Journal of Paleontology, 92(1), 2018, p. 59–70 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.47


Occurrence of Microdictyon from the lower Cambrian Xinji Formation along the southern margin of the North China Platform


Bing Pan,1,2 Timothy P. Topper,3 Christian B. Skovsted,4 Lanyun Miao,1 and Guoxiang Li1


1State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China ⟨bpan@nigpas.ac.cn⟩, ⟨hlmiao@nigpas.ac.cn⟩, ⟨gxli@nigpas.ac.cn⟩ 2University of Science and Technology of China, Hefei 230026, China 3Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham DH1 3LE, United Kingdom


⟨timothy.topper@durham.ac.uk⟩ 4Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden ⟨christian.skovsted@nrm.se⟩


Abstract.—Disarticulated net-like plates of the lobopod Microdictyon had a near cosmopolitan distribution from the early to middle Cambrian but are yet to be documented from the North China Platform. Here we report isolated plates of Microdictyon from the lower Cambrian Xinji Formation (Stage 4, Series 2) of the North China Platform, extending the paleogeographic distribution of Microdictyon in the early Cambrian. The plates of Microdictyon from the Xinji Formation are similar to those of other species established on the basis of isolated plates but do bear some new char- acters, such as mushroom-shaped nodes with a single inclined platform-like apex and an upper surface that displays radial lines. However, the plates documented here are left under open nomenclature due to inadequate knowledge of intraspecific and ontogenetic variation and low specimen numbers. Through comparison of the node shapes of the isolated plates of different Microdictyon species, we consider that low mushroom-shaped nodes could be a primitive and conservative character of Microdictyon while tall mushroom-shaped nodes may be a derived character. Subtle differences in shape and number of node apices may also represent intraspecific or ontogenetic variation.


Introduction


Phosphatic or phosphatized small skeletal fossils are generally dominant components among early Cambrian faunal assemblages. These fossils, including spicules, tubes, plates, cap-shaped shells, and disarticulated sclerites, are usually milli- metric in size and are collectively described as ‘small shelly fossils (SSFs)’ (Matthews and Missarzhevsky, 1975). Although the taxonomic affinities of many disarticulated sclerites remain unresolved, the discovery of extraordinarily well-preserved, articulated, multiplated, soft-bodied specimens in Burgess Shale–type Lagerstätten can provide significant insights into the functional morphology and phylogenetic position of SSF taxa. Perhaps the most influential examples are the descriptions of Microdictyon sinicum Chen, Hou, and Lu, 1989 from the Chengjiang Biota (Chen et al., 1995), Halkieria evangelista Conway Morris and Peel, 1995 from North Greenland (Conway Morris and Peel, 1990, 1995; Vinther and Nielsen, 2005), and Wiwaxia corrugata (Matthew, 1899) from the Burgess Shale (Conway Morris, 1985; Smith, 2014). Minute net-like phosphatic plates were first reported by


Matthews and Missarzhevsky (1975) from the Strenuella Limestone of Comley, England, and were described as proble- matic SSFs. The generic name, Microdictyon Bengtson, Matthews, and Missarzhevsky, 1981 in Missarzhevsky and Mambetov, 1981, was formally introduced on the basis of iso- lated reticulated sclerites from the lower Cambrian of Malyi


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Karatau, South Kazakhstan. To date, isolated Microdictyon plates have been reported from South Australia (Bengtson et al., 1986, 1990; Gravestock et al., 2001; Topper et al., 2009, 2011), South China (Hao and Shu, 1987; Shu and Chen, 1989; Tong, 1989; Li and Zhu, 2001; Zhang and Aldridge, 2007; Yang et al., 2015), Kazakhstan (Missarzhevsky and Mambetov, 1981; Bengtson et al., 1986; Dzik, 2003), Uzbekistan (Bengtson et al., 1986), Mongolia (Esakova and Zhegallo, 1996), Siberia (Bengtson et al., 1986; Missarzhevsky, 1989; Demidenko, 2006; Varlamov et al., 2008; Kouchinsky et al., 2011, 2015), Turkey (Sarmiento et al., 2001), England (Shropshire, Avalonia; Matthews and Missarzhevsky, 1975; Bengtson et al., 1986; Hinz, 1987), Baltica (Bengtson et al., 1986), Northeast Green- land (Laurentia; Skovsted, 2006), Canada (British Columbia, Laurentia; Bengtson et al., 1986), United States (New York, California, Nevada, Utah, Laurentia; Bengtson et al., 1986; eastern Massachusetts, Avalonia; Landing, 1988), and Mexico (Laurentia; McMenamin, 1984; McMenamin and McMenamin, 1990). Although Microdictyon had a near cosmopolitan dis- tribution in the lower and middle Cambrian, the large majority of species have been established exclusively on the basis of disarticulated sclerites; Microdictyon sinicum from the Cheng- jiang Biota (Chen et al., 1995; Hou and Bergström, 1995) and Microdictyon sp. from the Kaili Biota (Zhao et al., 1999, 2002, 2005) remain the only well-known articulated species. These delicate specimens from the Chengjiang Lagerstätte show that Microdictyon is an elongated caterpillar-like animal


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