B. Pan et al.—Microdictyon plates from the lower Cambrian of North China
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Figure 1. Locality map of sampled sections and the lithostratigraphic column of Shuiyu section. (1) Locality map showing the positions of Shuiyu section and Shangwan section; (2) lithostratigraphic column of the Xinji Formation at Shuiyu section, Ruicheng County, Shanxi Province. LQ F. = Luoquan Formation; LJY F. = Longjiayuan Formation.
Limestone (Stansbury Basin) and Mernmerna Formation (Arrowie Basin) in South Australia (Bengtson et al., 1990; Gravestock et al., 2001; Topper et al., 2009; Betts et al., 2016, 2017). Judging from the trilobite and small shelly fossil assemblages, and similarities with South Australian assem- blages, the Xinji Formation can be roughly correlated with Cambrian Series 2, Stage 4 (mid-late Botoman age in Siberian terminology).
Materials and methods
One well-preserved plate and four fragmented plates of Microdictyon were retrieved from calcic phosphoric fine quartz sandstone samples collected from a horizon 3.4m above the base of the Xinji Formation at the Shuiyu section, Ruicheng County, Shanxi Province. Only one poorly preserved fragment of Microdictyon plate was retrieved from phosphoric sandy limestone samples collected from the base of the Xinji Forma- tion at the Shangwan section, Luonan County, Shaanxi Province.
Samples collected from the Shuiyu section were processed
by digestion in 6% acetic acid, and samples from the Shangwan section were processed by digestion in 10% acetic acid. The selected specimens were placed on stubs, gold coated, and photographed using the scanning electron microscopy facility (LEO 1530VP) at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing.
Repository and institutional abbreviation.—All the illustrated specimens are housed and cataloged at storage facilities of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), Nanjing.
Taxonomy of isolated eoconchariid plates
The Cambrian net-like isolated plates, including Microdictyon, are usually described under the family Eoconchariidae (Chen et al., 1995; Demidenko, 2006; Zhang and Aldridge, 2007; Topper et al., 2009), which consists of three genera: Microdictyon Bengtson, Matthews, and Missarzhevsky, 1981, in Missarzhevsky andMambetov, 1981, Quadratapora Hao and Shu, 1987, and Fusuconcharium Hao and Shu, 1987. Isolated eoconchariid plates retrieved from acid residues are perforated, and each hole is surrounded by a series of nodes on the external surface. Different genera are predominantly recognized on the basis of plate shape, perforation size and arrangement, and node morphology. For example, Microdictyon exhibits hexagonally arranged holes with mushroom-shaped or spiky nodes; Quadratapora exhibits tetragonally arranged holes with sharply crested walls; Fusuconcharium exhibits a disorderly hexagonal hole arrangement with spine-like or rounded nodes (Zhang and Aldridge, 2007). Demidenko (2006) established the genus Onychomicrodictyon due to its plates combining features of Microdictyon (net-like perforated plates with five to seven mushroom-shaped nodes around one hole) and Onychodictyon Hou, Ramsköld, and Bergström, 1991 (strong spine-like process) and assigned it to Eoconchariidae. However, Steiner
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