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Journal of Paleontology 92(1):49–58
Remarks.—The new genus currently includes the type species only. Despite its overall dimensions and gross morphological similarity to some reported paleoscolecid fossils, Shaanxiscolex differs notably from other genera by the exclusive cuticular ornamentation that includes Hadimopanella-type plates bearing one or two circles of nodes on the surface and with a mosaic pattern of microplates occurring between the plates and within the intersegmental furrows. The differentiation of cuticular sclerites (e.g., plates, platelets, and microplates) has been suggested as being useful for taxonomic purposes of paleosco- lecids (Botting et al., 2012), and Paleozoic paleoscolecids from different localities and/or horizons often exhibit different scleritomes (Yang, 2016). Accordingly, the taxonomic resolu- tion attainable for the Yanwangbian material, in our opinion, recognizes a genus-level variation and thereby is described as a new genus, Shaanxiscolex.
Shaanxiscolex xixiangensis new genus new species urn:lsid:
zoobank.org:act:172F65C8-C047-44A4- 869D-347677703EB0 Figures 2, 3, 4.1, 5.1–5.3
Holotype.—SXYY-P 008a/b (part and counterpart; Fig. 2.1–2.3), an incomplete specimen with tessellating phosphatic plate array; trilobite Palaeolenus Zone of the Cambrian Yanwangbian Formation at the Yangjiagou section near Xixiang County, Shaanxi Province, China.
Paratypes.—Atotal of 12 compression fossils (SXYY-P 001a/b, 002, 004, 005, 009a/b, 010, 011, 012, 013, 014, 016, 017a/b) from the same level and locality as the holotype.
Diagnosis.—As for the genus. Occurrence.—As for the type species.
Description.—Specimens are preserved as flattened trunk compressions lacking both oral and aboral terminations, and most show a curved attitude (Fig. 2). A midfold along the trunk present in an individual (Fig. 2.4) is probably taphonomical. The trunk is slender and cylindrical, with preserved length ranging from 8.9 to 30.7mm; its width is 1.3 to 3.0mm. The fossils are often preserved in slight relief, which may be attributed to postmortem contraction; two cuticle layers are observed in some instances (Fig. 2.9). The entire trunk is traversed by a series of raised annuli,
with about 4–7 annulations per millimeter. Each annulus is typically covered with two rows of Hadimopanella-type plates that lie parallel to the annulations. The plates are generally positioned close to the borders of the annulations and are isolated from one another; they tend to be separated by a mosaic pattern of intervening microplates (Figs. 2, 3). The plates are round or slightly ovate discs in outline, mostly showing severe abrasion, with the upper series of node crowns invariably scattered (Fig. 3) due to taphonomic processes. The majority of plates are 65 to 95 μm in diameter with two circles of 12–18 nodes and an occasional central node on the convex surfaces (Fig. 3.4–3.7). A few smaller plates (40 to 50 μm in diameter) adorned the surfaces with a single circle of approximately five
nodes and an occasional central node (Fig. 3.8). Microplates are irregular in shape and size (approximately rhombic pattern to completely irregular), forming a mosaic and intervening pattern, separating the plates and filling the intersegmental furrows (Fig. 3.1–3.4). The platelet is invisible. A black, narrow, and straight gut trace is occasionally present and runs the length of the body (Fig. 2.6).
Etymology.—After Xixiang County of Shaanxi Province, where the Yanwangbian assemblage to which the new material belongs is located.
Remarks.—Since macroscopic and compressed paleoscolecids collected from shales often superficially resemble one another in gross morphology (Conway Morris, 1997; Harvey et al., 2010; Yang and Zhang, 2016), the rare preservation of their cuticular ornamentations prevents straightforward comparisons with Shaanxiscolex xixiangensis n. gen. n. sp. However, the arrangement of scleritomes of S. xixiangensis is reminiscent of some previously reported paleoscolecid taxa (Fig. 4). For example, Wudingscolex sapushanensis Hu et al., 2012 from the Guanshan Biota in Yunnan has two rows of plates and numer- ous platelets in each annulus, and each plate is covered by three circles of nodes outside and one or two central nodes (Hu et al., 2012, fig. 3A–G). Wudingscolex lacks comparable microplates of Shaanxiscolex and bears many more nodes (36 to 42) on the surface of the plates (Hu et al., 2012). Guanduscolex minor Hu et al., 2008 is another species known from the Guanshan Biota characterized by each annulus having three rows of smaller plates (about 30 μm in diameter), each plate with a circle of nine to 10 nodes and about five central nodes (Hu et al., 2008, pl. 1–10; Fig. 4.3). By comparison, each annulus of S. xixiangensis typically has two plate rows with a mosaic pattern of microplates between the plates (Fig. 4.1, 4.3). Wronascolex Ivantsov and Zhuravlev, 2005 is a fairly
species-rich and cosmopolitan genus extending from Cambrian, Stage 4 to Drumian (Yang and Zhang, 2016), characterized by two Hadimopanella-type plate rows in each annulus and adorning plates with 4 to10 nodes arranged in a single circle and occasionally with a central node on the upper surfaces (García-Bellido et al., 2013; Yang and Zhang, 2016; Fig. 4.2). A significant difference between Wronascolex and Shaanxiscolex, however, is that the latter has intervening microplates on the surfaces of annuli and its plates are always adorned with one or two circles of nodes (Fig. 4.1–4.2). The scleritome pattern of Shaanxiscolex is also similar to the
patterns of some phosphatized microscopic taxa. For example, a phosphatized trunk fragment (SAMP 43301) under open nomenclature was recently reported from the lower Cambrian Mernmerna Formation of South Australia (Topper et al., 2010). Each annulus of SAMP43301 consists of two rows (occasionally with a developed third row) of irregularly alternating plates (with one to three conical median nodes) positioned close to the borders, and with the interannular furrows occupied by irregular
microplates and occasional smaller platelets (Topper et al., 2010, fig. 4; Fig. 4.4). However, S. xixiangensis lacks platelets and its plates generally bear many more nodes (12 to18). These differences argue against a close relationship between the two taxa. Another comparable phosphatized taxon is the
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