine conditions persisting throughout much of the remaining Mesozoic (e.g., Hesselbo et al., Reference Hesselbo, Robinson and Surlyk2004). The transition from the Triassic red beds of the Mercia Mudstone Group to the black mudstones, limestones and sandstones of the Penarth Group and the Lias Group exemplifies the transition from continental to restricted and open marine environments (Plant et al., Reference Plant, Jones and Haslam1999; Newell, Reference Newell2018).
The Early Jurassic sedimentary record in the Cheshire Basin extends up to the Pliensbachian, and it is preserved in a small segment adjacent to the bounding Wem Fault in the southeastern area known as the Wem-Audlem Sub-basin (Evans et al., Reference Evans, Rees and Holloway1993; Ullmann et al., Reference Ullmann, Damaschke, Hesselbo, Jiang, Lawrence, Leng, Mattioli, Bancalin, Page, Pudal and Ruhl2025). Located on the eastern margin of Pangaea, this basin lay adjacent to elevated terrains, which contributed to its sedimentary dynamics during the Early Jurassic (Figure 1 – Plant et al., Reference Plant, Jones and Haslam1999).
1.b. Lithostratigraphy
1.b.1. Penarth group – Westbury formation (602.19 to 598.03 metres corrected core depth – mccd)
The Westbury Formation in Prees 2 is composed of laminated to massive-bedded mudstones (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023). Thin beds of limestone and siltstone are present, along with bivalve shell beds and fossiliferous, phosphatic and sand-rich layers. In some intervals, cross-laminated, convoluted or pyritic strata are observed. Organic matter content reaches up to ∼4% TOC (Figure 2). Three samples were taken from this unit (as shown in Figures 2 to 4). The Prees 2 core shows lithofacies typical of the Westbury Formation in other parts of the UK (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023).
Figure 2. Overview of palynomorph abundances for Prees 2. This figure presents palynological sampling data, samples in bold underwent oxidative treatment. Geochemical measurements (TOC and δ13C) for the studied section are from Hesselbo et al., (Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023). mccd = metres corrected core depth. Detailed lithostratigraphy and ammonite biozonation were produced by the JET project team (see Hesselbo et al. (Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023) for full documentation). Dashed line represents uncertain boundary, and shaded interval indicates there is no ammonite evidence. Additionally, the figure presents the total percentage of each palynomorph group. The median raw counts contributing to these totals are: pollen (154), acritarchs (104), spores (15.5), and dinoflagellate cysts (12). The spore-to-pollen ratio, defined as total spores divided by the sum of total pollen and spores (Σspores / (Σpollen + Σspores)), and plotted absolute abundance curves of polygonomorphs acritarchs, and dinoflagellate cysts. Absolute abundances of the total palynomorph assemblage were calculated using Lycopodium markers. Pale red squares indicate the CIEs present. X indicates samples with observed malformations in acritarchs.
1.b.2. Penarth group – Lilstock formation (598.04 to 592.33 mccd)
The Lilstock Formation is 5.70 m thick, with one sample taken from this interval (as shown in Figure 2). It consists of mudstones, siltstones and silty limestone, frequently exhibiting convoluted bedding. Heterolithic wavy to lenticular bedding with scours is also common. The upper part of the formation contains pale-grey or pale-brown siltstone or limestone beds, with mm-scale silt or limestone laminae, potentially representing microbial silty limestone. The lower part of the formation is referred to as the Cotham Member (598.04 to 592.51 mccd), and the overlying unit is identified as the Langport Member (592.51 to 592.33 mccd). These latter uppermost 18 cm consist of pale olive-green or grey faintly laminated limestone and mudstone beds, with scattered bioclasts and thin siltstone laminae at the base. Across the UK, the Cotham Member is characterized by features such as convoluted bedding, desiccation cracks and localized oolitic limestones, though these were not observed in the Prees 2 core (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023).
1.b.3. Lias group – Redcar mudstone formation (592.32 to 456.28 mccd)
The Redcar Mudstone Formation in Prees 2 contains carbonate concretions and is rich in macrofossils and trace fossils. Organic matter is notably enriched in the lowermost Hettangian, around 590.00 mccd, where dark-grey laminated silty limestones or mudstones with >4% TOC occur (Figure 2; Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023). The name ‘Redcar Mudstone’, originating from the Cleveland Basin, has been applied due to the similarity with successions in northeast England. The lower part of the succession at Prees 2 shares some features with the Blue Lias Formation but lacks its characteristic fully developed limestone beds and high-TOC shales. The Redcar Mudstone at Prees also shows signs of some basin restriction and distal storm influences (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023).
1.c. Biostratigraphy
Ammonites serve as the primary biostratigraphic correlation tools for marine Jurassic sequences, including the section of Prees 2 (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023). The base of the Jurassic is currently defined by the first appearance (FAD) of Psiloceras spelae (Hillebrandt et al., Reference Hillebrandt, Krystyn, Kürschner, Bonis, Ruhl, Richoz, Schobben, Urlichs, Bown, Kment and McRoberts2013). However, there is commonly a gap in ammonite occurrences between the last/highest occurrence (LO/HO) of the Triassic ammonite genus Choristoceras and the first/lowest occurrence (FO/LO) of the Jurassic genus Psiloceras (the earliest species of the latter genus are also commonly absent; Lindström et al., Reference Lindström, Van de Schootbrugge, Hansen, Pedersen, Alsen, Thibault, Dybkjær, Bjerrum and Nielsen2016). To better constrain this boundary, the pollen Cerebropollenites thiergartii is used as an accessory marker, appearing approximately 6 m below the FO of P. spelae at the Global Stratotype Section and Point (GSSP) at Kuhjoch, Austria.
Magnetostratigraphy and Carbon Isotope Excursions (CIEs) also help to further constrain the relative age and to characterize these boundaries (Lindström et al., Reference Lindström, Van de Schootbrugge, Hansen, Pedersen, Alsen, Thibault, Dybkjær, Bjerrum and Nielsen2016). For example, Ullmann et al. (Reference Ullmann, Damaschke, Hesselbo, Jiang, Lawrence, Leng, Mattioli, Bancalin, Page, Pudal and Ruhl2025), using the Prees 2 and nearby Wilkesley cores in the Cheshire Basin, effectively combined ammonite zonation with carbon isotope stratigraphy, significantly reducing uncertainties in zonal boundary definitions. The ammonite zone boundaries adopted here are derived from their refined study.
Pulses of CAMP volcanism coincide with the negative CIEs during the ETME (Davies et al., Reference Davies, Marzoli, Bertrand, Youbi, Ernesto and Schaltegger2017; Heimdal et al., Reference Heimdal, Jones and Svensen2020). The first disruption of the carbon cycle, as evidenced by CIEs in organic matter and carbonate rocks, is marked by the upper Triassic Precursor negative CIE (also known as the Marshi CIE), followed by the Spelae/Initial negative CIE around the Triassic–Jurassic (Tr–J) boundary and lastly the Tilmanni/Main negative CIE in the lowermost Jurassic (Hesselbo et al., Reference Hesselbo, Robinson, Surlyk and Piasecki2002; Guex et al., Reference Guex, Bartolini, Atudorei and Taylor2004; Ruhl et al., Reference Ruhl, Kürschner and Krystyn2009; Lindström et al., Reference Lindström, Van de Schootbrugge, Hansen, Pedersen, Alsen, Thibault, Dybkjær, Bjerrum and Nielsen2016; Bos et al., Reference Bos, Lindström, Van Konijnenburg-van Cittert, Hilgen, Hollaar, Aalpoel, Van der Weijst, Sanei, Rudra and Sluijs2023). Volcanic CO2 outgassing is widely considered the primary driver of the CIEs (Hesselbo et al., Reference Hesselbo, Robinson, Surlyk and Piasecki2002), with the release of methane hydrate from seafloor warming acting as a potential positive feedback mechanism (Beerling & Berner, Reference Beerling and Berner2002). Other studies suggest that ecosystem changes from sea-level fluctuations contributed to the Spelae/Initial CIE by promoting the growth of microbial mats (Fox et al., Reference Fox, Cui, Whiteside, Olsen, Summons and Grice2020). Additionally, recent findings highlight the role of CAMP sill emplacements in Paleozoic sequences, which may have released thermogenic carbon and further intensified the carbon cycle disruption (Heimdal et al., Reference Heimdal, Jones and Svensen2020).
2. Methods
A total of 92 samples (Figure 2) were processed at the palynological laboratory of Utrecht University. The samples were oven-dried at 60°C and then crushed, with ∼4 g of material processed on average. A Lycopodium tablet, with an average of 19,855 spores, was added to obtain an absolute count. The palynological processing protocol includes consecutive treatments with 10% hydrochloric acid (HCl) and 38%–40% hydrofluoric acid (HF) at room temperature to eliminate carbonate and silicate minerals. After each HF treatment, a 30% HCl wash was used to prevent calcium fluoride (CaF2) formation. Samples indicated in Figures 3 and 4 (in bold) required additional oxidative maceration with KClO3 and 65% HNO3 due to their high content of amorphous organic matter.
Figure 3. Quantitative stratigraphic distribution: focusing on the aquatic palynomorphs. The diameter and shading of the dots represent the relative abundance (%) of a given species. The symbol X indicates identifiable taxa in samples that underwent oxidative treatment but were too damaged for counting.
Figure 4. Quantitative stratigraphic distribution: focusing on pollen and spores. The diameter and shading of the dots represent the relative abundance (%) of a given species. The symbol X indicates identifiable taxa in samples that underwent oxidative treatment but were too damaged for counting.
The residual material was passed through a nylon sieve with a 7 µm mesh size. One drop of the collected residues was then permanently affixed onto two glass slides using a mix of 5% Polyvinyl Alcohol (PVA) solution and glass adhesive. Counting of approximately 300 palynomorphs per sample was conducted using light microscopy (Zeiss Axio Imager A1 microscope equipped with a Zeiss AxioCam MRc5 camera), under a magnification of 1000× (using oil immersion). Abundance data were calculated as percentages of the total palynomorph assemblage. For comparative purposes, the totals reflect an integrated counting strategy rather than high-resolution sums targeted at specific groups, particularly dinoflagellates. The count data are presented in the Supplementary Table S1.
One sample (SSK116452; 474.44 mccd.; Upper Hettangian) was selected for the isolation of dinoflagellate cyst specimens. The cysts were handpicked from the residue using a Zeiss Discovery V20 stereomicroscope. Selected specimens were mounted on an aluminium stub, coated with gold and palladium and analyzed using a TESCAN TIMA3-X GMU field emission scanning electron microscope. High-resolution images were taken to aid in the detailed morphological characterization of the specimens.
3. Results
3.a. Systematic palaeontology of selected taxa
Here, we highlight the most relevant acritarchs and dinoflagellate cysts observed at Prees 2, emphasizing taxa with consistent occurrences, new species and diagnostic clarifications for frequently encountered taxa.
Algae Incertae Sedis
Group: ACRITARCHA Evitt, Reference Evitt 1963
Genus: Micrhystridium Deflandre, 1937
Type Species: Micrhystridium inconspicuum (Deflandre, 1937) Deflandre, 1937 (by original designation)
Micrhystridium fragile Deflandre, Reference Deflandre 1947 ( sensu Wall, Reference Wall 1965 )
Figure 5. Microphotographs of acritarchs. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Micrhystridium fragile, SSK 116447 (469.18 mccd); (b) Micrhystridium fragile, SSK 116447 (469.18 mccd); (c) Micrhystridium fragile, SSK 116460 (482.42 mccd); (d) Micrhystridium stellatum, SSK 116527 (549.28 mccd), morphotype approximate to M. fragile; (e) Micrhystridium stellatum, SSK 116463 (485.33 mccd); (f) Micrhystridium fragile/stellatum, SSK 116447 (469.18 mccd), this specimen contains one process with an expanded base as is typical for M. stellatum, showing a possible transition between morphotypes; (g) Micrhystridium fragile/stellatum, SSK 116463 (485.33 mccd), this specimen contains one process with an expanded base as is typical for M. stellatum, showing a possible transition between morphotypes; (h) Micrhystridium stellatum, SSK 116530 (552.36 mccd); (i) Micrhystridium stellatum, SSK 116538 (559.53 mccd); (j) Micrhystridium stellatum, SSK 116444 (466.21 mccd); (k) Stellinium? sp., SSK 116527 (549.28 mccd); (l) Stellinium? sp., SSK 116434 (456.28 mccd); (m) Dorsennidium? simplex, SSK 116530 (552.36 mccd), distal focus; (n) Dorsennidium? simplex, SSK 116530 (552.36 mccd), proximal focus; (o) Dorsennidium? simplex, SSK 116536 (558.42 mccd); (p) Dorsennidium? simplex, SSK 116447 (469.18 mccd); (q) Dorsennidium formosum, SSK 116530 (552.36 mccd); (r) Dorsennidium formosum, SSK 116532 (554.36 mccd); (s) Dorsennidium formosum, SSK 116530 (552.36 mccd); (t) Dorsennidium formosum, SSK 116527 (549.28 mccd); (u) Dorsennidium rhombodinium, SSK 116527 (549.28 mccd); (v) Dorsennidium rhombodinium, SSK 116438 (459.84 mccd); (w) Dorsennidium rhombodinium, SSK 116527 (549.28 mccd); (x) Dorsennidium rhombodinium, SSK 116447 (469.18 mccd).
Original Description (translated, Deflandre Reference Deflandre 1947 , p. 8): The test is spheroidal, more or less regular and adorned with a small number (approximately 10 to 15) of thin, pointed, generally flexuous processes. This length of the processes typically reaches or exceeds the test diameter. Occasionally, they can be shorter, but rarely do they measure less than half the diameter. Holotype dimensions: diameter of approximately 10.5 μm without processes and nearly 28 μm with processes. Dimensions of the most characteristic paratypes: diameter of 8–10 μm without processes and 12–24 μm with processes. Angular aspects observed in some specimens (e.g., figs. 14 and 17) are likely the result of fossilization, as the species appears prone to deformation. The best-preserved individuals show inclusions (gaseous?) filling the vesicle’s interior.
Description of Prees 2 material: Vesicle subcircular in outline. Eilyma psilate exhibits a thin and smooth wall. The processes are hollow, flexible and acuminate distally; bifurcation is rare. The vesicle’s lateral profile is marginally convex due to the slightly enlarged process bases. This results in a circular to weakly subpolygonal optical outline.
Dimensions: Vesicle diameter 9–17 µm (mean 12.6 µm). Process length 4–14 µm (mean 7.5 µm). Processes visible in optical section 8–17 (n = 152).
Distribution: Hettangian – Planorbis to Bucklandi zones.
Remarks: Deflandre’ (Reference Deflandre1947) concept shows a circular vesicle outline with a possibly faintly granulate eilyma. In contrast, Wall (Reference Wall1965), whilst not providing a formal emendation, depicted the specimens with a relatively thinner wall, hollow processes and conspicuous spine bases. Because the specimens examined here display the latter set of features, the taxon is used sensu Wall (Reference Wall1965).
Micrhystridium lymensis Wall, Reference Wall 1965
Figure 6. Microphotographs of non-pollen palynomorphs. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Baltisphaeridium infulatum var. macroinfulatum, SSK 116438 (459.84 mccd), focusing on the base of the process; (b) Baltisphaeridium infulatum var. macroinfulatum, SSK 116434 (456.28 mccd); showing medial split; (c) Baltisphaeridium infulatum var. macroinfulatum, SSK 116444 (466.21 mccd); showing medial split; (d) Baltisphaeridium infulatum var. macroinfulatum, SSK 116438 (459.84 mccd), degraded shagrinated wall; (e) Micrhystridium setasessitante, SSK 116548 (568.53 mccd), close focus on the short hyaline process; (f) Micrhystridium setasessitante, SSK 116463 (485.33 mccd); (g) Micrhystridium setasessitante, SSK 116566 (584.46 mccd); (h) Micrhystridium setasessitante, SSK 116556 (575.18 mccd); (i) Micrhystridium lymensis, SSK 116542 (564.55 mccd); (j) Micrhystridium lymensis, SSK 116489 (511.47 mccd); (k) Micrhystridium lymensis, SSK 116559 (577.9 mccd); (l) Micrhystridium lymensis, SSK 116566 (584.46 mccd); (m) Tasmanites sp., SSK 116517 (539.39 mccd); (n) Crassosphaera hexagonalis, SSK 116550 (570.51 mccd); (o) Cymatiosphaera sp. 01, SSK 116438 (459.84 mccd); (p) Cymatiosphaera sp. 02, SSK 116438 (459.84 mccd); (q) Scabrate cyst, SSK 116575 (593.18 mccd); (r) Scabrate cyst, SSK 116447 (469.18 mccd); (s) Leiosphaeridia sp., SSK 116575 (593.18 mccd); (t) Leiosphaeridia sp., SSK 116532 (554.36 mccd); (u) Leiosphaeridia sp., SSK 116438 (459.84 mccd); (v) Foraminiferal test lining – trochospiral type, SSK 116572 (549.28 mccd); (w) Foraminiferal test lining – biserial type, SSK 116472 (494.43 mccd); (x) Halosphaeropsis liassica, SSK 116564 (581.76 mccd).
Original Description, Wall (Reference Wall1965), p. 157: Holotype: K1/3/2, Blue Lias, bed H87 of Lang (1924, p. 181) (A. liasicus Zone), Pinhay Bay, Lyme Regis, Dorset. A small acritarch species characterized by its globular shape, moderately thick, sometimes double-layered walls and exogenous solid spines (9–30 in number), ranging in length from 15% to 100% of the test diameter.
Dimensions: subspecies Micrhystridium lymensis var. lymensis has vesicle diameters varying between 11 to 17 µm and process lengths varying from 7 to 12 µm. Subspecies Micrhystridium lymensis var. gliscum has vesicle diameters varying between 14 and 18 µm and process lengths from 5.5 to 8 µm. Subspecies Micrhystridium lymensis var. rigidum has vesicle diameters varying between 10 and 20 µm and process lengths from 2 to 6 µm. Distribution: Rhaetian to Sinemurian.
Remarks: This is the most prevalent acritarch in the core. The specimens exhibit infraspecific variation, with three subspecies defined by their differences in test diameter, spine number and spine length. In the current study, we identified specimens smaller than 10 μm in vesicle diameter (i.e., normally 7–9 µm); probably this is a result of using a 7 µm mesh sieve as opposed to a larger mesh sieve used in the original study (Wall, Reference Wall1965, p. 157).
Micrhystridium setasessitante Jansonius, Reference Jansonius 1962
Original Description: Holotype: pl. 16, fig. 50; text fig. 3 -d; Imp. 2631-2-124 × 42.4 (Jansonius, Reference Jansonius1962). Vesicle circular in outline; numerous setose spines, evenly disposed, not crowded, along the outline (30–40 in number); spines ½ μ wide, 2–4 μ long, implanted on low hyaline warts (1 μ). Near the tip, the spines are slightly thickened and rounded; spines are flexible and usually gently curved. Vesicle diameter: 15–25 μm. Stratigraphic Occurrence: Lower Triassic.
Description of Prees 2 material: Vesicle subcircular in outline, with a diameter ranging between 13 and 22 μm. The holomorphic processes exhibit a bristle-like or filamentous structure, typically hyaline in appearance. The processes terminate simply, without complex branching. Processes range in length from 1 to 6 μm (mean 3.5 µm); 20 to 35 processes are visible (n = 56). Rare forms may exhibit as few as 1–5 processes. Excystment by splitting (partial rupture) of the vesicle wall is commonly observed. Distribution: Tilmanni to Bucklandi zones.
Remarks: Van de Schootbrugge et al. (Reference Van de Schootbrugge, Tremolada, Rosenthal, Bailey, Feist-Burkhardt, Brinkhuis, Pross, Kent and Falkowski2007) documented an acritarch peak in the Hettangian of St Audrie’s Bay, which they attributed to Micrhystridium microspinosum. However, the specimens they figured differ from the original diagnosis of M. microspinosum (Schaarschmidt, Reference Schaarschmidt1963), which describes a circular outline of 14–16 µm diameter, densely covered with very short (∼1 µm), conical processes. Our examination suggests that these specimens more closely resemble Micrhystridium setasessitante. Accordingly, we refer to comparable material in the present study by this designation.
Algae Incertae Sedis
Group: ACRITARCHA Evitt, Reference Evitt 1963
Genus: Dorsennidium Wicander, emend. Sarjeant & Stancliffe, Reference Sarjeant and Stancliffe 1994
Type species: Dorsennidium patulum, Wicander ( Reference Wicander 1974 )
Originally erected by Wicander (Reference Wicander1974) based on Devonian material, acritarchs assigned to the genus Dorsennidium were described as small subangular vesicles bearing processes of variable sizes. The type species was transferred to the genus Veryhachium by Wicander & Loeblich (Reference Wicander and Loeblich1977) due to the triangular outline of the vesicle. This transfer made Dorsennidium a junior synonym of Veryhachium. The genus was reinstated and emended by Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994), who restricted Veryhachium to polygonal forms with only three or four processes and reassigned taxa with four to ten processes and polygonal vesicles, modified by processes directly connected to the vesicle interior, to Dorsennidium. With this distinction, the authors aimed to make Dorsennidium more useful in biostratigraphic and palaeoenvironmental studies. Servais et al. (Reference Servais, Vecoli, Li, Molyneux, Raevskaya and Rubinstein2007) criticized this emendation, stating that the strict distinction of Veryhachium was based solely on a literature review and excluded over half of the validly described species. Notwithstanding this debate, the more complex morphology of the Hettangian species, particularly the variability in spine number and arrangement, fits the emended definition of Dorsennidium. Therefore, this emendation reconciles the morphological diversity observed in Lower Jurassic acritarch assemblages with the taxonomic framework provided by Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994).
Dorsennidium? simplex (Wicander, Reference Wicander 1974 ) Sarjeant & Stancliffe, Reference Sarjeant and Stancliffe 1994
Original Description, Wicander (Reference Wicander1974), p. 20: Vesicle spherical, 21 μm in diameter, wall thin, laevigate. Processes: 5 simple, laevigate processes, 27 μm long, 2.7 μm wide at the base, tapering to a sharp point, distinct from the vesicle, opening into and communicating freely with the vesicle interior. Method of excystment not observed.
Remarks: Dorsennidium? simplex was described as morphologically intermediate between Micrhystridium and Dorsennidium Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994). The spherical vesicle with simple processes shows characteristics of both genera. D.? simplex was marked as tentative due to these intermediate features. The species’ sides exhibit a markedly convex profile, further highlighting its transitional morphology. In this study, the vesicle is subcircular to subpolygonal in outline, varying from 9 to 14 µm in diameter. Typically, it has five primary simple processes in free communication with the vesicle interior; the processes are flexuous, tapering from slightly broadened bases to acuminate tips; occasional shorter accessory processes may be present.
Dorsennidium irregulare (Jekhowsky, Reference Jekhowsky 1961 ) Sarjeant & Stancliffe, Reference Sarjeant and Stancliffe 1994
Figure 7. Microphotographs of acritarchs and malformed palynomorphs. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Micrhystridium malformed, SSK 116566 (584.46 mccd); (b) Micrhystridium malformed, SSK 116463 (485.33 mccd); (c) Leiofusa jurassica, SSK 116538 (559.53 mccd); (d) Leiofusa jurassica, SSK 116538 (559.53 mccd); (e) Metaleiofusa diagonalis, SSK 116557 (575.93 mccd); (f) Dorsennidium europaeum, SSK 116434 (456.28 mccd), proximal focus; (g) Dorsennidium europaeum, SSK 116434 (456.28 mccd), distal focus; (h) Dorsennidium europaeum, SSK 116456 (478.43 mccd); (i) Dorsennidium irregulare, SSK 116553 (572.5 mccd); (j) Dorsennidium irregulare, SSK 116438 (459.84 mccd); (k) Dorsennidium irregulare, SSK 116484 (486.33 mccd); (l) Acritarch indet. reworked, SSK 116438 (459.84 mccd); (m) Micrhystridium fragile malformed, SSK 116560 (578.91 mccd); (n) Micrhystridium fragile malformed, SSK 116465 (487.33 mccd); (o) Micrhystridium fragile malformed, SSK 116447 (469.18 mccd); (p) Dorsennidium irregulare malformed, SSK 116447 (469.18 mccd); (q) Classopollis malformed, SSK 116550 (570.51 mccd); (r) Classopollis classoides malformed, SSK 116575 (593.18 mccd); (s) Malformed spore, SSK 116575 (593.18 mccd); (t) Malformed spore, SSK 116501 (523.28 mccd).
Original Description (translated, Jekhowsky, Reference Jekhowsky 1961, p. 208): The central body diameter ranges between 10 and 20 µm, with a globular, ovoid or subpolyhedral contour. It possesses 4 to 6 appendages, which range from 5 to 15 µm in length and 2 to 5 µm in width at the base. These appendages are relatively robust, conical and hollow, communicate with the central body and have closed ends. The shape and size of these appendages can vary significantly within a single individual. This species is highly polymorphic, exhibiting four principal morphotypes with diffuse boundaries but distinct morphological centres:
Forma subhexaedron: central body subhexahedral and convex, with six appendages (one at each ‘vertex’).
Forma subtetraedron: central body subtetrahedral and convex, with four appendages (one at each ‘vertex’).
Forma irregulare: central body subspherical, with five to six appendages.
Forma pirula: central body more or less ovoid, with one prominent appendage at one ‘pole’ and three to five shorter appendages in a crown around the opposite pole.
Dimensions: Central body diameter: 10–20 µm. Spine length: 5–15 µm. Spine width at base: 2–5 µm.
Remarks: Originally described as Veryhachium? irregulare by Jekhowsky (Reference Jekhowsky1961), the polymorphic nature, with distinct morphotypes, led to taxonomic uncertainties, as noted by Wall & Downie (Reference Wall and Downie1963) and Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994). The transitional morphology between globular forms (Micrhystridium) and polygonal forms (Veryhachium) highlights the challenges of assigning this species to a genus. Of the four morphotypes recognized by Jekhowsky (Reference Jekhowsky1961), our material includes only forma subtetraedron (convex, subtetrahedral vesicle with four appendages, Figures 7i–j) and forma irregulare (subspherical vesicle with five to ten appendages, Figure 7k).
Dorsennidium formosum (Stockmans & Willière, 1960) Sarjeant & Stancliffe, Reference Sarjeant and Stancliffe 1994
Original Description: Test outline triangular with convex sides, bearing five to six strong, curved spines. The spines range in length from 100% to 200% of the test size and taper from narrow bases to pointed tips. The vesicle is laevigate, with smooth walls. Dimensions: Vesicle diameter: 9.5–14.5 μm. Spine length: up to 200% of the vesicle size.
Remarks: Dorsennidium formosum was originally described as Veryhachium formosum by Stockmans & Willière (Reference Stockmans and Willière1962) and later refined by Wall (Reference Wall1965), who identified and described several forms, including V. formosum forma ancorastrum, which was distinguished by its triangular test/vesicle outline and convex sides bearing five to six curved processes. Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994) transferred V. formosum to the genus Dorsennidium, placing it in Category 5, which includes species with small vesicles and long processes. The emended diagnosis emphasizes the open, hollow spine bases and the laevigate vesicle walls. Wall (Reference Wall1965) highlighted the morphological variation within D. formosum, noting differences in vesicle inflation and spine distribution. However, Sarjeant & Stancliffe (Reference Sarjeant and Stancliffe1994) considered these variations insufficient for species-level distinction and instead emphasized the diagnostic features of the genus Dorsennidium.
Gen. et sp. Indet 01
Figure 8. Microphotographs of dinoflagellate cysts. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Rhaetogonyaulax rhaetica, SSK 116582 (600.18 mccd); (b) Beaumontella langii, SSK 116542 (564.55 mccd); (c) Beaumontella langii, SSK 116444 (466.21 mccd); (d) Beaumontella langii, SSK 116452 (474.44 mccd); (e) Valvaeodinium hirsutum?, proximal view, SSK 116489 (511.47 mccd); (f) Valvaeodinium hirsutum?, distal view, SSK 116489 (511.47 mccd); (g) Beaumontella? caminuspina, SSK 116570 (588.08 mccd); (h) Multiplicisphaeridium dendroideum?, SSK 116558 (576.92 mccd); (i) Umbriadinium mediterraneense, SSK 116542 (564.55 mccd); (j) Umbriadinium mediterraneense, SSK 116542 (564.55 mccd); (k) Umbriadinium mediterraneense, SSK 116438 (459.84 mccd); (l) Dapcodinium priscum, SSK 116575 (593.18 mccd); (m) Beaumontella? delicata, SSK 116509 (531.04 mccd); (n) Beaumontella? delicata, SSK 116509 (531.04 mccd); (o) Gen. et sp. indet. 02, SSK 116471 (493.43 mccd); (p) Gen. et sp. indet. 02, SSK 116476 (498.37 mccd); (q) Gen. et sp. indet. 01, SSK 116457 (479.43 mccd); (r) Gen. et sp. indet. 01, SSK 116452 (474.44 mccd); (s) Gen. et sp. indet. 01, SSK 116452 (474.44 mccd); (t) Gen. et sp. indet. 01, SSK 116452 (474.44 mccd).
Diagnosis: Chorate cyst elongate, outline elliptic–rectangular. Processes long, with expanded bases and simple distal tips. Processes are arranged in distinct latitudinal rows suggestive of tabulation, but the archaeopyle has not been observed. The central equatorial band is unornamented and interpreted as a paracingulum. Symmetry is broadly holomorphic. When divided into five latitudinal zones (bands), the specimen exhibits a pattern of process distribution. While the orientation (apical vs. antapical) remains uncertain, the most process-dense zone (interpreted as the base of the cysts in Figures 8q to 8t) typically contains 9–16 processes. The central equatorial band (Band 3) is consistently unornamented and likely corresponds to the paracingulum. The uppermost zone (Band 5), when visible in full, frequently shows 9 processes arranged with bilateral symmetry (e.g., 4–1–4 or 4–2–3).
Dimensions: Visible process counts range from 26 to 42 per specimen. Process length varies from 8 to 14 µm. Cyst body length ranges from 28 to 43 µm; width from 11 to 17 µm. Measurements in 10 specimens.
Remarks: These specimens match the ‘Gen. et sp. indet.’ form described by Feist-Burkhardt (Reference Feist-Burkhardt2009) from the Wutach region of Germany, Upper Sinemurian, and may represent the same or a closely related morphotype.
Gen. et sp. Indet 02
Diagnosis: Chorate cyst characterized by an exceptionally high process density (ca. 70–90 per specimen). The processes are solid, thin and often twisted (very flexible), measuring between 7 and 13 µm in length. Each spine has a simple tip that may occasionally (bi)furcate; they start slightly thicker at the base and taper to a thin point. Cyst body elliptical to subspherical, 18–26 µm in diameter. Measurements in 10 specimens. Only one specimen, depicted in Figure 8s, displays a visible aperture, potentially indicating an intercalary archeopyle in a lateral view.
3.b. Palynology
The Prees 2 record stands out for its high diversity of marine palynomorphs. Despite poor preservation, within intervals showing elevated levels of amorphous organic matter and pyritization, the microplankton remains remarkably diverse. Aquatic palynomorphs range from 16% to 76% of the total assemblage and display distinct peaks and cyclic fluctuations. Figure 2 illustrates the overall composition of marine vs. terrestrial palynomorphs throughout the investigated succession. For consistency, in the next section, we apply the term ‘common’ for a taxon whenever it constitutes more than 10% of the total palynomorph assemblage. If a taxon exceeds 30% of the assemblage, it is considered to be ‘abundant’. Conversely, a ‘rare’ occurrence denotes any taxon that represents less than 1% of the assemblage, and a taxon in the 1%–10% range is designated here as ‘low abundance’. A ‘sporadic’ occurrence applies to a taxon that only appears intermittently. These categories help clarify how often certain taxa appear and distinguish significant changes in the assemblages.
3.b.1. Triassic–Rhaetian, Westbury formation (602.19–598.18 mccd)
This interval is characterized by common occurrences of Rhaetipollis germanicus and Ricciisporites tuberculatus (Figure 9). Other important taxa include Ovalipollis pseudoalatus, Ovalipollis ovalis and Cryptopalynites pseudomassulae, a genus erected by Gravendyck et al. (Reference Gravendyck, Coiffard, Bachelier and Kürschner2023) previously referred to as Tsugaepollenites. Classopollis spp. are abundant. Spore taxa such as Deltoidospora sp. (Figure 10) and Reticulatisporites sp. are noteworthy. Among the undifferentiated bisaccates (∼2%), Lunatisporites rhaeticus is identified. Overall, this pollen-dominated assemblage reflects a primarily terrestrial input.
Figure 9. Microphotographs of pollen. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Ricciisporites tuberculatus, SSK 116582 (600.18 mccd); (b) Ovalipollis pseudoalatus, SSK 116582 (600.18 mccd); (c) Quadraeculina anellaeformis, SSK 116515 (537.39 mccd); (d) Cycadopites sp., SSK 116575 (593.18 mccd); (e) Chasmatosporites sp., SSK 116542 (564.55 mccd); (f) Chasmatosporites hians, SSK 116566 (584.46 mccd); (g) Perinopollenites elatoides, SSK 116575 (593.18 mccd); (h) Alisporites robustus, SSK 116516 (538.39 mccd); (i) Pinuspollenites minimus, SSK 116444 (466.21 mccd); (j) Alisporites sp., SSK 116553 (572.5 mccd); (k) Granuloperculatipollis rudis, SSK 116575 (593.18 mccd); (l) Chasmatosporites apertus, SSK 116542 (564.55 mccd); (m) Rhaetipollis germanicus, SSK 116582 (600.18 mccd); (n) Araucariacites australis, SSK 116530 (552.36 mccd); (o) Sciadopityspollenites macroverrucosus, SSK 116444 (466.21 mccd); (p) Sciadopityspollenites thiergartii, SSK 116546 (566.97 mccd); (q) Classopollis classoides tetrad, SSK 116516 (538.39 mccd); (r) Classopollis simplex, SSK 116434 (456.28 mccd).
Figure 10. Microphotographs of spores. Following the taxon name is the sample number, followed by depth in brackets. Scale bar 20 µm, unless otherwise noted. (a) Deltoidospora toralis, SSK 116530 (552.36 mccd); (b) Deltoidospora sp., SSK 116516 (538.39 mccd); (c) Dictyophyllidites mortonii, SSK 116566 (584.46 mccd); (d) Dictyophyllidites mortonii, SSK 116489 (511.47 mccd); (e) Osmundacidites wellmanii, SSK 116528 (550.36 mccd); (f) Calamospora tener, SSK 116575 (593.18 mccd); (g) Kraeuselisporites reissingerii, SSK 116542 (564.55 mccd); (h) Acanthotriletes varius, SSK 116575 (593.18 mccd); (i) Conbaculatisporites spinosus, SSK 116527 (549.28 mccd); (j) Concavisporites jurensis, SSK 116566 (584.46 mccd); (k) Punctatisporites sp., SSK 116515 (537.39 mccd); (l) Todisporites minor, SSK 116575 (593.18 mccd); (m) Densoisporites fissus, SSK 116472 (494.43 mccd); (n) Polypodiisporites polymicroforatus, SSK 116575 (593.18 mccd); (o) Retusotriletes mesozoicus, SSK 116575 (593.18 mccd); (p) Triancoraesporites sp., SSK 116580 (598.18 mccd); (q) Foraminisporis jurassicus , SSK 116503 (525.24 mccd); (r) Anapiculatisporites spiniger, SSK 116526 (548.28 mccd); (s) Kyrtomisporis sp., SSK 116534 (556.44 mccd); (t) Lycopodiacidites rugulatus, SSK 116471 (493.43 mccd); (u) Retitriletes austroclavatidites, SSK 116470 (492.44 mccd); (v) Baculatisporites sp., SSK 116575 (593.18 mccd); (x) Trachysporites asper, SSK 116575 (593.18 mccd); (w) Striatella seebergensis, SSK 116541 (563.55 mccd).
3.b.2. Triassic–Jurassic transition, Lilstock formation – Cotham member (593.18 mccd)
This assemblage is marked by the common occurrence of Araucariacites (FO) and Perinopollenites elatoides (FO), with the latter forming an important accessory pollen type (8%). Other relevant pollen taxa include Sciadopityspollenites thiergartii (FO), Sciadopityspollenites mesozoicus (FO) and Granuloperculatipollis rudis. The genus Cerebropollenites was revised and emended to Sciadopityspollenites (Gravendyck et al., Reference Gravendyck, Coiffard, Bachelier and Kürschner2023). Key spores are Polypodiisporites polymicroforatus, Calamospora tener (FO) and Acanthotriletes varius. The spores and pollen in this assemblage show darkening.
3.b.3. Jurassic, Hettangian, Redcar mudstone formation (592.19 to 461.83 mccd)
Within the Redcar Mudstone Formation, Classopollis spp. dominates (16 to 68% of the total assemblage; average 42.5%), with Sciadopityspollenites thiergartii is sporadic ranging from low to common abundance (up to 14.5%). Chasmatosporites spp. also appears sporadically as an important accessory pollen. Around the Tilmanni zone, high levels of amorphous organic matter result in TOC values of up to 4.8% (Figure 2). Some heavily oxidized samples were difficult to count but still contributed to the biostratigraphy chart (Figures 3 and 4). From the Tilmanni to Planorbis zones, several first occurrences (FO) are noted: Trachysporites asper, Foraminisporis jurassicus, Quadraeculina anellaeformis and Kyrtomisporis sp., along with Pinuspollenites minimus and Vitreisporites pallidus among the undifferentiated bisaccates. Bisaccates are sporadic (∼2% average, up to 8%). Kraeuselisporites reissingerii is the most common spore; it appears sporadically (up to 6.7%) throughout the core. Lycopodiacidites rugulatus has its FO in the Liasicus ammonoid zone.
3.b.4. Jurassic, Sinemurian, Redcar mudstone formation (463.73–456.28 mccd)
Overall, the palynofloral assemblage remains similar to that of the Hettangian but with slight shifts: Classopollis spp. still dominate, while taxa such as Chasmatosporites spp. and Trachysporites asper become more abundant. The base of the Sinemurian, defined at 463.73 mccd, is tightly constrained by combining ammonite occurrences from Prees 2 and Wilkesley (Ullmann et al., Reference Ullmann, Damaschke, Hesselbo, Jiang, Lawrence, Leng, Mattioli, Bancalin, Page, Pudal and Ruhl2025). Two metres below, at 461.84 mccd, lies the FO of Sciadopityspollenites macroverrucosus, a pollen taxon recently formalized as an alternative marker for the base of the Sinemurian (Lindström et al., Reference Lindström, Pedersen, Vosgerau, Hovikoski, Dybkjær and Nielsen2023). This close stratigraphic relationship between ammonite and pollen events strengthens the biostratigraphic framework for the Lower Sinemurian more widely.
3.c. Aquatic palynomorphs
In the Subboreal Province of Northwest Europe, two key dinoflagellate cyst biozones define the Upper Triassic–Lower Jurassic transition: the Rhaetogonyaulax rhaetica Zone (upper Rhaetian) and the Dapcodinium priscum Zone (Hettangian) (Mangerud et al., Reference Mangerud, Paterson and Riding2019). In the Prees 2 core, Rhaetogonyaulax rhaetica is absent or rare (about 1%) and occurs within the Westbury Formation. Alongside Dapcodinium priscum, Beaumontella langii, acanthomorph acritarchs and green algae, these aquatic palynomorphs constitute roughly 20% of the assemblage in this interval (602.19–598.18 mccd).
Within the Triassic–Jurassic transition, in the Cotham Member (593.18 mccd), the non-pollen palynomorphs recorded include Dapcodinium priscum (17%), Leiosphaeridia spp. (5%), scabrate cysts (4%) and Tasmanites spp. (3%).
Moving into the Redcar Mudstone Formation, the marine component generally increases upwards. Within the Tilmanni and Planorbis ammonoid zones (592.19–562.55 mccd), aquatic palynomorphs range from 12.8% to 37.6%, with high levels of amorphous organic matter and oxygen-restricted laminated facies (Figure 2). Acanthomorph acritarchs dominate this interval, particularly Micrhystridium setasessitante (common to sporadic, up to 23.5%) and Micrhystridium lymensis (sporadic to common, up to 16.4%). The ‘Tilmanni/Main isotope excursion event’ (Hesselbo et al., Reference Hesselbo, Robinson, Surlyk and Piasecki2002; Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023) also occurs in this interval, beginning at around 588 mccd. Several taxa make their first appearances, including Micrhystridium fragile, Micrhystridium stellatum and Dorsennidium species. Additional sporadic forms (up to ∼7%) include Beaumontella langii, Umbriadinium mediterraneense (FO), foraminiferal test linings (FO) – uniserial, biserial and coiled – and Tasmanites spp., while Halosphaeropsis liassica (FO) and Cymatiosphaera spp. appear rarely.
Above the Planorbis, into the Liasicus zone, aquatic palynomorphs can surpass 50% of the total assemblage (Figure 2). Notable peaks of polygonomorphs and acritarchs (especially Dorsennidium) occur at 556 and 554 mccd (Figure 2). During this phase, Micrhystridium fragile shifts from sporadic to common, and Micrhystridium stellatum to sporadic, while Micrhystridium setasessitante declines. The lowest occurrences of Gen. et sp. Indet 01 and Gen. et sp. Indet 02 are also here, along with Valvaeodinium hirsutum?. From the top Liasicus to the Angulata zone (523.28–464.41 mccd), foraminiferal test linings (especially biserial types) increase to 12%. No new lowest or highest occurrences are noted, but the abundance of microalgae larger than 20 µm (e.g., B. infulatum var. macroinfulatum) rises markedly.
4. Discussion
4.a. Terrestrial assemblages
4.a.1. Triassic – Rhaetian (602.19–598.18 mccd)
The taxa observed in this interval exemplify a typical uppermost Rhaetian flora, similar to ‘phase 3’ described by Schuurman (Reference Schuurman1977) in outcrops from France and Luxembourg, and are characterized by the dominance of pollen such as Classopollis alongside Ovalipollis spp., Ricciisporites tuberculatus and Rhaetipollis germanicus. The palynoflora also resembles shallow marine or coastal successions in northern and eastern Spain – assemblage PA1 of Barrón et al. (Reference Barrón, Gómez, Goy and Pieren2006) – as well as the assemblage H1 in northern Austria of Bonis et al. (Reference Bonis, Kürschner and Krystyn2009). The assemblage also shows a close correspondence with the miospores and pollen of the Westbury Formation elsewhere in the UK (e.g., Assemblage SAB01 of Warrington (Reference Warrington2005) and Bonis et al. (Reference Bonis, Ruhl and Kürschner2010)). Detailed comparisons of each zone are provided in supplementary material S2.
Ovalipollis ovalis is assigned to voltzialean conifers. It exhibits xerophytic features, but its preferred growth and drainage habitat remain unclear (Lindström et al., Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017). Rhaetipollis germanicus is a gymnosperm of unknown habitus (Lindström et al., Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017) that might be a teratological variation of another pollen type due to its thick exine (Vajda et al., Reference Vajda, Mcloughlin, Slater, Gustafsson and Rasmusson2023). While Ricciisporites tuberculatus is characterised by an herbaceous ruderal life habit (Kürschner et al., Reference Kürschner, Mander and McElwain2014) or may have occupied a mid-canopy layer in mire settings (Lindström et al., Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017). Recent in-situ work interprets R. tuberculatus as an aberrant pollen produced by the seed fern Lepidopteris ottonis (Vajda et al., Reference Vajda, Mcloughlin, Slater, Gustafsson and Rasmusson2023; Reference Vajda, Mcloughlin, Slater, Gustafsson and Rasmusson2024), although a recent critique based on wall ultrastructure favours unrelated parent plants (Zavialova, Reference Zavialova2024).
Classopollis spp. and Granuloperculatipollis rudis, both Cheirolepidiaceae conifers, are interpreted as xerophytic and adapted to well-drained, coastal habitats (Figure 11a; Alvin, Reference Alvin1982; Vakhrameev, Reference Vakhrameev1991). Overall, these upper Rhaetian floras reflect drought-tolerant communities thriving in warm, semi-arid conditions, with gymnosperm forests prevalent near coastal environments (Bonis & Kürschner, Reference Bonis and Kürschner2012).
Figure 11. Idealized depositional model illustrating palaeoenvironmental changes and highlighting the distribution of acritarchs and dinoflagellate cysts in the Cheshire Basin (not to scale). Green represents coastal plain deposits; reddish-brown the Permian–Triassic continental clastic deposits; greys are the marine deposits. Sequence-stratigraphic interpretations follow Hesselbo et al. (Reference Hesselbo, Robinson and Surlyk2004). (a). Rhaetian terrestrial-dominated interval with abundant conifer pollen (xerophytic traits), reflecting warm, semi-arid conditions. Euryhaline aquatic palynomorphs in a shallow environment. (b). Heterolithic facies with erosional features and microbial facies. Spores indicate increased moisture. Low diversity aquatic palynomorphs indicating stressed conditions and suggesting marine incursions in a proximal environment at the onset of the Hettangian. (c). Dominance of xerophytic pollen (Cheirolepidiaceae) and cosmopolitan acritarchs in a low-oxygen shallow water setting with (relative) high TOC. Opportunistic taxa blooming under stressful conditions. (d). Higher aquatic diversity and reduced terrestrial input indicate a more distal, shelf environment. Declining short-spined acanthomorph acritarchs, alongside rising polygonomorph acritarchs and dinoflagellate cysts, signify a shift to a relative deeper inner-shelf condition.
4.a.2. Triassic–Jurassic transition (593.18 mccd)
Araucariacites australis represents the upper canopy of well-drained coastal environments, while the taxodiaceous conifer P. elatoides is associated with upper canopy vegetation in mires and riverbanks, indicating coastal and riverine environments with varying moisture levels (Balme, Reference Balme1995; Lindström et al., Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017). Calamospora tener and Acanthotriletes are indicative of ground cover in mires and riverbank settings, suggesting diverse undergrowth in these habitats (Bos et al., Reference Bos, Lindström, Van Konijnenburg-van Cittert, Hilgen, Hollaar, Aalpoel, Van der Weijst, Sanei, Rudra and Sluijs2023). Barrón et al. (Reference Barrón, Gómez, Goy and Pieren2006) indicate that dry-tolerant species were dominant in the Rhaetian, followed by a short humid period, allowing for the recovery of vascular cryptogams (ferns and conifers). Also, Abbink et al. (Reference Abbink, Targarona, Brinkhuis and Visscher2001) used Perinopollenites elatoides as a climate signal for wetter (Figure 11b) and cooler conditions in a lowland setting during a period with climate fluctuating between arid, tropical and subtropical.
This sample shows a mixed palynofloral assemblage from the Tr–J, representing a transitional zone. The high abundance of P. polymicroforatus, often found as a Rhaetian ‘fern spike’, can be traced across European basins (Larsson, Reference Larsson2009; Lindström et al., Reference Lindström, Van de Schootbrugge, Hansen, Pedersen, Alsen, Thibault, Dybkjær, Bjerrum and Nielsen2016; Van de Schootbrugge et al., Reference Van de Schootbrugge, Van der Weijst, Hollaar, Vecoli, Strother, Kuhlmann, Thein, Visscher, Van Konijnenburg-van Cittert, Schobben and Sluijs2020; Bos et al., Reference Bos, Lindström, Van Konijnenburg-van Cittert, Hilgen, Hollaar, Aalpoel, Van der Weijst, Sanei, Rudra and Sluijs2023). While the transition to the Hettangian is marked by the FO of Sciadopityspollenites thiergartii, a significant marker for the Lower Jurassic, here (Figure 2) it occurs coeval with the ‘Spelae/Initial’ CIE (starts at 593.98 mccd and peaks at 592.78 mccd – Ullmann et al., Reference Ullmann, Damaschke, Hesselbo, Jiang, Lawrence, Leng, Mattioli, Bancalin, Page, Pudal and Ruhl2025) and lies below the Tr–J boundary (592.33 mccd) proposed by Hesselbo et al. (Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023).
The disappearance of canopy vegetation (Ovalipollis, Riccisporites and Rhaetipollis) coincides with the initial CIE. This is also documented in St Audrie’s Bay, indicating a shift to a warmer and/or wetter climate (Bonis et al., Reference Bonis, Ruhl and Kürschner2010). Since most miospore taxa in our study are identified only at the genus level and the Rhaetian interval has low resolution, it is challenging to track species-specific extinctions. Nevertheless, our data reveal the disappearance of bisaccate pollen and spore taxa as well.
Several taxa were not permanently extirpated but still had a decline in abundance before showing relative recovery in the Hettangian. This ‘mass rarity phase’ is a key characteristic of the ETME (Lindström, Reference Lindström2021). These severe taxonomic losses proceeded in two distinct phases across the ETME, which are closely associated with the three negative CIEs (Lindström, Reference Lindström2021; Gravendyck et al., Reference Gravendyck, Schobben, Bachelier and Kürschner2020; Wignall & Atkinson, Reference Wignall and Atkinson2020). In cores from Denmark, Lindström (Reference Lindström2021) also identified the second rarity event just before the Spelae/Initial CIE, marked by the extirpation of several taxa and the proliferation of disaster species filling newly vacated niches. Similarly, Wignall & Atkinson (Reference Wignall and Atkinson2020) documented a first extinction event in the lower Cotham Member that wiped out many bivalves and ostracods and a second extinction at the top of the Langport Member, which eliminated additional bivalves, ostracods and the last conodonts. These marine extinction phases align with a Polypodiisporites polymicroforatus abundance interval. Similar results were reported in the Bonenberg section (Germany) using diversity indices (Gravendyck et al., Reference Gravendyck, Schobben, Bachelier and Kürschner2020).
In the Prees-2 record, spore abundance is generally low (mean spore-to-pollen ratio, S/P = 0.1), punctuated by periodic increases (S/P > 0.2–0.3) as illustrated in Figure 2. A simpler, non-climatic explanation for the absence of a pronounced fern spike during the ETME interval in the Cotham Member is limited sampling resolution; for example, Bonis et al. (Reference Bonis, Ruhl and Kürschner2010) documented two fern spikes within a 13-sample range (assemblage SAB2). Also, given that spores consistently appear in small quantities within marine sediments, Bonis & Kürschner (Reference Bonis and Kürschner2012) indicate the presence of Cheirolepidiaceae-dominated forests in the interior of Pangaea, dominated by low-diversity C. meyeriana. In contrast, C. torosus had a coastal habitat, alongside an increase in spore-producing plants near the Tethys Ocean coast. These authors hypothesized that the vegetation variations were a result of a warming climate (caused by the increase in the greenhouse effect), which enhanced land-sea temperature contrasts and intensified monsoons. The result was a drier continental interior and more intense precipitation around the margins of Tethys.
4.a.3. Jurassic, Hettangian (592.19 to 461.83 mccd)
This assemblage is indicative of the Jurassic and clearly shows the transition into Hettangian flora with several long-ranging species. It is comparable with the assemblage SAB4 of Bonis et al. (Reference Bonis, Ruhl and Kürschner2010) and PA-3 in Spain of Barrón et al. (Reference Barrón, Gómez, Goy and Pieren2006) and resembles phase 5 described by Schuurman (Reference Schuurman1977).
The pollen types in this interval span various ecological roles and plant groups. According to Lindström et al., (Reference Lindström, Erlström, Piasecki, Nielsen and Mathiesen2017), Chasmatosporites spp. (cycads/ginkgoes) likely occupied upper-canopy niches in mire habitats, while Quadraeculina anellaeformis is an unknown gymnosperm with xerophytic traits but unclear growth habits. Pinuspollenites minimus (a Pinaceae conifer) was xerophytic and well-suited to upper-canopy settings in well-drained environments. Deltoidospora spp. (Ferns from the Dipteridaceae or Dicksoniaceae) occupied understory habitats in either mire or relatively drier patches. Kraeuselisporites (lycophytes) formed ground cover in mire settings, while Trachysporites (another fern group) also favoured ground cover in similar wetland conditions.
The continued dominance of drought-tolerant Cheirolepidiaceae conifers suggests that these taxa remained resilient and adapted to the semi-arid conditions from the Rhaetian into the Jurassic. The Hettangian assemblage within the Blomidon Formation (eastern Canada), as described by Fowell & Traverse (Reference Fowell and Traverse1995), is marked by a significant dominance of Classopollis, often constituting more than 60% of the total palynoflora. A similar pattern is observed in Australia, where this Classopollis acme is referred to as the Cheirolepidiacean phase (Bomfleur et al., Reference Bomfleur, Schöner, Schneider, Viereck, Kerp and Mckellar2014). This persistence phase across different time intervals highlights the ecological success of the Classopollis producer and its dominance during this interval.
4.b. Marine palaeoecology
In southern British basins, the Westbury Formation is conspicuous by abundant Rhaetogonyaulax rhaetica (Warrington, Reference Warrington2005; Bonis et al., Reference Bonis, Ruhl and Kürschner2010). Classically considered as a Rhaetian indicator and regional marker for a maximum flooding event recorded in upper Rhaetian strata across Europe and Australia (Lindström & Erlström, Reference Lindström and Erlström2006), this taxon is considered typical of relatively deeper open marine environments (Courtinat & Piriou, Reference Courtinat and Piriou2002). The high relative abundance and concentrations of this taxon up to the lower Cotham Member and association with high TOC (values up to 12%) black shale lithology suggest open marine conditions for these basins (Hesselbo et al., Reference Hesselbo, Robinson and Surlyk2004; Bonis et al., Reference Bonis, Ruhl and Kürschner2010).
In Prees 2 samples conversely, R. rhaetica is rare to absent. The aquatic assemblage is dominated instead by Beaumontella langii and Dapcodinium priscum (an euryhaline taxon capable of tolerating variable conditions (Courtinat & Piriou, Reference Courtinat and Piriou2002)). According to these authors, D. priscum is found in both high- and low-energy settings, while R. rhaetica is associated with low-energy conditions. These taxa occur coeval with heterolithic lenticular bedding and TOC up to 5%, suggesting a relatively low-energy, shallow marine setting (Figure 11a).
Studies in Northern Ireland document similar conditions to Prees 2. In the Lough Foyle and Larne basins, R. rhaetica occurs in significantly lower abundances (1–9 counts and 5–14%, respectively) (Boomer et al., Reference Boomer, Copestake, Raine, Azmi, Fenton, Page and O’Callaghan2021; Raine et al., Reference Raine, Fenton, Boomer, Azmi and Copestake2021) than in southern UK basins. These reductions are attributed by Boomer et al. (Reference Boomer, Copestake, Raine, Azmi, Fenton, Page and O’Callaghan2021) to salinity fluctuations. Although in the Prees 2 samples from the Westbury Formation contains the second largest absolute amount of dinocysts in the entire section, with values reaching >30,000 cysts/g (Figure 2), the salinity levels in this region might have been inadequate for R. rhaetica proliferation.
The overlying Lilstock Formation reflects a transition to brackish-marine conditions, evident in its convoluted beds, heterolithic facies with erosional surfaces and microbial silty limestones (Hesselbo et al., Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023), representing a regressive succession (Hesselbo et al., Reference Hesselbo, Robinson and Surlyk2004). Wignall & Bond (Reference Wignall and Bond2008), following earlier authors, describe the Lilstock Formation as quasi-marine units, lacking fully marine taxa such as brachiopods, ammonoids and bryozoans, indicating fluctuating salinity reaching evaporitic conditions.
The upper Cotham Member contains the Spelae/Initial CIE identified by Hesselbo et al. (Reference Hesselbo, Robinson, Surlyk and Piasecki2002, Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023). The Initial/Spelae CIE broadly overlaps a depauperate benthic-marine interval and is associated with shoaling of euxinic waters into the photic zone across the Tr–J boundary (Jaraula et al., Reference Jaraula, Grice, Twitchett, Böttcher, Lemetayer, Dastidar and Opazo2013; Atkison & Wignall, Reference Atkinson and Wignall2019). The sample of Prees 2 coeval with the initial/Spelae CIE peak coincides with reduced marine diversity (monospecific dinoflagellate cyst assemblage, with only D. priscum and green algae identified), suggesting a synchronous response to environmental perturbations, potentially linked to changes of water influx, temperature and variable salinity.
The dominance of Cymatiosphaera sp. (a green alga) and D. priscum has been linked to the “Spelae/Initial CIE”, as observed at the Tr–J boundary in the Eiberg Basin in Austria (Bonis et al., Reference Bonis, Kürschner and Krystyn2009). This assemblage of D. priscum and green algae have also been observed in other locations in the UK (Warrington, Reference Warrington2005; Bonis et al., Reference Bonis, Ruhl and Kürschner2010). Similar occurrences in Northern Ireland point to low-energy sedimentation beneath a restricted water column with bottom-water oxygen deficiency (Boomer et al., Reference Boomer, Copestake, Raine, Azmi, Fenton, Page and O’Callaghan2021). Overall, the assemblage at the Cotham Member suggests intermittent marine incursions into a proximal restricted environment (Figure 11b).
Moving upward, the Blue Lias Formation (equivalent to the Redcar Mudstone Formation) reflects a mid-shelf setting with moderate water depths (Hesselbo et al., Reference Hesselbo, Robinson and Surlyk2004). In Prees 2, however, the lower part of the Redcar Mudstone (Tilmanni to Planorbis ammonoid zones) is dominated by both simple, short-spined, cosmopolitan acanthomorphic acritarchs (Micrhystridium setasessitante and Micrhystridium lymensis). Tyson (Reference Tyson1995) associates these characteristics with inshore, shallow-water conditions.
The assemblage through the Tilmanni to Planorbis ammonoid zones includes several first occurrences, such as that of Umbriadinium mediterraneense and foraminiferal test linings, which offer insights into environmental conditions. U. mediterraneense is linked to anoxic events and may indicate marine transgressive phases associated with poorly oxygenated environments, particularly during periods of organic enrichment, as also in the Toarcian (Bucefallo-Palliani & Riding, Reference Bucefallo-Palliani and Riding1999). This interpretation is further supported by the presence of foraminiferal linings, commonly found in marine low-oxygen, stress-tolerant ecosystems (Stancliffe, Reference Stancliffe1989; Clémence et al., Reference Clémence, Bartolini, Gardin, Paris, Beaumont and Page2010) but also including brackish hypoxic deltaic ecosystems (Mudie & Yanko-Hombach, Reference Mudie and Yanko-Hombach2019).
Additionally, there is wavy-wrinkled bedding at the base of the unit (first 5 metres); Hesselbo et al. (Reference Hesselbo, Al-suwaidi, Basker, Ballabio, Belcher, Bond, Boomer, Bos, Bjerrum, Bogus and Boyle2023) point to tidal or wave influence, underscoring shallow, ‘inshore’ deposition (Figure 11c). ‘Wrinkle structures’ can indicate the former presence of microbial mats and are commonly found in intertidal to lower supratidal zones, particularly within settings that exhibit low hydrodynamic energy, typically associated with heterolithic successions (Porada & Bouougri, Reference Porada and Bouougri2007), reflecting a shallow depositional environment. However, the absence of mudcracks through this part of the succession is evidence of subtidal conditions.
The earliest Hettangian record in Western Europe shows that dysoxic conditions, interspersed with periodic anoxia and photic zone euxinia (PZE), as well as ocean acidification, were prevalent (Jaraula et al., Reference Jaraula, Grice, Twitchett, Böttcher, Lemetayer, Dastidar and Opazo2013; Atkinson & Wignall, Reference Atkinson and Wignall2019; Bond et al., Reference Bond, Dickson, Ruhl, Bos and Van de Schootbrugge2023; Trudgill et al., Reference Trudgill, Rae, Whiteford, Adloff, Crumpton-banks, Van Mourik, Cuperus, Corsetti, Doherty, Gray and Greene2025). Furthermore, Ullmann et al. (Reference Ullmann, Damaschke, Hesselbo, Jiang, Lawrence, Leng, Mattioli, Bancalin, Page, Pudal and Ruhl2025) report a significant size reduction in Schizosphaerella spp., a calcareous nannofossil common in the lower Jurassic, coinciding with the ‘Tilmanni/Main CIE’ in Prees 2. The dominance of these acritarchs (Figures 2 and 3) is attributed to shallow marine basins becoming salinity stratified, inducing anoxic conditions, driven by elevated CO2 levels during the ‘Tilmanni/Main CIE’ event; these environmental stressors likely triggered blooms that favoured opportunistic taxa, as discussed by Van de Schootbrugge et al. (Reference Van de Schootbrugge, Tremolada, Rosenthal, Bailey, Feist-Burkhardt, Brinkhuis, Pross, Kent and Falkowski2007).
The palaeoecological data from the Planorbis and Liasicus ammonoid zones around the UK suggest relatively stable, albeit suboptimal, environmental conditions, following the disrupted conditions after the ETME (Mander et al., Reference Mander, Twitchett and Benton2008). At St Audrie’s Bay, persistent photic zone euxinia established around the base of the Blue Lias Formation (pre-Planorbis Zone) weakens gradually over time (Jaraula et al., Reference Jaraula, Grice, Twitchett, Böttcher, Lemetayer, Dastidar and Opazo2013). The palynological microscope slides of this interval in Prees 2 are full of amorphous organic matter, and the relatively elevated TOC (∼4%) values reinforce the notion of restricted circulation favouring organic matter preservation.
The assemblages in the upper Planorbis to Liasicus ammonoid zones are characterized by a decline in the dominance of short-spined acanthomorph acritarchs and an increase in polygonomorph acritarchs (Figure 2 – largest spike at 552.36 mccd, with 36,800 cysts/g, representing 29.8% of the total assemblage) and more polygonal-shaped Micrhystridium species, such as M. fragile and M. stellatum, along with new dinocyst species. The rise of relatively longer-spined Micrhystridium genera, polygonomorphs and Baltisphaeridium forms suggests an inner proximal shelf (Figure 11d) environment (Tyson, Reference Tyson1995).
Absolute palynomorph counts decline above the Planorbis zone (Figure 2). Additionally, bioturbation is observed from 552 mccd to the top of the section (Figure 2), likely accelerating organic matter degradation (Jessen et al., Reference Jessen, Lichtschlag, Ramette, Pantoja, Rossel, Schubert, Struck and Boetius2017). A cyclicity in the terrestrial-to-marine ratio (T:M) indicates increasing marine influence towards the Hettangian–Sinemurian boundary (Figure 2). This suggests either a more distal setting relative to land or reduced terrestrial input. The concurrent decline in miospore concentrations, increased phytoplankton diversity and higher proportions of opaque plant debris support an inner shelf environment (Figure 11d).
The Angulata Zone may hypothetically mark the return to normally functioning marine ecosystems following the mass extinction, as evidenced by fossil macroinvertebrates (Mander et al., Reference Mander, Twitchett and Benton2008). According to these authors, there are limited studies on the Blue Lias Formation, but the size, depth and diversity of trace fossil assemblages imply a well-oxygenated environment, more suitable for benthic faunal life compared to preceding intervals. Paulsen & Thibault (Reference Paulsen and Thibault2023) also observed that more stable environmental conditions were established during the Angulata biochron, based on calcareous nannofossils from the Mochras core, NW Wales (UK). Palaeobotanical evidence further suggests that atmospheric CO2 levels returned to pre-extinction values sometime in the Hettangian, likely during the Angulata Zone, though the precise timing remains biostratigraphically unconstrained (Mander et al., Reference Mander, Kürschner and McElwain2013).
4.c. Malformations
A notable feature of the Prees 2 samples is the occurrence of malformed palynomorphs (Figure 7). As our understanding of mass extinctions has deepened, the focus has shifted from simply identifying species disappearing to recognizing the importance of diversity loss and the presence of malformations as indicators of environmental stress (Hull et al., Reference Hull, Darroch and Erwin2015; Gravendyck et al., Reference Gravendyck, Schobben, Bachelier and Kürschner2020; Lindström, Reference Lindström2021). Malformed pollen and spores (Figures 7q to 7t) are recorded during the end-Permian mass extinctions, where volcanic halogen emissions likely led to ozone depletion and heat stress in vegetation (Visscher et al., Reference Visscher, Looy, Collinson, Brinkhuis, Van Konijnenburg-van Cittert, Kürschner and Sephton2004; Benca et al., Reference Benca, Duijnstee and Looy2022; Liu et al., Reference Liu, Peng, Marshall, Lomax, Bomfleur, Kent, Fraser and Jardine2023). Recent studies have also linked sedimentary mercury (Hg) anomalies in Tr–J boundary sections in Sweden, Denmark and Germany to vegetation collapse and the proliferation of malformed fern spores (Lindström et al., Reference Lindström, Sanei, Van de Schootbrugge, Pedersen, Lesher, Tegner, Heunisch, Dybkjær and Outridge2019; Bos et al., Reference Bos, Zheng, Lindström, Sanei, Waajen, Fendley, Mather, Wang, Rohovec and Navrátil2024). Further, greenhouse warming might have caused heat stress, leading to polyploidy, abnormal meiosis and cytokinesis in extant angiosperm pollen (Kürschner et al., Reference Kürschner, Batenburg and Mander2013).
Marine palynomorphs, such as dinoflagellate cysts and acritarchs, are classified as morphospecies and demonstrate a wide range of morphological variability. This polymorphism is often regarded as typical and may reflect distinct genotypes or ecophenotypical morphotypes arising from a single phytoplankton species (Munnecke et al., Reference Munnecke, Delabroye, Servais, Vandenbroucke and Vecoli2012).
In documenting the atypical or aberrant acritarchs shown in Figures 2 and 7, we have adopted a cautious approach, focusing on specimens that display significant deviations from typical forms and from the morphological variability we described in the previous paragraph. These malformations include dilated or irregularly ramified processes (Figures 7m to 7p) and vesicles with pronounced, tumour-like inflations extending from the vesicle (Figures 7a and 7b). These extreme forms are rare and represent less than 1% of the counts. We also observed a subtle trend of variations among the morphotypes M. fragile and M. stellatum, as shown in Figures 5f and 5g.
While abnormal acritarch morphologies are rarely described in the literature, especially in the Mesozoic, they are hypothesized to be linked to environmental stressors associated with disturbances in the global carbon cycle during key extinction intervals (Delabroye et al., Reference Delabroye, Munnecke, Servais, Vandenbroucke and Vecoli2012; Munnecke et al., Reference Munnecke, Delabroye, Servais, Vandenbroucke and Vecoli2012; van Soelen et al., Reference van Soelen, Twitchett and Kürschner2018). These stressors, potentially mediated by feedback mechanisms such as changes in salinity, nutrient availability and water column stratification, reflect broader disruptions in ocean chemistry. Thus, acritarch malformations may serve as indirect indicators of such factors, particularly increased carbon loading and its cascading environmental effects. Nonetheless, further research is essential to fully understand the mechanisms underlying these morphological anomalies and their potential connections to large-scale biogeochemical changes.
4.d. Acritarchs process length variation
In the Prees 2 borehole, intervals with a greater proportion of acanthomorph acritarchs represent shallow water conditions in the Hettangian (Figure 11c). This interval is marked by acritarchs with short, hyaline processes and often only a few spines present. Process length can deviate from the ‘norm’, and this length variation has been associated with salinity fluctuations. Dinoflagellate cysts show changes in assemblages over salinity gradients, and some species also show changes in morphology in response to lowered salinity. Mertens et al. (Reference Mertens, Ribeiro, Bouimetarhan, Caner, Nebout, Dale, De Vernal, Ellegaard, Filipova and Godhe2009) demonstrated that the process length of the modern dinoflagellate cyst Lingulodinium machaerophorum serves as a useful palaeosalinity proxy. Studies from modern surface sediment assemblages and culture studies demonstrated that, broadly, processes are typically longer in higher salinity environments and reduced in lower salinity environments (Ellegaard et al., Reference Ellegaard, Dale, Mertens, Pospelova and Ribeiro2017).
Similarly, during the late Permian biotic crisis, acritarch process lengths are observed to have shortened, an adaptation interpreted as evidence of decreased salinity in shallow marine environments by van Soelen et al. (Reference van Soelen, Twitchett and Kürschner2018). These authors suggest that changes in palaeoenvironment from open marine (before the extinction) to nearshore conditions during and after the crisis, due to a period of sea-level rise, leading to a noticeable change in acritarchs distribution and morphology. This change is attributed to a reduction in salinity that can only be explained by increased run-off. Increased nutrient availability, possibly in combination with soil erosion, is thus ultimately responsible for the development of anoxia in the basin as a consequence of global warming.
A decrease in salinity by at least 3 Practical Salinity Units (PSU) in water masses of the Blue Lias Fm. was inferred from changes in oyster oxygen isotope values and Sr/Ca elemental ratios (Van de Schootbrugge et al., Reference Van de Schootbrugge, Tremolada, Rosenthal, Bailey, Feist-Burkhardt, Brinkhuis, Pross, Kent and Falkowski2007). A compatible scenario is a relative change in water mass conditions and is most likely related to increased freshwater runoff. Although this remains speculative and requires further morphometric and geochemical testing to confirm.
5. Conclusions
Prees 2 provides a unique record of aquatic microorganisms and vegetation dynamics through and, especially, following the ETME. The assemblages observed here can be correlated with established palynostratigraphic schemes from other locations in the UK and Europe, confirming and supporting findings from previous studies on both core and outcrop materials related to the ETME and subsequent Jurassic successions.
The Westbury Formation contains a typical Rhaetian palynoflora. However, unlike other southern UK basins, there is not a dominance of R. rhaetica. Instead, the assemblage is dominated by B. langii and D. priscum dinoflagellate cysts. Coupled with sedimentological evidence, this suggests deposition in a relatively shallow marine setting, spanning a broad shelf area characterized by lower energy conditions.
The Cotham Member represents a transitional interval marked by the abrupt disappearance of some key Rhaetian canopy taxa. This shift occurs within a stratigraphic interval that coincides with the peak of the initial/Spelae CIE and the ETME acme interval. This unit contains a mixture of miospore and dinoflagellate cyst taxa more commonly associated with Jurassic assemblages while also preserving elements linked to Late Triassic communities. These include proxies for regional events seen across European basins, such as the P. polymicroforatus zone, the ‘darkening spore zone’ and the ‘fern spike’. These features illustrate a gradual transition through distinct stages of ecological succession. The aquatic palynomorphs suggest possible stressed and restricted depositional conditions.
In the Redcar Mudstone Formation, terrestrial palynomorph assemblages are dominated by Classopollis spp. and other long-ranging taxa similar to those observed in other Early Jurassic settings. The aquatic assemblages reflect adaptations to frequently stressed conditions, such as low oxygen levels. Acritarchs from this formation, particularly short-spined acanthomorph types, are typical of brackish, marginal successions. These low-diversity, high-dominance assemblages reflect an inshore shallow-water environment that transitions over time to a more diverse open marine shelf setting.
The detailed examination of aquatic palynomorphs, including the identification of potential new species, significantly enhances our understanding of Jurassic marine ecosystems. The refinement of regional and global chronostratigraphic frameworks is significantly enhanced by the integration of palynological data with existing lithostratigraphic, biostratigraphic and carbon isotope geochemical data. This study highlights the significance of aquatic alongside terrestrial microfossils to reconstruct palaeoenvironmental conditions and the dynamics of marine ecosystems during periods of global environmental change.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756825100459.
Acknowledgements
The PhD research project and conference travel related to the preparation of this manuscript were funded by the Human Frontier Science Program (HFSP) under grant RGP0066/2021 to JCFR, TRAV and BvdS. This is a contribution to the Early Jurassic Earth System and Timescale Project (JET) funded under the auspices of the International Continental Scientific Drilling Program (ICDP). SPH acknowledges funding from the UK Natural Environment Research Council (NERC) grant NE/N018508/1. SEM images were generated using research infrastructure funded through FWO grant I013118N. Staff at the BGS National Geological Repository, British Geological Survey, Keyworth, are warmly thanked for their longstanding support for this research programme. Special thanks go also to Sofie Lindström. We also appreciate the technical support from Giovanni Dammers and Natasja Welters at Utrecht University.
Funding Statement
Open access funding provided by Utrecht University.
Competing interests
The authors declare that they have no competing interests.
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