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Journal of Paleontology 92(1):59–70
et al. (2012) consider that hole size and the overall plate dimensions of Onychomicrodictyon together with one unnamed plate documented by Bengtson (1991) are identical with Onychodictyon ferox Hou, Ramsköld, and Bergström, 1991. Therefore, Onychomicrodictyon was considered to be a junior synonym of Onychodictyon (Steiner et al., 2012; Topper et al., 2013). Because of the anatomy of soft-bodied specimens, Onychodictyon was assigned to Onychodictyidae Hou and Bergström, 1995 (Hou and Bergström, 1995; Liu et al., 2008). After the discovery of the soft-bodied Microdictyon
sinicum, it has been well known that the trunk of this species has nine pairs of plates along sides of the caterpillar-like body. The plates show a regular net-like morphology with a hexagonal hole arrangement but vary distinctly in size and outline. Chen et al. (1995) delineated four morphotypes: elongate (the anterior pair), round (the second pair), ovoid (the third to eighth pairs), and rhomboidal (the posterior pair). Microdictyon sinicum demonstrates that the plate outline is variable along the trunk of a single animal, indicating that plate outline is not a reliable taxonomic character. The soft-bodied specimens were preserved in fine siliciclastic rocks, and unfortunately the details of node morphology and microstructure have been obscured by diage- netic processes, complicating detailed comparisons with iso- lated phosphatic plates retrieved from acid residues. Zhang and Aldridge (2007) have demonstrated that node microstructure is relatively consistent at species level according to a large col- lection of isolated plates from southern China. Zhang and Aldridge (2007) do report some morphological variation within species, and it remains to be seen whether the degree of variation represents intraspecific variation or warrants the erection of a separate species. Conjoined plates of Microdictyon and Onychodictyon that are interpreted as having preserved the molting process display identical ornamentation, indicating that plate ornamentation did not vary during ontogeny (Zhang and Aldridge, 2007; Topper et al., 2013). However, these specimens only represent a single molt stage, and the entire ontogenetic sequence of plate-bearing lobopodians remains unclear. With the information currently at our disposal, the description of Microdictyon plates must focus on the arrange- ment of holes and microstructure of the nodes, rather than on plate outline and size. To date, there are eight Microdictyon species that were
erected on the morphology of isolated plates (only M. sinicum was established on soft-bodied preservation). All the soft- bodied fossils of Microdictyon recovered from the Chengjiang Biota at different sections in eastern Yunnan belong to Microdictyon sinicum. Without knowledge of the detailed plate morphology of M. sinicum, it is entirely possible that many of these species based solely on isolated plates may be plate mor- photypes of M. sinicum. All specimens recovered from the Xinji Formation possess
the typical hexagonal meshwork arrangement and mushroom- shaped nodes that characterize Microdictyon. The plates described herein show similarities to some formally established species but also bear some new characters. Due to the small sample set, and without detailed information regarding scleri- tome configuration and intraspecific and ontogenetic variation (see Topper et al., 2011), the authors here take a cautious approach and describe the Microdictyon specimens under open
nomenclature at species level. The morphological character- istics of the plates will be described in detail.
Systematic paleontology
The terminology used to describe the Microdictyon plates follows that of Zhang and Aldridge (2007). Phylum Lobopodia Snodgrass, 1938
Class Xenusia Dzik and Krumbiegel, 1989
Order Archonychophora Hou and Bergstrӧm, 1995 Family Eoconchariidae Hao and Shu, 1987 Genus Microdictyon Bengtson, Matthews, and
Missarzhevsky, 1981 in Missarzhevsky and Mambetov, 1981
Type species.—Microdictyon effusum Bengtson, Matthews, and Missarzhevsky, 1981 in Missarzhevsky and Mambetov, 1981.
Other species.—M. rhomboidale Bengtson, Matthews, and Missarzhevsky, 1986; M. robisoni Bengtson, Matthews, and Missarzhevsky, 1986; M. chinense Hao and Shu, 1987; M. sphaeroides Hinz, 1987; M. sinicum Chen, Hou, and Lu, 1989; M. depressum Bengtson in Bengtson et al., 1990; M. fuchengense Li and Zhu, 2001; M. jinshaense Zhang and Aldridge, 2007.
Remarks.—The plates of Microdictyon, Quadratapora, and Fusuconcharium show similarity in meshwork arrangement, with each perforation surrounded by a number of protruding nodes. Genera are distinguished by the arrangement of holes and the microstructure of the nodes. Microdictyon plates exhibit a regular hexagonal meshwork that is distinct from Quadratapora plates, with a regular tetragonal meshwork, and Fusuconcharium plates, which display a slightly more irregular meshwork. Only Microdictyon is well known from soft-bodied preservation. The isolated plates of Quadratapora and Fusuconcharium have only been reported from the Shuijingtuo Formation in Zhenba, Shaanxi, South China (Hao and Shu, 1987), where they co-occur with Microdictyon (Zhang and Aldridge, 2007). In the Shuijingtuo Formation, the number of plates belonging to Quadratapora (63) and Fusuconcharium (28) is quite small in comparison to that of the documented plates of Microdictyon (577; data from Zhang and Aldridge, 2007), and it is possible that the plates of Quadratapora and Fusuconcharium are aberrant forms or representatives of other Microdictyon species. Microdictyon rozanovi Demidenko, 2006 was erected on the basis of two broken plates that display a char- acteristic extended edge (Demidenko, 2006). This lateral extension is somewhat similar to the edge of M. rhomboidale, but we con- sider this subtle variation of the plate edge as insufficient evidence to erect a new species and, on the basis of node morphology, we suggest that these specimens most likely represent interspecific variants of M. rhomboidale. McMenamin (1984) informally described a new species, M. multicavus, from the lower Cambrian of Sonora, Mexico. However, due to the poor preservation of the specimens illustrated and the lack of a subsequent formal description, we do not recognize the species as valid, pending further study.
Microdictyon sp. Figures 2–4
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