Fulu Biota, a New Exceptionally Preserved Cambrian Fossil Assemblage from the Longha Formation in Southeastern Yunnan

Fulu biota, a new exceptionally preserved Cambrian fossil assemblage from the Longha Formation in southeastern Yunnan

Shanchi Penga, b*, Xianfeng Yangc, Liu Yuc, Xuejin Zhua, b, Haijing Sund, Samuel Zamorae, Yingyan Maoa, and Yuchen Zhanga

aState Key Laboratory of Palaeobiology and Stratigraphy (Chinese Academy of Sciences), Nanjing 210008, China bNanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China cYunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, ChinaeNational Maritime Museum of China, Tianjin 300467, China dNational Maritime Museum of China, Tianjin 300467, China eInstituto Geológico y Minero de España, C/ Manuel Lasala, 44 - 9º B, 50006 Zaragoza, Spain

Abstract A new exceptionally preserved Cambrian biota, with fossils belonging to multiple phyla, has recently been found from the middle part of the Longha Formation, near Fulu Village, southeastern Yunnan, South China, and is named the Fulu biota. Groups recovered so far include polymerid trilobites, agnostoids, large bivalved arthropods, bradoriid arthropods, echinoderms, brachiopods, priapulids (worms), hyoliths, macroscopic algae, and trace fossils. The arthropods dominate and are highly diverse with echinoderms second in diversity. The associated agnostoids provide a precise late Guzhangian Age (late Miaolingian Epoch) for the biota, indicating that the biota occurs stratigraphically about halfway between the occurrences of the Kaili and the Guole biotas of South China, from both of which the new Fulu biota differs significantly in faunal components. Guzhangian biotas preserving poorly mineralized tissues and complete echinoderms were previously unknown in Gondwana until now, providing a unique window into life at this critical time interval.

Keywords: Fulu biota; Fossils of multiple phyla; Longha Formation; Guzhangian; Yunnan, South China

1. Introduction We report the discovery of an important new fossil biota, the Fulu biota, that shows exceptional preservation of some invertebrate groups at a critical but poorly known time interval – the “late” Cambrian. It is also located in the southeastern corner of Yunnan Province at a much younger horizon than the famous Chengjiang and Guanshan biotas, both of which are in eastern Yunnan and are early Cambrian in age (Hou et al., 2004, 2017; Hu et al., 2013). Until the Fulu biota was discovered, there were only two of such biotas found from the upper half of the Cambrian of China, the Kaili biota from basal Miaolingian Series (Zhao, 2011) in eastern Guizhou Province and the Guole biota from middle Furongian Series in western Guangxi Province (Zhu et al., 2016), occuring at about 508 Ma and 491.5 Ma respectively. As the third exceptionally preserved biota from upper half of Cambrian in China, the Fulu biota which is from upper Guzhangian Stage at about 498.5 Ma in age, is sandwiched stratigraphically between the older Kaili biota and the younger Guole biota, close to the halfway, and therefore fills an important gap in the knowledge of Cambrian faunas (Fig.1). This deposit is also of comparable age to the important “orsten” fauna of Scandinavia, and the Weeks Formation of North America, providing an opportunity to compare contemporary faunas from different environments, paleocontinents, and styles of preservation.

[Figure 1 is about here]

Figure 1. Global and Chenese chronstratigraphic subdivisions of Cambrian System and stratigraphic positions of Cambrian major fossil Lagerstätten (solid stars) and the new Fulu lagerstätte (open star). Fm. = Formation; Sh. = Shale.

2. Locality and Geological Setting As a natural outcrop on the north side of the Y028 rural highway, the fossil site of the new exceptionally preserved Fulu biota lies about 1 km northwest of Fulu Village (Fig. 2D) or about 15 km southeast of the Tianpeng Township (Fig. 2C, D). From either Fulu or Tianpeng, the Y028 highway leads to the fossil locality. The Cambrian outcrop widely in Tianpeng-Fulu area, which lies at the southeastern corner of Yunnan Province, and exclusively consists of Cambrian formations belonging to upper half of the system, i.e. Miaolingian and Furongian series. No pre-Miaolingian succession are known from this area. The Tianpeng-Fulu area is regarded as one of the classical areas in Yunnan for the Cambrian stratigraphic studies, known as the Funing Stratigraphic Subregion of the Southeast Yunnan Startigraphic Region (Luo et al., 2009), and holds a number of lithostratigraphic unit stratotypes of formations that include, in ascending order, the Tianpeng, Longha, Tangjiaba, and Bocaitian formations. These formations formed the southwestern flank of an anticline, the Zhongzhai-Nanon Anticline near the Fulu Village, and are unconformably overlain by Devonian succession, including the Yujiang and Ping’en formations (Fig. 2D).

[Figure 2 is about here]

Figure 2. Maps showing the locations of Cambrian exceptionally preserved biotas in Yunnan and Guangxi provinces (A‒C) and the geology of Fulu area, Tianpeng, Funing, southeastern corner of Yunnan (D).

The Fulu biota is from the middle part of the Longha Formation. Lithologically, this formation in the Tianpeng-Fulu area consists of carbonate successions with siliciclastic interbeds and is about 2000 meters thick. The formation is divided into three members (Yao et al., 2018). The lower Member exclusively consists of carbonate rocks, dominated in thin to thick-bedded dolomite and interbedded with dolomitic limestone and ribbon limestone. The middle and upper members consist of thin-bedded ribbon limestone, thin-bedded dolomitic limestone, and thin-bedded mudstone, interbedded with middle-thick bedded dolomite, and thin-bedded siltstone. Difference of those two members lies in that the middle Member bears mainly interbeds of dolomite and a few interbeds of mudstone and silty mudstone and the upper Member bears more interbeds of mudstone and silty mudstone but a few interbeds of dolomite. In general, the lithofacies indicate a continuously deepening depositional environment for the Longha Formation.

3. Material and methods All specimens were collected from a 2.5-m-thick interval of dark grey marl in the lower part of the Middle Member of the Longha Formation, which is usually weathered into yellow mudstone. Fossils from the mudstone preserve original profile with a more or less distortion and little compaction. A great number of echinoderms and arthropods are articulated. Most of specimens are photographed with digital camera Nikon D70, and a few of specimens are photographed under a Zeiss stereomicroscope (Model Axio Zoom V16) having a digital head (AxioCamMrM) and a circle light around the lens for providing uniform lighting. Images are processed by the functions of automatic color, contrast, and tonality of Adobe Photoshop CS6 software in order to show, as closely as possible, specimen’s natural color.

4. Fulu Biota The Fulu Biota is dominated by arthropods, of which the polymerid trilobites are abundant and the most diverse group with more than 14 genera. This fossil group is dominated particularly by damesellids (Bergeronites, Palaeadotes, Teinistion, Damesops, Blackwelderia etc., including a possible new damesellid genus) (Fig. 3A‒ E), forming a unique feature in faunal composition for Fulu biota as no damesellids are known from either the older Kaili or the younger Guole biotas. Other polymerids such as Paracoosia, Liostracina (Fig. 3N, O), Monkaspis and three small-sized forms including Torefera (Fig. 3F), an undetermined shumardiid (Fig. 3G) and an undetermined leiostigiid taxon, are common and are also unknown from the Kaili and Guole biotas. Trilobites from the Fulu biota are usually preserved articulated as complete exoskeletons, some of which have the hypostome attached in situ, impressed beneath the glabella (Fig. 3D, E). The complete exoskeleton is recorded for the first time for a number of described polymerid taxa, revealing their thoracic or thoracopygonal features and helping to clarify the concepts of some genera. For instance, Palaeadotes Öpik, 1967 and Bergeronites Sun in Guo, 1965 were considered to be synonymous by Lu and Zhu (1980). The new material of complete exoskeletons of both genera in Fulu biota (Fig. 3A, B) showed they are closely similar but certainly belong to different genera. Bergeronites has more anteriorly placed palpebral lobes and a proportionally smaller pygidium with narrower (tr., exsag.) pleural fields that bear entire pleural or interpleural furrows. In clear contrast, Palaeadotes has posteriorly placed palpebral lobes and a proportionally larger pygidium with broader (tr., exsag.) with pleural fields that bear incomplete pleural or interpleural furrows. In addition, Bergeronites is much smaller than Palaeadotes in size. When fully developed, the mature Palaeadotes reaches to about 90 mm in length but the maximum size of Bergeronites is only about 20 mm long. Cooper et al. (1996) further synonymized Bergeronites and Palaeadotes with Neodrepanura (then called Drepanura). However, Neodrepanura differs from Bergeronites and Palaeadotes greatly as it lacks general spine in cephalon and macropleual spines in thorax (Liu and Lei, 2011), supporting recognizing these are three as separate genera (Zhang and Jell, 1987; Peng et al., 2004). Except for a few new taxa, the polymerid trilobites of Fulu biota are known from formations of either the slope facies in South China or the platform facies in North China, providing new evidence for correlating Guzhangian succession of both regions. Agnostoids follow the polymerid trilobites in diversity. They include at least six genera, all of which are also not known from both Kaili and Guole biotas. Although agnostoids are less abundant than polymerid trilobites, this fossil group contains some important species such as Hadragnostus (=Formosagnostus) modestus, Kormagnostus minutus, Ammagnostus wangcunensis, Clavagnostus spinosus, and Agnostus sp. (Fig. 3H‒M), all of which, except for Agnostus sp., are known from the Proagnostus bulbus and Linguagnostus reconditus zones in western Hunan (Peng and Robision, 2000; Peng, 2009). Some of these agnostoid taxa occur also in Guzhangian formations of Laurentia, Siberia, Australia, Sweden, and Kazakhstan. The agnostoid assemblage precisely constrains the late Guzhangian age (late Miaolingian Epoch) for the biota. This age indicates that the Fulu biota is a new exceptionally preserved Cambrian fossil assemblage of South China, occurring above the Kaili Lagerstätte of eastern Guizhou Province (Wuliuan, early Miaolingian) (Zhao et al., 2005) and below the Guole Lagerstätte of western Guangxi Province (Jiangshanina, middle Furongian) (Zhu et al., 2016). The Fulu biota differs in having a much more diverse agnostoid assemblage, almost completely different from that of the Kaili and Guole biotas. Although the Kaili and Guole biotas are also highly diversified in polymerid trilobites, but they are differentiated again by lacking identical agnostoid taxa. The large bivalved (Fig. 4K) and bradoriid arthripods (Fig. 4L) are rare in the Fulu biota, each represented by a single species. The large bivalved arthropod is assigned to an undetermined species of Canadaspis and the bradoriid referred to Anabarochilina australis (Hinz-Schallreuter, 1993). The specimen of Canadaspis sp. has sub-oval-shaped carapace valves with the lowermost part of the left carapace valve slightly damaged so that the right valve could be partially exposed. It has a straight hinge in horizontal aspect and the maximum breadth at about mid-length of the hinge. Canadaspis was first known from the Burgess Shale, Canada (Walcott, 1912; Briggs, 1978) and was recently reported from the Chengjiang and Kaili biotas, South China (Hou and Bergström, 1997; Zhao, 2011) with a stratigraphic range of lower Nangaoan through middle Wuliuan. The new material of Canadaspis from the Fulu biota added a new locality in South China for the species and extended its range to upper Guzhangian Stage. The bradoriid arthropod is characterized by having postplete valves with a highly convex anterior node lying slightly anteroventrally and a less prominent triangular-shaped anterodorsal node. It is confidently assigned as Anabarochilina australis (Hinz-Schallreuter, 1993), an intercontinental form known from Gondwana (Australia, India) (Collette et al., 2011), Laurentia, and Kazakhstan (Abdulin et al., 1990; Siveter and Williams, 1997; Melnikova, 2003; Jones and Laurie, 2006). The genus Anabarochilina is even more widespread, known from Baltica, Avalonia, and Siberia (Melnikova, et al., 1997). Previously the range of A. australis is known from the latest Miaolingian Ptycnostatus gibbus Zone through earliest Guzhangian Lejopyge laevigata Zone (Jones and McKenzie, 1980; Siveter and Williams, 1997), the new material of the Fulu biota extended its geographic distribution to South China and its stratigraphic range up to lower Liguagnostus recongitus Zone. Echinoderms (Fig. 4A‒F) are rich and diverse in the Fulu biota, and are dominated by undetermined eocrinoids and stylophorans. None of these echinoderm species are recorded from either the Kaili or Guole Lagerstätten in China or from the Weeks Formation Lagerstätte in Utah, USA, which has a nearly equivalent, or may be slightly older age than the Fulu Biota. They likely represent new taxa recorded for the first time from siliciclastic environments of Guzhangian age. Echinoderms from Guzhangian formations are very rare worldwide, with a few taxa described from only carbonate facies in Laurentia (Nevada), Siberia, and Gondwana (Iran) (Sprinkle, 1973; Zamora et al., 2013b; Guensburg and Rozhnov, 2014; Lefebvre and Lerosey- Aubril, 2018). Contrary to other Guzhangian Lagerstätten like the Weeks Formation in which echinoderms only represent a small portion of total diversity (Lerosey- Aubril et al. (2018), echinoderms are more diverse in the Fulu biota. Eocrinoids include a new undetermined form with large marginal spines mounted on thecal plates (Fig. 4C); and another taxa with large polyplated theca composed of numerous tiny plates and a holomeric stem (Fig. 4B). Stylophorans are dominated by cothurnocystids (Fig. 4A, E) and rare mitrocystitid mitrates (Fig. 4D). A possible dibrachicystid rhombiferan (Fig. 4F) is also present. The echinoderm assemblage, at least the stylophorans, are more or less similar to those from the Furongian formations of Korea and South China (Guolo biota) (Lee et al., 2005; Zamora et al., 2013a). In contrast, eocrinoids are very different from these and other Cambrian formations. Brachiopods of Fulu biota may consist of three species, among which one belongs to Lingulella (Fig. 4H), and two others may each represent an obolid and an orthiid species respectively (Fig, 4I, J). Guzhangian brachiopods from Yunnan and other places in China remain unstudied except for at Paibi, Hunan, South China (Engelbretsen and Peng, 2007; Percival et al., 2016) and Wanxian, Hebei, North China (Mei, 1993). The Fulu biota bears no shared species with the documented Guzhangian brachiopods from South and North China. Obollellids in the Fulu biota are common with about two dozen specimens of dorsal and ventral carapace valves in current collection. This species resembles, in shape and smooth larval shell, Experilingula large from the Ptychagnostus atavus-Linguagnostus reconditus zones of Huaqiao Formation, Paibi (Percival et al., 2016) but differs in having thicker, widely spaced concentric lines and in the larger size of the valve (about 10 mm via 3~4.8 mm in width). Six brachiopod specimens in the current collection are characterized by having dense ornamentation of radial ridges, and are questionably assigned as undetermined orthiids because no interior structures of the valves are available for generic identification. The “worms” from the Fulu biota are moderately rare and are represented by a single palaeoscolecid species. Material includes three incomplete and four complete specimens with the body 21–29 mm long and 1.5–2.3 mm wide. Palaeoscolecid also found from the Kaili and Guole biotas each includes only one species. The species from the Kaili Lagerstätte was described as Palaeoscolex ratcliff Robison, 1969 (Zhao et al., 1999), whereas the species from the Guole Lagerstätte remain unassigned (Zhu et al., 2016). Hyoliths (Fig. 4N), trace fossils (Fig. 4O) and macroalgae (Fig. 4G) of the Fulu biota are very rare, each is represented by only one or two specimens. The hyolith specimen characterized by calcareous conical shell (Fig. 4N) is an undetermined species probably belonging to hyolithides. The filled trace fossil on bedding surfaces in burrow is represented by a single undetermined ichnospecies that is thin in diameter, gently curved, and likely branched. The figured specimen of macroalgae is branched continuously and assigned to Doushantuophyton sp. This genus is widely distributed in the Ediacaran formations of the Yangtze Platform, reported from the Toushantuo (=Doushantuo) Formation of Hubei and Anhui provinces, South China (Chen and Xiao, 1991; Yuan et al., 1995), and in the Vendian formations of the northwestern East European Platform of Russia (Golubkova et al., 2018). It also recorded from the Kaili Formation, extending its range to the Oryctocephalis indicus Zone of the lower part of Wuliuan Stage, basal Miaolingian (Yang et al., 1999; Wu et al., 2010). The discovery of the species in southeastern Yunnan further extends its range to the Linguganostus reconditus Zone of the Guzhangian Stage, upper Miaolingian.

[Figs 3, 4 are about here]

Fig. 3. Polymerid trilobites and agnostoid arthropods from the Fulu biota, southeastern Yunnan, South China. All in dorsal view; all scale bars = 2 mm. (A) Bergeronites sp., external mould of exoskeleton, NIGP 171327; (B) Palaeadotes hunanensis (Yang in Zhou et al., 1977), early holaspid exoskeleton, NIGP 171328; (C) Damesops sp., exoskeleton, NIGP 171329; (D) Teinistion sp., exoskeleton, NIGP 171330; (E) A new damesellid genus and species, exoskeleton, NIGP 171331; (F) Terifera sp., exoskeleton, NIGP 171332; (G) Shumadiidae gen. et sp., external mould of exoskeleton, NIGP 171333; (H) An agnostoid exoskeleton assigned tentatively to Agnostus sp., NIGP 171334; (I) Kormagnostus minutus (Schrank, 1975), exoskeleton, NIGP 171335; (J) Clavagnostus spunosus (Resser, 1938), cephalon, NIGP 171336; (K) Hadragnostus modestus (Lochman in Lochman and Duncan, 1944), external mould of exoskeleton, retrodeformation showing inferred strain ellipse, NIGP 171337; (L, M) Ammagnostus wangcunensis, cephalon and pygidium, NIGP 171338, 171339; (N) Paracoosia asiatica (Mansuy, 1916)., exoskeleton, NIGP 171340; (O) Liostracina sp., external mould of exoskeleton, NIGP 171341.

Fig. 4. Echinoderm, macroalgae, brachiopod, bivalve, large bivalved arthropod, bradoriid, worm, hyolith, and trace fossil from the Fulu biota, southeastern Yunnan, South China. All scale bars = 2 mm. (A, E) Conthurnocystid gen. and sp. undetermined, NIGP 171342, 171343; (B) Eocrinoid gen. and sp. undetermined, NIGP 171344; (C) Spiny eocrinoid gen. and sp. undetermined, NIGP 171345; (D) Primitive mitrocystitid mitrate, NIGP 171346; (F) Dibrachicystid? gen. and sp. undetermined, NIGP 171347; (G) Doushantuophyton sp., NIGP 171348; (H) Lingulella sp., plan view of pedicle valve external, NIGP 171349; (I) Obolid gen. and sp. undetermined, plan view of dorsal valve external, NIGP 171350; (J) Othriid? gen. and sp. undetermined, plan view of dorsal valve external, NIGP 171351; (K) Canadaspis sp., lateral view, NIGP 171352; (L) Anabarochilina australis (Hinz-Schallreuter, 1993), complete carapace in lateral view, NIGP 171353; (M) Palaeoscolecid gen. and sp. undetermined, NIGP 171354; (N) Hyolithid gen. and sp. undetermined, NIGP 171355; Trace fossil, igen. and isp. undetermined, NIGP 171736.

5 Discussion The moderately diverse agnostoids in the Fulu biota are all known from the Huaqiao Formation of northwestern Hunan except for one species assigned tentatively to Agnostus (Peng and Robision, 2000). In northwestern Hunan these agnostoids are exclusively Guzhangian in age, among which Hadragnostus modestus ranges from the later Lejopyge laevigata Zone though early Linguagnostus reconditus Zone, Ammagnostus wangcunensis and Kormagnostus minutus range from the later Proagnostus bulbus Zone through early Linguagnostus reconditus Zone, and the range of Clavagnostus spinosus is restricted to the lower Linguagnostus reconditus Zone (Peng and Robision, 2000, figs. 3, 4). Therefore their co-ranges suggest the interval bearing the Fulu Biota should be restricted to the lower Linguagnostus reconditus Zone, or at most from the Proagnostus bulbus Zone to lower Linguagnostus reconditus Zone. It means the Fulu biota is likely early late Guzhangian in age. Agnostus has never been found in China, the present exoskeleton assigned tentatively to the genus has a prominent preglabellar median furrow, a posteriorly slightly expended posteroglabella with centered medial glabella node in the cephalon and a pygidial axes with effaced F1 and F2 furrows and slightly expended posteroaxis with accurately round end that does not reach to the poster border furrow. All these features are reminiscent of some specimens assigned to Agnostus pisiformis from Sweden and England (Westergård, 1946; Rushton, 1978), which is associated with Lionguagnostus reconditus (Rushton, 1978; Ahlberg and Ahlgren, 1996; Axheimer et al., 2006) and is also Guzhangian in age. The Fulu biota is characterized by a faunal assemblage with exceptionally preserved organisms belonging to multiple fossil groups. Its discovery opens a new window for the understanding of evolutionary processes of Cambrian life. Together with the Kaili and Guole biotas, the Fulu biota forms a successive series of Lagersttäten in the upper half of Cambrian in South China. All the three biotas are exclusively dominated by polymerid trilobites, both in abundance and diversity, but the Fulu biota is remarkably different in faunal components because it bears unique assemblage of damesellid trilobites and has no identical genera and species known from other two biotas. The first recorded the complete or nearly complete exoskeletons of polymerid trilobites from the Fulu Lagersttäte help to explore the entire exoskeletal morphology for a number of incompletely known taxa in morphology. The agnostoids in the Fulu biota are also unique and more diverse in Fulu biota than in two others. The Fulu Lagersttäte yields rich and diverse echinoderms that differ from the Guzhangian echinoderm species from carbonate facies as well as those from the siliciclastic facies of both Kaili and Guole Lagersttäten, and may represent some hitherto unknown species. The discovery of new echinoderms in such critic time interval where only little information was available (Zamora et al., 2013b) improves our understanding of Cambrian faunal composition and fills an important gap between older well known Drumian taxa and the poorly understood Furongian assemblages. Globally, the Fulu biota is more or less identical in age with the fauna from “upper Weeks Formation” Lagersttäte (Proagnostus bulbus Zone) in Utah, Great Basin (Robison et al., 2015; Lerosey-Aubril et al., 2018) the youngest Cambrian exceptionally preserved biota of western UAS, which seems different in bio- and lithofacies and has different taphonomy features. It is also more or less identical in age with the fauna of Orsten Lagersttäte (Agnostus pisiforms Zone) from upper Alum Shale, southern Sweden (Waloszek and Müller, 1997), which is clearly different in preserved type and faunal composition. Intensive studies on the Fulu biota of South China and the Weeks and Orsten faunas are required in order to have a detailed comparison of the Guzhangian faunal and taphonomic features on those biotas occurring on the widely separated paleocontinents.

Acknowledgements This work was supported in part by State Key Laboratory of Palaeobiology and Stratigraphy (20191101), the Institute of Geological Survey of China and the National Commission on Stratigraphy of China (DD20190009), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB 26000000). We are grateful to Yin Bao’an and Mo Xixing from the Guangxi Institute of Geological Survey and Liu Feng and Liu Zhefu, the villagers from Balang Village of Jianhe County, Guizhou Province for their assistance with the field work. This is a contribution to the IGCP 668 project ‘The stratigraphic and magmatic history of Early Paleozoic equatorial Gondwana and its associated evolutionary Dynamics.’ XY is support by National Natural Sciences Foundation of China (grant number 41562001), and SZ is supported by the Spanish Ministry of Science, Innovation and Universities (grant number CGL2017-87631), co-financed by the European Regional Development Fund and project “Aragosaurus: recursos geológicos y paleoambientales” (ref: E18_17R) funded by the Government of Aragón.

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