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There are morphological similarities between the species documented from China and Australia; for example, nodes with multiple-tip apices only occur in the plates of M. chinense and M. jinshaense from South China (Zhang and Aldridge, 2007) and M. jinshaense from South Australia (Topper et al., 2009, 2011) and the plates illustrated herein. This could indicate that North China had a close paleogeographic position with South Australia and South China in the early Cambrian. The early occurrence (Stage 2), wide distribution, and long stratigraphic range of Microdictyon suggest that the genus may have been a pioneering member of Cambrian communities displaying a strong adaptability to a variety of environments during the Cambrian explosion.
Morphological variation of the nodes of the isolated Microdictyon plates.—It is obvious that some characters of the nodes of Microdictyon plates can occur in different Microdictyon species. For example, the short spiny nodes over the entire external surface of some medium-sized plates (possibly representing an inter- mediate ontogenetic growth stage) of Microdictyon chinense (Zhang andAldridge, 2007) are also observed at themargin of the plates of several other species, such asMicrodictyon cf.M.effusum, Microdictyon jinshaense (Zhang and Aldridge, 2007), Microdictyon cf. M. depressum (Skovsted, 2006), and the plates herein. Thus, the short spiny node, which occurs in particular positions on plates or potentially in selective ontogenetic stages of some species, may be an intraspecific or ontogenetic variable character. The mushroom-shaped nodes are also quite common among isolated Microdictyon plates. Low mushroom-shaped nodes occur in nearly all the species of Microdictyon, but tall mushroom-shaped nodes only occur in the plates of M. robisoni (see Kouchinsky et al., 2011). Because of the later occurrence (Cambrian, Stage 5) of M. robisoni with the tall mushroom- shaped nodes, this charactermight be considered a derived feature, and low mushroom-shaped nodes may be a primitive and con- servative character. As discussed, the character of the node apices is critical to distinguishing species of Microdictyon when pre- sentedwith only isolated plates; however, a cautious approach has been advocated (Zhang and Aldridge, 2007; Topper et al., 2009, 2011).As noted in the preceding, nodeswith single-tip apices and multiple-tip apices can coexist in a single plate (Fig. 3.1, 3.2, 3.7) and possibly represent intraspecificorontogeneticvariation. Comparing with other types of single-tip apices documented
in the literature, the oblique platform-shaped apices of the nodes figured herein are quite distinct but bear subtle similarities to the apices of the tall mushroom-shaped nodes of M. robisoni from Siberia described by Kouchinsky et al. (2011, figs. 28, 29). The apices of M. robisoni from Siberia, however, also typically bear several flattened tubercles that are not observed in the specimens herein. Due to the small sample set, it is not known whether the single oblique platform-shaped apices are a stable character of Microdictyon plates from North China, and the significance of this character for the taxonomy of the isolated Microdictyon plates is uncertain. The radial surface lines described herein on one specimen (Fig. 4.5, 4.6) may represent an aberrant form similar to the strong spine of Microdictyon cf.M. effusum and Microdictyon jinshaense (Zhang and Aldridge, 2007), or it may simply be a preservational artefact.
Conclusion
Microdictyon plates are reported for the first time from the lower Cambrian Xinji Formation (Stage 4, Series 2) of the North China Platform, extending the Cambrian paleogeographic range of Microdictyon. The plates described herein are characterized by low mushroom-shaped nodes and a single oblique platform- like apex. Some morphological variations do exist, with nodes in some plates characterized by multiple apices, and the upper surface of the nodes on another plate bears radial lines that extend from the apex to the rim of the node. These radial lines are a new nodular microstructure and may represent a case of intraspecific variation or a preservational artefact. Due to uncertainties regarding intraspecific and ontogenetic variation and the small sample size, the specimens herein have been left in open nomenclature. Microdictyon species described from iso- lated plates have in the past been established on the combination of several characters. However, studies dealing with the taxo- nomy of isolated plates should comprehensively analyze whe- ther the morphological variability observed in the assemblage could represent intraspecific or ontogenetic variation. Much of this can only be done when examining large data sets. It appears possible that low mushroom-shaped nodes may represent a primitive character of the genus while the tall mushroom-shaped nodes may be a more derived character. It is likely that the minor morphological variations observed in this small assemblage represent intraspecific or ontogenetic varia- tion; however, in the absence of additional material, this remains unresolved.
Acknowledgments
This work was supported by grants from the National Natural Science Foundation of China (41372021), the Chinese Academy of Sciences (XDB10010101), the Ministry of Science and Technology of China (2013CB835000), and a COFUND fellowship from Durham University. M. Zhu, F. Zhao (Nanjing), and L. Zhang (Xi’an) are thanked for the field guidance and assistance. G. Brock (Macquarie University), an anonymous reviewer, and the editors are also thanked for their careful work and constructive reviews that much improved the manuscript.
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