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Journal of Paleontology 92(1):59–70
variation, with one hole surrounded by four nodes and 13 other holes respectively surrounded by five and seven nodes in plate S1 (square in Fig. 2.1, 2.2). In most cases, each node connects three walls; however, a few nodes can be observed connecting four walls in plate S1 (arrows in Fig. 2.1, 2.2, 2.6). Most of the nodes are low and mushroom-shaped, with an ovate to sub- triangular outline. The length of the nodes ranges from 29.24 to 160.94 µm with the average of 77.26 µm and decreases toward the periphery of the plate. The majority of nodes show a single acentric low apex that protrudes from the surface. The apices roughly point in a single direction (Figs. 2.2, 2.3, 3.1, 3.10). It is noteworthy that many apices have a distinct circular or sub- circular rim that forms a slightly concave platform at an oblique angle with the surface of the mushroom-shaped nodes (Fig. 2.3, 2.7–2.10). The apex of the node in plate S3 is quite distinct, with a relatively planar surface (Fig. 4.1, 4.2, 4.4, 4.5). The diameter of the apices ranges from 10.00 to 33.29 µm with the average of 21.52 µm. Toward the periphery of the plate, the mushroom- shaped nodes of plates S1 and S5 deform, giving the appearance of a single sharp tubercle with an indistinct rim (Figs. 2.2–2.4, 3.9), and some apices of nodes of plate S2 separate into multiple little tips (Fig. 3.1, 3.2, 3.7). The multiple tips only cover half of the apex of one node in plate S2 (Fig. 3.6). The upper surfaces of the nodes in plate S3 bear radial lines that originate from the apex and extend outward to the rim of the node (Fig. 4.4–4.6), possibly extending into the base of the nodes (Fig. 4.5). The mushroom-shaped nodes are hollow and sealed by one capping layer (Figs. 2.5, 3.8, 4.8, 4.9). On the inner surface, each hole has a cylindrical tubular wall with a basal opening (Figs. 2.6, 3.3). The peripheral girdle has a slightly inclined outer edge and an upright inner edge (Fig. 3.9). The plate microstructure con- sists of one basal framework layer (Fw) and one upper capping layer (Cl) attaching on the framework (Figs. 2.11, 3.5, 3.8, 4.8, 4.9). One poorly preserved fragment retrieved from the Shang- wan section shows that holes are surrounded by regularly hexa- gonally arranged mushroom-shaped nodes (Fig. 4.7).
Remarks.—The isolated plates of Microdictyon from the North China Platform show similar characteristics to previously reported isolated plates of Microdictyon. The ovoid, strongly convex plate and low mushroom-shaped nodes are quite similar to the type species, M. effusum (Bengtson et al., 1986), but the single oblique platform-like or relatively protruding planar apices of the plates from North China contrast with the single spiny apices of M. effusum. Multi-tip apices have been described in Microdictyon chinense and M. jinshaense (Zhang and Aldridge, 2007; Topper et al., 2009), but this represents a rather pervasive character over the entire plate, rather than the rare peripheral occurrences observed in the North China material. The spiny nodes of the plates from North China occur in many species of Microdictyon, such as Microdictyon chinense, Microdictyon cf. M. effusum, Microdictyon jinshaense (Zhang and Aldridge, 2007), and Microdictyon cf. M. depressum (Skovsted, 2006). But the plates described herein can be easily distinguished from the plates of these species by the combina- tion of characters listed in the preceding. The outline and arrangement and microstructure of the nodes on the North China Microdictyon plates also show some similarities to the plates of M. depressum (Bengtson et al., 1990), but the plates of
M. depressum are quite flat and their single-tip apices are low. Topper et al. (2011) reported 6 types of plates from the Ajax Limestone in South Australia. Like the specimens documented herein, some of them also bear multi-tip apices, but the shape and combination of the tips are obviously different. Both assemblages are relatively small (herein: five specimens; Topper et al., 2009: six specimens), impeding a detailed comparison.
The radial line on the surface of the nodes is here reported for the first time on Microdictyon plates. Discussion
Stratigraphical range and distribution of Microdictyon.— Microdictyon has a cosmopolitan geographic distribution from the Cambrian uppermost Stage 2 to uppermost Stage 5 (Figs. 5, 6). Complete soft-bodied specimens however have only been discovered from South China. Microdictyon is here documented from the lower Cambrian Xinji Formation of the North China Platform for the first time, extending the paleogeographic range of the genus in the late early Cambrian (Stage 4). The earliest occurrence of Microdictyon is from the lower Micrina etheridgei Zone (below the Abadiella huoi Zone) of the Ajax Limestone in section AJX-M from South Australia (Topper et al., 2011; Betts et al., 2016), which may represent the uppermost Cambrian Stage 2. Bengtson et al. (1986) also reported Microdictyon? tenuiporatum revised as Quadratapora tenuiporatum by Zhang and Aldridge (2007) from the Tom- motian Stage (pretrilobite stratum). In Stage 3, Microdictyon is reported from the Abadiella huoi Zone in Australia (Bengtson et al., 1990; Topper et al., 2011) that correlates with the oldest trilobite zone, Parabadiella Zone in South China (Betts et al., 2016, 2017). This may be approximately coeval to the Eofal- lotaspis Zone in Morocco and the Profallotaspis jakutensis Zone in Siberia, as suggested by Yuan et al. (2011). The genus then rapidly dispersed to many paleocontinents, with the majority of described species documented from the Eoredlichia- Wutingaspis Zone in South China (Chen et al., 1995; Li and Zhu, 2001; Zhang and Aldridge, 2007), the lower Nevadella Zone in Laurentia (British Columbia [Canada], New York, California, Nevada, Utah [United States], Mexico: Bengtson et al., 1986; McMenamin, 1984), the Callavia Zone in Avalonia (Avalonian part of eastern Massachusetts [United States]: Bengtson et al., 1986; Landing, 1988; Shropshire [England]: Hinz, 1987), and the upper Delgadella anabara Zone to the Judomia Zone in Siberia (Kouchinsky et al., 2015). Some iso- lated plates of Microdictyon are also reported in the uppermost strata of Stage 3, in localities such as Kazakhstan (Bengtson et al., 1986; Dzik, 2003), Uzbekistan (Bengtson et al., 1986), Siberia (Bengtson et al., 1986; Varlamov et al., 2008), and Mongolia (Esakova and Zhegallo, 1996). In Stage 4, Micro- dictyon has been documented from the Pararaia bunyerooensis Zone in Australia (Topper et al., 2009), the Lermontovia grandis Zone of Siberia (Demidenko, 2006), the Bonnia-Olenellus Zone in Northeast Greenland (Skovsted, 2006), the Elliptocephala asaphoides assemblage in the United States (Laurentia part of eastern New York; Bengtson et al., 1986), the Strenuella Limestone and Protolenus Limestone in Shropshire (England/ Avalonia; Bengtson et al., 1986; Hinz, 1987), and North China (herein). There is a sharp decrease in distribution and diversity
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