Y. Cai & H. Hua comment: Zhuravlev, Gámez Vintaned & Ivantsov (Reference Zhuravlev, Gámez Vintaned and Ivantsov2009) reported the problematic Ediacaran fossil Gaojiashania annulucosta in Siberia and they considered that this is the first find of Gaojiashania outside China, since Gaojiashania had previously only been reported from the Gaojiashan Member of the middle Dengying Formation in the Ningqiang area, southern Shaanxi Province, South China. However, we believe that the so-called Siberian Gaojiashania was mis-identified, and what was described as Gaojiashania annulucosta by Zhuravlev, Gámez Vintaned & Ivantsov (Reference Zhuravlev, Gámez Vintaned and Ivantsov2009) is more appropriately ascribed to Shaanxilithes ningqiangensis, another problematic Ediacaran fossil that has also been known from the Gaojiashan Member in Shaanxi Province of South China (Chen, Chen & Lao, Reference Chen, Chen and Lao1975; Xing et al. Reference Xing, Ding, Luo, He and Wang1984), as well as the stratigraphically equivalent Taozichong Formation in Guizhou Province (Hua, Chen & Zhang, Reference Hua, Chen and Zhang2004) and the Jiucheng Member (Dengying Formation) in Yunnan Province of South China (Zhu & Zhang, Reference Zhu, Zhang, Peng, Babcock, Zhu and Li2005), the Zhoujieshan Formation in Qinghai Province (Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007), and the Zhengmuguan Formation in Ningxia Hui Autonomous Region of North China (Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007).

Here we illustrate diagnostic features of Gaojiashania and Shaanxilithes from their type localities (the Gaojiashan section and Lijiagou section, respectively) in southern Shaanxi in order to clarify their identification.

1. Gaojiashania

The genus Gaojiashania Yang, Zhang & Lin, Reference Lin, Zhang, Zhang, Tao and Wang1986 (not Yin, Zhang & Lin, as cited in Zhuravlev, Gámez Vintaned & Ivantsov, Reference Zhuravlev, Gámez Vintaned and Ivantsov2009) was established in Lin et al. (Reference Lin, Zhang, Zhang, Tao and Wang1986; see also Zhang, Reference Zhang1986). According to Lin et al.'s (Reference Lin, Zhang, Zhang, Tao and Wang1986) and Zhang's (Reference Zhang1986) initial description, the type species Gaojiashania cyclus (Fig. 1a–f) is a tubular body fossil and is characterized by densely articulated body rings. The rings can be disarticulated and preserved as isolated pieces (see pl. 1, fig. 15 of Lin et al. Reference Lin, Zhang, Zhang, Tao and Wang1986). The fossils can be straight (Fig. 1a) or sinuous (Fig. 1d). The tubes of Gaojiashania are typically 50–60 mm in length (although most specimens are incompletely preserved, and the longest specimen we have observed is >200 mm in length) and 7–9 mm in diameter, and the rings (Fig. 1e, f) are 1–2 mm in thickness (Lin et al. Reference Lin, Zhang, Zhang, Tao and Wang1986; Zhang, Reference Zhang1986). Gaojiashania cyclus occurs in fine-grained sediments, primarily siltstone/calcisiltite and mudstone/calcilutite beds. It is typically pyritized and preserved approximately parallel to bedding surface.

Figure 1. Gaojiashania cyclus and Shaanxilithes ningqiangensis from the Gaojiashan Member at the type localities (a–j) and Taozichong Formation in Guizhou (k–l). (a) Three-dimensionally pyritized and well-preserved Gaojiashania cyclus; (b) close-up of ‘aperture’ part in (a); (c) close-up of (a) showing thickened body rings; (d) G. cyclus showing sinuous tube (possibly due to post-mortem bending and twisting); (e) disarticulated body rings of G. cyclus preserved on the bedding surface of siltstone/mudstone beds in the Gaojiashan Member; (f) isolated and pyritized body ring of G. sp. (g, h) Shaanxilithes ningqiangensis from the Gaojiashan Member (Dengying Formation) at the Lijiagou section (type section; Xing et al. Reference Xing, Ding, Luo, He and Wang1984), southern Shaanxi Province, South China. Note three S. ningqiangensis specimens (arrows in g), with different sizes, are preserved in slightly different layers (g). (i, j) S. ningqiangensis from the Gaojiashan Member (Dengying Formation) at the Gaojiashan section (~20 km southeast of the type section, Lijiagou section), southern Shaanxi Province, South China. Note that the ribbon compressions overlap but do not cross-cut (arrows in i). Also note the mass accumulation (specimens are preserved in slightly different layers) and varying degrees of fragmentation (j). (k) S. ningqiangensis from the Ediacaran Taozichong Formation, Guizhou Province, South China. Note sinuous ribbons. (l) Close-up of S. ningqiangensis showing closely spaced annulations. All scale bars represent 1 cm, except in (l) 5 mm. A colour version of this figure is available at http://www.cambridge.org/journals/geo.

Gaojiashania annulucosta Zhang, Li & Dong (Zhang et al. Reference Zhang, Dong, Tian, Ding, Ding, Zhang, Li and Dong1992; Li et al. Reference Li, Ding, Zhang, Dong, Chen, Ding, Zhang, Li and Dong1992) was identified from thin-sections of limestones in the Gaojiashan Member. According to the initial description, the tubes are slightly curved and preserved parallel to bedding plane, with a length of 4.5–6.2 cm, diameter 8–12 mm. The body rings, with a thickness of 0.8 mm, can be clearly identified in thin-sections and average spacing between body rings is approximately 2 mm. The size variations in the diameter of body rings are due to obliquely cut thin-sections. The tubes are calcified and filled with microsparitic calcites, and the surrounding matrices are lime muds.

After examining the type specimens of G. cyclus and G. annulucosta, we believe that their basic morphology and size are similar. The only difference is their taphonomy: G. cyclus is pyritized and preserved in siltstone and mudstone, whereas G. annulucosta is calcified and preserved in limestone. We think that these two species are taphonomic variants and should be ascribed to one species, that is, the type species, Gaojiashania cyclus Yang, Zhang & Lin, 1986 (in Lin et al. Reference Lin, Zhang, Zhang, Tao and Wang1986; Zhang, Reference Zhang1986).

Gaojiashania has always been interpreted as a body fossil since its publication. The interpretation is supported by the observation that some of the body rings of Gaojiashania can be disarticulated (Fig. 1e, f). Such disarticulation is inconsistent with a trace fossil preservation.

2. Shaanxilithes ningqiangensis

Chen, Chen & Lao (Reference Chen, Chen and Lao1975) first described cf. Sabellidites sp. from the Gaojiashan Member in southern Shaanxi and they interpreted it as a body fossil. Similar fossils from the same stratigraphic horizon at the Lijiagou section were later described as Shaanxilithes ningqiangensis (Fig. 1g–l) Xing, Yue & Zhang, Reference Xing, Ding, Luo, He and Wang1984 (in Xing et al. Reference Xing, Ding, Luo, He and Wang1984). In Chen, Chen & Lao's (Reference Chen, Chen and Lao1975) description, cf. Sabellidites sp. is a slender ribbon-shaped and annulated fossil, with a length from 20 to 30 mm, width from 1 to 3 mm (up to 4–5 mm), and 60–90 annulations spaced at ~0.34 mm intervals (see Fig. 1i). Although Xing et al. (Reference Xing, Ding, Luo, He and Wang1984) described Shaanxilithes ningqiangensis in the chapter on trace fossils, they interpreted it as an impression (thus not necessarily a trace fossil) of an annulated ribbon-shaped organism. According to Xing et al.'s (Reference Xing, Ding, Luo, He and Wang1984) description, Shaanxilithes ningqiangensis is a ribbon-shaped impression with a constant width (1–6 mm or more) and is characterized by a series of closely spaced transverse annulations. The observed length ranges from 25 to 60 mm. Since no completely preserved specimens have yet been reported, the full morphology of Shaanxilithes ningqiangensis is uncertain. Subsequently, Shaanxilithes ningqiangensis has also been described from the Taozichong Formation in Guizhou Province, South China (Hua, Chen & Zhang, Reference Hua, Chen and Zhang2004; Fig. 1k, l) and the Zhoujieshan Formation in Qinghai Province and the Zhengmuguan Formation in Ningxia Hui Autonomous Area of North China (Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007).

3. Morphological and taphonomic comparisons between Gaojiashania and Shaanxilithes

Based on our observation of the type materials and new materials collected from the type localities, the key differences between Gaojiashania cyclus (or G. annulucosta) and Shaanxilithes ningqiangensis are their different size, thickness and density of transverse structures (rings and annulations), and how the transverse structures are disposed. Quantitative measurements (Chen, Chen & Lao, Reference Chen, Chen and Lao1975; Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007) show that Shaanxilithes ningqiangensis possesses 2–3 annuli per millimetre (Fig. 1h, l). However, the thickness of a body ring of Gaojiashania cyclus often exceeds 1 mm (Fig. 1f) and their spacing can be up to 3 mm. Furthermore, Shaanxilithes ningqiangensis is more sinuous (typically more than 2 bends; Fig. 1k) than Gaojiashania cyclus or G. annulucosta fossils (typically 1–2 bends; Fig. 1d). More importantly, Gaojiashania cyclus body rings appear to be thickened and articulated by a very thin membrane, which can be degraded easily. In a sense, Gaojiashania cyclus is morphologically analogous to an accordion-like tube, with the thickened rings held together by a thin cylindrical membrane. When this membrane is degraded before pyritization, the rings can be preserved as disarticulated and isolated rings (Fig. 1e, f). Of course, the ribbons of Shaanxilithes ningqiangensis could originally have been cylindrical tubes as well (Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007), but these tubes are relatively thin and the annulations on the tubes are closely spaced and not strongly thickened. Additionally, Gaojiashania cyclus (or G. annulucosta) and Shaanxilithes ningqiangensis from the Gaojiashan Member are preserved differently, although it is unclear whether their different taphonomic styles reflect any differences in underlying biological attributes. Nonetheless, Gaojiashania fossils are typically pyritized in siltstones/mudstones or preserved as weakly skeletonized fossils in limestones. In contrast, Shaanxilithes ningqiangensis are typically preserved as flattened ribbons or more commonly as impressions. Finally, they have different biostratigraphic ranges: at the Gaojiashan section (Fig. 2) and other sections in the Ningqiang area where both Shaanxilithes ningqiangensis and Gaojiashania cyclus are present, the occurrence of Shaanxilithes ningqiangensis is invariably below that of Gaojiashania cyclus.

Figure 2. Biostratigraphic occurrences of Gaojiashania, Shaanxilithes, and other Ediacaran fossils in the Gaojiashan Member, middle Dengying Formation, at the Gaojiashan section, southern Shaanxi Province, South China.

Among Chinese specimens in our collections, some Guizhou specimens (Fig. 1k–l) are of primary importance for morphological reconstruction because they have preserved the best three-dimensional resolution of the tube structures. Fossils from other localities were subjected to varying degrees of compression (e.g. Fig. 1g–j). The Siberian fossil specimens sometimes show isolated torus-like structures with clear central openings that are similar to Gaojiashania. However, the inner tube structure is unknown, although some of the specimens (Fig. 1g) may be funnel-shaped as suggested by Zhuravlev, Gámez Vintaned & Ivantsov (Reference Zhuravlev, Gámez Vintaned and Ivantsov2009). It is remarkable that Shaanxilithes are preferentially preserved as fragmented pieces with different sizes, but the annulations are still connected; Gaojiashania are preferentially disarticulated rather than fragmented. We argue that this may indicate that these two taxa may have possessed distinctive tube structures, at least in terms of connective tissues which made the two taxa preserve in different taphonomic ways.

Some of the newly reported Siberian Shaanxilithes (Zhuravlev, Gámez Vintaned & Ivantsov, Reference Zhuravlev, Gámez Vintaned and Ivantsov2009, fig. 2a, c) are preserved with a relatively lower width/length ratio than Chinese specimens, most of which are often preserved as fragmented pieces. This may be due to different hydrodynamic conditions during deposition of the fossils, with the Chinese specimens preserved in higher energy environments and the Siberian specimens in lower energy environments. We also argue that the five genera and five species of ribbon-like fossils reported by Shen et al. (Reference Shen, Xiao, Dong, Zhou and Liu2007) from North China would be ascribed to one taxon, Shaanxilithes, and the different preservations of specimens are due to varying degrees of taphonomic resolution. This is supported by Shaanxilithes from the Gaojiashan section (southern Shaanxi, South China, Fig. 2) that are preserved as fragmented pieces with different sizes and different morphological resolutions on the same bedding plane (e.g. Fig. 1j). In Shen et al.'s (Reference Shen, Xiao, Dong, Zhou and Liu2007) descriptions, well-preserved specimens that were ascribed to Shaanxilithes cf. ningqiangensis share very similar tube structures to those of Siberian fossil specimens, particularly in terms of ornaments and ribbon sizes.

Considering these morphological differences between Gaojiashania cyclus and Shaanxilithes ningqiangensis, it is clear that the Siberian fossils reported by Zhuravlev, Gámez Vintaned & Ivantsov (Reference Zhuravlev, Gámez Vintaned and Ivantsov2009) are more similar to Shaanxilithes ningqiangensis in size and shape. Additionally, the preservational style of the Siberian fossils is particularly similar to Shaanxilithes ningqiangensis from Guizhou (Fig. 1k–l). Although Shaanxilithes ningqiangensis fossils from southern Shaanxi Province, Qinghai Province and the Ningxia Hui Autonomous Area are preserved in fine-grained siliciclastic sediments with varying carbonate contents, they do share the diagnostic features (compression or impression of thin ribbons with closely spaced annulations) and certain taphonomic features (dense accumulation on bedding surfaces, varying degree of fragmentation, ribbons overlapping but not penetrating each other). Importantly, Shaanxilithes ningqiangensis is not preserved as disarticulated rings like those of Gaojiashania cyclus.

We also argue that both Gaojiashania and Shaanxilithes are body fossils, not trace fossils. The disarticulated rings of Gaojiashania unambiguously suggest that it is a body fossil, since trace fossils cannot be disarticulated in this way. Shaanxilithes only show overlapping (Fig. 1i) but not cross-cutting relationships among ribbons, even in very dense populations preserved on bedding surfaces. The phylogenetic affinity of Shaanxilithes is still unknown, although they seem to represent some eukaryotic organisms with indeterministic growth (hence the very long ribbons). The Siberian find is an important extension of the geographic range of Shaanxilithes ningqiangensis and it therefore supports the biostratigraphic significance of Shaanxilithes ningqiangensis (Shen et al. Reference Shen, Xiao, Dong, Zhou and Liu2007).