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Research article Journal of the Geological Society Published Online First https://doi.org/10.1144/jgs2020-043
Brachiopod-dominated communities and depositional environment of the Guanshan Konservat-Lagerstätte, eastern Yunnan, China
Feiyang Chen1,2, Glenn A. Brock1,2, Zhiliang Zhang1,2, Brittany Laing2,3, Xinyi Ren1 and Zhifei Zhang1* 1 State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology, Northwest University, Xi’an, 710069, China 2 Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia 3 Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada FC, 0000-0001-8994-4187; GAB, 0000-0002-2277-7350; Z-LZ, 0000-0003-2296-5973; BL, 0000-0002-0874-8879; Z-FZ, 0000-0003-0325-5116 * Correspondence: [email protected]; [email protected]
Abstract: The Guanshan Biota is an unusual early Cambrian Konservat-Lagerstätte from China and is distinguished from all other exceptionally preserved Cambrian biotas by the dominance of brachiopods and a relatively shallow depositional environment. However, the faunal composition, overturn and sedimentology associated with the Guanshan Biota are poorly understood. This study, based on collections through the best-exposed succession of the basal Wulongqing Formation at the Shijiangjun section, Wuding County, eastern Yunnan, China recovered six major animal groups with soft tissue preservation; brachiopods vastly outnumbered all other groups. Brachiopods quickly replace arthropods as the dominant fauna following a transgression at the base of the Wulongqing Formation. A transition from a botsfordiid-, eoobolid- and acrotretid- to an acrotheloid-dominated brachiopod assemblage occurs up-section. Four episodically repeated lithofacies reveal a relatively low- energy, offshore to lower shoreface sedimentary environment at the Shijiangjun section, which is very different from the Wulongqing Formation in the Malong and Kunming areas. Multiple event flows and rapid obrution are responsible for faunal overturn and fluctuation through the section. A detailed lithofacies and palaeontological investigation of this section provides a better understanding of the processes and drivers of faunal overturn during the later phase of the Cambrian Explosion. Supplementary material: Composition and comparison of the Malong Fauna and the Guanshan Biota is are available at: https://doi.org/10.6084/m9.figshare.c.5080799 Received 10 March 2020; revised 29 June 2020; accepted 2 August 2020
Discoveries of spectacular soft-bodied animal assemblages from other Cambrian Konservat-Lagerstätten, which are dominated (in Cambrian Konservat-Lagerstätten around the world have provided terms of diversity and relative abundance) by euarthropod groups. incredible insights into the anatomy, behaviour, ecology and early Faunal overturn between the Chengjiang, Malong and Guanshan evolution of complex metazoans (Paterson et al. 2011; Hu et al. biotas suggests that the sessile benthic members of the assemblages 2013; O’Brien and Caron 2016; Aria and Caron 2017; Hou et al. are affected by the same factors that affect mobile trilobites (Luo 2017; Yang et al. 2018; Liu et al. 2020; Z.F. Zhang et al. 2020a). et al. 2008; Chen et al. 2019). Furthermore, the Wulongqing Early Cambrian Konservat-Lagerstätten from China – such as the Formation is characterized by bioturbated, thinly bedded sand- Niutitang Fauna, Chengjiang Biota, Guanshan Biota, Shipai Biota, stones, siltstones and mudstones, which crop out widely in eastern Balang Fauna, Kaili Biota and the newly discovered Qingjiang Yunnan, South China (Fig. 1) and represent a transgressive systems Biota (Peng et al. 2005; Zhao et al. 2005; Hu et al. 2013; J. Liu et al. tract directly after the Hongjingshao Formation (Hu et al. 2010). 2016; Hou et al. 2017; Fu et al. 2019) – span a wide range of Previous, very generalized, sedimentological work on the geological time and provide a unique opportunity to map changes in Wulongqing Formation suggests a relative shallow (shoreface to early Cambrian ecological communities over time (Chen et al. offshore transitional) depositional environment (Hu et al. 2010; 2019). The Guanshan Biota (Cambrian Series 2, Stage 4) Chen et al. 2019), which is distinct from the generally deeper water supplementary material, one of the oldest Konservat-Lagerstätten (in some cases slope to basin) setting of most other early Cambrian from South China, occurs in the Wulongqing Formation (Hu et al. deposits that preserve soft tissues (Ivantsov et al. 2005; Collom et al. 2013) in eastern Yunnan. Younger than the famous Chengjiang and 2009; Peel and Ineson 2011). Malong biotas (Cambrian Series 2, Stage 3), but older than the Kaili Continuous exploration and research in the Guanshan Biota has and Burgess Shale biotas (Miaolingian Series, Wuliuan Stage), the led to the discovery of multiple new localities and increased Guanshan Biota is a significant evolutionary bridge in our systematic descriptions of the fossil taxa (Hu et al. 2010, 2013; understanding of the chronology of the Cambrian radiation and its J. Liu et al. 2012a, 2016; Hopkins et al. 2017; Li et al. 2017; Chen aftermath (J. Liu et al. 2012a; Hu et al. 2013). Recent intensive, et al. 2019)(Fig. 1), including documentation of one of the oldest although preliminary, excavations reveal that the Guanshan Biota is examples of kleptoparasitism in the fossil record (Z.F. Zhang et al. composed of 14 major animal groups and various ichnotaxa (Hu 2020a). The Wulongqing Formation is generally poorly exposed at et al. 2010, 2013; Chen et al. 2019). Uniquely, the Guanshan Biota most sites and artificial cover by urban landscaping has obscured is dominated by brachiopods, which serves to distinguish it from all many of the classic flat-lying sites. There has been a dearth of even
© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
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Fig. 1. Localities of the Guanshan Biota and distribution of lower Cambrian outcrops in eastern Yunnan, South China. The Shijiangjun section in Wuding county is represented by locality 1. basic ecological analyses of the faunal assemblages from the The uppermost Hongjingshao and lower Wulongqing formations are Guanshan Biota, and the detailed sedimentology and lithology of exposed in the new section with a very clear conformable stratigraphic the succession are very poorly resolved. contact (Figs 2 and 3). This provides an opportunity to document This paper aims to comprehensively document the lithofacies and temporal changes in the faunal composition and sedimentary sedimentology of the basal part of the Wulongqing Formation hosting environments at the centimetre scale based on lithological, soft-bodied fossils at the Shijiangjun section, the best-exposed sedimentological, palaeontological and ichnological evidence. This succession in Wuding county, eastern Yunnan (Fig. 1). These data detailed study enables an interpretation of the depositional environ- will help to decipher the relationships between microfacies, ment associated with the lower Wulongqing Formation and facilitates sedimentary events and faunal overturn after transgression and how a better resolution of the process and drivers of faunal overturn that fluctuations in depositional environments affect the faunal compos- distinguish the Guanshan faunas from the Wuding, Malong and ition during the later stages of the Cambrian evolutionary radiation. Kunming areas (Hu et al. 2013; Chen et al. 2019). Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
Guanshan Konservat-Lagerstätte, eastern Yunnan
Fig. 2. Stratigraphic column, sedimentology, facies, structures, bioturbation index and pie charts of relative faunal abundance in the lower Wulongqing Formation at the Shijiangjun section. The 41 rock samples taken for cutting and polishing are marked on the left-hand side of the column. The datum point (0 cm) is the boundary between the upper Hongjingshao and lower Wulongqing formations. Four facies (F1, F2, F3 and F4) were recognized. The bioturbation index for each polished rock samples was evaluated based on the extent to which bioturbation disrupted the primary bedding. The composition of the fossil assemblage is shown by pie charts at the phylum level and the brachiopod genus level. Ass., assemblages; BI, bioturbation index; F., facies. HJS, Hongjingshao Formation; Sam., sample number; WLQ, Wulongqing Formation.
Materials and methods four-week field season sequentially and independently from ten contiguous siltstone and mudstone layers varying in thickness from The Shijiangjun section (25° 35′ 11″ N, 102° 22′ 22″ E) was 6 to 110 cm (Fig. 2; Table 1). Whole fossils were identified and measured through the uppermost Hongjingshao and lower classified to the phylum level and, where applicable, brachiopod Wulongqing formations and large-scale sedimentary features were genera were identified. Faunal relative abundances are based on all noted. A total of 2988 fossil specimens were collected in one the well-preserved fossils, whereas trace fossils and fragmentary and Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
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Fig. 2. Continued. unidentifiable specimens, as well as all shell concentrations, were sedimentary structures. Scanning of the polished slabs was achieved excluded. using an Epson V370 photo-scanner at Macquarie University, Lithological samples (n = 41) in oriented plaster jackets were Australia. Following the methodology outlined by Dorador et al. collected at intervals from mudstone and sandstone layers through (2014) and Dorador and Rodríguez-Tovar (2018), Adobe the section (Fig. 2; Table 2). All the samples collected for rock slabs Photoshop was used to digitally improve the visibility (contrast) and thin sections were cut and polished at the Shaanxi Key of the sedimentary and ichnological structures. Sedimentary Laboratory of Early Life and Environments, China and revealed the characteristics, including grain size, lithology, sedimentary struc- vertical internal organization of the physical and biogenic tures and vertical bioturbation intensity were recorded (Fig. 2; Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
Guanshan Konservat-Lagerstätte, eastern Yunnan
Fig. 3. Field photographs of the lower Wulongqing Formation at the Shijiangjun section. Yellow upper case letters mark the layers yielding a fossil assemblage in accordance with Figure 2.(a) General view of the lower part of the section. The yellow line on the bottom of the section indicates the lithological contact between the Hongjingshao and Wulongqing formations. (b) Load casts at the bottom of the sandstone deposits. (c) Wavy bedding structure above layer C. (d) Normal graded bedding from fossil-yielding layer D, scale bar: 1 cm. (e) Gutter casts, lenticular bedding and wavy ripples at the Shijiangjun section. (f) Plan view of gutter casts from fossil-yielding layer F. (g) The simplified palaeoenvironmental reconstruction for the Guanshan Biota from Wuding area. HJS, Hongjingshao Formation; WLQ, Wulongqing Formation.
Table 2). The percentage bioturbation in each sample was evaluated These percentages were then used within the bioturbation index (BI) using Adobe Photoshop (Cao et al. 2015; Gougeon et al. 2018). The scheme of Taylor and Goldring (1993). All the rock samples and bioturbation area was selected using the lasso tool and recorded fossil specimens investigated are deposited in the Early Life through the measurement log in pixels. This was then divided by the Institute (ELI) and the Department of Geology, Northwest total area in pixels to determine the percentage of bioturbation. University, Xi’an China. Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
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Table 1. Relative abundance and diversity of fossil taxa collected from the lower Wulongqing Formation at the Shijiangjun section, Wuding county
Animal groups Taxa A B C D E F G H I J Total Brachiopoda Neobolus 16 96 238 23 48 386 3 2 3 815 2030 Eoobolus 16 49 63 54 30 155 109 70 25 16 587 Westonia 4 9 5 192 93 39 64 147 553 Linnarssonia 12 43 3 3 61 Kutorgina 88 Nisusia 516 Arthropoda Trilobites 72 260 80 2 1 87 4 33 34 573 699 Guangweicaris 931 3 43 Panlongia 2 226 Isoxys 2 14 1 13 1 1 7 39 Tuzoia 1 1 237 bradoriids 14 15 29 Leanchoilia 112 Hyolitha Linevitus 1 12 142 25 6 7 24 7 3 227 227 Anomalocaridids 112 2 Vetulicola 2 4 1 1 8 8 Priapulida Palaeoscolecidans 1 2 6 4 1 8 22 22 Total 132 512 530 111 91 748 318 139 174 233 2988
Results ﬂows (Bouma 1962; Kneller 1995). The medium maturity of the sand/silt laminations probably indicates a certain degree of Geological setting, locality and section winnowing and transportation. The stratigraphic section is 8 m thick and composed of distinctive Interpretation. The interbedded mudstone and sandstone reflect intercalated beds of thin to thick (5–60 cm), very fine to very coarse an alternation of quiet water sediment fallout (low energy) sandstone, siltstone and mudstone (Fig. 3a). Rare gravels and combined with relatively high-energy flows (Buatois et al. 2012; isolated pebbles occur in sandstone samples S2, S3, S4, S5, S6, S9 Majid et al. 2017). and S16, in addition to two layers of purple muddy medium to coarse sandstone (S15 and S16), which contained 3–5% oolite Facies 2: Silty mudstones grains (Fig. 2; Table 2). Commonly developed primary sedimentary Facies 2 is represented by uniform mudstones with occasional structures include massive bedding, normal graded bedding millimetre-scale silt laminations (≤1mm)(Fig. 4c). The silt grains are (Fig. 3d), lenticular bedding and wavy bedding (Figs 2, 3c, e). moderately sorted, angular to subrounded (low content) and of low The contacts between the sandstones and mudstones are sharp. The sphericity (Fig. 5c). Interestingly, the M10 layer (Fig. 2) contains a most common local erosion structures include gutter casts, erosional higher concentration of muscovite than any other layer. Fragmentary scour and low ripple marks (Figs 2, 3b, e, f). The measured section shelly fossils are often present and are preserved parallel or oblique to has an overall low level of bioturbation, with some highly bedding, with a particularly high concentration of trilobite fragments bioturbated beds occurring in the middle part of the section (3.3– documented in layer M25. Bioturbation is rare (BI = 0), with the 5.3 m) accompanying the only identified trace fossil Teichichnus? percentage bioturbation never exceeding 1% (Fig. 2; Table 2). isp. (Fig. 2). Based on lithological, sedimentary and ichnological The absence of rheological surfaces on the silty mudstone features, the section is divided into four distinct facies that repeat packages indicates a relatively low-energy hydrodynamic system and cycle throughout the section, as shown in Figures 2–5. (Zavala et al. 2012). Abundant sub-parallel to oblique brachiopod and/or trilobite fragments within the mudstone indicate transporta- Lithofacies identification and interpretation tion by currents (Fürsich et al. 1992). Facies 1: low–medium bioturbated and interbedded Interpretation. High rates of fallout or other unobservable environmental stressors (e.g. oxygen, salinity or temperature) may mudstones–siltstones/sandstones be responsible for the relative absence of bioturbation. As a result, Facies 1 consists of thinly bedded mudstone with thin to thick the relatively structureless silty mudstone packages are interpreted laminated siltstone and/or very fine sandstone (Figs 4a, b and 5a). as deposited from rapid fallout from suspension during quiet periods The silt and sand grains are medium to well-sorted, mainly angular of fair weather conditions (Maceachern et al. 1999). to subrounded, low to high sphericity with increasing sphericity up- section (Fig. 5a). Fine to medium sandstone intercalations occur as lenticular and wavy bedding (Fig. 4a). Laterally discontinuous Facies 3: low to highly bioturbated glauconitic sandstones millimetre-scale (mainly 3–5 mm with some c. 1 mm) silt Facies 3 consists of very fine to very coarse sandstone with rare laminations are common. Graded laminations (4–10 mm) manifest granule- to pebble-sized clasts (Fig. 4d–g). The granules and either as a sharp horizontal contact or an erosional base (sole marks) pebbles predominately occur in samples S2–S6, S9 and S16 (Figs 2 (Figs 4b and 5b). The contact between sand and mud is nearly and 4e; Table 2). The medium- to very coarse-grained sand beds always sharp. Bioturbation is generally indistinct and unidentifiable, from the lower and upper part of this section are characterized by with Teichichnus? isp. documented in two samples (Fig. 2). The very poorly to poorly sorted grains distributed within the intervals bioturbation index ranges from 0 to 3, with a predominant index of 0–2.1 m and 4.6–5.0 m (Figs 4d, e and 5d, g). Coarse grains are 0–1 (up to 4.89% disturbance). More heavily bioturbated beds exist mainly angular to subrounded and dominated by low to medium locally (M5 and M20) with indexes of 2–3 recording up to 40% sphericity (Fig. 5d, g). Although the very fine- to medium-grained sedimentary fabric disturbance (Fig. 2; Table 2). The graded sand beds from interval 2.2–4.2 m are mainly moderately sorted laminations and erosive bases suggest deposition from decelerating (Fig. 4f), few beds show medium to high sphericity. Two beds (S15 Table 2. Summary of lithology, sedimentary structures, taphonomic characters, fossil concentrations, bioturbation index and occurrence of soft tissues observed in the Wulonqging Formation from the Shijiangjun section Downloaded from Sample Sample thickness Bioturbation Soft number (mm) Lithology Sedimentary structures Taphonomy Fossil concentrations index tissue M25 40 Yellowish green silty mudstone Parallel laminations, discontinuous Abundant fragments are sub-parallel, A large number of fossil fragments are oblique to 0 silt laminations parallel or oblique to the bedding the bedding plane plane
M24 21 Yellowish green mudstone intercalated with Weakly normal graded lamination, Few fossil (fragments) are parallel to Trilobite fragments concentration, high 0 http://jgs.lyellcollection.org/ brick red silty–very fine sandstone, sand grains sole marks, wavy lamination, sub-parallel to the bedding plane fragmentation are moderately sorted, angular to sub-angular, erosion bottom surface, top and within mud background few subrounded grains, low–medium bottom boundary of sandstone are sphericity sharp M23 39 Greyish green mudstone intercalated with brick Weakly normal graded lamination, Few fossils (brachiopods and 1×
red silty–very fine (feldspathic) sandstone sole marks, wavy lamination unknown) are sub-parallel to oblique Yunnan eastern Konservat-Lagerstätte, Guanshan to the bedding plane within mud background M22 27 Purple silty mudstone, some spicular mineral Massive bedding No fossils 0 M21 49 Yellowish green silty mudstone Tiny sole marks, parallel lamination Many fossils (brachiopods and Westonia concentration, shells within mudstone 0 √ (silt), massive mudstone bed unknown fragments) are parallel to are low fragmentation, random orientation, sub-parallel to the bedding plane poorly sorted, parallel–oblique to the bedding byguestonOctober1,2021 plane M20 25 Lavender mudstone interbedded with purple silt– Massive bedding (bioturbated) Few fossils (brachiopods and Eoobolus concentration, Westonia concentration 3 √ very fine sandstone, sand grains are well- unknown) are parallel to sub-parallel sorted, angular to subrounded, medium–high to the bedding plane within mud sphericity background, one shell concave-up S16 40 Purple, muddy, very coarse sandstone, c. 2% Massive bedding 0× granules (up to 3 mm), 1% ooid, grains very poorly sorted, angular to subrounded, low– medium sphericity, grains’ long axis are usually vertical–oblique to bedding plane; grains are spherical, equant, square, triangle, rod-like and irregular shape S15 35 Purple matrix-supported muddy medium–coarse Truncated and scoured tops of mud Shell fragments distributed close to the 2 sandstone, 3–5% oolite, 1– 2% glauconite, with parallel oolite (elongated– boundary between mud and sand grains are moderately sorted, angular to sub- rounded), mud clasts angular, low sphericity; grains are square, triangular, rod-like and irregular shape; glauconite grains are subrounded S14 32 Greyish green muddy glauconitic sandstone, Massive bedding (bioturbated), rip 4 grain size from fine to coarse, 3% coarse up clasts (microbial mats?) glauconite grains, grains are moderately sorted, angular to sub-angular, low sphericity, triangular, square, round, tabular, rod-like shapes M19 46 Greyish mudstone interbedded with brick red silt Normal graded bedding, lenticular Few fossil (brachiopods and unknown) 0 √ and very fine sandstone lamination, sole marks are parallel to sub-parallel to the bedding plane within mud background (continued) Table 2. (Continued)
Sample Downloaded from Sample thickness Bioturbation Soft number (mm) Lithology Sedimentary structures Taphonomy Fossil concentrations index tissue S13 50 Brick red very fine–fine sandstone interbedded Wavy lamination, lenticular 3× with greyish-green mudstone, 1–2% medium lamination, massive bedding glauconite, elongate minerals, feldspathic (bioturbated) sandstone, grains are moderately sorted, angular to subrounded, low–medium sphericity, triangular, square, round, tabular, http://jgs.lyellcollection.org/ rod-like shapes M18 42 Greyish green mudstone interbedded with silt– Weakly normal graded bedding, Few fossil (brachiopods and unknown) Eoobolus concentration, Westonia concentration 0 very fine sandstone, grains are moderately lenticular bedding (very fine are parallel–oblique to the bedding sorted, angular to subrounded, medium sandstone), sole marks plane within mud background, one sphericity shell concave-down S12 45 Yellowish green muddy very fine–fine Massive bedding (bioturbated) 5× sandstone, 1% medium-grained glauconite, grains are moderately sorted, angular to subrounded, high sphericity S11 50 Brick red muddy very fine sandstone, few fine– Massive bedding (bioturbated) 4 medium sand grains, few glauconite grains, byguestonOctober1,2021 grains are well-sorted, high sphericity, most of Chen F. them are sub-angular M17 35 Yellowish green, thin-bedded silty mudstone Lenticular lamination (very fine Few fossils (conjoined brachiopod Westonia and Eoobolus and Neobolus 0 √
with very fine sandstone sand), tiny sole marks, low angle valves and unknown) are parallel or concentration, shells are random orientation, al. et wavy lamination (non-parallel) oblique to the bedding plane within low fragmentation, poorly sorted (1–12 mm), mud background parallel to sub-parallel to the bedding plane, conjoined; few Neobolus preserved with tubeworm M16 36 Yellowish green mudstone Discontinuous, parallel silt Several fossils are parallel to the Larval or juvenile Neobolus concentration 0 laminations bedding plane distributed within the mud background M15 25 Yellowish green mudstone intercalated with grey Lenticular lamination, sole marks Few fossil (brachiopods and unknown) Westonia concentration, larval or juvenile 1 siltstone and very fine sandstone, sand grains are parallel to sub-parallel to the Neobolus concentration, shells are low are well-sorted, medium sphericity, angular to bedding plane within mud fragmentation, parallel to the bedding plane, subrounded, tabular grains are usually parallel background poorly sorted (1–13 mm), random orientation, to bedding, few oblique to the bedding plane conjoined M14 43 Greyish green mudstone interbedded with brick Normal graded lamination, Fossils are parallel to sub-parallel to the Westonia concentration, Neobolus concentration, 0 red siltstone lenticular bedding, parallel bedding plane, a few shells concave- shells are parallel to the plane, poorly sorted, lamination, sole marks down most are conjoined and a few are disarticulated M13 45 Greyish green, very thin-bedded mudstone Normal graded lamination, A few fossils are parallel to the bedding Neobolus concentration, shells are low 0 interbedded with very fine sandstone and lenticular bedding, sole marks plane fragmentation, parallel to the bedding plane, siltstone conjoined, poorly sorted (0.5–4 mm), random orientation S10 65 Brick red, thin-bedded, very fine sandstone, few Massive bedding, mud clasts? 0× glauconite grains, grains are well-sorted, angular–rounded, medium–high sphericity M12 26 Dark green, thin-bedded mudstone intercalated Normal graded lamination One fossil oblique to the bedding plane 0 × with medium laminated grey siltstone S9 48 Brick red, muddy, fine–medium sandstone, Massive bedding (bioturbated?) 0×
grains are poorly sorted, angular to Downloaded from subrounded, low–medium sphericity; coarse grains up to c. 10%, granules are c. 1–2%, quartz grains > dark minerals > feldspar grains > lithic fragments; spherical, rod-like, triangular, square and irregular shapes M11 35 Yellowish green silty mudstone, few fine Massive bedding Few fossils (brachiopod and trilobites? Neobolus and Eoobolus and Westonia and 0× sandstone, grains are poorly sorted, angular to hyoliths, unknown rounded?) are hyoliths and Redlichia and Palaeolenus
sub-angular, low sphericity; spherical, square, parallel to the bedding plane fragment concentration, fragments are poorly http://jgs.lyellcollection.org/ tabular, irregular shapes sorted, oblique to perpendicular to the bedding plane S8 30 Brick red, medium sandstone, few very coarse Weakly normal graded lamination 0× quartz grains, poorly sorted, angular–sub- angular, few subrounded, low sphericity, grains, long axis are usually oblique–parallel to Yunnan eastern Konservat-Lagerstätte, Guanshan the bedding plane, quartz > black minerals > feldspar S7 35 Brick red, medium–coarse sandstone, grains are Weakly normal graded lamination, 1 poorly sorted, angular to subrounded, low– sole marks? medium sphericity, spherical, tabular, rod-like shapes byguestonOctober1,2021 S6 45 Brick red, coarse–very coarse sandstone, few Massive bedding 0 granules, grains are poorly–very poorly sorted, angular to subrounded, low sphericity; quartz, feldspar, lithic fragment, dark minerals S5 35 Brick red coarse sandstone, granules up to 10– Sharp boundary, massive 0 20%, small pebbles up to 5%, very poorly lamination/bedding sorted, sub-angular–rounded, medium sphericity, spherical and bladed shapes; the lower part is thin very fine–fine sandstone (very coarse sand grains up to 5%, mud up to 5%) and mudstone M10 25 Greyish green silty mudstone, silt grains are dark Massive bedding Few fossils (round, flat, crescent- Trilobite fragment concentration 0 √ minerals (spherical, tabular, bladed and rod- shaped, irregular) are sub-parallel to like shapes), parallel–oblique to bedding oblique to the bedding plane plane, few are vertical, angular–subrounded (low content), low sphericity, higher content of white mica than other beds M9 33 Greyish green silty mudstone, silt grains are dark Massive bedding Few fossils are sub-parallel to oblique Neobolus and Redlichia and Palaeolenus and 0 minerals (spherical, tabular, bladed and rod- to bedding plane hyoliths fragment concentration like shapes), parallel–oblique to bedding plane, angular–subrounded (low content), low sphericity M8 35 Yellowish green mudstone intercalated with Weakly normal graded bedding, Many fossils (brachiopod shells and Neobolus and Redlichia and Palaeolenus and 0 white siltstone, silt grains are dark minerals parallel lamination, lenticular unknown fragments) are preserved hyoliths fragment concentration (spherical, tabular, bladed and rod-like lamination, tiny sole marks parallel–sub-parallel to the bedding shapes), parallel–oblique to bedding plane, plane angular–subrounded (low content), low sphericity (continued) Table 2. (Continued)
Sample Sample thickness Bioturbation Soft Downloaded from number (mm) Lithology Sedimentary structures Taphonomy Fossil concentrations index tissue S4 40 Dark green fine–medium sandstone, few Massive bedding (sandstone), sole 0× granules, grains are poorly sorted, angular to mark, sharp boundary subrounded, most of them are angular to sub- angular; low sphericity; c. 90–95% are quartz grains, followed by feldspar and others
M7 30 Yellowish thin-bedded mudstone Weakly normal graded bedding Many fossils (one conjoined and many Palaeolenus fragment concentration 0 √ http://jgs.lyellcollection.org/ dorsal or ventral brachiopod shells and unknown fragments) are preserved parallel–sub-parallel to the bedding plane M6 40 Yellowish green mudstone Weakly normal graded bedding Plentiful fossils (brachiopod shells, Palaeolenus and Redlichia concentration, parallel 0 trilobites and unknown fragments) to the bedding plane, Palaeolenus low are preserved parallel to sub-parallel fragmentation to the bedding plane, few are oblique, few shells concave-up M5 50 Yellowish green mudstone interbedded with grey Lenticular lamination (fine Several fossils are preserved parallel– Palaeolenus and Redlichia fragment 2 fine sandstone, grains are medium sorted, the sandstone), sole marks sub-parallel to the bedding plane, concentration byguestonOctober1,2021
quartz grains are sub-angular to subrounded, few fossils are oblique to the bedding Chen F. low sphericity plane, two conjoined fossils M4 50 Yellowish green mudstone interbedded with grey Weakly normal graded bedding, Fossils are preserved parallel to sub- 1 fine–medium sandstone, poorly sorted, the wavy lamination, lenticular parallel to the bedding plane, few quartz grains are angular to subrounded, most lamination, sole marks shells concave-up al. et are angular to sub-angular, low sphericity S3 53 Greyish green medium–coarse sandstone, few Fining-upwards 0 granules, poorly sorted, the granules are angular to subrounded; low–medium sphericity; 95% are quartz grains, very few are detritus and feldspar S2 63 Purple granules, very coarse sandstone, some Massive bedding 0× interstitial mud layer on the top, some granules, grains are poorly sorted, the granules are sub- angular to subrounded; medium–high sphericity; c. 85–90% are quartz grains, and followed by detritus and feldspar M3 45 Greenish mudstone interbedded with medium Weakly normal graded lamination, Few fossils (on top close to the upper Linnarssonia concentration, low fragmentation, 0 √ grey fine sandstone, grains are poorly sorted, wavy lamination, lenticular sandstone) are parallel to the bedding poorly sorted angular–rounded, medium–high sphericity lamination (0.1–0.4 mm thick), plane within silt or clay, few shells sole marks concave-down M2 40 Greenish silty mudstone, unidentified spicules Tiny rip-up clasts Fossils are parallel to the bedding plane 0 (mineral or sponge?) M1 40 Yellowish mudstone interbedded with purple Lenticular lamination Fossils are sub-parallel to the bedding 1 fine–medium sandstone, grains are medium plane, few shells concave-down sorted, sub-angular–subrounded, low sphericity S1 55 Grey fine–medium sandstone, moderately sorted, Massive bedding 0 subrounded–well rounded, spherical Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
Guanshan Konservat-Lagerstätte, eastern Yunnan
Fig. 4. Polished slabs of the lower Wulongqing Formation at the Shijiangjun section, with facies classification and sample numbers in parentheses. (a, b) Slabs of Facies 1 showing wavy laminations, graded lamination, lenticular lamination and erosive base. (c) Silty mudstone without sedimentary structures representing Facies 2. (d–g) Massive sandstone deposits representing Facies 3. (d, e) Poorly sorted, angular to sub-angular clasts with few granules. (f, g) Highly bioturbated sandstone with glauconite grains. (h) Mudstone without structures (Facies 4). Scale bars 5 mm. and S16) contain 1–5% elongate ooids. Most of the ooids are oval and replaced by iron oxides (mostly hematite and goethite). These and few are rounded. grains occur in every sandstone interbed at relatively low contents The sandstone beds are either characterized by a homogeneous (Fig. 2; Table 2). The grains are usually medium sorted, subrounded uniform grain size or high bio-disturbance, which has destroyed the to rounded and of medium sphericity (Fig. 5e). Although glauconite original sedimentary structures. Only levels S7 and S8 show weakly cannot be used as a specific environmental indicator (Mcrae 1972; normal graded bedding. Sand beds S11–S15 show a relatively Chafetz and Reid 2000; Chafetz 2007), it is commonly associated higher content of mud and a higher percentage of bio-disturbance with transgressive systems tracts (Delamette 1989; Garzanti et al. (BI = 2–5) (Fig. 2; Table 2). The bioturbation index and bio- 1989; Amorosi et al. 2012; Banerjee et al. 2012, 2017; Rudmin disturbance reach a peak of BI = 5 and 98.76% within S12 (Figs 4f et al. 2017). Different types of glauconite (i.e. autochthonous, and 5f), followed by S11 (80.88%) and S14 (76.16%) (Fig. 4g). parautochthonous and detrital) can be determined based on the However, more than half of the sandstones below S11 show scarce criteria proposed by Amorosi (1997). The glauconite that usually or no bioturbation (Fig. 2; Table 2). occurs in detrital granular and sand facies lacks a diffuse green The occurrence of syngenetic glauconite grains within the pigmentation, which often alternates between glauconite-rich and sandstones of Facies 3 is unique (Fig. 4f,g;Table 2). These glauconite-free layers, and can be interpreted to indicate an grains were identified based on their green colour, random allochthonous (e.g. parautochthonous or detrital) origin (Amorosi microcrystalline internal texture and aggregate polarization 1997; Baioumy and Boulis 2012). By contrast, the low compos- (Baioumy and Boulis 2012). They are, in some instances, coated itional and structural maturity of Facies 3, as well as a lack of Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
F. Chen et al.
Fig. 5. Photomicrographs of thin sections from the lower Wulongqing Formation at the Shjiangjun section showing four lithofacies types. (a) Medium sorted irregular grains from Facies 1. (b) Graded laminations with an erosional base from Facies 1. (c) Mudstone with low content of well-sorted silt grains from Facies 2. (d) Poorly sorted grains with low sphericity from Facies 3. (e) Common glauconite grains within Facies 3; note the iron oxides within grains (black arrows). (f) Highly bioturbated sandstone from Facies 3. (g) Poorly sorted sandstone from Facies 3, coarse grains are irregular with low sphericity. (h) Uniform mudstone of Facies 4. All photomicrographs were taken with parallel light except (d), which is under cross-polarized light. Scale bars 1 mm.
glauconite in the older Hongjingshao Formation, implies a contacts at the lower and upper boundaries between the sandstones parautochthonous origin (Amorosi 1997; Baioumy and Boulis and mudstones show that each sandstone layer represents a single 2012), in which the autochthonous glauconites have been event. However, the changing grain size inside the thin sandstone transported a short distance from their original location by waves, units shows an unstable hydrodynamic environment. Facies 3 is storm currents and/or gravity ﬂow processes. interpreted to have been deposited within lower shoreface zone Local observations of Facies 3 show that these sandstones have a formed near the storm wave base and was affected by multiple low compositional and textural maturity, which suggests that the pulses of gravity flows. sediments were deposited with minimal traction and clast collisions from a proximal sediment source. Therefore the clasts retain their immature, angular texture (Henstra et al. 2016). Storm deposits are Facies 4: mudstones generally understood to consist of well-sorted sand with a fining- Facies 4 represents mudstones with occasional interbedded wisps of upwards sequence that reflects the waning storm waves (Swift et al. silt (Figs 4h and 5h). The mud layers are considerably thicker (2.5– 1983; Saito 1989; Duke et al. 1991; Cheel and Leckie 1993; Meldahl 3 cm) than in other facies (Fig. 5h). The silt laminations are fairly 1993). The storm ﬂow usually converts to a turbidity current as the thin (0.3–1 mm) with sharp erosive bases and a crudely micro- power of the storm ﬂow weakens near the storm wave base, resulting graded lower part and structureless upper part. Shelly fossils in the suspended mud and gravel depositing together with ﬁne preserved within Facies 4 are usually parallel to sub-parallel to the suspended sediments during recessive periods (X. Liu et al. 2012b). bedding plane. The bio-disturbance within Facies 4 is the lowest Interpretation. The common occurrence of poor bedding and among the four facies, only up to 0.15%, resulting in a low disordered accumulation indicate fairly rapid suspension fall out bioturbation index (BI = 0) (Fig. 2; Table 2). without winnowing (Stow 2005), probably affected by gravity flow Interpretation. These sedimentary features, along with the soft deposition in relatively deeper water (Wu et al. 2016). The sharp tissue preservation associated with Facies 4, suggest a mainly rapid Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
Guanshan Konservat-Lagerstätte, eastern Yunnan
Fig. 6. Exquisitely preserved soft-bodied fossils from the lower Wulongqing Formation at the Shijiangjun section. (a) Brachiopod Linnarssonia concentration from assemblage A (ELI-SJJ-001). (b) Trilobite Palaeolenus concentration from assemblage B (ELI-SJJ-002). (c, d) Brachiopods Neobolus and Westonia concentrations from assemblage F (ELI-SJJ-003, ELI-SJJ-003-2). (e) Brachiopod Neobolus with well-preserved parasitic tubeworms, indicated by arrows (assemblage B, ELI-SJJ-004). (f) Brachiopod Westonia preserved with mantle canals (assemblage F, ELI-SJJ-005). (g) Rare brachiopod Nisusia sp. (assemblage J, ELI-SJJ-006). (h) Well-preserved coiled palaeoscolecidan (assemblage F, ELI-SJJ-007). (i) Posterior part of an indeterminate vetulicolian (assemblage A, ELI-SJJ-008). ( j) Trilobite Palaeolenus preserved with the rare digestive system (assemblage B, ELI-SJJ-009). Scale bars: (a, e–h and j) 2 mm; (b–d, i) 1 cm. deposition (obrution) of suspended muds settling from weak storm specimens (Table 1; Fig. S1). Even though the anomalocaridiids, flows in a relatively low-energy environment (Zhu et al. 2001). vetulicolians and priapulids are rare in this section, they are very important elements of Cambrian Burgess Shale-type Lagerstätten Composition and relative abundance of fossil assemblages (Zhao et al. 2010; Paterson et al. 2011; Smith 2015). Four genera of organophosphatic brachiopods, including Neobolus, Eoobolus, Thousands of well-preserved fossils spanning six key animal groups Westonia (Luo et al. 2008), Linnarssonia (Duan et al. 2020) and (n = 2988) were collected from the lower Wulongqing Formation at two calcareous taxa (Kutorgina and Nisusia) occur throughout the the Shijiangjun section during one four-week field season. The taxa section. Neobolus is the most abundant genus (40.2%), followed by include brachiopods, arthropods, hyoliths, priapulids, vetulicolians Eoobolus (28.9%) and Westonia (27.2%). However, arthropods and anomalocaridiids in descending order of rank abundance remain the most diverse group, composed of trilobites, bradoriids, (Table 1). All these taxa are also found in the Wulongqing Guangweicaris, Panlongia, Isoxys, Tuzoia and Leanchoilia. Among Formation from the Kunming and Malong areas (Hu et al. 2013; these, trilobites are the most abundant taxon (82%) (Table 1). Chen et al. 2019). Brachiopods, arthropods and hyoliths form the Fossil data from every mudstone layer was obtained during four three main components, with up to 98.9% of the total number of weeks of intensive fieldwork in 2018 (Table 1). The fossil Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
F. Chen et al.
Fig. 7. Stratigraphic fluctuation in the relative abundance of the community at the (a) phylum level and (b) brachiopod genus level from the Shijiangjun section. composition within assemblages A and B is similar, consisting of C, whereas Neobolus and Eoobolus together exceed 97%. five animal groups, while faunal diversity decreases in assemblages Assemblages C and D are mainly composed of Eoobolus (20.5 C–F. This is followed by an increased diversity associated with and 62.8%, respectively) and Neobolus (77.3 and 26.7%, faunal assemblages G–J. Faunal assemblage I has the highest respectively) with minor Westonia. By contrast, Westonia reaches diversity, with almost all taxa known from the entire section a higher relative abundance (26.2%) in assemblage F. Eoobolus concentrated in this assemblage. Assemblage F has the greatest dominates assemblages G and H (52.4 and 64.2%, respectively), abundance of fossils (n = 748) accounting for 25% the total number where Westonia also reaches a higher proportion of the assemblage of individuals, followed by assemblages C, B, G and J (Fig. 2; (44.7% in G). Assemblages I and J are both dominated by Westonia, Table 1). with 61.5 and 88% relative abundance, respectively; Eoobolus (24 The relative abundance of individual specimens from ten and 9.6%, respectively) ranks second in these assemblages. The rare sampling layers was obtained (Fig. 2; Table 1) to gauge the calcareous brachiopods Kutorgina and Nisusia are restricted to the baseline assemblage structure. Assemblages A and B are dominated upper part of the section in assemblages I and J (Figs 6g and 7b). by arthropods, accounting for 63.6 and 59.8%, respectively. The lower part of the Wulongqing Formation (0–6 m) at the Brachiopods dominate all other assemblages from layers C–J, Shijiangjun section also contains distinctive brachiopod and with some fluctuation of composition in the relative abundance trilobite fossil concentrations (Fig. 2; Table 2). The concentrations between brachiopod taxa. The abundance of brachiopods reaches a preserved in coarser sandy deposits (especially Facies 3) are highly peak within assemblage F. Hyoliths, a common early Cambrian fragmented (although also fragile and thin) and moderately well- group, occur throughout the entire section, except for assemblage sorted, which indicates a relatively high level of energy and G. Anomalocaridiids, vetulicolians and priapulids are interspersed transportation (Table 2). Some well-preserved shell concentrations irregularly within the assemblages. are also preserved within thin mud beds (e.g. Facies 1 and 4), The relative abundance of six genera of brachiopods throughout occasionally restricted to single bedding planes, and in a relative the section is very instructive (Fig. 2; see also Fig. 7b). Assemblage sense these thin shells are characterized by low levels of A is composed, almost equally, of three genera (Neobolus, 36.4%; fragmentation, poor sorting, low to medium disarticulation, and Eoobolus, 36.4%; and Linnarssonia, 27.2%), whereas assemblage occur sub-parallel to bedding planes with a high ratio (>50%) of B contains a higher proportion of Neobolus (51.1%), with the conjoined brachiopod shells with more or less soft tissue relative abundance of the remaining two taxa 26.1 and 22.8%, preservation. These taphonomic proxies indicate a relatively rapid respectively. Westonia occurs as a small proportion of assemblage obrution deposit and minimal transportation (Figs 2 and 6a–d; Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
Guanshan Konservat-Lagerstätte, eastern Yunnan
Table 2). The shell concentrations from the Shijiangjun section are coarse deposits indicates a complex hydrodynamic environment, either monospecific or paucispecific, dominated by brachiopods or with less frequent waves and distal storms (Zhu et al. 2001; Buatois trilobites (Fig. 6a–d). These concentrations are nearly always and Mángano 2011; X. Liu et al. 2012; Michalík et al. 2013). restricted to specific layers. For example, abundant Palaeolenus are Periodic subaqueous gravity flows resulted in the deposition of exclusively found within layer M6 in assemblage B (Fig. 6b), distinctive centimetre-scale sandstone interbeds (Facies 3) at the whereas a concentration of Linnarssonia shells is known within Shijiangjun section. Hence the sedimentary environment of the layer M3 in assemblage A (Fig. 6a). The brachiopod concentrations lower Wulongqing Formation in the Wuding area is largely the from assemblage F are most abundant and mainly composed of result of fluctuating wave energy, distal storms and gravity flows. monospecific layers of Neobolus (Fig. 6c)orWestonia (Fig. 6d), The centimetre-scale conglomerates characterized by high respectively. The Eoobolus and Westonia shell concentrations sphericity reported from the Wulongqing Formation at Malong extend to the upper part of the section (Fig. 2). Throughout the and Kunming represent high-energy channels, probably proximal to section, brachiopod concentrations are completely restricted to the shoreface (Hu et al. 2010; Chen et al. 2019). The absence of Facies 1 and 2, whereas trilobite concentrations are mainly basal conglomerates and the occurrence of medium to very coarse associated with Facies 4, which is restricted to assemblage B sandstones with few granules at the base of the Wulongqing (Fig. 2). Formation in the Wuding area (Fig. 4e) suggest a relatively deeper Remarkable soft tissue preservation occurs in all assemblages and low-energy clastic sedimentary environment than that in the except D and E, demonstrating the high preservation potential Malong and Kunming areas (Hu et al. 2010; Chen et al. 2019), within facies at the Shijiangjun section of the Wulongqing although this remains to be tested because detailed continuous Formation in the Wuding area. Tube-dwelling organisms encrusting successions of the Wulongqing Formation have not been studied to Neobolus shells (see Z.F. Zhang et al. 2020a) (with exceptionally sedimentologically. Overall, the depositional environment here is preserved setae and soft viscera) are fairly common within the lower interpreted as offshore to lower shoreface as defined by Martin et al. part of the section within mudstone beds (layers A, B, C and F) (2016) and the offshore zone of Buatois and Mángano (2011), (Fig. 6e). Abundant specimens of Westonia display high-quality which slightly extends below the storm wave base (Fig. 3g). soft tissue preservation from assemblage F, including setal fringes and mantle canals (Fig. 6f). Palaeoscolecidan worms, as an Faunal overturn and assemblage composition important component of lower Paleozoic soft-bodied assemblages, were found throughout the section, except for assemblage C The baseline time series of the fossil data recovered from the lower (Fig. 6h). Relatively rare vetulicolians occur at the base and in the Wulongqing Formation at the Shijiangjun section reveals a unique upper part of the section (assemblages A, B, I and J) (Figs 2 and 6i). transition in the structure of the benthic community over time Anomalocaridiids are the rarest element in the section, only (Fig. 2; Table 1). The relative abundance of six key animal groups, preserved as isolated frontal appendages in assemblages I and including six brachiopod genera, from ten sampled layers J. The rare oldest known digestive system of trilobites (Hopkins demonstrates gradual replacement, overturn and fluctuation in the et al. 2017) have also been preserved in the Wuding area, but only in faunal composition (Figs 2 and 7). Although arthropods dominate assemblage B (Fig. 6j). the base (0–1.1 m) of the section (assemblages A and B), the proportion of brachiopods gradually increases, replacing arthropods as the dominant fauna in assemblages C–J, reaching peak Discussion abundance (97.99%) within assemblage F (Figs 2 and 7a). Depositional environment Although there is a fluctuation in the relative abundance of brachiopods through assemblages G–J(c. 60–80%), arthropods Heterolithic successions consisting of sandstone beds interbedded maintain a relatively low, but stable, percentage. with mudstones are usually deposited below the fair weather wave There is no doubt that trilobites dominated early Cambrian base and above the storm wave base (Dott and Bourgeois 1982; benthic communities in terms of diversity and abundance, which is Myrow and Southard 1996; Dumas and Arnott 2006; Bullimore demonstrated well in the older Chengjiang Lagerstätte (Zhao et al. et al. 2008; Buatois and Mángano 2011; Eide et al. 2015). These 2010; Hou et al. 2017; Paterson et al. 2019) and the Malong Fauna beds are commonly described as tabular (Elliott 1978; Coe et al. (Luo et al. 2008; Chen et al. 2019). The latter is characterized by 2003) and often show abundant erosive gutter casts (Eide et al. extremely abundant and diverse trilobites yielding from the 2015). The alternation of mudstone (Facies 1, 2 and 4) and underlying Hongjingshao Formation (Fig. S1; Table S1). sandstone (Facies 3) layers – in addition to graded lamination/ However, detailed fossil data from the Guanshan Biota in bedding, wavy bedding, ripple marks and gutter casts from the Wuding and Malong areas reveals a community structure that is Shijiangjun section – suggests a depositional environment close to unique for early Cambrian Konservat-Lagerstätten, with brachio- the storm wave base, which underwent multiple depositional events pods dominating the benthic community in abundance, if not and episodic cycles (Shanmugam 2002; Hu et al. 2010; Buatois and diversity, and often forming distinctive concentrations of shell Mángano 2011). beds in the lower Cambrian Stage 4 of the Wulongqing Formation Previous studies have interpreted the sedimentary environment (Figs 2 and 7a; Fig. S1). The ecological transition from trilobite- associated with the Guanshan Biota as mainly offshore transition to brachiopod-dominated communities occurs widely across with common storm events (Hu et al. 2010; Chen et al. 2019), shallow marine clastic environments across the South China which is comparable with the Cambrian Stage 4 Emu Bay Shale Platform (Fig. 1), coinciding with well-documented transgression from Australia (Paterson et al. 2016) and the Ordovician Fezouata events during Cambrian Age 4 (Luo et al. 2008; Hu et al. 2010, Biota (Martin et al. 2016). However, typical storm-generated 2013; Chen et al. 2019). Thus organophosphatic brachiopods structures such as hummocky cross-stratification, an indicator of diversify and become superabundant across the broad ‘shallow’ oscillatory combined flows reflecting deposition under high-energy shelf of the Yangtze Platform during the final stage of the storm conditions (Arnott and Southard 1990; Cheel 1990; Southard Cambrian Explosion (Z.F. Zhang et al. 2020a). The rise of et al. 1990; Cheel and Leckie 1993; Yokokawa et al. 1999; Dumas organophosphatic brachiopods as the numerically dominant et al. 2005) are absent in the Wuding succession. element in the lower Cambrian Stage 4 Wulongqing Formation The occurrence of erosive bases, ripple marks, wavy bedding, is the oldest brachiopod-dominated soft substrate community fine-graded bedding, gutter casts and multiple massive fine to known in the fossil record and represents a precursor to more Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
F. Chen et al. complex community tiering and brachiopod-dominant benthic preservational trap that typifies the Burgess Shale-type deposits communities during the Great Ordovician Biodiversification Event (Gaines and Droser 2005). (Bassett et al. 2002; Servais and Harper 2018; Topper et al. 2018; The relatively shallow sedimentary environment (lower offshore Z.L. Zhang et al. 2018, 2020b; Z.F. Zhang et al. 2020a). in Martin et al. 2016 or offshore in Buatois and Mángano 2011)of The brachiopods recovered from the section include lingulides the Guanshan Biota also separates it from most other Cambrian (Eoobolus, Neobolus and Westonia), an acrotretide (Linnarssonia) Lagerstätten worldwide except, perhaps, for the early Cambrian and calcareous kutorginides (Kutorgina and Nisusia)(Table 1). Emu Bay Shale from Australia, which is interpreted to have been Lingulides occur in high abundance and also form many shell deposited in a nearshore micro-basin setting adjacent to an active concentrations within several assemblages (Fig. 2). The number of tectonic margin that generated continual syndepositional faulting brachiopod concentrations (at least ten thin mud beds) far exceeds and slumping (Paterson et al. 2016). The Guanshan Biota is also those produced by trilobites (only one mud bed). The composition comparable with the Ordovician Fezouata Biota, both in terms of of brachiopod taxa within each assemblage shows a rapid transition depositional environment and shelly faunal composition (Van Roy through time (Figs 2 and 7b). Neobolus is predominant in the lower et al. 2015; Saleh et al. 2018). The latter was deposited mainly in an part of the section (assemblages A–C, E and F), with Eoobolus offshore to lower shoreface setting (Martin et al. 2016). (lingulides) and acrotretides common, but subordinate (Figs 2 and Gravity, traction and turbiditic flows are responsible for the 7b). The relative abundance of the acrotheloid brachiopods, earlier transitions from arthropod- to brachiopod-dominated assemblages referred to as ‘Westonia’ gubaiensis, increases gradually up-section, from the lower part of the Wulongqing Formation at the Shijiangjun replacing, in part, the lingulides (Eoobolus and Neobolus) and section. The depositional environment between the fair weather acrotretides. This is partly attributed to the fact that the brachiopods wave base and the storm wave base is usually affected by frequent of Eoobolus and Linnarssonia had a much smaller shell (c. 2–5mm event flows, such as oscillatory and gravity flows (Kooi and Groen in maximum length) than Westonia. In addition, Westonia has a very 2001; Majid et al. 2017), which helps to mix oxygen-enriched wide and circular shell in outline, which is potentially adapted to the surface water with stagnant bottom water, providing favourable shallowing seawater environment. In general, the linguliform (e.g. nutrient-rich conditions for the development of the benthic lingulides and acrotretides) brachiopods show a strong control on community (X. Liu et al. 2012b). Transportation from a nearby assemblage dominance, whereas calcareous forms (kutorginides) source, rapid fall out from suspension and the resuspension of remain rare (Fig. 7b). seston provides a high nutrient load for suspension feeders such as Fossil concentrations, although common throughout geological brachiopods to flourish. time (Li and Droser 1997; Damborenea and Lanés 2007; Mancosu The limited amount of bioturbation throughout most of the et al. 2015; El-Sabbagh and El Hedeny 2016; García-Ramos and section seems to indicate conditions unfavourable for burrowing, Zuschin 2019), are rarely reported from Burgess Shale-type resulting from high turbidity, high or low salinity, or the relatively Lagerstätten (Han et al. 2006). The dominance of brachiopods low oxygen content, perhaps explaining the dominance of relatively within the Guanshan Biota, compared with other Cambrian small, physiological simple filter-feeding brachiopods. The increase Lagerstätten, is unique (Luo et al. 2008; Zhao et al. 2010; in the bioturbation index in the middle part of the section (3–6m Paterson et al. 2016; Strang et al. 2016; Fu et al. 2019). The in situ above the basal contact) is coincident with assemblages G–I, preserved brachiopod concentrations in the Wuding area also occur indicating more favourable conditions, probably a result of the in the Malong and Kunming areas, which indicates a wide relatively shallower depositional environment or fluctuating oxic geographical distribution (c. 6000 km2) after the rapid transgression conditions (Gaines and Droser 2005). The frequent overturn of at the base of the Wulongqing Formation (Hu et al. 2013; Chen et al. fossil assemblages, especially brachiopods, may be attributed to 2019; Z.F. Zhang et al. 2020a). frequent environmental fluctuations and the episodic input of Overall, the fossil data show that brachiopods quickly replaced coarser sediments, which probably periodically interrupt the arthropods as the dominant fauna following a transgression that led benthic suspension assemblages. to the deposition of the Wulongqing Formation at Wuding (Figs 2 Detailed analysis of the sedimentology, lithology and structures and 7a; Fig. S1). Different brachiopod genera dominated different facilitates the identification of distinct lithofacies associated with assemblages and, in places, formed distinctive shell concentrations transgressive systems tracts that directly affected the composition, (Figs 2 and 7b). diversity and relative abundance of faunal assemblages in the transition from the Hongjingshao to the Wulongqing deposits (Luo et al. 2008; Chen et al. 2019). Microfacies analysis, the degree of Guanshan Biota and its environment bioturbation and the faunal composition at the lower part of the Wulongqing Formation provide a new understanding of how The Guanshan Biota is an exceptionally preserved Konservat- fluctuations in the depositional environment influenced the faunal Lagerstätte, uniquely characterized by brachiopod-dominated early overturn in the Guanshan Biota across the Yangtze Platform in Cambrian communities, substantially different from the arthropod- eastern Yunnan. dominated Konservat-Lagerstätten such as the Chengjiang and ’ Burgess Shale biotas (Zhao et al. 2010; O Brien and Caron 2016; Conclusions Hou et al. 2017; Nanglu et al. 2020). Although the preservation of soft tissues within biomineralized and sclerotized exoskeletons is This is the first detailed report of the lithofacies, depositional common, which is at least partly attributable to the high number of environments and associated relative faunal abundance in the brachiopods, trilobites and hyoliths, completely soft-bodied organ- Cambrian Age 4 Guanshan Biota. The new Shijiangjun section isms (e.g. ctenophores) are absent in the Shijiangjun section, which through the basal part of the Wulongqing Formation in the Wuding is similar to the Ordovician Fezouata Biota (Saleh et al. 2020). This area, eastern Yunnan reveals fossil assemblages composed of six phenomenon is possibly related to preservation bias because the bilaterian groups (Brachiopoda, Arthropoda, Hyolitha, Priapulida, brachiopods, trilobites and hyoliths are more resistant to decay and Vetulicola and Anomalocaridiids). Detailed sedimentological, much more readily preserved within this Konservat-Lagerstätte, lithological and ichnological characteristics of the section indicate which might lead to an underestimation of the diversity (Saleh et al. that: (1) hydrodynamic conditions are fluctuating, with episodic 2020) of the Guanshan Biota in the Wuding area. The lack of changes in energy and current regimes producing periodically completely soft-bodied taxa may be due to the lack of an exaerobic coarse sand beds (Facies 3); (2) the sediments are derived from a Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021
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