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Preliminary Notes on Soft- Bodied Fossil Concentrations from the Early

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Preliminary Notes on Soft- Bodied Fossil Concentrations from the Early ARTICLES Chinese Science Bulletin 2006 Vol. 51 No. 20 2482—2492 curate horizons for some genera, and findings of excep- DOI: 10.1007/s11434-005-2151-0 tionally well-preserved fossil concentrations or enrich- ment layers, which is a rare phenomenon of taphonomy and preservation. Field work by the Early Life Institute Preliminary notes on soft- (ELI), Northwest University has confirmed these fossil bodied fossil concentrations concentrations in those sections and has discovered many new such horizons, thus providing further data to from the Early Cambrian investigate the depositional palaeoenvironment of this Chengjiang deposits region in the Early Cambrian. The term ‘fossil concentration’ usually refers to HAN Jian1, SHU Degan1,2, ZHANG Zhifei1, skeletal concentration, meaning dense stacking of the LIU Jianni1, ZHANG Xingliang1 & YAO Yang1 fossils in spite of classification, preservation and de- formation of the hardparts[34]. Hardpart concentrations, 1. Department of Geology and Key Laboratory for Continental Dynam- which are very common in the fossil record, are usually ics, Northwest University, Xi’an 710069, China; controlled by sedimentation and deposition, and partly 2. School of Earth Sciences and Resources, China University of Geo- [35] sciences, Beijing 100083, China affected by diagenesis during fossilization . Fossil Correspondence should be addressed to Han Jian (email: concentrations can be made up of fossils of various [email protected]) phyla. Particularly, a fossil concentration is sometimes Received June 15, 2005; accepted September 15, 2005 composed of one species or a fossil assemblage that is Abstract The efforts of labor-intensive collecting dominated overwhelmingly by a single species; these in the Early Cambrian Chengjiang deposits in eastern types are termed ‘monospecific’ or ‘paucispecific’ con- Yunnan Province, China led to the discovery of many centration, respectively, and this phenomena of fossil horizons containing exceptionally well preserved concentration is mediated strikingly by ecology[34]. soft-bodied fossil concentrations, many of which can Such types of fossil concentrations, such as of Chuan- be assigned to either monospecific concentrations or dianella ovata, Kunmingella douvillei and hyolithids, paucispecific concentrations. The features of these are common at various sites in the Chengjiang depos- fossil concentrations support the hypothesis that its[36]. It is important to distinguish fossil concentrations frequent storm events producing tempestites mainly from fossil aggregates; the latter is characterized typi- contributed to the preservation of abundant soft- cally by a well-defined size range, a definite shape, and bodied fossils in the Chengjiang deposits, and indi- a boundary representing something like fecal pellets cate that the balance of the ecological web in this and fecal residues, or representing a recurrent composi- region was probably frequently destroyed or upset by tion in exoskeletal elements[36]. such storm events during that geological time. Ani- Comparatively, a fossil concentration with well- mals in a fossil assemblage in such fossil concentra- tions probably occupied similar ecological biotopes. preserved soft tissues is termed herein a ‘soft-bodied fossil concentration’ (SFC). Compared with hardpart Keywords: Early Cambrian, Chengjiang fauna, preservation, soft- concentrations, the SFC is characterized by numerous bodied fossil concentration (SFC). soft-bodied individuals occurring in the same bedding The Chengjiang fauna has played a pivotal role in plane or thin bedding layer, and can be viewed as a our understanding of the origin and diversification of specific status of fossil concentration with peculiar metazoans since its discovery in 1984. To date, this preservation, taphonomy and deposition. In particular, fauna has been documented as consisting of 121 genera an SFC fossil assemblage is sometimes also composed with 140 species belonging to 24 phyla[1－27]. The geo- of a single species or very few species with a extremely graphic distribution of the Chengjiang fauna has been dominant taxa richness; these two types are respec- expanded from Chengjiang County to eastern Yunnan tively termed ‘soft-bodied monospecific concentration’ Province including Haikou, Malong, Yiliang, Anning (SMC) and ‘soft-bodied paucispecific concentration’ and elsewhere[25－33], with about 20 localities in to- (SPC) corresponding to monospecific and paucispecific tal[25,33]. Successive excavation in recent years at the concentration sensu Kidwell[34]. Compared with hard- Ercaicun section and the Jianshan section in Haikou part concentrations, an SFC holds the promise of figur- Town, Kunming City led to the discovery of many ac- ing out comprehensively and precisely a picture of the 2482 Chinese Science Bulletin Vol. 51 No. 20 October 2006 ARTICLES biota in the palaeoocean The SFC might therefore re- SFCs of Cricocosmia jinningensis, Haikouella lanceo- flect faithfully the features of a natural community, lata, Phlogites longus and of Kunmingella douvillei even the population structure at that time, and thus can from the Ercaicun section, Haikou. Our field survey has provide more information on population age range identified that bedding planes with an SFC are more within a short interval. common in eastern Yunnan Province, especially in the Jianshan section (Fig. 1). Part SFCs are listed below 1 Stratigraphy with short descriptions: The Early Cambrian Heilinpu (Chiungchussu) For- (1) Porifera. A sponge, Choia sp. SFC occurs in mation was originally divided in ascending order into the upper part of the Maotianshan Mudstone Member the Shiyantou Member and the Yu’anshan Member[37]; in the Tanglipo section of Dabanqiao, Kunming. The the latter can be subdivided into a lower black shale sponge bodies of various sizes are paved on the bed- unit and an upper ‘Maotianshan Shale unit’[38] yielding ding plane. the Chengjiang Biota. Recent classification renamed A Protospongia sp. SFC occurs in the lower part of the ‘Yu’anshan Member’ as the Yu’anshan Formation Maotianshan Mudstone Member in the Haoyicun sec- consisting of four members, i.e. lower Black Siltstone tion of Xianjie, Anning. Some of specimens are com- Member, lower Black Shale Member, upper Maotian- plete, and the sponge body and large stuatins are visi- shan Shale Member and upper Siltstone Member[29,39]. ble. Because all the SFCs presented in this paper occur in (2) Discoidal fossils. Stellostomites eumorphus the later two members, we prefer the second classifica- SMC occurs in the lower part of the Maotianshan Mud- [32] tions for convenience of description in detail, and we stone Member in the Maotianshan section , propose the name ‘Maotianshan Mudstone Member’ to Shankoucun and the Haoyicun sections of Anning, Da- replace ‘Maotianshan Shale Mb. (Maotianshan Shale yucun section of Chenggong, Tanglipo section of Da- Member)’ for the lithology is dominated by mudstones banqiao as well as the middle part of the Jianshan sec- intercalated with siltstones rather than shale. tion of Haikou. Most individuals are preserved roughly parallel to the bedding planes. Most of them are com- 2 Method and materials plete adults, but occasionally include a very few Each fossil concentration in a section is exposed in smaller ones. S. eumorphus is rather sparse in other an area of approximately 2 m×10 m and the slabs ac- horizons and usually co-occurs with other types of fos- sils in this section. However, it is very rare in other sec- quired are generally less than 20 cm×20 cm in square tions of the Haikou region, such as Mafangcun, Er- because of the constricted conditions of fossil collec- caicun, Dazicun, and Yunlongsi. For Yunnanomedusa tion. Thus, the fossil concentration is characterized by eleganta, of the same group as S. eumorphus, its SFC thousands of slabs containing a similar fossil assem- has not been found in the Kunming and Chengjiang blage that were acquired in a thin bedding or layer by regions. It should be mentioned that the discoidal fos- successively excavating its horizons in the field. Exca- sils S. eumorphus and Pararotadiscus guizhouensis vation in a neighbor section can help recognize the dis- from the Middle Cambrian Kaili Fauna, Guizhou Prov- tribution of a fossil concentration in space. The density ince, China have been interpreted as pelagic gregarious of the fossil assemblage in a fossil concentration is av- animals[41]. eraged from part slabs that have been acquired because (3) Brachiopods. The Heliomedusa orienta mono- the densities on different slabs are not always even. specific concentration occurs in the middle part of the Illustrated specimens in this paper have been deposited Maotianshan Mudstone Member in the Jianshan section at the ELI, Northwest University, Xi’an, China. (Fig. 1); all the specimens are complete individuals similar in size. 3 Description of soft-bodied fossil concentrations The Lingulellotreta malongensis SPC with occa- Little attention has been focused on the SFC even sional Lingulella chengjiangensis occurs in the up- though this phenomenon has been widely reported for per-middle part of the Maotianshan Mudstone Member years in the localities of eastern Yunnan, such as in the Jianshan section (Fig. 1). Different individuals of Chengjiang, Malong and Wuding. Chen et al.[28－30] various sizes usually form a cluster, which is character- briefly mentioned this aspect. Wang et al.[40] reported ized by several individuals attaching their distal pedun- www.scichina.com www.springerlink.com
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