Phosphate replicated and replaced microstructure of molluscan shells from the earliest Cambrian of China WEIMIN FENG and WEIGUO SUN Feng,W.M. and Sun,W.G. 2003. Phosphate replicated and replaced microstructure of molluscan shells from the earliest Cambrian of China. Acta Palaeontologica Polonica 48 (1): 21–30. The earliest Cambrian Meishucunian phosphoritic succession in eastern Yunnan,China,contains well−preserved mollus − can shells that offer insights into the early evolution of skeletonization. Phosphate internal moulds,phosphate replaced originally carbonate shells,and phosphate coatings show lamello−fibrillar structure,prismatic structure,and regularly fo − liated structure. The lamello−fibrillar structure appears earlier in the fossil record than laminar structures such as nacreous or foliated structures. It has been identified in fossil mollusks,which occur in China as early as the lower phosphate layer of the Zhongyicun Member of the Meishucunian. Therefore,the lamello−fibrillar structure appears to be primitive in mol − lusks. The lamello−fibrillar and prismatic aragonite is the most common shell material of molluscan skeletons in the Early Cambrian Meishucunian and equivalents around the world. Although the early molluscan microstructure is not so diverse as that of extant mollusks,it may be of use in high rank taxonomic classification as shown by the early conchiferan mol − lusks discussed here. These mollusks are characterized by the horizontal fibrillae that are layered and parallel,and thereby differ from hyoliths,in which the horizontal fibrillae appear to be in the form of the bundles of fibres that can branch or anastomose. Key words: Mollusca,small shelly fossils,microstructure,biomineralisation,Cambrian,Meishucunian Stage,China. Weimin Feng [[email protected]] and Weiguo Sun [[email protected]], Nanjing Institute of Geology and Paleon− tology, Academia Sinica, 39 East Beijing Road, 210008 Nanjing. Introduction many phyla,such as the Mollusca,Brachiopoda,and Hyo− litha. Qian and Bengtson (1989) in a comprehensive review The apparently sudden appearance of abundant and diverse attributed the Meishucunian small skeletal fossils to a cata− small skeletal fossils in rock successions near the base of the logue of morphological groups. Cambrian is an expression of the “Cambrian explosion”, Those different major groups may have their own diag− a long−standing enigma in both geology and biology. The nostic characters of skeletonization,which can be used as ev − most spectacular difference between the latest Precambrian idence for identification and classification. Although most of and the earliest Cambrian faunas is the advent of biomineral the Early Cambrian small skeletal fossils are problematic due skeletonization. For this reason,research on the biominerali − to incomplete preservation or unusual morphology,they may zation characters of the early small skeletal fossils may pro− help in recognizing ancestors of later major taxa and reveal vide significant information about the origin and early diver− their phylogenetic relationships. sification of the major groups of the Metazoa. Mollusks,as a good example to start with,contributed Although in the last three decades Early Cambrian small a very important part to the earliest Cambrian small skeletal skeletal fossils have been found in basal Cambrian rocks at fossils. Pioneering work by Runnegar (1983,1985a,b,1989; numerous localities throughout the world,little is known about their biomineralization and diagenesis (Bengtson and Runnegar and Jell 1976) on the microstructures of Cambrian Conway Morris 1992; Lowenstam 1989; Runnegar 1989). mollusks and works of other researchers (Müller 1975; Mis− Previous studies commonly concentrated on morphological sarzhevsky 1989; Brasier 1990; Bengtson et al. 1990; Carter characters of “small shelly fossils”. More and more small and Hall,1990; Bengtson 1992; Li and Chen 1992; Kou − skeletal fossils,such as Microdictyon (see Bengtson et al. chinsky 1999,2000b; Feng 1998; Feng et al. 2000,2001) 1986; Chen et al. 1989), Wiwaxia (see Bengtson and Conway have provided very useful information about early molluscan Morris 1984; Butterfield 1990),and Halkieria (see Conway biomineralization. Our investigations are based on mollus− Morris and Peel 1990,1995),have been proven to be dis − can fossil materials from the Meishucunian in eastern persed hard parts (sclerites) covering bodies of relatively Yunnan,southwestern China. The aim of this paper is to use large animals. Their original taxonomic affiliation based on diagenetic phosphate replication of the original mineral tis− morphology of single sclerites appeared incorrect. Other sues to determine the original shell structure of early mol− Early Cambrian small skeletal fossils have been attributed to lusks. Acta Palaeontol. Pol. 48 (1): 21–30, 2003 http://app.pan.pl/acta48/app48−021.pdf 22 ACTA PALAEONTOLOGICA POLONICA 48 (1),2003 Material and methods 75 90 105 120 135 45 The Meishucunian is the basalmost Cambrian chronostrati− Beijing Dahai Yulu graphic unit in the eastern Yunnan of China and corresponds, China in ascending order,to Xiaowaitoushan,Zhongyicun,Dahai 30 Members of the Dengying Formation and the Shiyantou Kunming 26 N Member of the Yuanshan Formation. Small skeletal fossils abruptly appear in abundance in the Zhongyicun Member. 15 The fossil material dealt with in this paper was collected in 1996 and 1999 from the Zhongyicun and Dahai Members Baizai of the Early Cambrian Meishucunian at Beideng of Anning County,Baizhai of Xundian County,and Dahai and Yulu of 25 N Huize County (Fig. 1). The rock samples were carbonates and phosphorites. Most of the fossil specimens were obtained Kunming by washing weathered rock with water. Acid maceration with 5–8% acetic acid was also applied. Observations were done under SEM (JSM−6300) in addition to Figs. 2B and 5B,C, Anning taken with Philips X−30 at Uppsala University,Sweden. This paper adopts the standardized microstructure terminology set 24 N up by Carter et al. (1990). Molluscan fossils used for the research on microstructure include the monoplacophorans Ramenta cambrina Jiang, 102 E 103 E 104 E 1982 (Luo et al. 1982), Latouchella cf. korobkovi Vostokova, 1962, Papilloconus explanatus Feng et al.,2000, Ilsanella cf. Fig. 1. Location of studied exposures of the Meishucunian in the Yunnan orectes (Jiang,1982), Ilsanella? rozanovi Wang,1994, Wat− Province. sonella yunnanensis (He and Yang,1982),and the para− gastropod Archaeospira ornata Yu,1979. Watsonella is con− 1989). Information about early molluscan microstructures is sidered to be a monoplacophoran based on the most complete obtained mostly from the internal moulds of conchs (Table 1). phylogenetic analysis available (Carter et al. 2000). It is the As typical for secondarily phosphatized small shelly fos− link between the stenothecid monoplacophorans and the sils,the Meishucunian specimens usually show phosphoritic Bivalvia. It lacks a true pegma,which,as indicated by Pojeta internal moulds,calcite−replaced original shell,and a phos− and Runnegar (1974),is necessary for membership in the phate coating of its exterior. In acid treated materials the Rostroconchia,and its microstructure is in part much like that calcitic shell is removed and represented by a narrow empty of Pojetaia and Fordilla (see Carter et al. 2000). All these cavity between the mould and coating. The coating then easily specimens are housed in the Nanjing Institute of Geology and exfoliates. Frequently the shell itself is also phosphatized. Palaeontology, Academia Sinica, abbreviated NIGP. Each of these three kinds of phosphatic structures is poten− tially a source of information for original shell microstructure. The external phosphatic coating,usually only roughly Mode of preservation replicates the external shell morphology,but it can be some − times recovered and observed from the inside (e.g.,Olemp − ska 1994; Hinz−Schallreuter 1995). It shows very fine details Phosphatization,so common in strata near the Precambrian– of the external shell surface in negative. In the case of mol− Cambrian boundary around the world,played a key role in lusks these may be details of organic periostracum,if pho − preserving information on the original shell structure (Bengt− sphatization was early enough. Frequently,internal moulds son and Conway Morris 1992; Brasier 1990; Dzik 1994; of cavities within the shell wall are attached to the coating. Runnegar 1985a; Runnegar and Bengtson 1990). As a gen− Mostly these are channels etched by endolithic cyanobacteria eral rule,phosphatic replacement or filling occurs only in (e.g., Runnegar 1985b; Olempska 1986). small specimens,whereas silicification affects specimens of Phosphoritic internal moulds provide additional informa− all sizes (Yochelson 1999). Phosphate fillings of small shells tion,in this case replicating the internal shell structure (e.g., can replicate very fine structures,up to 100 nm (Runnegar Runnegar 1985a). and Pojeta 1985). This allows determination of both the to− In both of the above cases the information is rather reliable pography of the shell surface and its microstructure (Run− and relatively easy to interpret. Much more difficulty arises negar 1989). Crystal morphologies replicated on internal when the secondarily phosphatized shell wall is considered. moulds were reported to be rather common in phosphate and The phosphatization,which may be late diagenetic in some carbonate successions of the earliest Cambrian (Runnegar cases,may replace the