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Journal of Paleontology 92(1):59–70


possessing paired uniramous lobopod limbs with nine pairs of reticulated plates positioned along both sides of the sub- cylindrical trunk (Chen et al., 1989, 1995; Ramsköld and Hou, 1991; Bergström and Hou, 2001; Hou et al., 2004). These soft- bodied specimens indicate that Microdictyon is quite similar to other fossils with uniramous lobopods, including Aysheaia Walcott, 1911 (Whittington, 1978), Hallucigenia Walcott, 1911 (Conway Morris, 1977), Xenusion Pompeckj, 1927 (Dzik and Krumbiegel, 1989), Luolishania Hou and Chen, 1989, Onychodictyon Hou, Ramsköld, and Bergström, 1991, and Cardiodictyon Hou, Ramsköld, and Bergström, 1991. In general, these fossils are assigned toLobopodia (Hou and Bergström, 1995; Ramsköld and Chen, 1998), and comparisons of soft-part anatomy show great similarity to extant onychophorans (Ramsköld and Hou, 1991; Ramsköld and Chen, 1998; Smith and Ortega- Hernández, 2014). Some lobopodianswithmosaic features of both lobopods and arthropods, such as Miraluolishania haikouensis Liu et al., 2004, Jianshanopodia decora Liu et al., 2006, and Diania Liu et al., 2011, indicate thatwell-preserved lobopodian fossils can potentially shed light on the origin of arthropods (Liu et al., 2004, 2006, 2011; Ma et al., 2009). The high diversity and morphological disparity of dis-


articulated phosphatic lobopodian plates represent an excellent complement to soft-bodied Lagerstätte specimens. Despite their near cosmopolitan distribution in the Cambrian, Microdictyon plates had not been previously documented from the North China Platform. Here we report the first occurrence of Microdictyon plates from the Xinji Formation on the North China Platform, extending the paleogeographic distribution of this genus. The aim of this paper is to describe and illustrate the isolated plates from the North China Platform and to provide an overview of the paleogeographic and stratigraphic distribution of Microdictyon and taxonomic problems associated with these net-like sclerites.


Geologic setting


Specimens of Microdictyon described herein were collected from the Xinji Formation at the Shuiyu section, Ruicheng County, Shanxi Province, and at the Shangwan section, Luonan County, Shaanxi Province (Fig. 1.1), along the south margin of the North China Platform (Li et al., 2014). The Xinji Formation is regionally the lowermost Cambrian


strata, disconformably overlying Precambrian strata. It consists of siliciclastic rocks intercalated with carbonates and is mainly distributed on the southwestern to southern margin of the North China Platform (Liu et al., 1991; Liu et al., 1994). The thickness of the Xinji Formation decreases from the south to the north along the south margin of the North China Platform, and the lithology varies (Liu, 1986; Liu et al., 1991; Yan et al., 1993). Due to its diverse and well-preserved small shelly fossils, the Xinji Formation has attracted considerable attention (He et al., 1984; He and Pei, 1985; Feng et al., 1994; Pei and Feng, 2005; Li et al., 2014; Pan et al., 2015; Li et al., 2016; Skovsted et al., 2016), though isolated plates of Microdictyon were not recov- ered previously. Trilobites were also reported from the Xinji Formation (Zhang and Zhu, 1979; Zhang et al., 1979). In the Shangwan section, the Xinji Formation dis- conformably overlies the Luoquan Formation (Ediacaran),


which is composed predominantly of conglomerates, gravel- bearing siltstones, and shales (for stratigraphic column, see fig. 1 in Li et al., 2014). The Xinji Formation is approximately 20 m thick and can be subdivided into two parts. The 11mthick lower part of the formation consists mainly of sandy phosphoric limestones, shales, and siltstones. The ~ 0.8m thick basal bed consists mainly of phosphoric sandy limestones and contains abundant small shelly fossils. One isolated plate of Microdictyon described herein was collected from this bed. The upper part of the Xinji Formation consists mostly of calcareous sandstones and dolomitic limestones with trilobites, but scarce small shelly fossils. The Xinji Formation is conformably over- lain by the Zhushadong Formation (Li et al., 2014). In the Shuiyu section, the Xinji Formation disconformably


overlies the Precambrian strata, either the Luoquan Formation (Ediacaran, predominantly conglomerates) or the Longjiayuan Formation (Paleoproterozoic or Mesoproterozoic, gray dolos- tones) (Fig. 1.2). There is a karst surface between the Luoquan Formation and the Longjiayuan Formation. According to a local geological survey report, the Xinji Formation at Shuiyu section is about 39.8mthick and can be subdivided into two parts: the lower part is dominated by gray-black phosphatic conglomerates, purple- red shale, and phosphoric sandstone, and the upper part consists of red quartz sandstone intercalated with argillaceous dolostone (Bureau of Geology and Mineral Resources of Shanxi Province, 1989). Here, only the basal part (10.7m) has been observed, with the upper part of the section covered by vegetation. There are abundant trace fossils in the basal rocks, and the trace fossil assemblage belongs to a typical Cruziana ichnofacies (Miao and Zhu, 2014), indicating a subtidal environment for the basal Xinji Formation at the Shuiyu section. Several isolated plates of Microdictyon described herein were collected from a strongly weathered bed of calcic phosphoric quartz siltstone, 3.4m above the base of the Xinji Formation (Fig. 1.2). The Xinji Formation yields a small trilobite fauna, con-


sisting of Estaingia (Bergeroniellus) lonanensis Hsiang, 1965 in Lu et al., 1965, Estaingia (Hsuaspis) houchiuensis Chang, 1963 in Hsiang, 1963, and Redlichia cf. R. nanjiangensis Zhang and Lin (Zhang and Zhu, 1979; Zhang et al., 1979; Miao, 2014). Judging from the trilobite assemblage, the Xinji Formation was correlated with the Drepanuloides Zone of the middle Tsan- glangpuan stage (Cambrian Stage 4) on the Yangtze Platform (Zhang and Zhu, 1979; Zhang et al, 1979; Yu et al., 1984; He and Pei, 1985; Miao, 2014). The Estaingia trilobite assemblage can also be compared with the Pararaia janeae Zone of South Australia (Paterson and Brock, 2007; Miao, 2014). In addition, numerous small shelly fossils were discovered from the Xinji Formation, such as chancelloriid sclerites, sponge spicules, hyoliths, Pojetaia runnegari Jell, 1980, Mackinnonia rostrate Zhou and Xiao, 1984, Stenotheca drepanoida He and Pei, 1984 in He et al., 1984, Pelagiella madianensis Zhou and Xiao, 1984, Cupitheca holocyclata Bengston, 1990 in Bengston et al., 1990, C. costellata Xiao and Zhou, 1984, Yochelcionella chinensis Pei, 1985, Cambroclavus absonus Conway Morris, 1990 in Bengston et al., 1990, and Apistoconcha cf. A. apheles Conway Morris, 1990 in Bengston et al., 1990, and so on. (He et al., 1984; Yu et al., 1984; Pei, 1985; Feng et al., 1994; Li et al., 2014; Pan et al., 2015; Li et al., 2016; Skovsted et al., 2016). Most of these fossils have been reported from the Parara


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