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Journal of Paleontology 92(1):49–58
the studied material as seemingly persistent elements of phos- phatized forms suggests that Cambrian paleoscolecids achieved a widespread geographic distribution in South China and exploited a sophisticated life habit in the early Cambrian seafloor.
Geologic setting and age constraints
The described fossil material was collected from yellowish- green silty mudstone of the lower Yanwangbian Formation at the Yangjiagou section near Xixiang County, southern Shaanxi Province of China, bordering the north margin of the Yangtze Block (Fig. 1.1–1.3). In the study area, the Yanwangbian Formation is overlain by dolostones of the Kongmingdong Formation, in which fossils are rare (Chen and Zhang, 1987), and it conformably overlies the Xiannudong Formation, a suc- cession of limestone of archaeocyathan reef facies (Hicks and Rowland, 2009; Yang et al., 2016). Lithologically, the Yanwangbian Formation at the Yangjiagou section is about 298m thick and consists mainly of thick-bedded sandstones interbedded with yellowish-green shales or mudstones (Fig. 1.4), with common ripple marks (Fig. 1.5) and bioturba- tions (Chen and Zhang, 1987; Zhang et al., 2008; Zhu, 2010). It therefore represents sediments deposited in a littoral facies environment (Zhu, 2010). Surprisingly, the interbedded shales from the lower part of the formation (Fig. 1.6) yield a relatively productive fossil assemblage. This Lagerstätte is dominated by abundant shelly fossils such as trilobites, hyolithids, bradoriids, and brachiopods (e.g., Chen and Zhang, 1987; Chen, 1999; Liu et al., 2008); soft-bodied paleoscolecids have also been collected (Zhang et al., 2008, fig. 4B). In terms of trilobite biostratigraphy, Chen and Zhang
(1987) proposed that the upper and lower parts of the Yanwangbian Formation at the Yangjiagou section were com- parable with the Wulongqing and Hongjingshao members of the Canglangpu Formation in eastern Yunnan, respectively. Whereas the occurrence of the trilobite Palaeolenus in the lower Yanwangbian Formation (e.g., Cheng et al., 1980; Chen and Zhang, 1987) indicates that the fossil horizons are generally correlated with the Canglangpuan regional stage for South China (Zhang et al., 2008) and Series 2, Stage 4 of the provi- sional stratigraphic scheme for the Cambrian (Peng et al., 2012). It is therefore coeval with the Shipai fauna (Zhang and Hua, 2005; Yang and Zhang, 2016) and the Guanshan biota (Luo et al., 2006; Hu et al., 2008), also from South China, as well as the Emu Bay Shale from South Australia (Paterson et al., 2008). Clearly then, the Yanwangbian assemblage is a new Burgess Shale–type biota, implying a unique faunal realm from the north margin of the Yangtze Platform during the early Cambrian.
Materials, preservation, and methods
The new taxon is represented by twelve specimens, all of which are long annulated trunks (Fig. 2) from the trilobite Palaeolenus Zone of the Yanwangbian Formation near Xixiang County, Shaanxi Province, China. All specimens investigated in this study are obtained by splitting mudstone slabs, meaning that fossils are generally flatted and compressed on bedding sur- faces. However, several specimens are preserved in slight relief
(Fig. 2.1–2.6), possibly due to postmortem contraction. Intriguingly, individuals either were obtained by splitting the longitudinal section of the rock slab (Fig. 2.6) or display a penetrative attitude into the substrate (Fig. 2.1), revealing dra- matic information regarding life habit (see the following). These fossils can also be treated as evidences of in situ preservation of the material. Studied material is generally preserved in a curved attitude (Fig. 2.1, 2.5–2.8); thus, the concave side is considered ventral because paleoscolecids are always coiled with the ventral side inward (Huang, 2005). Extremities at both ends are unfortunately not preserved, so it is difficult to determine an anterior-posterior direction for these fossils. Although none of the fossils are preserved together with a lined burrow, the pre- sence of black gut trace (Fig. 2.6) shows that the fossils are decayed remains and not molts. Fine cuticular structures (plates and associated micro-
structures) are well preserved in most specimens (Figs. 2, 3). EDX elemental mapping concurs with a common view that the plates of paleoscolecids are usually composed of calcium phosphate. The specimens were photographed using a Canon 5D
Mark II camera with auxiliary light to increase the color contrast of the morphological structures. A FEI Quanta 500 scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX) was used at the State Key Laboratory of Continental Dynamics of Northwest University, China. Line drawing and diagrammatic figures were made using Coreldraw X6 and Photoshop CS4 software.
Repository.—Fossils investigated in this study were housed in the collection of the Early Life Institute,Northwest University,China.
Systematic paleontology
Terminology.—The definitions of morphological terms are a combination of Conway Morris and Robison (1986), Müller and Hinz-Schallreuter (1993), Conway Morris (1997), and Topper et al. (2010).
Class Palaeoscolecida Conway Morris and Robison, 1986 Family Palaeoscolecida Whittard, 1953 Genus Shaanxiscolex new genus
urn:lsid:
zoobank.org:act:37B6571C-E363-4869-B50B- B597C88F88AD
Type species.—Shaanxiscolex xixiangensis new genus new species, from the Palaeolenus Biozone of the Cambrian Yan- wangbian Formation at the Yangjiagou section near Xixiang County, Shaanxi Province, China.
Diagnosis.—Macroscopic worm, with slender, cylindrical body consisting of uniform, serial annuli repeated along entire trunk, each annulation with two rows of alternating Hadimopanella- type plates positioned close to borders and with a mosaic pattern of intervening microplates separating the plates and filling intersegmental furrows. Plates round or slightly ovate discs, 40 to 95 μm in diameter and generally isolated from one another; each plate covered with one or two circles of five to 18 nodes and an occasional central node on surface.
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