Early Jurassic Microbial Mats—A Potential Response to Reduced Biotic Activity in the Aftermath of the End-Triassic Mass Extinction Event

Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85

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Palaeogeography, Palaeoclimatology, Palaeoecology

journal homepage: www.elsevier.com/locate/palaeo

Early Jurassic microbial mats—A potential response to reduced biotic activity in the aftermath of the end-Triassic mass extinction event

Olof Peterffy a, Mikael Calner a, Vivi Vajda a,b,⁎ a Department of Geology, Lund University, SE-22362 Lund, Sweden b Swedish Museum of Natural History, P.O. Box 50007, SE-104 05 Stockholm, Sweden article info abstract

Article history: Wrinkle structures are microbially induced sedimentary structures (MISS) formed by cyanobacteria and are com- Received 6 June 2015 mon in pre-Cambrian and Cambrian siltstones and sandstones but are otherwise rare in the Phanerozoic geolog- Received in revised form 4 December 2015 ical record. This paper reports the first discovery of Mesozoic wrinkle structures from Sweden. These are Accepted 22 December 2015 preserved in fine-grained and organic-rich heterolithic strata of the Lower Jurassic (Hettangian) Höganäs Forma- Available online 30 December 2015 tion in Skåne, southern Sweden. The strata formed in a low-energy, shallow subtidal setting in the marginal parts fi Keywords: of the Danish rift-basin. Palynological analyses of ne-grained sandstones hosting the wrinkle structures show Hettangian that the local terrestrial environment probably consisted of a wetland hosting ferns, cypress and the extinct co- Mass extinction nifer family Cheirolepidaceae. Palynostratigraphy indicates a Hettangian age, still within the floral recovery phase Microbial mat following the end-Triassic mass extinction event. The finding of wrinkle structures is significant as the presence Cyanobacteria of microbial mats in the shallow subtidal zone, (in a deeper setting compared to where modern epibenthic mi- Wrinkle structures crobial mats grow) suggests decreased benthic biodiversity and suppressed grazing in shallow marine environ- Sweden ments in the early aftermath of the end-Triassic mass extinction event. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Modern microbial mats are restricted to marginal marine environments, such as tidal flats and salt marshes where burrowing and grazing Microbes and eukaryotic algae dominate the Precambrian fossil re- is minimal. Rapid fluctuations in salinity, water depth, currents and cord, but with the evolution of grazing metazoans and increasing rates temperature form unfavourable conditions for benthic organisms, leav- of bioturbation in the early Phanerozoic, cyanobacteria became restrict- ing the extremophile organisms to flourish. Fenchel (1998) showed that ed to more marginal marine environments (Lan, 2015). However, a four month old mat can be completely ingested in a time span of during time intervals of severe environmental stress with extreme weeks by gastropods and other grazers. Post-burial burrowing may fur- conditions, microbes could expand over a wider area of the shelf and ther obliterate mat laminae (e.g., Hagadorn and Bottjer, 1997; Mata and contribute to the development of anachronistic facies (Sepkoski et al., Bottjer, 2009). 1991; Mata and Bottjer, 2012; Forel et al., 2013), atypical of Microbial mats are formed by cyanobacteria, generally by multilayer metazoan-dominated Phanerozoic sea floors. This has been document- microbial communities composed of several cyanobacterium species ed in association with the Late Devonian (Wood, 2000) and end- (Ramsing et al., 2000). The morphology of cyanobacterial mats is a Triassic mass extinction events (Pruss et al., 2004; Kershaw et al., result of a combination of factors, such as: environment, sediment char- 2012; Forel et al., 2009), and the late Silurian Lau Event, a taxonomically acteristics and dominant cyanobacterial species (Ramsing et al., 2000). small but ecologically important crisis (Calner, 2005, 2008). Younger An advanced biofilm of extracellular polysaccharide forming a laterally examples of this phenomenon in association with mass extinctions extensive, organic layer may be called a microbial mat (Noffke, 2010). have not been known until recently, when Ibarra et al. (2014) docu- Microbial mats form through the cooperation of individual microbes, mented unusually extensive microbialites from the Triassic–Jurassic which secrete a biofilm of strongly adhesive extracellular polymeric boundary interval in southwestern United Kingdom, and suggested a substance (EPS; Stoodley et al., 2002). Physically, they behave like visco- linkage to the end-Triassic mass extinction event. elastic fluids, in that they may both undergo elastic, reversible deformation as well as deform irreversibly if subject to sufficiently high shear stress for a certain amount of time (for most mats around 18 min; Thomas et al., 2013). These films can develop on almost any surface ⁎ Corresponding author at: Swedish Museum of Natural History, P.O. Box 50007, SE-104 05 Stockholm, Sweden. on Earth and their complexity even resembles that of eukaryotic tissue, E-mail address: [email protected] (V. Vajda). facilitating optimal temperature, salinity and nutrition levels for the

http://dx.doi.org/10.1016/j.palaeo.2015.12.024 0031-0182/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85 77 constituent microbes (Costerton et al., 1995; Noffke, 2010). Biofilms and suggested that wrinkle structures reflect the actual crinkled upper EPS not only hamper erosion by binding sediment grains together; the surface of a living mat, whereas Noffke et al. (2002) and Noffke smooth upper surface also has the effect of reducing the shear stress (2010) argued that they form due to post-burial deformation of the exerted by eroding currents (Paterson, 1994). mat by the pressure exerted by the overlying sediments. The The preservation-potential of microbes differs depending on the overload causes dewatering as the liquid bound in the mat escapes depositional context. In carbonate depositional environments, stromat- laterally, thus crinkling the mat. Porada et al. (2008) on the other olites and oncoids may be formed. Owing to early cementation, these hand, proposed a model with tidally induced oscillations in ground are rigid, generally easily distinguished structures with a long history water, which liquefy and reorganize sediments trapped under the of research (e.g., Walter, 1976; Tewari and Seckbach, 2011). Their sealing layer of a microbial mat. This effect would be strongest in counterparts in siliciclastic sediments are less well known, since the the shallow subtidal zone. Laboratory experiments with viscoelastic preservation potential of microbes on those substrates is lower and films carried out by Thomas et al. (2013) developed this model identification is more challenging leading to the under-representation further. As the water and the biofilm both have different viscosities of fossil cyanobacteria mats from siliciclastic settings in the literature. and behave like two immiscible fluids, they respond differently to Nevertheless, over the past two decades, more research has focused shear stress. A harmonic so called Kelvin–Helmholtz instability is in- on traces of microbial activity in siliciclastic settings as these are duced at the interface, giving rise to the characteristic Kinneyia ripple especially interesting for astrobiologists studying extra-terrestrial pattern in the biofilm. analogues. This study aims to describe Early Jurassic wrinkle structures from Wrinkle structures have been attributed to the sediment-stabilizing shallow marine, siliciclastic strata at the Kulla Gunnarstorp coastal cliff ability of these microbial mats and is an umbrella term used for so called section (Fig. 1), in the southernmost province of Sweden. We further in- runzelmarken, Kinneyia ripples and “elephant skin” (Hagadorn and terpret the depositional environment and, through palynological analy- Bottjer, 1997, 1999). They are microbially induced, commonly over- ses, investigate the temporal relationship between these wrinkle steepened surface irregularities developed on sand- and siltstones structures and the end-Triassic mass extinction event. The global record (Hagadorn and Bottjer, 1997, 1999). Kinneyia-type wrinkle structures of Mesozoic MISS is very sparse, and Swedish MISS have thus far been (or just Kinneyia structures) feature minute flat-topped, winding crests documented only from Palaeozoic strata (e.g., Martinsson, 1965; separated by troughs or pits, resembling small scale interference ripples Calner, 2005; Calner and Eriksson, 2011). Thus the Early Jurassic struc- (Martinsson, 1965; Porada et al., 2008). Hagadorn and Bottjer (1997) tures of this study are the youngest yet described from Sweden, and

0 100 200 300 m N

Road 111

Norway

Sweden Fennoscandian Border Zone Denmark Kulla Gunnars- torp Helsing- borg Kulla Gunnarstorp coastal cliff section RomeleåsenLund Fault Zone Fyledalen Fault Zone

Vomb Trough Malmö N Jurassic strata

20 km

Fig. 1. Geological map of southern Sweden (with Sweden inset), Jurassic deposits highlighted in blue. The beach exposure at Kulla Gunnarstorp is marked with an arrow. 78 O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85 among the first in relatively close temporal connection with the end- 3. Geological setting and stratigraphy Triassic mass extinction event. Upper Triassic–Lower Jurassic strata of Sweden are restricted to the southernmost province, Skåne and to adjacent offshore areas where 2. Material and methods they form the marginal parts of the Danish Basin (Norling et al., 1993; Ahlberg et al., 2003; Vajda and Wigforss-Lange, 2009; Pott and The wrinkle structures described herein derive from the lower part McLoughlin, 2011; Vajda et al., 2013; Fig. 1). The succession includes of the Helsingborg Member (topmost Höganäs Formation) at Kulla three distinct formations together representing a gradual transition Gunnarstorp northwest of Helsingborg (Fig. 1; Pieńkowski, 1991a, b). from continental to fully marine depositional conditions (Fig. 2). The The studied specimens derive from strata immediately below and lowermost of these formations, the Kågeröd Formation (Norian), is com- above a distinct marker bed at the southernmost end of the Kulla posed of immature sandstones, arkoses and abundant extraformational Gunnarstorp coastal cliff section. Additional rock samples from the conglomerates formed through the accumulation of debris flows and stratigraphic interval containing the wrinkle structures were analysed stream processes in proximal alluvial environments under low and for grain size and mineralogical content by light microscopy. The rock episodic precipitation regime. The overlying Höganäs Formation slabs hosting the wrinkle structures illustrated herein are stored in the (Rhaetian–Hettangian) is dominated by mudstone and fine-grained reference collections at the Department of Geology, Lund University, sandstone with subordinate palaeosols and coal seams, reflecting an Sweden. unprecedented change in climate and run-off in the latest Triassic A single sandstone sample hosting wrinkle structures on the upper (Pieńkowski, 2002). This environmental change, evident in the lithology, bedding plane, and with the cyanobacterial film preserved, was has been attributed to the break-up of the supercontinent Pangea analysed palynologically with the aim to date the bed and to aid (Ahlberg et al., 2003). palaeoenvironmental interpretation. The sample was processed accord- The Kulla Gunnarstorp coastal cliff section is exposed along the ing to standard palynological procedures at Global Geolab Ltd., Canada. shore c. 10 km north of the city of Helsingborg and belongs to the Approximately 10 g of sediment was first treated with 40% hydrochloric uppermost part of the Höganäs Formation. The Höganäs Formation is acid (HCl) to remove potential calcium carbonate, and further macerat- sub-divided into three members, namely the Triassic Vallåkra and ed in 45% hydrofluoric acid (HF). The organic residue was sieved using a Bjuv members and the Jurassic Helsingborg Member (Fig. 2).The 10 μm mesh and mounted in epoxy resin on two microscopic slides. Vallåkra Member is composed mainly of claystone, and reflects the Three hundred pollen and spores were counted and the percentage of transition from the arid Pangaean Kågeröd redbeds to more humid each palynomorph taxon was calculated (Table 1). The slides and conditions. The Bjuv Member comprises floodplain deposits and hosts residues are deposited at the Swedish Museum of Natural History, a prominent coal seam at its base (known as the B-seam). The youngest Stockholm, Sweden and illustrated specimens are identified by member of the Höganäs Formation is the Helsingborg Member, which S-numbers. in its lowermost part comprises coarse-grained and poorly sorted sandstones, the Boserup beds (Larsson, 2009). Palynological assemblages from the Boserup beds show a transition from typical Rhaetian to typical Hettangian palynomorphs, and were thus inferred by Lindström and Erlström (2006) to include the T–J boundary. Overly- Table 1 ing deposits reflect a transgressive phase and are predominately Distribution of palynomorphs in the examined Kulla Gunnarstorp wrinkle structure sam- composed of deltaic sediments, with some tidal marine influences ple with relative abundance in % for each taxa. (Norling et al., 1993). These are the sediments investigated in this Palynomorphs Number of specimens % study, at the Kulla Gunnarstorp section. The section has previously Bryophytes been described by Troedsson (1951), Pieńkowski (1991a), Norling Foraminisporis jurassicus 2 0.7 et al. (1993) and Ahlberg (1994). Palynostratigraphical work has Stereisporites antiquasporites 2 0.7 been carried out by Larsson (2009). Troedsson (1951) and Ahlberg Stereisporites psilatus 2 0.7 (1994) attributed the section to the Hettangian, whereas both Total bryophyte spores 6 2.0 Pieńkowski (1991a; using sequence stratigraphy) and Larsson (2009; Lycophytes palynostratigraphy) concluded that the outcrop is of Sinemurian age. Acanthotriletes varius 2 0.7 Larsson's (2009) age assessment was based upon the occurrence Retitriletes austroclavatidites 7 2.3 Retitriletes semimuris 6 2.0 of the key taxon Cerebropollenites macroverrucosus and the spore Uvaesporites spp. 2 0.7 Retitriletes semimuris. Total lycophyte spores 17 5.7 The basal part of the overlying Rya Formation (Sinemurian –

Ferns Aalenian) is composed of poorly consolidated and cross-bedded quartz Deltoidospora spp. 66 22.0 arenites (Döshult Member), including an extraformational conglomer- Osmundacidites wellmanii 8 2.7 ate with crystalline basement clasts, overlain by mudrocks with abun- Conbaculatisporites mesozoicus 3 1.0 dant ammonites, the latter implying deposition in a fully marine, Cyathidites australis + C. minor 51 17.0 although near-shore environment. Laevigatosporites ovatus 2 0.7 Striatella scanica 1 0.3 Trachysporites fuscus 6 2.0 4. Results Total fern spores 137 45.7

Gymnosperms 4.1. Sedimentology Alisporites spp. 8 2.7 Araucariacites australis 4 1.3 The Kulla Gunnarstorp locality exposes an approximately 250 m Classopollis chateaunovi 24 8.0 long and 2–3 m high coastal cliff section (Fig. 1) with interbedded, Perinopollenites elatoides 91 30.3 ﬁ Podocarpidites spp. 7 2.3 organic-rich, dark grey to black shale, mudstone and very ne grained Ricciisporites tuberculatus 5 1.7 quartz arenite lenses. The strata dip slightly to the south so that at Vitreisporites pallidus 1 0.3 least 7–8 m of stratigraphy is accessible. The sedimentary facies Total gymnosperm pollen 140 46.7 comprise rhythmically bedded heteroliths displaying primarily wavy Total % 300 100.0 to lenticular bedding (Fig. 3a), and subordinate ﬂaser bedding. O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85 79

System Stage Formation Member Miospore Zone Toarcian Rydebäck Member Spheripollenites-Leptolepidites Rya Formation Pliensbachian Katslösa Member Chasmatosporites

Pankarp Member Early Sinemurian C. macroverrucosus Döshult Member

Helsingborg Mb Pinuspollenites-Trachysporites 201.6 Hettangian Transitional Spore-spike Interval Höganäs Formation Bjuv Member Rhaetian Vallåkra Member Corollina - Ricciisporites

Undefined

Triassic Jurassic Norian Kågeröd Formation

Fig. 2. Stratigraphic column of the Triassic–Jurassic units of Skåne correlated with local pollen zones (from Koppelhus and Nielsen, 1994; Larsson, 2009).

Subaquatic synaeresis cracks (Fig. 3b) occur in the lenticular facies, but Trace fossils are notably abundant in the upper bedding plane true, subaerial desiccation cracks have not been found anywhere in the (Fig. 4f) and preserved as up to 16 mm thick and several centimetres section. long epichnial furrows, filled either with clay or medium- to even Wrinkle structures were found in situ in what Pieńkowski (1991a) coarse-grained sand. Backfill structures are common and occur in sever- referred to as Unit 5 (Fig. 3c–d). This unit consists of wavy bedded al of the furrows. All trace fossils in the upper bedding plane post-date heteroliths hosting trace fossils, such as Rhizocorallium (Fig. 3e), togeth- the formation of the wrinkle structures. er with prominent drainage channels with intraformational basal conglomerates. The sand beds are generally fine-grained, but one 3–4cm 4.3. Palynology—stratigraphical and palaeoenvironmental implications thick, grey quartz-wacke forms a laterally continuous layer in this unit and can serve as a marker bed (Fig. 3d). It consists of well-sorted, The sandstone sample containing the MISS studied for palynology coarse-grained sandstone with a white, silty matrix that weathers to a hosts a well-preserved terrestrial palynoflora of relatively low diversity, rusty yellowish colour. It is weakly cemented and has a high effective and 15 spore and seven pollen taxa were identified (Table 1). The spores porosity. Mineralogically, the bed is dominated by quartz (c. 95%), but attain c. 53% of the assemblage and gymnosperm pollen grains c. 47%. No feldspars and smaller grains of heavy minerals also occur. Thin coatings algae or dinoflagellates were encountered. Fern spores dominate the of pyrite are evident on some grains. spore component, which in turn are strongly dominated by the trilete spores Cyathidites spp. (18%) and Deltoidospora spp. Key taxa, such as 4.2. Wrinkle structures Trachysporites fuscus and Striatella scanica occur in low numbers. Lycophyte spores reach a relative abundance of 5% including the Early Ju- The wrinkle structures are preserved exclusively on the upper bed- rassic key taxa Retitriletes semimuris and Retitriletes austroclavatidites. ding planes of 1–3 cm thick beds of current-rippled quartz arenites Bryophyte spores reach 2%, represented by Stereisporites spp. and (Fig. 4a–f). The arenites are very well sorted and fine grained, with rel- Foraminisporites jurassicus (Fig. 5). The gymnosperm component is atively high contents of mica. Epichnial, endichnial and hypichnial trace strongly dominated by Perinopollenites elatoides, followed by Classopollis fossils (Diplocraterion and Scolithos) are relatively common with biotur- chateaunovi (Fig. 5). Based on the composition of the flora (Table 1), bation indices ranging from 2 to 4 (sensu Droser and Bottjer, 1986), such as the abundance of specific taxa together with key-species, a which means that the primary sedimentary structures are disturbed Hettangian age is inferred for the beds hosting the wrinkle structures. but the sediment is not homogenized and individual burrows are clearly The assemblage is similar to that identified by Larsson (2009) al- discernible. The lower bedding plane also hosts horizontal furrows to- though less diverse, possibly due to the fact that Larsson sampled the gether with slightly irregular hypichnial protrusions, interpreted as more productive mudstones that occur between the microbial mat- resting traces and scour marks. bearing sandstones within the sampled succession. Larsson (2009) The wrinkle structures cover flat surfaces of an area ranging from 30 interpreted the Kulla Gunnarstorp strata as Sinemurian based chiefly to 60 cm2 and are all less than 1 mm high. Smooth current ripple on the presence of the miospore taxa C. macroverrucosus and R. foresets occur adjacent to the wrinkles, sloping down into topographi- semimuris, taxa that were also recovered in this study. However, ranges cally lower, wrinkle-free areas (Fig. 4b–c). of Cerebropollenites macroverucosus and Retitrilites semimuris extend at The wrinkles occur either as elongate or pitted forms. The former least from the base of the Hettangian (Koppelhus, 1991; Vajda, 2001; features up 1–2 mm wide, semi-continuous, commonly bifurcated and Cirilli, 2010; Lund, 1977; Morbey, 1975; Pedersen and Lund, 1980). subparallel ridges, which are separated by slightly wider (1–2mm) Therefore, none of these taxa serves as a marker for the Sinemurian. A round-based troughs (Fig. 4d). The crests are flattened and slopes Hettangian age for the studied assemblage is instead supported by com- over-steepened. The pitted forms on the other hand, have round bases parison with assemblages of similar composition, e.g., with the and are generally oval, with their short axes ≤1 mm and the long axes Munkerup assemblage from Bornholm, Denmark of Koppelhus 1–3mm(Fig. 4e). In many cases they are slightly interconnected with (1991), characterized by high relative abundances of P. elatoides, each other, showing a transition to more elongate or kidney-shaped Pinuspollenites minimus, Deltoidospora toralis, Calamospora, forms. Slopes are steep or overhanging, and the crests form an irregular Chasmatosporites spp. and Alisporites spp. together with low numbers network. Their morphology is concordant with the description of of C. macroverrucosus. Koppelhus (1991) referred the Munkerup assem- Kinneyia-type wrinkle structures of Porada et al. (2008).Sincetheun- blages to Lund's (1977) Hettangian Pinuspollenites–Trachysporites derlying bedforms are indiscernible below the wrinkles (non-transpar- Zone. Cirilli's (2010) comprehensive work on northern European ent wrinkle structure of Noffke, 2000), these were probably formed by a Triassic–Jurassic palynological revealed that C. macroverrucosus ranges thick epibenthic mat (cf. Noffke, 2010). from the topmost Triassic (where it is sparse), through the Hettangian 80 O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85

Fig. 3. Illustrations of outcrop features at Kulla Gunnarstorp locality. (A) Tidally influenced heterolithic bedding with bi-directional ripples. (B) Trace fossils of the Skolithos ichnofacies and numerous synaeresis cracks (subtidal desiccation cracks) on the upper bedding plane of current-rippled, fine-grained sandstone (some of the synaeresis marks marked with white arrows). (C) Overview of strata in the southern end of the studied section. Note variation in heterolithic facies and erosional down-cutting between the wavy- and flaser-bedded facies units. (D) Close up of in situ wrinkle structures. The rust-coloured bed in the top right is the marker bed. (E) Rhizocorallium ichnofossil immediately above marker bed.

(where it reaches low but consistent relative abundances), and up into 2011; Steinthorsdottir and Vajda, 2015; Sha et al., 2015); more than the Sinemurian. The presence of the pollen Ricciisporites tuberculatus is double that of modern post-industrial levels. further indicative of an assemblage older than Sinemurian and rather The palaeoenvironmental proxies at the Kulla Gunnarstorp locality indicates an early to mid-Hettangian age. indicate the presence of coastal wetland communities with ferns and Taxodium (Perinopollenites) representing the major part of the ﬂora. Taxodium is included in the Cupressaceae family (cypress) and grows 5. Discussion presently in a broad range of warm moist habitats. Their co-existence with ferns (spores making up over 50% of the assemblages) and the 5.1. Palaeoenvironment lack of the drought-resistant Pinaceae indicate that the Taxodium pollen grains identiﬁed within the studied sample were probably produced by The Early Jurassic was characterized by a hot climate with an atmo- forms with wetland adaptations similar to the modern bald cypress. The spheric carbon dioxide concentration (pCO2) around 900–1000 ppm extinct Cheirolepidiaceae represent a group of gymnosperms that dur- (Beerling and Royer, 2002; Bonis et al., 2010; Steinthorsdottir et al., ing the Jurassic were widespread globally. They were possibly related O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85 81

Fig. 4. Illustrations of wrinkle structures found at Kulla Gunnarstorp. All scale bars = 1 cm. (A) Elongate wrinkle structures cut by epichnial tracefossils. Smooth area in the lower, middle part represents a ripple foreset. (B) Close-up of elongate, subparallel ridges. Note the steep slopes of wrinkles, far exceeding the normal angle of repose for sand. (C) Pitted and reniform morphology. (D) The wrinkle structure ﬁlls out the area between the ripple foresets, forming a leveling structure. (E) The one specimen found in situ, also forming a leveling surface. The degree of bioturbation is not evident from the photo, but endichnial and hypichnial traces are abundant (bioturbation index 3). (F) Close-up of epichnial trace fossil shown in Fig. 4A, formed by a vermiform invertebrate, cutting the ridges of the wrinkle structure. 82 O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85

Fig. 5. Light micrographs of representative pollen and spores from the investigated samples, scale bar = 10 μm, same magnification for all illustrated specimens. Taxa, sample number and England Finder Reference (EFR) and S- numbers. (A) Stereisporites antiquasporites (Wilson and Webster, 1946) Dettmann, 1963, WRS-1B, EFR-P40/3, (S085450). (B) Retitriletes semimuris (Danze-Corsin and Laveine, 1963) McKellar, 1974, WRS-1B, EFR-Q31/1, (S085451). (C) Retitriletes austroclavatidites (Cookson, 1953) Döring et al., in Krutzsch 1963, WRS-1A, EFR-L23/2, (S085452). (D) Deltoidospora spp. WRS-1A, EFR-S38/2, (S085453). (E) Deltoidospora spp., WRS-1A, EFR-D22/3, (S085454). (F) Conbaculatisporites mesozoicus Klaus, 1960, WRS-1A, EFR- E20/1, (S085455). (G) Foraminisporis jurassicus Schultz, 1967, WRS-1B, EFR-O20/3, (S085456). (H) Cyathidites australis Couper, 1953, WRS-1A, EFR-O27/2, (S085457). (I) Acanthotriletes varius Nilsson, 1958, WRS-1B, EFR-Q16/1, (S085458). (J) Striatella scanica (Nilsson) Filatoff and Price, 1988, WRS-1B, EFR-U16/2, (S085459). (K) Classopollis chateaunovi Reyre, 1970, WRS- 1B, EFR-K11, (S085460). (L) Perinopollenites elatoides Couper, 1958, WRS-1B, EFR-Q15/3, (S085461). (M) Vitreisporites pallidus (Reissinger) Nilsson, 1958, WRS-1B, EFR-D43/3, (S085462). (N) Ricciisporites tuberculatus Lundblad, 1945, WRS-1A, EFR-L41/1, (S085463). to the Araucariaceae or Cupressaceae (Watson, 1988; Mehlqvist et al., interpret the presence of the disaster taxon R. tuberculatus to indicate 2009) and were represented by a range of growth forms from to large that the vegetation was still within the recovery phase after the Tr–Jex- trees to small shrubs (Kürschner et al., 2013). Although they seem to tinction event and close to the system boundary. have been adapted to a wide range of ecological settings, from arid to The presence of several (up to 10 m wide) channel sandstones with wet environments including mangroves (Watson, 1988; Kürschner basal intraformational conglomerates and a mudstone facies strongly et al., 2013), the xeromorphic morphological characters of their leaves, enriched in organic material, along with bi-directional current ripples have generally been interpreted as an adaptation to an arid, coastal en- in the heteroliths, suggests an extremely shallow, marginal marine vironment. We envisage the Classopollis-producing conifers to have environment with a pronounced palynological influence from nearby inhabited a coastal setting, closer to the shore compared to the other wetland vegetation communities. There is no convincing evidence for plants, growing on a drier, sandy substrate enriched in salts, a habitat repeated subaerial exposure of the succession, as neither palaeosols not suitable for the otherwise arid-tolerant pines. Importantly, we nor rootlets have been identified. Subaquatic synaerisis cracks O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85 83

(Fig. 3b) are abundant in certain beds but true desiccation cracks have Permian mass extinction events, microbial mats spread into environ- not been observed. ments not occupied prior to the event, and started to dominate in the The ichnofauna at Kulla Gunnarstorp has been interpreted previous- environments that they already inhabited (Forel et al., 2013; Mata and ly as representing mainly marine conditions, but authigenic minerals Bottjer, 2012). (pyrite and siderite) precipitated during early diagenesis suggest brack- The end-Triassic mass extinction is one of the less studied major mass ish water influences (Pieńkowski, 1991a; Ahlberg, 1994). For the above extinction events with regard to the precise timing and magnitude. Based reasons, the exposed strata are herein interpreted as representing a very on U–Pb zircon dating of marine strata, the extinction has been dated at shallow subtidal setting subjected to tidal flat progradation and 201.6 ± 0.3 Ma by Schaltegger et al. (2008) and 201.31 ± 0.43 Ma by shallowing only rarely resulting in subaerial exposure. Schoene et al. (2010). The event resulted in extinction of conodonts and a severe decline of ammonoids and brachiopods among other groups, 6. Astrobiological implications but also led to dinosaurs taking over niches previously occupied by am- phibians or mammal-like reptiles on land (Tanner et al., 2004; Akikuni Due to their 3.4-billion-year fossil record, cyanobacterial mats are et al., 2010). Several extinction mechanisms have been proposed, includ- important targets in astrobiological research (Noffke, 2015 and refer- ing widespread volcanism (Central Atlantic Magmatic Province, CAMP) ences therein). Studying fossil microbial mats provides information associated with the break-up of Pangea (Marzoli et al., 2004; Tanner about early Earth's ecosystems and might serve as an analogue for et al., 2004). early life on other planets and celestial bodies in our solar system. Nonetheless, unlike the end-Permian mass extinction and despite a Photosynthetic cyanobacteria (commonly preserved as stromatolites) decrease in ichnofauna, diversity and bioturbation rates during the were a driving force in the evolution of oxygen- dependent life Rhaetian (Twitchett and Barras, 2004), a significant microbial response (Kershaw et al., 2012)andthesemicrobesinfluenced and changed the associated with the end-Triassic mass extinction has so far been report- composition of the atmosphere. ed only from localities in the southwestern United Kingdom (Ibarra Phanerozoic microbially induced sedimentary structures (MISS) are et al., 2014). The general lack of stromatolites following the Tr–Jevent normally associated with tidal deposits, in which the fluctuating salinity has been attributed to a biocalcification crisis (van de Schootbrugge and water levels form extreme environments limiting burrowing and et al., 2007; Mata and Bottjer, 2012; Vajda and Bercovici, 2014). Howev- grazing by invertebrates, thus facilitating the formation and preserva- er, such a scenario would not exclude microbial growths in siliciclastic tion of microbial mats. In modern tidal flats, differing hydraulic condi- settings. We propose an explanation in which the expansion of tions and moisture levels on the tidal flat will lead to the development cyanobacterial mats in siliciclastic subtidal settings is a response to the of diverse types of MISS depending on where in the tidal zone they form. end-Triassic mass extinction, with lower faunal diversity and with a de- The upper intertidal zone is drained and flooded daily by tidal cur- pleted invertebrate gene pool with inferior abilities to adapt to extreme rents, whereas the lower intertidal belt is dry only during neap tides re- conditions. We propose that reduced grazing and bioturbation due to curring every two weeks. The subtidal zone is always submerged. In the mass extinction event explain the presence and preservation of mi- modern settings, microbial mats grow thicker between the beach and crobial mats in this Early Jurassic shallow subtidal zone. This is further the upper intertidal/lower supratidal zone, only to thin out substantially supported by the palynological results revealing that the enigmatic towards the subtidal zone (Noffke and Krumbein, 1999; Bose and “disaster” pollen taxon R. tuberculatus is present in the assemblage, Chafetz, 2009). although in low relative abundance. The lower supratidal conditions allow for favourable mat forming conditions, owing to a delicate balance between inundation and desic- 7. Conclusions cation. The fluctuation of water levels causes an environmental stress for grazers, while still supplying the mats with moisture. However, if This study describes the first microbially mediated sedimentary the mats are perpetually wet, they become soft and more susceptible structures in the Mesozoic succession of Sweden and one of the first to grazing (Bose and Chafetz, 2009). This balance is one explanation of global occurrences of earliest Jurassic microbial mats. The studied wrin- why modern microbial mats dramatically decrease in thickness in the kle structures are of Kinneyia-type and these form within the sediments lower intertidal zone and seaward. owing to the cohesiveness of microbial mats, although the exact mode Even though there is broad agreement that Kinneyia-type wrinkle of formation remains unclear. Importantly, the studied Kinneyia struc- structures form beneath thick, cohesive microbial mats, their exact tures derive from the shallow subtidal or lower intertidal zone, i.e., in mode of formation is still debated. As discussed earlier, modern a deeper environment compared to where modern mats develop. epibenthic mats mainly grow in the supratidal to intertidal zones, but Biostratigraphy indicates a late Hettangian (earliest Jurassic) age of according to the tidal hydrodynamic and ground water flow model ad- the deposits, i.e., relatively soon after the Late Triassic mass extinction vanced by Porada et al. (2008), Kinneyia-type wrinkle structures would event. Since perpetually wet, and thus soft, microbial mats are suscepti- preferentially form in the subtidal zone. ble to predation. The presence of wrinkle structures may be explained It is notable that the wrinkle structures described herein occur in by lower bioturbation and grazing associated with the mass extinction. strata lacking rootlets or other signs indicative of prolonged subaerial exposure. It is striking that even desiccation cracks, which form in a List of taxa matter of hours in tidal settings, are absent. Therefore we interpret the strata to have been deposited in a very shallow, low-energy subtidal, Bryophyta (mosses) i.e., below low-tide level, setting. This signifies that they were formed Foraminisporis jurassicus Schultz 1967 in a deeper setting compared to where modern epibenthic microbial Stereisporites antiquasporites (Wilson & Webster 1946) Dettmann mats grow, which all occur above low-tide level. Regardless of which 1963 model is invoked for the genesis of wrinkle structures, the palaeosetting Stereisporites psilatus (Ross 1943) Pflug in Thomson & Pflug 1953 in the subtidal zone requires an explanation. Lycopsida (club mosses) Acanthotriletes varius Nilsson 1958 6.1. MISS and their relation to the end-Triassic mass extinction event Retitriletes austroclavatidites (Cookson 1953) Döring et al. in Krutzsch 1963 Based on statistical analyses of the stratigraphic ranges of marine Retitriletes semimuris (Danze-Corsin & Laveine 1963) McKellar 1974 taxa, Raup and Sepkoski (1982) delimitated five major mass extinctions Uvaesporites spp. in Earth history. In the aftermath of both the late Devonian and the end- Filicopsida (ferns) 84 O. Peterffy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 464 (2016) 76–85

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