Preliminary notes on soft-bodied fossil concentrations ... - Springer Link

ARTICLES Chinese Science Bulletin 2006 Vol. 51 No. 20 2482—2492

DOI: 10.1007/s11434-005-2151-0

Preliminary notes on softbodied fossil concentrations from the Early Cambrian Chengjiang deposits HAN Jian1, SHU Degan1,2, ZHANG Zhifei1, LIU Jianni1, ZHANG Xingliang1 & YAO Yang1 1. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi’an 710069, China; 2. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China Correspondence should be addressed to Han Jian (email: elihanj@nwu. edu.cn) Received June 15, 2005; accepted September 15, 2005

Abstract The efforts of labor-intensive collecting in the Early Cambrian Chengjiang deposits in eastern Yunnan Province, China led to the discovery of many horizons containing exceptionally well preserved soft-bodied fossil concentrations, many of which can be assigned to either monospecific concentrations or paucispecific concentrations. The features of these fossil concentrations support the hypothesis that frequent storm events producing tempestites mainly contributed to the preservation of abundant softbodied fossils in the Chengjiang deposits, and indicate that the balance of the ecological web in this region was probably frequently destroyed or upset by such storm events during that geological time. Animals in a fossil assemblage in such fossil concentrations probably occupied similar ecological biotopes. Keywords: Early Cambrian, Chengjiang fauna, preservation, softbodied fossil concentration (SFC).

The Chengjiang fauna has played a pivotal role in our understanding of the origin and diversification of metazoans since its discovery in 1984. To date, this fauna has been documented as consisting of 121 genera － with 140 species belonging to 24 phyla[1 27]. The geographic distribution of the Chengjiang fauna has been expanded from Chengjiang County to eastern Yunnan Province including Haikou, Malong, Yiliang, Anning － and elsewhere[25 33], with about 20 localities in total[25,33]. Successive excavation in recent years at the Ercaicun section and the Jianshan section in Haikou Town, Kunming City led to the discovery of many ac2482

curate horizons for some genera, and findings of exceptionally well-preserved fossil concentrations or enrichment layers, which is a rare phenomenon of taphonomy and preservation. Field work by the Early Life Institute (ELI), Northwest University has confirmed these fossil concentrations in those sections and has discovered many new such horizons, thus providing further data to investigate the depositional palaeoenvironment of this region in the Early Cambrian. The term ‘fossil concentration’ usually refers to skeletal concentration, meaning dense stacking of the fossils in spite of classification, preservation and deformation of the hardparts[34]. Hardpart concentrations, which are very common in the fossil record, are usually controlled by sedimentation and deposition, and partly affected by diagenesis during fossilization[35]. Fossil concentrations can be made up of fossils of various phyla. Particularly, a fossil concentration is sometimes composed of one species or a fossil assemblage that is dominated overwhelmingly by a single species; these types are termed ‘monospecific’ or ‘paucispecific’ concentration, respectively, and this phenomena of fossil concentration is mediated strikingly by ecology[34]. Such types of fossil concentrations, such as of Chuandianella ovata, Kunmingella douvillei and hyolithids, are common at various sites in the Chengjiang deposits[36]. It is important to distinguish fossil concentrations from fossil aggregates; the latter is characterized typically by a well-defined size range, a definite shape, and a boundary representing something like fecal pellets and fecal residues, or representing a recurrent composition in exoskeletal elements[36]. Comparatively, a fossil concentration with wellpreserved soft tissues is termed herein a ‘soft-bodied fossil concentration’ (SFC). Compared with hardpart concentrations, the SFC is characterized by numerous soft-bodied individuals occurring in the same bedding plane or thin bedding layer, and can be viewed as a specific status of fossil concentration with peculiar preservation, taphonomy and deposition. In particular, an SFC fossil assemblage is sometimes also composed of a single species or very few species with a extremely dominant taxa richness; these two types are respectively termed ‘soft-bodied monospecific concentration’ (SMC) and ‘soft-bodied paucispecific concentration’ (SPC) corresponding to monospecific and paucispecific concentration sensu Kidwell[34]. Compared with hardpart concentrations, an SFC holds the promise of figuring out comprehensively and precisely a picture of the Chinese Science Bulletin

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ARTICLES biota in the palaeoocean The SFC might therefore reflect faithfully the features of a natural community, even the population structure at that time, and thus can provide more information on population age range within a short interval. 1

Stratigraphy

The Early Cambrian Heilinpu (Chiungchussu) Formation was originally divided in ascending order into the Shiyantou Member and the Yu’anshan Member[37]; the latter can be subdivided into a lower black shale unit and an upper ‘Maotianshan Shale unit’[38] yielding the Chengjiang Biota. Recent classification renamed the ‘Yu’anshan Member’ as the Yu’anshan Formation consisting of four members, i.e. lower Black Siltstone Member, lower Black Shale Member, upper Maotianshan Shale Member and upper Siltstone Member[29,39]. Because all the SFCs presented in this paper occur in the later two members, we prefer the second classifications for convenience of description in detail, and we propose the name ‘Maotianshan Mudstone Member’ to replace ‘Maotianshan Shale Mb. (Maotianshan Shale Member)’ for the lithology is dominated by mudstones intercalated with siltstones rather than shale. 2

Method and materials

Each fossil concentration in a section is exposed in an area of approximately 2 m×10 m and the slabs acquired are generally less than 20 cm×20 cm in square because of the constricted conditions of fossil collection. Thus, the fossil concentration is characterized by thousands of slabs containing a similar fossil assemblage that were acquired in a thin bedding or layer by successively excavating its horizons in the field. Excavation in a neighbor section can help recognize the distribution of a fossil concentration in space. The density of the fossil assemblage in a fossil concentration is averaged from part slabs that have been acquired because the densities on different slabs are not always even. Illustrated specimens in this paper have been deposited at the ELI, Northwest University, Xi’an, China. 3

Description of soft-bodied fossil concentrations

Little attention has been focused on the SFC even though this phenomenon has been widely reported for years in the localities of eastern Yunnan, such as － Chengjiang, Malong and Wuding. Chen et al.[28 30] [40] reported briefly mentioned this aspect. Wang et al. www.scichina.com

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SFCs of Cricocosmia jinningensis, Haikouella lanceolata, Phlogites longus and of Kunmingella douvillei from the Ercaicun section, Haikou. Our field survey has identified that bedding planes with an SFC are more common in eastern Yunnan Province, especially in the Jianshan section (Fig. 1). Part SFCs are listed below with short descriptions: (1) Porifera. A sponge, Choia sp. SFC occurs in the upper part of the Maotianshan Mudstone Member in the Tanglipo section of Dabanqiao, Kunming. The sponge bodies of various sizes are paved on the bedding plane. A Protospongia sp. SFC occurs in the lower part of Maotianshan Mudstone Member in the Haoyicun section of Xianjie, Anning. Some of specimens are complete, and the sponge body and large stuatins are visible. (2) Discoidal fossils. Stellostomites eumorphus SMC occurs in the lower part of the Maotianshan Mudstone Member in the Maotianshan section[32], Shankoucun and the Haoyicun sections of Anning, Dayucun section of Chenggong, Tanglipo section of Dabanqiao as well as the middle part of the Jianshan section of Haikou. Most individuals are preserved roughly parallel to the bedding planes. Most of them are complete adults, but occasionally include a very few smaller ones. S. eumorphus is rather sparse in other horizons and usually co-occurs with other types of fossils in this section. However, it is very rare in other sections of the Haikou region, such as Mafangcun, Ercaicun, Dazicun, and Yunlongsi. For Yunnanomedusa eleganta, of the same group as S. eumorphus, its SFC has not been found in the Kunming and Chengjiang regions. It should be mentioned that the discoidal fossils S. eumorphus and Pararotadiscus guizhouensis from the Middle Cambrian Kaili Fauna, Guizhou Province, China have been interpreted as pelagic gregarious animals[41]. (3) Brachiopods. The Heliomedusa orienta monospecific concentration occurs in the middle part of the Maotianshan Mudstone Member in the Jianshan section (Fig. 1); all the specimens are complete individuals similar in size. The Lingulellotreta malongensis SPC with occasional Lingulella chengjiangensis occurs in the upper-middle part of the Maotianshan Mudstone Member in the Jianshan section (Fig. 1). Different individuals of various sizes usually form a cluster, which is characterized by several individuals attaching their distal pedun2483

ARTICLES cle ends intertwisted with a roughly consistent orientation (Fig. 2(a)). Sometimes they are also scattered randomly on the bedding plane.

(4) Priapulids. The priapulid worms, which inhabit the sediment or the interface of sediment-water, are present in all soft-bodied fossil localities in eastern

Fig. 1. The biostratigraphic column of the Jianshan section, Haikou, Kunming. Black block represents soft-bodied fossil concentration. Th., thickness; Coe, coelenterates; ch., chaetognaths. 1, Megaspirellus houi Chen & Erdtmann, 1991; 2, Yuknessia sp.; 3, Sinocylindra yunnanensis Chen & Erdtmann, 1991; 4, Fuxianospira gyrata Chen & Zhou, 1997; 5, Halichondrites sp.; 6, Triticispongia diagonata Mehl & Reitner in Steiner et al., 1993; 7, Protospongia sp.; 8, Saetaspongia densa Mehl & Reitner, 1993; 9, Paraleptumitella globula Chen et al., 1989; 10, Leptomitella conica Chen et al., 1989; 11, Choia xiaolantianensis Hou et al., 1999; 12, Crumillospongia sp.; 13, Quadrolaminiella diagonalis Chen et al., 1990; 14, Allonnia phrixothrix Bengtson & Hou, 2001; 15, Xianguangia sinica Chen & Erdtmann,1991; 16, Maotianshania cylindrica Sun & Hou, 1987; 17, Cricocosmia jinningensis Hou & Sun, 1988; 18, Palaeoscolex sinensis Hou & Sun, 1988; 19, Sicyophorus rara Luo & Hu, 1999; 20, Xiaoheiqingella peculiaris Hu in Chen et al., 2002; 21, Paraselkirkia sinica sinica Luo & Hu, 1999; 22, Tylotites petiolaris Luo & Hu, 1999; 23, Corynetis brevis Luo & Hu, 1999; 24, Microdictyon sinicum Chen et al., 1989; 25, Onychodictyon ferox Hou et al., 1991; 26, Cardiodictyon catenulum Hou et al., 1991; 27, Paucipodia inermis Chen et al., 1995; 28, Hallucigenia fortis Hou & Bergström, 1995; 29, Luolishania longicruris Hou & Chen, 1989; 30, Miraluolishania haikouensis Liu & Shu, 2004; 31, Megadictyon haikouensis Luo & Hu, 1999; 32, Facivermis yunnanicus Hou & Chen, 1989; 33, Retifacies abnormalis Hou et al., 1989; 34, Squamacula clypeata Hou & Bergström, 1997; 35, Kuamaia lata Hou, 1987; 36, Skioldia aldna Hou & Bergström, 1997; 37, Eoredlichia intermedia Lu, 1940; 38, Yunnanocephalus yunnanensis Mansuy, 1912; 39, Kuanyangia pustulosa Lu, 1941; 40, Xandarella spectaculum Hou et al., 1991; 41, Pygmaclypeatus daziensis Zhang et al., 2000; 42, Naraoia sp.; 43, Naraoia spinosa Zhang & Hou, 1985; 44, Misszhouia longicaudata Chen et al., 1996; 45, Pectocaris eurypetala Hou et al., 2004; 46, Priminocaris subconigera Hou & Bergström,1989; 47, Isoxys auritus Jiang, 1982; 48, Isoxys paradoxus Hou, 1987; 49, Leanchoilia illecebrosa Hou, 1987; 50, Kunmingella douvillei Mansuy, 1912; 51, Chuandianella ovata Lee, 1975; 52, Clypecaris pteroidea Hou, 1999; 53, Fuxianhuia protensa Hou, 1987; 54, Jianfengia multisegmentalis Hou, 1987; 55, Combinivalvula chengjiangensis Hou, 1987; 56, Ercaia minuscula Chen & Huang, 2001; 57, ?Tuzoia sinensis Pan, 1957; 58, Cindarella eucalla Chen et al., 1996; 59, Urokodia aequalis Hou et al., 1989; 60, Acanthomeridion serratum Hou et al., 1989; 61, Fortiforceps foliosa Hou & Bergström, 1997; 62, Yunnanocaris megista Hou, 1999; 63, Branchiocaris? yunnanensis Hou, 1987; 64, Mafangocaris multinodus Luo & Hu in Chen et al., 2002; 65, Jianshania furcatus Luo & Hu, 1999; 66, Anomalocaris saron Hou et al., 1995; 67, Amplectobelua symbrachiata Hou et al., 1995; 68, Parapeytoia yunnanensis Hou et al., 1995; 69, Burithes yunnanensis Hou et al., 1999; 70, Ambrolinevitus ventricosus Qian, 1978; 71, Glossolites yunnanensis Hou et al., 1999; 72, Linevitus opimus Yu, 1974; 73, Heliomedusa orienta Sun & Hou, 1987; 74, Lingulella chengjiangensis Jin et al., 1993; 75, Lingulellotreta malongensis Rong, 1974; 76, Diandongia pista Rong, 1974; 77, Botsfordia? sp.; 78, Longtancunella chengjiangensis Hou et al., 1999; 79, Eognathacantha ercainella Chen & Huang, 2002; 80, Phlogites longus Luo & Hu, 1999; 81, Vetulocystis catenata Shu et al., 2004; 82, Vetulicola rectangulata Luo & Hu, 1999; 83, Vetulicola cuneatus Hou, 1987; 84, Xidazoon stephanus Shu et al., 1999; 85, Heteromorphus confusa Luo & Hu 1999; 86, Pomatrum ventralis Luo & Hu, 1999; 87, Beidazoon venustum Shu, 2005; 88, Haikouella lanceolata Chen et al., 1999; 89, Haikouella jianshanensis Shu et al., 2003; 90, Haikouichthys ercaicunensis Luo et al. in Shu et al., 1999; 91, Zhongjianichthys rostratus Shu, 2003; 92, Stellostomites eumorphus Sun & Hou, 1987; 93, Batofasciculus sp.; 94, Dinomischus venustus Chen et al., 1989; 95, Calathites spinalis Luo & Hu, 1999; 96, Petalilium latus Luo & Hu, 1999; 97, Cambrotentacus sanwuia Zhang & Shu, 2001; 98, Archotuba conoidalis Hou et al., 1999.

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Fig. 2. Soft-bodied fossil concentrations (SFC) from the Early Cambrian Chengjiang Lagerstätte. Scale bar represents 1mm. (a) Lingulellotreta malongensis from the Jianshan section of Haikou, Kunming; (b) Cricocosmia jinningensis from the Jianshan section; (c) Sicyophorus rara from the Shankoucun section of Anning; (d) Pectocaris eurypetala from the Anshan section of Haikou; (e) Ercaia inuscule from the Jianshan section; (f) Haikouella jianshanensis from the Jianshan section; (g) Haikouichthys ercaicunensis from the Jianshan section; (h) Batofasciculus sp. From the Jianshan section.

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ARTICLES Yunnan Province. Thus the priapulid worm is generally applied as an index fossil for estimating the preservation quality of fossils at a locality. To date, 11 genera and 11 species have been reported[13], but only Maotianshania cylindrica, Cricocosmia jinningensis, Palaeoscolex sinensis, Paraselkirkia sinica and Sicyophorus rara are found with fossil concentrations. An M. cylindrica SMC occurs in the Maotianshan section. Nearly all individuals are complete adults oblique to the bedding planes occasionally with dragging traces (see Fig. 19 in ref. [29] and Fig. 30 in ref. [38]). Three horizons of soft-bodied C. jinningensis concentrations have been recognized in the Chengjiang Lagerstätte: the first C. jinningensis SPC is located in the lower part of the Maotianshan Mudstone Member near the basal horizon of the Chengjiang fauna. It occurs in a yellowish muddy siltstone with approximately 8cm in thickness in the Haoyicun section, where all specimens are complete individuals with their elongate bodies twisted to some degree and lying oblique to the bedding planes, approximately 20 to 30 individuals per square decimeter, displaying a high ability of burrowing concerning the compression ratio of the mudstone. Usually only a small area of the body is exposed when the slab is split open, but they prove consequently to be complete after preparation. Whereas the occurrence of the C. jinningensis SPC in the Jianshan section is similar to that in the Haoyicun section, both of them are preserved in siltstone, but the latter co-occurs with Leanchoilia illecebrosa and P. sinensis. In the Tanglipo section, C. jinningensis and P. sinensis SPC occur in a layer of mudstone about 5cm in thickness. The second C. jinningensis SPC occurs in the Yu’anshan Member of Haikou region, usually located in a 10 cm-thick bed about 10 cm above the siltstone, ranging from approximately 10 to 20 individuals to the square decimeter. Most of them are adult individuals oblique to the bedding planes. C. jinningensis is found co-occurring with a few of the lobopods such as Microdictyon sinicum in the Ercaicun section, with P. sinensis in the Tanglipo section and with a few Paraselkirkia sinica in the Jianshan section. In non-concentration horizons, the body of C. jinningensis is parallel or sub-parallel to the bedding plane, and the hind parts of their trunks are curved to some degree, and very few of them are straight. The third C. jinningensis SPC, associated with a few P. sinensis, numerous fragments of algae and other 2486

elements of organisms parallel to the bedding plane (Fig. 2(b)), occurs at the top of the Maotianshan Mudstone Member in the Jianshan section. Both kinds of worms lack their proboscises and their bodies are curved to some degree. A Paraselkirkia sinica fossil concentration occurs in a layer of mudstone about 5mm in thickness at the lower part of the Maotianshan Mudstone Member in the Shankoucun section. Their bodies, with remarkable traces of guts and obscure proboscises, are almost completely preserved roughly parallel to the bedding planes[30,42]. Associated species are mineralized shells of Kunmingella douvillei, fragments of Calathites spinalis Luo and Hu[32] and Cambrotentacus sanwuia Zhang and Shu[33], isolated sclerites of lobopod Microdictyon sinicum and priapulid worm Tabelliscolex hexagonus[9] as well as a kind of brachiopod. Chen[30] has proposed that Paraselkirkia sinica is of gregarious habit and that the fossil assemblage in this concentration was buried in a disaster event. However, seen form the status of fossil preservation, this proposal is rather questionable. A Sicyophorus rara SPC with a few associated M. cylindrica and Xiaoheiqingella peculiaris occurs in a thin bedding layer about 2cm thick in the Shankoucun section, and all specimens are complete individuals[42]. Half of their bodies are parallel to the bedding planes and the others are oblique or perpendicular to the bedding planes, having approximately 3－5 individuals per square decimeter (Fig. 2(c)). (5) Lobopods. A Cardiodictyon catenulum SMC occurs in the Jianshan section and all of the specimens are complete individuals parallel to the bedding planes. A Facivermis yunnanicus monospecific SMC occurs in the Jianshan section. All of the specimens are completely preserved with their bodies being curved or twisted to some degree oblique to the bedding planes, exhibiting a feature of preservation similar to the SMC of M. cylindrica and the second SMC of C. jinningensis but less in richness. (6) Arthropods. A Naraoia spinosa-Naraoia sp. paucispecific concentration occurs in the Kuangshan section of Malong, with the specimens weathering grayish white in color, and different from the general appearance of the Chengjiang fossils in other localities. The length of specimens ranges from 7 to 10 mm, smaller than the average size of the specimens in other regions. The appendages and trace of guts are not found, whereas the non-mineralized dorsal carapaces, which Chinese Science Bulletin

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ARTICLES occasionally are detached into isolated head shields and trunk shields, display a flattened appearance. The richness of specimens is hard to estimate because of the extreme similarity in their carapaces and poor preservation. A Misszhouia longicaudata SMC occurs in the Anshan section of Haikou. All of these specimens are complete individuals longer than 5 cm in length; their bodies are generally dorso-ventrally compressed roughly parallel to the bedding layer, with about 1－2 individuals per square decimeter. A Pectocaris eurypetala[43] SMC is present in a muddy bedding layer about 10 mm in thickness and just 30 cm above a thick layer of 40 cm-thick sandstone in the Anshan section. The eyes, the inner and outer appendages and the trunks are visible beneath the carapaces. Most of them are laterally compressed, complete adults, with about 1 to 3 individuals per square decimeter (Fig. 2(d)). A Pisnnocaris subconigera SMC is found in the middle part of Maotianshan Mudstone Member in the Tanglipo section in Kunming; most of the specimens are complete individuals with appendages occasionally preserved, with less than 10 individuals per square meter. An Ercaia minuscule[5] SPC, consisting of complete individuals of this taxon and very few complete adults of Leanchoilia illecebrosa, occurs in the Jianshan section. The bodies of E. minuscule, about 2－4 mm in length and apparently less than that of L. illecebrosa, are laterally compressed with a set of well-preserved appendages (Fig 2(e)). This SPC has proximately 5－8 individuals per square decimeter on a slab. (7) Yunnanozoans. A Yunnannozoon lividum SFC occurs in the Ma’anshan section of Chengjiang County. The specimens show a typical leaden color, and most of them are laterally compressed parallel to the bedding plane with their bodies curved to some degree. On some slabs they somewhat overlap (see Fig. 1(a) in ref. [44]). A Haikouella lanceolata SMC occurs in a thin bedding of mudstone above a 3－5 cm-thick sandstone in the lower part of the Maotianshan Mudstone Member in the Haikou region (Fig. 1). The 300 specimens of H. lanceolata mentioned by Chen et al.[4,45] were collected from this horizon. These specimens are complete animals but their posterior part is hard to expose by preparation[45]. Most specimens are flattened whereas few specimens with their gill arches are preserved in www.scichina.com

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three-dimensions. These specimens are almost preserved at various angles with their long body axis roughly parallel to the bedding plane. Animals of various sizes are found overlapping. In the Ercaicun section, individuals numbering less than 1－3 per square decimeter can be recognized on the bedding plane when the rock is split out, but severely weathered fragments of the rocks from this horizon display a greater density, about 10－20 individuals per square decimeter. The density seems to decrease to the east as manifested in the Jianshan section. Mallatt andChen[45] have proposed that the Haikouella animals lived in schools that were buried by the mudslides responsible for their fossilization. The horizon of Haikouella jianshanensis SPC (BA layer in Fig. 1, BA represents ‘black arthropod’, referring to Haikouella jianshanensis SPC in the field work) in the Jianshan section is lower than that of H. lanceolata. Most of the specimens are laterally compressed and sagittally parallel to the bedding plane with uniform size. Some of them are randomly distributed on the bedding plane and sometimes with an identical orientation. The anterior part of the animals, especially the gill arches, are characterized by the typical leaden or dark grayish color of this group, and their posterior parts, however, are almost invisible with a color the same as the matrix (Fig 2(f)). Occasionally, the whole bodies including their gill arches are weathered close to the color of the matrix, indicating that weathering has played an important role in the appearance of the Chengjiang fossils[3, 46]. The associated animals include a few adult individuals of the Vetulicolians Pomatrum ventralis, which is a swimming filter-feeding animal similar to H. jianshanensis in lifestyle. In total there are approximately 7－10 individuals to a square decimeter. These animals are almost evenly distributed on the bedding plane in a restricted excavated area in the Haikou region. The same horizon in the Anshan section is similar in appearance but with the dorsal segments of H. jianshanensis being better preserved than that in the Jianshan section. (8) Vetulicolians and vetulocystids. A Xidazoon stephanus SPC occurs in the Jianshan, Ercaicun and the Anshan sections. A Didazoon haoae SPC occurs in the Tanglipo section of Dabanqiao, with complete individuals. It is worthy to note that D. haoae, X. stephanus and P. ventralis occur in a thin bed deep red in color in the Tanglipo, Anshan and the Jianshan sections. The SPC of Beidazoon venustum[47]-Vetulocystis 2487

ARTICLES catenata[23] occurs in a thin bedding layer with numerous fragments of algae at the middle part of the Maotianshan Mudstone Member in the Jianshan section. Sometimes they cannot be easily distinguished from each other because these two kinds of animals are almost the same in size and most of them are fragments having a flattened appearance, and which are grayish dark in color. The density of them on this bedding plane is not as great as the SMC of Yunnannozoon lividum. Both animals are extremely sparse in the horizon of BA (Fig. 1). A Vetulicola sp. SMC occurs below that of Haikouichthys ercaicunensis[17] in the Jianshan section, and the posterior parts of the body are usually preserved, of similar size. A few individuals have their anterior parts preserved but these are poor in quality. (9) Vertebrates. The horizon of the Haikouichthys ercaicunensis SFC can be found in the middle part of the Maotianshan Mudstone Member in the Jianshan section, and it occurs in a lamina of mudstone about 5 mm in thickness just above a 5 mm-thick sandstone with grading laminae among a suite of intercalated beddings of siltstone and mudstone below the horizon of the SPC of Beidazoon venustum[47]-Vetulocystis catenata. Most of them are adult individuals being laterally flattened sub-parallel to the bedding planes roughly with a preferential orientation (Fig. 2(g)). Surprisingly, only the anterior parts are preserved, while their hind parts are hard to separate from the matrix, displaying a fashion similar to that of the Haikouella jianshanensis. This horizon contains a few specimens of Zhongjianichthys rostratus[22] displaying also a similar feature in preservation, and containing additional approximately 4% complete adults of Leanchoilia illecebrosa. In total, the density in this concentration is about 10－20 individuals per square decimeter. Other specimens of H. ercaicunensis found in other sections are complete but rather rare. (10) Batofasciculus sp. The SMC of this taxon can be found in the upper part of the Maotianshan Mudstone Mb. in the Jianshan section, and all of the specimens appear as discrete fragmental straight branches parallel to the bedding plane with a consistent orientation (Fig. 2(h)). This concentration can also be found in the Anshan section of Haikou (Hu S., personal communication). Several complete specimens of B. ramificans have been found in the Maotianshan[38] and the Shankoucun sections, and morphologically, its arc-like branches differ from the straight forms of B. sp. 2488

Additional examples of SFC beddings of undetermined phyla will be described elsewhere. As noted above, SFCs are widely distributed in eastern Yunnan Province. Every locality with preserved soft-bodies fossils is associated with SFCs; therefore, SFC can be regarded as an important feature of the Chengjiang Lagerstätte. All these SFCs, except those of Stellostomites eumorphus and C. jinningensis have not been found contemporaneously in more than two regions although they are almost evenly distributed on the bedding plane in a limited area, indicating a spatially and temporally restricted character. The widespread occurrence of the SFCs in the Haikou region is somewhat related to the intensity of field work done because SFCs are usually confined to a thin bedding less than 10cm in thickness, and successive excavation can find more SFCs in other regions especially in the Chengjiang region. Most SFCs except those of C. jinningensis are not completely comparable among different regions. However, SFCs in neighboring sections, such as the Shankoucun and the Haoyicun sections in the Anning region, and the Ercaicun, Jianshan and the Anshan sections in Haikou, Kunming region can be used as a tool for precise biostratigraphic correlation. The Early Cambrian Diandong Basin can be divided into three sedimentary zones[33,40]. The distribution of the SFCs might also reflect this kind of lithological difference, such as in the Chengjiang and Kunming regions, where the SFCs can be found throughout the Maotianshan Mudstone Member, whereas in the Anning region, only a few of them occur in its lower part, and its upper part is dominated by siltstones or sandstones containing abundant fragmentary remains of a variety of taxa, representing a high-energy environment. The fossil concentration of Allonnia phrixothrix[11], which is preserved as complete individuals or fragments, can only be found in the Sapushan section of the Wuding region. Palaeogeographically, the Qujing region is the deepest area in the Diandong Basin with less abundance and richness in fossil, and only the Naraoia spinosa fossil concentration have been found in that region. Thus, the SFCs are abundant in the southwest of the basin, corroborating the palaeoenvironmental analysis that the basin is a restricted bay opening to the south as inferred from the Xiaojiang fault[25]. However, the differences in sedimentary facies among the regions in eastern Yunnan Province do not constrain a species in a single section. A species that is common in a section can also be rare or absent in another section Chinese Science Bulletin

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ARTICLES despite the intensity of labor. For example, in the Chengjiang region, Vetulicola cuneata is comparably common, whereas its sister species Vetulicola rectangulata has never been reported; the latter species is common in the Haikou and Anning regions, where only a few specimens of V. cuneata have been found recently. Therefore, it could be argued that the difference in the various sedimentary facies did not stand as barriers to isolate the species, but it apparently mediated the richness of the animals. Our intense field survey over a large area supports the fact that more than one species are sometimes found in a concentration, which in turn reveals that a concentration first regarded as an SMC is actually an SPC. 4

Preservation and taphonomy

The mechanisms and processes of the Chengjiang Lagerstätte in preservation and taphonomy have been － widely hypothesized and are still in debate[29,48 50]. Evidence coming from lithology supports the hypothesis that tempestite rocks can account for the abundant preservation and wide distribution of soft-bodied Chengjiang fossils[35]. Most of the Chengjiang fossils are preserved as aluminosilicate films, similar to the host rocks[29]; some of them have been completely pyritized, and organic carbon can be occasionally traced on some hardpart fossils[50]. Zhang and Shu[51] argued that the rapid mineralization of the body surface rather than rapid burial was the key factor in inhibiting the decay of the body. No matter which factor has promoted the soft-bodied preservation, all Chengjiang fossils known come from the mudstone of the Maotianshan Mudstone Member, while the siltstone has only provided hardparts. However, the finding of the soft-bodied C. jinningensis concentrations in the Jianshan and Haoyicun sections indicates that muddy siltstone can also be the host rock for exceptional fossil preservation. Whether the Chengjiang fossils have been deposited in situ or were transported from elsewhere is still controversial[52,53]. The finding of SFCs seems to support the storm hypothesis: some SFCs except that of the infauna are current-aligned along the bedding plane (Fig. 2(a), (d), (g), (h)), indicating a phenomenon of transporting; but the distance of transport and whether they have been sorted are hard to discern. Additional evidence is that these SFCs lack any trace of bioturbation. Some dragging traces that have been found around the bodies of priapulid worms are very short[42], most likely indicating a phenomenon of death struggle after www.scichina.com

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rapid burial. Taking this together with the lithological evidence of the tempestites[35,39], it stands to reason that the widespread occurrence of the SFCs was formed during frequent storms with rapid burial. Some SFCs, for example, the third one of C. jinningensis in the Jianshan section (Fig. 2(b)) and that of Paraselkirkia sinica in the Shankoucun section, are characterized by incomplete individuals, with poor quality in preservation, and a fossil assemblage with various fragments of organism, etc., and were possibly formed by slow stacking rather than rapid burial, thus they are not included in discussion below. 5

Palaeoecology

5.1 The fossil assemblage occurring at an SFC most likely representing a statistical community Hallam[54] has characterized the concept of a statistical population to distinguish it from the normal fossil populations that are formed by the gradual stacking of hardparts in a stratigraphical interval. A statistical population is usually formed by a catastrophic event that leads to a simultaneous burial of all living things in that region, and generally can be found in thin beddings or on a single bedding plane containing very few species, and the adjacent horizons are almost non-fossiliferous[54], consisting with the features of SFCs from the Chengjiang deposits. On the one hand, different individuals of the same taxon in the same SFC are generally uniform in size and in quality of preservation, indicating that they had undergone the same transporting or burial process, whereas individuals belonging to different taxa sometimes are different in size and preservation probably because their bodies exhibit different responses or reactions to the taphonomic processes. For example, Leanchoilia illecebrosa fossils in the Haikouichthys ercaicunensis SPC are complete whereas most hind bodies of H. ercaicunensis are poorly preserved; Pomatrum ventralis fossils are better qualified than H. jianshanensis in completeness in the H. jianshanensis SFC. On the other hand, these SFCs containing few species generally occur in thin bedding planes or a single bedding plane and lack fossils in adjacent horizons. Combined with evidence of tempestites[35,39], it is reasonable to regard the same species in a fossil assemblage in such an SFC as a statistical population mostly close to the original picture of the living population; the fossil assemblages in an SFC can closely represent a regional ancient community given 2489

ARTICLES the limitations of fossil preservation; the overwhelming species of this assemblage probably represents a naturally dominant species in that region. Taxa of a fossil assemblage in an SFC generally possess a similar type of locomotion, and some of them occupy the same or adjacent ecological niches. The mobile pelagic forms are illustrated by the SFCs of H. jianshanensis and H. ercaicunensis noted above; the infaunal forms by P. sinensis in the C. jinningensis SFC as well as Xiaoheiqingella peculiaris in the Sicyophorus rara SPC. However, a few of the individuals of Leanchoilia illecebrosa in the C. jinningensis SPC probably are exceptional; they are heterogeneous with the associated C. jinningensis because these two groups strikingly differ from each other in lifestyle. 5.2 The dominant species in an SFC probably possessing a gregarious habit The richness of a species in a section is mediated by spatio-temporal factors and taphonomy. Fossil concentrations caused by slow transportation of hardparts like a skeleton concentration can not be applied as evidence of gregarious habit of the overwhelming species in the fossil assemblage; on the contrary, a single species occurring exclusively in an SPC formed by rapid burial can be regarded as strong evidence to support its gregarious habit, partly because the prerequisite for an SFC is that there was a flourishing natural population or a swarm of a species once living in the ancient ocean. On the contrary, a rare species in a section could also not support its solitary nature, for example, C. jinningensis is extremely rich in the Anning and Haikou regions, but very rare in the Chengjiang region. Seen from the known SFCs and based on our extensive survey, we suggest that the gregarious habit was common in Early Cambrian ocean communities. 5.3 An SFC probably representing firstly a group demise event and a recovery event in a region The muddy tempestite rocks between SFCs and the siltstones are usually non-fossiliferous. Geochemical analysis results show that these rocks are affected by freshwater[39]. The periodical storms have frequently destroyed local ecosystems[25,32]. The abundant fossils in the SFC might reflect the mass demise events to some extent. Hagadorn[55] has proposed that some pelagic forms could escape from storm disaster. Possibly the infaunal forms could also escape from the disaster events by burrowing. However, fossil SFCs illustrated 2490

in this paper are known from sponges, coelenterates, priapulid worms to arthropods, lobopods, lophophorans as well as deuterostomes preferring diverse life styles such as infaunal habit, epibenthic sessile habit, benthic crawling and benthic swimming, as well as mobile pelagic habits. Therefore, most populations at that time could hardly escape from storms. The pioneer species after storms would have quickly recovered and were able to flourish in that region due to lack of predators or other competitors, and were quickly buried as an SFC by the storms later on. The dominant taxon at an SFC might represent an opportunistic species during the recovery of their ecosystem. The episodic storms would have prompted the refreshing of the local dominant species. The non-fossiliferous horizons illustrate that the reconstruction of a food web in the ecological pyramid needs a certain span of time. Probably the infaunal burrowing taxa and the nektonic filtering forms will immigrate first into a virgin environment followed by the benthic forms; the appearance of the giant predators, which generally are regarded as a representative of equilibrium species occupying the top of the ecological pyramid, represents the complete recovery of their ecosystem[29,32]. But this process is possibly mediated by the nature of the substrate. For example, the benthic filter sponges and chancellorids could anchor in the soft muddy sediment, whereas the benthic sessile forms such as Cheungkongella ancestralis, Phlogites longus, Archotuba conoidalis, Dinomischus venustus and the linguloids apply the ecdysis of trilobites, especially the cheeks, or the shells of brachiopod Diandongia pista and hyloliths[20,26] as a hard supporting base anchoring in the soft muddy deposits. A kind of small epibiont of problematic affinity is also found attaching to the surface of these hardparts[42], indicating that these forms appeared later during the ecological restoration. Chen et al.[29,30] estimated that there are less than 20 recovering events through the Maotianshan Mudstone Member in eastern Yunnan. Concerning the nature of SFCs that could promise to provide accurate data in this aspect, we conceive that there are less than 16 such events in the Jianshan section where up to 16 horizons of SFCs have been found. The BA layer in the Jianshan section, which is located just above the lowest horizon of the soft-bodied Chengjiang fossils and the underlying black shale (see Fig. 1), most likely represents the first flourish of the Chengjiang fauna on account of the vast majority of taxa including some representatives of Chinese Science Bulletin

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ARTICLES deuterostomes found in that horizon. Vannier and Chen[56] have proposed that the living space of the Chengjiang biota is not constrained to the lower level of the water, and that some pelagic forms have exploited the midwater environment of the sea. Our analysis on SFCs is consistent with this idea, but the actual case would be more complicated concerning many active pelagic forms of arthropods such as Isoxys auritus, Branchiocaris yunnanensis, Naraoia spinosa that appeared below the horizon of the Haikouichthys SPC, but none of them have been found in this SPC. As mobile swimmers, the yunnaozoans, vetulicolians and Vetuloicystids, H. ercaicunensis, as well as the chordates Cathyamyrus diadexus and Zhongxiniscus inter－ － － media[1 4,17 19,21 23,25,45,57] are filter feeders, but they did not appear in the same SFC; and some of them flashed in only once in the strata records, so that the questions above need further consideration. Seen from the appearances of other types, one storm event cannot lead to the entire extinction of a dominant species in fossil concentrations. 6

Conclusions

(1) Wide distribution of SFCs in the Chengjiang Lagerstätte of eastern Yunnan supports the hypothesis that frequent storms were the main factor in preservation of abundant soft-bodied fossils. Storms in the Early Cambrian Diandong Basin were mainly concentrated in the south part of this area. The occurrence of SFCs is mediated by palaeogeography, palaeoenvironment and palaeoecology. (2) The fossil assemblage occurring at an SFC most likely represents a statistical community in a short interval; the dominant species in that assemblage most likely possessed a gregarious habit; all species in that assemblage can be speculated as probably occupying a similar ecological niche; an SFC layer probably represents periodically a group demise event and a recovery event in local ecosystems. Acknowledgements The authors offer their heartfelt thanks to Dr. S. Turner and Prof. K. Yasui for improving English. Thanks are given to Dr Hu S. of Yunnan Geological Institute and two anonymous experts for providing constructive advices. Thanks are also due to Cheng Meirong, Zhai Juanping for photography; and Guo Hongxiang and Ji Yanbing for fossil collections. This work was supported by the National Natural Science Foundation of China (Grant Nos. 40332016, 32070207), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). www.scichina.com

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Vol. 51 No. 20 October 2006