A new Rhaetian plant assemblage from Zilanba, the northern Sichuan Basin, South China

doi:10.21203/rs.3.rs-2951324/v1

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A new Rhaetian plant assemblage from Zilanba, the northern Sichuan Basin, South China

https://doi.org/10.21203/rs.3.rs-2951324/v1

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published 02 Mar, 2024

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Triassic-Jurassic strata are extensively developed and widely distributed in the Sichuan Basin, South China. In particular, the continental Upper Triassic Xujiahe Formation is well-exposed in this region, yielding rich assemblages of fossil plants. Here, a new fossil assemblage is reported from Zilanba in the Guangyuan area, northern Sichuan Basin. This locality yields 31 species of fossil plants belonging to 18 genera, newly collected from ‘member Ⅲ’ of the Xujiahe Formation. Based on the floral assemblages and previous magnetostratigraphic and palynostratigraphic studies, the host strata are considered lower Rhaetian in age. The early Rhaetian flora in the Guangyuan area is dominated by Cycadales and Bennettitales but also contains abundant ferns and low proportions of various sphenopsids, conifers and other gymnosperms. The occurrence of some climate-diagnostic plants, such as Dipteridaceae, Anthrophyopsis and Ptilozamites, indicate that this area experienced a humid and warm tropical or subtropical climate during the early Rhaetian.

early Rhaetian

fossil plants

new fossil locality

Sichuan Basin

South China

The end-Triassic mass extinction was one of the five most severe biotic crises in the Phanerozoic (Raup and Sepkoski 1982; Sepkoski 1996; McElwain et al. 1999; Heimdal et al. 2020). Extinctions or turnover patterns of marine organisms across the Triassic-Jurassic transition have been well documented (Tomašových and Siblík 2007; Whiteside and Ward 2011; Martindale et al. 2012; Lucas 2013; Atkinson et al. 2019). However, fewer studies dealing with the patterns of terrestrial floral change have been published. In the eastern Tethys region, Upper Triassic and Jurassic sequences are well-developed in China, and incorporate typical continental (fluvio-paludal), coal-bearing deposits in the Sichuan Basin (Wang et al. 2010), showing differences from the sedimentary successions of this age in western Tethys regions in Europe including Germany, Poland, Sweden and Greenland which are characterized by thin coals and deltaic to shallow marine deposits. The Upper Triassic strata represented by the Xujiahe Formation are well-exposed along the eastern and northeastern margins of the Sichuan Basin in southern China, and consist of coal-bearing, clastic rocks with abundant plant fossils (Wang et al. 2010). As floral variation and the distribution of ecological traits are crucial for understanding the regional responses of palaeovegetation to environmental oscillations (McElwain et al. 1999, 2007; Bonis and Kürschner 2012), the well-exposed, continuous plant-rich successions of the Sichuan Basin provide key data for investigating palaeoclimates and terrestrial palaeoenvironments preceding the Triassic-Jurassic transition.

The Late Triassic flora of the Xujiahe Formation in the Sichuan Basin has been investigated for more than a century since the early work of Schenk (1883, 1884). A series of investigations focusing primarily on the stratigraphy and fossil systematics in this region has provided a fairly comprehensive understanding of the gross composition of the Late Triassic flora in the Sichuan Basin (Li 1964; Yang 1978; Ye et al. 1986; Huang and Lu 1992; Huang 1992, 1995; Wu 1999; Wang et al. 2010; Zhou et al. 2021). In recent decades, additional reports of plant fossils and lithological profiles from the Xujiahe Formation have been published, providing new information on a variety of fields, such as plant diversity, fertile structures and foliar morphology (Wang et al. 2015; Zhou et al. 2016; Xu et al. 2021, 2022), palynofloras and palaeoecology (Liu et al. 2015a, 2015b; Li and Wang 2016; Li et al. 2016, 2018, 2020), petrified woods and their palaeoclimatic implications (Qian et al. 2010; Tian et al. 2016), lithostratigraphy and sequence stratigraphy (Lu et al. 2015, 2019, 2023; Jiang et al. 2017; Zhu et al. 2017), biomarkers for palaeowildfires (Pole et al. 2018; Song et al. 2020), intrinsic interactions between continental chemical weathering and volcanism (Shen et al. 2022), and magnetostratigraphy (Li M.S. et al. 2017). These studies have precisely dated the Xujiahe Formation as Norian–Rhaetian, thus providing a well-resolved temporal context for understanding the climatic and environmental variations prior to the Triassic-Jurassic transition in the Sichuan Basin.

The fossil flora of the Xujiahe Formation is represented mainly by leaf and stem compressions that, locally, have accumulated into thick coal seams (Li 1964; Huang 1992, 1995; Wu 1999; Yang 1978; Ye et al. 1986). Dominant plant groups in the flora include Cycadales (e.g., Ctenis, Pseudoctenis), Bennnettitales (e.g., Pterophyllum, Anomozamites, Otozamites) and ferns (e.g., Clathropteris, Dictyophyllum, Cladophlebis). Equisetales (e.g., Neocalamites and Equisetites), ‘seed ferns’ (e.g., Ptilozamites, Lepidopteris), Ginkgoales/Leptostrobales (e.g., Baiera, Glossophyllum and Czekanowskia) and conifers (e.g., Podozamites and Ferganiella) are also common elements of the Xujiahe flora, but are of lower diversity.

Detailed systematic descriptions of the broader Xujiahe Formation flora have been published in several previous studies. Recently, a new fossil locality at Zilanba was discovered in Guangyuan, northern Sichuan Basin, with diverse and well-preserved fossil plants in ‘member Ⅲ’ of the Xujiahe Formation. Here we document the composition of this new lower Rhaetian assemblage and assess its implications for interpreting the regional palaeoclimate and local palaeoenvironment prior to the Triassic-Jurassic transition. We also advance the prospect of reconstructing Late Triassic environmental oscillations in the context of a more precisely time-calibrated sedimentary succession in this region and developing improved mid-Mesozoic correlations for continental strata between eastern and western Eurasia.

The Sichuan Basin, also known as the Red Basin, is a sedimentary basin in Southwest China covering eastern Sichuan Province and Chongqing City. Geologically, the Sichuan Basin is located on the Yangtze Platform, characterized by well-developed Paleozoic–middle Mesozoic marine successions and, especially, terrestrial sequences of Late Triassic to Cretaceous age (Wang et al. 2010). Three major tectonostratigraphic units were recognized in this basin: the Upper Yangtze, Lijiang and Kangding units (Wang et al. 2010), of which Zilanba is located in Yangtze Unit.

The Indosinian movement during the Middle–Late Triassic led to the formation of the Chuanxi Depression and the tectonic uplift of the Longmen Mountains, which provided the dominant influence on sedimentation in the Sichuan Basin that resulted in a shift from marine to terrestrial deposition (Li et al. 2010; Liu et al. 2021). Within the Sichuan Basin, the Upper Triassic successions are well-developed and widely distributed in the northern, eastern and southern parts, where they are represented by thick fluvial-lacustrine, and coal-forming swamp deposits (Wang et al. 2010). The Upper Triassic strata assigned to the Xujiahe Formation, host rich coal and gas resources, in addition to abundant plants fossils (Li 1964; Ye et al. 1986; Wang et al. 2010).

Influenced by a large-scale regression during the Ladinian, the Xujiahe Formation unconformably overlies the dolomitic Middle Triassic Leikoupo Formation (Wang et al. 2010)—a transition especially well expressed in the Guangyuan area. The Xujiahe Formation is a clastic unit composed of sandstone, siltstone, mudstone and coal, and has been subdivided into seven informal members. In the Guangyuan area, the Xujiahe Formation is subdivided into just four members, of which, ‘member Ⅰ’ and ‘member Ⅲ’ are dominated by mudstones, and ‘member Ⅱ’ and ‘member Ⅳ’ are dominated by sandstones (Wang et al. 2010).

The new fossil material was collected from the Upper Triassic Xujiahe Formation along the Bailong River at Zilanba, close to Baolun Town, Guangyuan City, in the northern Sichuan Basin, South China (32°25′14.28″N, 105°39′45.24″E; Fig. 1). The Zilanba section is located on the western margin of the South China Block (Wang et al. 2010), and was situated in low middle-latitudes of the eastern Tethyan region (Li M.S. et al. 2017) during the Rhaetian. ‘Member Ⅲ’ of the Xujiahe Formation is considered to be Rhaetian in age based on magnetostratigraphic (Li M.S. et al. 2017) and palynological data (Li et al. 2020).

The newly discovered outcrops along the Bailong River at Zilanba expose a fairly continuous succession of the Xujiahe Formation, from ‘member Ⅰ’ to ‘member Ⅳ’ (Fig. 1), with typical continental features, overlying the Middle Triassic Leikoupo Formation, and being overlain by the Lower Jurassic Baitianba Formation. ‘Member Ⅰ’ of the Upper Triassic Xujiahe Formation is sporadically exposed, thin, and dominated by yellowish-brown mudstone interbedded with coal seams, whereas ‘member Ⅱ’ is minimally exposed. The lower part of ‘member Ⅲ’ is concealed by vegetation at the study site. However, its upper part is well exposed and contains abundant and well-preserved fossil plants. ‘Member Ⅳ’ is dominated by medium-grained sandstone and conglomerates, with common occurrences of fossil tree trunks in crevasse splay and channel-fill facies; a section of gray mudstone interbedded with coal seams exposed at the top of ‘member Ⅳ’, contains numerous plant (leaf) compressions (Fig. 1).

One hundred and fifty-seven plant fossil specimens (predominantly impressions) were collected from ‘member Ⅲ’ of the Upper Triassic Xujiahe Formation in this study. The fossil assemblage is generally well preserved in a grey–black shale matrix, providing detailed morphological characters. Some leaf fragments preserve cuticles that enabled recent systematic revisions of some selected taxa (Xu et al. 2021, 2022). The techniques used both in the field and laboratory were detailed by Popa (2011).

Photographs of plant fossils were taken with a Canon 60D camera equipped with an EF-S 18–135 mm f/3.5–5.6 IS lens. Standard graphics software (Adobe Photoshop® CC) was used only to enhance the contrast of images.

For some compression fossils, the cuticles were mechanically removed from the lamina, then treated with Hydrochloric acid (HCl) and Hydrofluoric acid (HF) to remove calcium carbonate and silica. The leaf fragments were then treated with Schultz reagent until the cuticle became translucent and brownish. After being washed with distilled water until neutral, the material was then treated with 5% ammonium hydroxide solution (NH₃·H₂O) for several minutes until remnant coalified mesophyll was removed from the cuticles. The cuticle was washed again with distilled water until neutral, and then mounted on glass slides and studied using a Zeiss V12 transmitted light (optical) microscope (LM) or LSM 710 Laser Scanning Confocal Microscope (LSCM). Some cuticles were glued on aluminium stubs, then gold-coated for Scanning Electronic Microscopy with a LEO 1530 VP SEM at the Lab Center of the Nanjing Institute of Geology and Palaeotology, Chinese Academy of Science, Nanjing, China (NIGPAS). All hand specimens, slides and SEM stubs are housed at the NIGPAS, illustrated specimens are catalogued as PB23147, PB23148, PB23150, and PB201969—PB201997 respectively; the other specimens which do not illustrated herein are recorded with field numbers as 201404-BL001—201404-BL157 respectively.

The fossil assemblage contains 31 species assigned to 18 genera, belonging to Equisetales, ferns, ‛seed ferns’, Bennettitales, Cycadales, Nilssoniales, Ginkgoales, conifers and unassigned gymnosperms. Most foliage of cycads and the pinnae of ferns are preserved as almost complete leaves in irregular orientation on the bedding planes, indicating hypautochthonous accumulation and burial condition. Since the identified species have all been formally described from other localities in the region, we simply provide a summary of the composition of the new fossil assemblage and include notes on key morphological features of some taxa.

Equisetopsida

Among pteridophytes, six species ascribed to four genera are recognized. Equisetales are the most abundant, represented by stems and nodal diaphragms with leaf sheaths assigned to Equisetites Sternberg, 1833, including Equisetites ferganensis Seward (Fig. 2a–b) and Equisetites sp. Axes attributed to Neocalamites Halle, 1908 possess obvious longitudinal ridges and furrows on the surface of the stems. However, without complete nodes preserved (Fig. 2e–f), these axes cannot be attributed to an established species. Radicites Potonié, 1893 roots are also common, with a central axis and lateral appendages (Fig. 2c). Annulariopsis Zeiller, 1903 is represented by several verticillate leaflets with spathulate outline and a single straight midrib (Fig. 2d).

Ferns

Ferns are represented by five species belonging to three genera and are dominated by Dipteridaceae (especially Clathropteris Brongniart, 1828 and Dictyophyllum Lindley et Hutton, 1834). The leaf fragments of Clathropteris meniscioides Brongniart are characterized by a straight midrib, with distinct secondary veins forming regular and rectangular networks, and the finest veins tend to form rectangular meshes enclosing blind vein-endings (Fig. 2i, k). A well-preserved leaf fragment of Dictyophyllum nathorsti Zeiller shows typical Dipteridacean architecture, with more than ten preserved pinnae along the leaf segments (Fig. 2j). The lamina lobes are alternate, triangular and slightly falcate with a conspicuous midrib zigzagging towards the apex, and secondary veins dichotomizing several times, connected to each other in irregular, rectangular or polygonal meshes (Fig. 2j). Cladophlebis Brongniart, 1849 is represented by several species at Zilanba, e.g., Cladophlebis denticulata (Brongniart) Nathorst, C. fangtzuensis Sze, and C. kaoiana Sze. Most of these are represented by pinnae fragments (Fig. 2g–h), with margins varying from entire (Fig. 2g) to denticulate (Fig. 2h).

‘Seed ferns’ (‘Pteridospermopsida’)

‘Pteridosperms’ (a polyphyletic grouping of cupulate seed plants) are rare in the Zilanba flora, and represented by a single taxon, Ptilozamites chinensis Hsü emend. Zhou, 1981 (Fig. 5d) (Xu et al. 2022). The material is well-preserved, retaining cuticles, which facilitated the recent re-investigation of the systematic placement, intraspecific variation and palaeoclimatic implications of this taxon (Xu et al. 2022).

Cycadopsida

Cycadopsida representatives in this fossil assemblage are here deemed to include Cycadales and Nilssoniales. Ten species ascribed to three genera were recognized (Fig. 3c–d, f–j), and these constitute the dominant group in the Zilanba assemblage. Cycadales (Fig. 3c–d, g–j) is represented by Pseudoctenis Seward emend. Harris, 1964 (Fig. 3c–d) and Doratophyllum Harris, 1932 (Fig. 3g–j). Nilssoniales is identified based on a single leaf fragment ascribed to Nilssonia sp. cf. N. orientalis (Fig. 3f).

Bennettitopsida

Bennettitales is represented mainly by leaf fragments with opposite to sub-opposite pinnae (Fig. 3a–b, e) attributed to Anomozamites Schimper, 1870 (e.g., Anomozamites loczyi Schenk and Anomozamites sp. cf. A. gracilis Nathorst). Although of low diversity, bennettitalean foliage is abundant.

Ginkgoopsida

A single specimen of Ginkgoites sp. was recorded from Zilanba, having wedge-shaped leaf lobes with obtuse apices (Fig. 4a). The cuticle is well-preserved, with clear epidermal cells and stomata (Fig. 4b–d). Stomata are haplocheilic and monocyclic, with guard cells usually surrounded by four to five subsidiary cells with robust anticlinal walls. Stomata are sunken, with guard cells about 35 µm long and 10 µm wide (Fig. 4b–d). Stenorachis Saporta, 1879 was also recorded, with a robust axis bearing spirally arranged appendages that are swollen distally (Fig. 4g–h).

Conifers

Conifers are represented mainly by abundant Pityophyllum Nathorst, 1899 and Podozamites Braun, 1843 leaves. Pityophyllum occurs as simple, dispersed needle-like leaves (Fig. 4j–k). Podozamites (Fig. 4i) has broader multiveined leaves. Podozamites expanded across eastern Asia in middle latitudes during the Late Triassic (Pole et al. 2016) and remained widely distributed in the Northern Hemisphere until the Early Cretaceous. The numerous occurrences of Podozamites in lower Rhaetian strata of the Sichuan Basin demonstrate the importance of this taxon in the Late Triassic vegetation of the region (Pole et al. 2016). Two leaf fragments of Storgaardia Harris, 1935 are also recorded, with linear or acerose leaves oppositely or sub-oppositely attached to the axis, with a single midrib (Fig. 3e–f). Storgaardia originated during the Late Triassic in East Asia, and arguably spread to Europe and Greenland, then experienced a rapid decline towards the end of the Tithonian (Li Q.J. et al. 2017). The occurrence of Storgaardia in the Xujiahe Formation at Zilanba is the first record from the Upper Triassic of the Sichuan Basin.

Gymnospermae Incertae sedis

Abundant leaf fragments of Anthrophyopsis Nathorst emend. Xu, Popa et Wang 2021 with various margin types were founded (Fig. 5a–c) (Xu et al. 2021). Anthrophyopsis is a Late Triassic leaf taxon with uncertain systematic affinities and poorly understood morphology and anatomy (Xu et al. 2021). Its systematic affinities might lie with Cycadopsida, Bennettitopsida or various other seed fern groups (Xu et al. 2021). The newly collected, abundant material of Anthrophyopsis from Zilanba recently facilitated a comprehensive revision of this genus, resulting in emended diagnoses at genus and species level (Xu et al. 2021).

Vegetation composition and biostratigraphy

Some of the taxa reported from the Zilanba section (excluding Anthrophyopsis venulosa and Storgaardia mentoukouensis) have been reported by previous researchers working in the Sichuan Basin (Li 1964; Ye et al. 1986; Wu 1999). The composition of Zilanba fossil assemblage has much in common at family and genus level (e.g., Anthrophyopsis, Anomozamites, Clathropteris, Dictyophyllum, Ptilozamites; Table 1) with the well-studied Rhaetian florals of Europe and Greenland (e.g., the Bayreuth flora in Germany; the Rhaetian flora of Upper Silesia and the Tatra Mountains in Poland; the Skåne Rhaetian (Bjuv Member) floras in Sweden and the Lepidopteris Zone in Greenland) (Kustatscher et al. 2018), despite its lower diversity and few shared species.

Table 1

Comparisons of the new Rhaetian plant assemblage found at Zilanba in the Sichuan Basin, South China, with those of the Rhaetian floras from Germany, Poland, Sweden and East Greenland (data from Kustatscher et al. 2018 and references therein).
	Germany	Poland	Sweden	East Greenland	This study
Anthrophyopsis				*	*
Anomozamites	*		*	*	*
Annulariopsis					*
Baiera			*	*
Brachyphyllum		*
Camptopteris			*
Cladophlebis	*	*	*	*	*
Clathropteris	*	*		*	*
Ctenis	*
Cycadolepis				*
Cyparissidium			*
Czekanowskia		*		*
Desmiophyllum	*
Dictyophyllum	*		*	*	*
Doratophyllum				*	*
Elatocladus	*		*	*
Equisetites	*		*	*	*
Ginkgo			*
Ginkgoites	*	*	*	*	*
Hausmannia				*
Lepidopteris	*	*	*	*
Marattia	*
Neocalamites		*	*		*
Nilssonia	*			*	*
Nilssoniopteris	*
Pachypteris	*			*
Pagiophyllum		*
Palissya	*	*	*
Peltaspermum		*
Phlebopteris	*		*
Pityophyllum					*
Podocarpites					*
Podozamites				*	*
Pseudoctenis	*			*	*
Pterophyllum	*	*	*	*
Ptilozamites	*		*	*	*
Radicites					*
Rhapidopteris	*
Rhinipteris				*
Sagenopteris			*
Schizolepis	*
Schmeissneria	*
Selaginellites	*
Sphenobaiera			*	*
Spiropteris				*
Stachytaxus	*		*	*
Stenorachis					*
Taeniopteris		*		*
Thaumatopteris	*		*
Todites	*		*
Voltzia	*
Weltrichia				*
Widdringtonites		*		*
Wielandiella				*
Williamsonia			*	*

During the Late Triassic, two main floristic provinces were defined in China: the Northern Floristic Province (NFP) which was marked by the “Danaeopsis-Bernoullia” macrofossil assemblage, and the Southern Floristic Province (SFP) which was characterized by the “Dictyophyllum-Clathropteris” plant association (Sun 1995). Within the Zilanba flora, Clathropteris meniscioides and Dictyophyllum nathorsti are typical elements of the SFP during the Late Triassic. Ptilozamites chinensis is widely reported from Upper Triassic strata in Sichuan, Hunan, Jiangxi, Fujian and Guangdong provinces, and with typical occurrences in the SFP (Xu et al. 2022). Anthrophyopsis has been described from Upper Triassic successions of Sweden, Greenland, China, Russia, Georgia, Tajikistan and Iran, and became especially common during the Rhaetian (Xu et al. 2021). Doratophyllum hsuchiahoense, originally established by Li (1964), has been reported from many fossil assemblages of the Upper Triassic Xujiahe Formation in the Sichuan Basin. This taxon appears to have been endemic to the Sichuan Basin.

The Late Triassic floras in the SFP can be divided into Carnian, Norian and Rhaetian associations based on their taxon representation and have been dated precisely using intercalated marine strata with age-indicative fauna (Sun 1995; Kustatscher et al. 2018). Among these, the Rhaetian succession is marked by the Ptilozamites–Anthrophyopsis assemblage defined from the Yangbaichong Formation of Shaqiao, Hunan Province (Zhou 1989), incorporating Clathropteris, Cycadocarpidium, Nilssoniopteris, Pterophyllum, Ptilozamites, Podozamites, Stalagma and Todites. This flora is highly similar to the Anyuan flora in Jiangxi Province (Xiong et al. 2009), Wenbinshan flora in Fujian Province (Zhou 1978), and is equivalent to the Lepidopteris Zone of eastern Greenland (Jameson Land), Sweden, Poland and Germany (Kustatscher et al. 2018), although the nominal taxon of this zone is absent from the Zilanba assemblage. Based on the combination of the newly collected material and the fossil assemblages reported elsewhere (Li 1964; Ye et al. 1986; Wu 1999), the common occurrences of Anthrophyopsis, Clathropteris, Ptilozamites, Todites, Pterophyllum and Cycadocarpidium indicate that the fossil flora from ‘member Ⅲ’ of the Xujiahe Formation belongs to the Ptilozamites–Anthrophyopsis assemblage and is, thus, Rhaetian in age. This is corroborated by magnetostratigraphic (Li M.S. et al. 2017) and palynostratigraphic (Li et al. 2020) data.

Palaeoclimate significance

The fossil flora of the Xujiahe Formation indicates that the Sichuan Basin was dominated by gymnosperms (especially cycads and bennettites) and ferns during the Late Triassic (Li 1964; Ye et al. 1986; Wu 1999). The newly collected fossil assemblage from ‘member Ⅲ’ of the Xujiahe Formation at Zilanba confirms a floral composition consistent with previous reports.

Ecologically, Equisetites and Neocalamites occupied riverbank and marsh habitats (Deng 2007) but their climatic preferences are ambiguous.

Extant representatives of Dipteridaceae (i.e., Dipteris Reinward 1825) are restricted to tropical regions (Kramer 1990), although the family may have had a broader climatic and biogeographic range during hyperthermal intervals of the Mesozoic (Corsin and Waterlot 1979). Fossil Dipteridaceae, such as Clathropteris and Dictyophyllum occur at Zilanba and, on this basis, hot and humid climates have been inferred in the region for the Rhaetian (Wang et al. 2015; Zhou et al. 2016). These taxa were probably understorey plants of forests, riverbanks and marshes (Deng 2007; Guignard et al. 2009; Wang et al. 2015; Zhou et al. 2016).

Among the gymnosperms, the most representative Late Triassic leaf genus Anthrophyopsis, is usually very well-preserved (Xu et al. 2021). Anthrophyopsis produced leaves in excess of 50 cm long and 20 cm wide (macrophylls in the leaf size scale of Raunkiaer 1934 and Webb 1959), and can be regarded as a warm and humid palaeoclimate indicator, since such large leaves are characteristic of modern tropical humid climates (Jacobs 1999; Xu et al. 2021). Ptilozamites is one of the most widespread genera of Mesozoic seed ferns, reaching its greatest diversity during the Late Triassic. According to the re-appraisal of Ptilozamites chinensis based on the newly collected specimen at Zilanba (Xu et al. 2022), and fossil records from other Mesozoic strata in China, this plant is generally reported from fluvio-deltaic-lagoonal marshes, terrigenous marine and fluvio-lacustrine or swampy settings, with a preference for warm and humid conditions (Xu et al. 2022).

Ginkgoopsida and conifers account for a small percentage of the Zilanba flora. Extant Ginkgoales are deciduous and limited mainly to subtropical valley floors and lower montane slopes, preferring cool conditions with moderate humidity (Deng 2007; Tang et al. 2012). However, Mesozoic Ginkgoales are abundant in coal-bearing strata over a much broader geographic range indicating much wider climatic and edaphic tolerances than the single extant species (Abbink et al. 2004; Deng 2007). Deciduousness appears to be a universal character within the clade suggesting a preference for moderate to strongly seasonal climates. Ginkgoales were secondary contributors to Triassic and Jurassic coal deposits in Europe and Greenland, co-occurring with abundant Bennettitales and ferns (Popa 2014), and they played a similar role in China along with other regions along the northern fringe of the Tethys Ocean (Sadovnikov 2020). Therefore, the presence of Ginkgoites at Zilanba is considered to denote a humid climate with seasonal variations, albeit that only a single specimen is available.

Extant conifers are widely distributed and adapted to various climatic conditions (Deng 2007). Some Mesozoic conifers bear close ecological resemblances to extant forms (Deng 2007). Among the new material from Zilanba, Podozamites is considered to be an indicator of humid climates owing to its broad leaves, and Storgaardia mentoukouensis is considered indicative of a warm-temperate to temperate climate (Li Q.J. et al. 2017).

The Guangyuan area was located in tropical-subtropical latitudes during the Late Triassic (early Rhaetian), consistent with a humid and warm palaeoclimate inferred from the majority of the leaf fossils. The occurrence of a fossil wood taxon Xenoxylon guangyuanense Tian, Wang et Philippe in ‘member Ⅱ’ of the Xujiahe Formation has been argued to indicate a short-term cooling event in the Sichuan Basin during the Norian-Rhaetian interval (Tian et al. 2016) but the climatic significance of this wood taxon with ambiguous affinities remains to be fully resolved. Collectively, the fossil data suggest that the palaeoclimate during deposition of the Xujiahe Formation was predominantly warm and humid but may have been punctuated by short-term cooling events. In general, the precisely dated Xujiahe flora at Zilanba provides a key datum for further palaeoclimate reconstructions for the Norian to Rhaetian interval in the lead-up to the end-Triassic biotic crisis.

Palaeoenvironmental significance

The upper part of ‘member Ⅲ’ of the Xujiahe Formation at Zilanba is well-exposed and yields abundant fossil plants. It is lithologically dominated by a grey-dark mudstone intercalated with coal seams and grey fine-gained sandstones, typical of organic-rich (peat-forming) paludal conditions with sporadic flooding events. Cross-bedding and large fossil transported trunks in sandstones indicate frequent crevasse-splay or local fluvial channels intersecting the mire environments representative of ‘member Ⅲ’ (Fig. 6).

Of the fossil material from ‘member Ⅲ’ at Zilanba, cycads, bennettites ferns and sphenopsids are most commonly preserved in dark-grey to black, silty mudstone, with layer seven (Fig. 1), a dark-grey silty mudstone, the richest in fossils. Most plant fossils are exquisitely preserved indicating hypautochthonous burial. Among these fossils, the occurrence of Anthrophyopsis indicates typical lakeside wetlands, delta plains and wet fluvial flood plain conditions. Most conifers (predominantly isolated fragmentary leaves of Podozamites) are preserved in dark-grey fine sandstones, indicating a modest degree of transport before burial.

In summary, the sedimentary environment during the deposition of ‘member Ⅲ’ of the Xujiahe Formation at Zilanba was a fluvial flood plain with local peat swamp development. The plant assemblage represents a lowland community with only minor contributors from plants growing on better-drained sites.

The new early Rhaetian fossil assemblage collected from ‘member III’ of the Xujiahe Formation at the new locality, Zilanba, Guangyuan area, northern Sichuan Basin, includes 31 species within 18 genera. The flora from the palaeo-subtropics is diverse and dominated by ferns, cycads, bennettites, and incorporates a range of other equisetalean and gymnosperm groups. The floral composition indicates a generally humid and warm climate with limited seasonality during the early Rhaetian in the Sichuan Basin.

Data availability statement

The dataset (all specimens) analyzed during the current study are available in the fossil herbarium of Nanjing Institute of Geology and Palaeotology, Chinese Academy of Science, Nanjing, China (NIGPAS), and open for public accessibility. If there is any difficulty for accessing, the corresponding author is willing to help. The serial number of every specimen was listed in the manuscript. http://bbg.nigpas.ac.cn/#/index.

Acknowledgments

We thank Mr. Huaxin Zhang, Mr. Yaoyu Li (Southwest Petroleum University), Dr. Xiaoqing Zhang and Mr. Hongyu Chen (NIGPAS) for support during the field work. This study is financially supported by the National Natural Science Foundation of China (grant numbers 42072009, 41972120, 42172129), the Stratigraphic Priority Program (B) of the Chinese Academy of Sciences (grant number XDB 2610302), the Programs from State Key Lab of Palaeobiology and Stratigraphy (20191103, 173131, 213112), and CSC (grant number 202204910396). S.M. is supported by grants from the Swedish Research Council (VR grant numbers 2018-04527 and 2022-03920).

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A new Rhaetian plant assemblage from Zilanba, the northern Sichuan Basin, South China

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