November 11, 2007
Problematic Macrofossils From Ediacaran Successions in the North China and Chaidam Blocks
By Shen, Bing Xiao, Shuhai; Dong, Lin; Zhou, Chuanming; Liu, Jianbo
ABSTRACT- Upper Neoproterozoic successions in the North China and nearby Chaidam blocks are poorly documented. North China successions typically consist of a diamictite unit overlain by siltstone, sandstone, or slate. Similar successions occur in Chaidam, although a cap carbonate lies atop the diamictite unit. The diamictites in both blocks have been variously interpreted as Cryogenian, Ediacaran, or Cambrian glacial deposits. In this paper, we describe problematic macrofossils collected from slate of the upper Zhengmuguan Formation in North China and sandstone of the Zhoujieshan Formation in Chaidam; both fossiliferous formations conformably overlie the aforementioned diamictites. Some of these fossils were previously interpreted as animal traces. Our study recognizes four genera and five species-Helanoichnus helanensis Yang in Yang and Zheng, 1985, Palaeopascichnus minimus n. sp., Palaeopascichnus meniscatus n. sp., Horodyskia moniliformis? Yochelson and Fedonkin, 2000, and Shaanxilithes cf. ningqiangensis Xing et al., 1984. None of these taxa can be interpreted as animal traces. Instead, they are problematic body fossils of unresolved phylogenetic affinities. The fundamental bodyplan similarity between Horodyskia and Palaeopascichnus, both with serially repeated elements, indicates a possible phylogenetic relationship. Thus, at least some Ediacaran organisms may have a deep root because Horodyskia also occurs in Mesoproterozoic successions.Among the four genera reported here, Palaeopascichnus Palij, 1976 and Shaanxilithes Xing et al., 1984 have been known elsewhere in upper Ediacaran successions, including the Dengying Formation (551- 542 Ma) in South China. If these two genera have biostratigraphic significance, the fossiliferous units in North China and Chaidam may be upper Ediacaran as well. Thus, the underlying diamictites in North China and Chaidam cannot be of Cambrian age, although their correlation with Ediacaran and Cryogenian glaciations remains unclear. As no other Neoproterozoic diamictite intervals are known in North China and Chaidam, perhaps only one Neoproterozoic glaciation is recorded in that area.
THE EDIACARAN Period is pivotal to the understanding of a possible Precambrian fuse to the Cambrian explosion and the igniter (e.g., global glaciations) of that evolutionary fuse. The late Ediacaran Period is characterized by classic Ediacara fossils that have been reported from most major continents (Narbonne, 1998, 2005), as well as simple trace fossils that have been widely used as direct evidence for the beginning of animal activities (Fedonkin, 1994; Droser et al., 2002; Dzik, 2005). However, recent studies indicate that many previously described trace fossils were incorrectly interpreted as such (Jensen, 2003; Jensen et al., 2005; Droser et al., 2005; Seilacher et al., 2003, 2005). According to a recently compiled list (Jensen et al., 2006), many previously named Ediacaran trace fossil taxa from Russia and China have been reinterpreted as body fossils or as problematic fossils. Thus it is important to reexamine these purported trace fossils. As part of this exercise, in this paper we focus on upper Neoproterozoic (likely Ediacaran) tracelike fossils from the North China and Chaidam blocks that have been previously interpreted as animal traces.
Upper Neoproterozoic outcrops in the North China Block are sporadic, and most occur in the southern and western margins. Typically, upper Neoproterozoic successions in North China consist of a single diamictite unit, unlike other continents such as the Quruqtagh area where up to three Neoproterozoic diamictite units are developed (Xiao et al., 2004). This diamictite unit is overlain by a siliciclastic unit, followed by lower Cambrian deposits. It can be traced along the western and southern margins and has been suggested to be Cryogenian (or equivalent to the ~635 Ma Nantuo glaciation in South China; X. Zhao et al., 1980; Condon et al., 2005), Ediacaran (Gao et al., 1980), or Cambrian (Y. Wang et al., 1980), although chronostratigraphic evidence in support of these correlations is tenuous at best.
The situation is no better in the Chaidam Block of northwestern China, where Neoproterozoic outcrops are few and isolated. Nonetheless, upper Neoproterozoic successions in Chaidam seem to be similar to those in North China; they also consist of a diamictite unit and overlying finer-grained siliciclastics, although a cap carbonate unit occurs atop the diamictite unit. The similar lithostratigraphic sequences have prompted many to correlate the diamictites in North China and Chaidam, although independent tests of this correlation have not been forthcoming.
In both North China and Chaidam, the finer siliciclastic units that overlie the diamictite units contain macroscopic fossils. The initial reports of these fossils were published more than 20 years ago (Y. Wang et al., 1980; Yang and Zheng, 1985), when such fossils were described under various taxonomic names and were uncritically interpreted as animal traces. In light of recent reevaluation of Neoproterozoic trace fossils (Jensen, 2003), there is a need to clarify the taxonomic confusion and to reassess the trace fossil interpretation of the North China and Chaidam fossils. Thus, the goals of this study are to redescribe previously reported trace fossils and to report two new forms from the upper Neoproterozoic successions in North China and Chaidam. A careful description of these fossils will provide a systematic basis on which we can more objectively assess Ediacaran ichnological diversity and test the biostratigraphic significance of certain Ediacaran fossils (e.g., Palaeopascichnus Palij, 1976). In addition, these fossils will help to constrain the age of the underlying diamictites in North China and Chaidam; these diamictites, if indeed they are of Cambrian or Ediacaran age (Gao et al., 1980; Y. Wang et al., 1980), would have wider implications for the extent of post-Cryogenian glaciations (Bertrand-Sarfati et al., 1995; Bowring et al., 2003).
Our samples were collected at two sections, the Suyukou section in the Helanshan area of Ningxia Hui Autonomous Region, northern China, and the Quanjishan Section in the Chaidam Basin of Qinghai Province, northwestern China (Fig. 1). The former is located on the western margin of the North China Block, and the latter on the Oulongbuluke microcontinent of the Chaidam Block (Lu, 2002).
Suyukou section.-Upper Neoproterozoic successions occur only on the western and southern margins of the North China Block; much of the rest of the block was exposed above sea level during the late Neoproterozoic (H. Wang, 1985). The thickness of upper Neoproterozoic successions on the western and southern margin is quite variable, although they always contain a diamictite unit overlain by a siltstone/sandstone/slate unit.
FIGURE 1-Geographic map, showing the location of the North China Block, South China Block, Tarim Block, and Chaidam Block. White dot denotes the Suyukou locality in the western margin of the North China Block. Star denotes the Quanjishan section in the Oulongbuluke microcontinent (Lu, 2002) lying on the northern margin of the Chaidam Block.
At Suyukou, late Neoproterozoic strata are represented by the Zhengmuguan Formation, which unconformably overlies dolostone of the Mesoproterozoic Wangquankou Group and underlies, also unconformably, phosphorite of the Lower Cambrian Suyukou Formation (Fig. 2). The lower member of the Zhengmuguan Formation is a 70 m thick diamictite unit, which grades into the 80 m thick grayish slaty siltstone of the upper member. The lower member is dominated by heterolithic diamictite with cobble- to boulder-sized clasts and mostly calcareous matrix. The 5 m thick transition between the two members is gradual, with the size and abundance of outsized clasts decreasing upsection. No radiometric dates are available to directly constrain the depositional age of the Zhengmuguan Formation, although it is believed to be Neoproterozoic to earliest Cambrian in age on the basis of stratigraphic relationships with the Wangquankou and Suyukou Formations.
The fossils reported in this paper were collected in fine- grained slaty siltstone of the uppermost 1 m of the Zhengmuguan Formation, although Yang and Zheng (1985) described five ichnogenera and six ichnospecies from several horizons in the upper member of the Zhengmuguan Formation.
Quanjishan section.-The Quanjishan section is located in northern Chaidam, which was recently recognized as the Oulongbuluke microcontinent (Lu, 2002). Here, Neoproterozoic successions occur in a few sporadic outcrops (Y. Wang et al., 1980; Lu, 2002). Unlike in the North China Block, upper Neoproterozoic successions in Oulongbuluke are typically more than 1 km thick. The Quanjishan section is one of the best known in this area. At Quanjishan, the Neoproterozoic sequence is represented by the Quanji Group, which overlies amphibolite-gneiss-migmatite of the early Proterozoic Delingha Group and underlies bioturbated dolostone of the Cambrian Xiaogaolu Group. The Quanji Group includes, in ascending order, the Mahuanggou, Kubaimu, Shiyingliang, Hongzaoshan, Heitupo, Hongtiegou, and Zhoujieshan Formations (Fig. 2). The 450 m thick Mahuanggou Formation consists of conglomerate, cross-bedded quartz sandstone. The 350 m thick Kubaimu Formation is composed of thick-bedded, medium to coarse sandstone, unconformably succeeded by 200 m thick light grey quartzite or quartz sandstone of the Shiyingliang Formation. The 300 m thick Hongzaoshan Formation represents the only carbonate-dominated unit in the Quanji Group, consisting mainly of dolostone with abundant dissolution structures. The Hongzaoshan dolostone is conformably overlain by the 120 m thick Heitupo shale and the <20 m thick Hongtiegou diamictite. The overlying Zhoujieshan Formation begins with a 7 m thick dolostone that may be considered as a cap carbonate conformably overlying the Hongtiegou diamictite (Fig. 3.1). The rest of the Zhoujieshan Formation is composed of reddish, fine sandstone and siltstone. No detailed sedimentary analysis of the Quanji Group has been published, although the fossiliferous Zhoujieshan Formation is believed to have been deposited in an intertidal environment (Y. Wang et al., 1980). The only radiometric constraint comes from a 738 +- 28 Ma zircon U-Pb (SHRIMP) date of a basaltic andesite unit in the lower Quanji Group, and thus the base of Quanji Group is estimated to be about 760 Ma (Lu, 2002). Previous authors (Y. Wang et al., 1980; Xing et al., 1985) illustrated several fossils from the Zhoujieshan Formation, but systematic description has not been published. We collected additional fossils from more than 10 horizons in the Zhoujieshan Formation, with the lowest occurrence being ~5 m above the cap carbonate unit in the basal Zhoujieshan Formation. Interpretation and correlation of diamictites.-Diamictites in the Hongtiegou and Zhengmuguan Formations have been traditionally interpreted as glacial in origin. Dropstones and striated clasts have been reported to occur in both Formations (Y. Wang et al., 1980; Z. Zheng et al., 1994). In Chaidam, the spatial distribution of the Hongtiegou and equivalent diamictites is poorly documented, but dropstones have been noted in the Hongtiegou diamictite (Fig. 3.2), which, like many Neoproterozoic glacial deposits, is immediately overlain by a cap carbonate (Fig. 3.1). In the North China Block, the Zhengmuguan and stratigraphically equivalent diamictites-for example, the Luoquan diamictite in Shaanxi and Henan provinces (Mu, 1981; Guan et al., 1986)-can be traced along the southern and western margins. They also contain unambiguous dropstones (Fig. 3.3, 3.4).
Although the Zhengmuguan and Hongtiegou diamictites in two distinct tectonic units are generally regarded as recording the same glaciation (Y. Wang et al., 1981), their correlation with other Neoproterozoic glacial deposits is less clear. They have been variously considered as representing the ~635 Ma Nantuo glaciation (Z. Zhao et al., 1980; Mu, 1981), an Ediacaran glaciation (Gao et al., 1980; Y. Wang et al., 1981; Lu et al., 1985; Guan et al., 1986; Z. Zheng et al., 1994) that may be equivalent to the Hankalchough glaciation in Quruqtagh (Xiao et al., 2004) or the Gaskiers glaciation in Newfoundland (Bowring et al., 2003) or a Cambrian glaciation (Y. Wang et al., 1980). The fossils described in this paper will be used to test these hypotheses.
Genus HELANOICHNUS Yang in Yang and Zheng, 1985, emended
Type species.-Helanoichnus helanensis Yang in Yang and Zheng, 1985.
Original diagnosis.-Primitive pascichnia made by soft-bodied organisms feeding along bedding surface. Burrow system can be complex. Inner part of trace consists of spiral or intertwining burrows, whereas outer part consists of circular, elliptical, or sinuous burrows. Inner burrows thinner than outer ones. Burrows 0.5- 1.0 mm in diameter. Burrow system covers an area of 5-10 cm^sup 2^. Burrow system appears to consist of entangling cords, but burrows separated from each other by certain distance (translated from Yang and Zheng, 1985).
Emended diagnosis.-Cylindrical fossil with no surface ornamentations. Fossils typically preserved as curved or looped ribbons on bedding surface. Ribbon width millimetric but can be variable within a single specimen. Ribbon length typically centimetric in mostly incomplete specimens. Ribbon margin can be smooth or irregular. When intersecting, ribbons show overlapping rather than self-overcrossing relationships. No side branches occur in the ribbons.
FIGURE 2-Stratigraphic columns of upper Neoproterozoic successions in (from left to right) the Helanshan area, North China; Quanjishan area, Chaidam; Ningqiang area, South China; and Yangtze Gorge area, South China. See Figure 1 for locality information. WQK: Wangquankou Gr.; SYK: Suyukou Fm.; MHG: Mahuanggou Fm.; KBM: Kubaimu Fm.; SYL: Shiyingliang Fm.; HZS: Hongzaoshan Fm.; HTP: Heitupo Fm.; HTG: Hongtiegou Fm.; ZJS: Zhoujieshan Fm.; XGL: Xiaogaolu Gr.; GJB: Guojiaba Fm.; SJT: Shuijintuo Fm.
Discussion.-This genus was originally interpreted as grazing traces (pascichnia) produced by soft-bodied organisms feeding along a bedding surface (Yang and Zheng, 1985). However, most specimens show significant within-specimen variations in their width (Fig. 4.2, 4.3, 4.7), inconsistent with grazing traces, which are expected to have a more or less uniform width along a single trace that corresponds to the width of the trace maker (Droser et al., 2005). The margin of a specimen can be smooth in some parts but irregular in other parts of the ribbon (Fig. 4.1). In addition, when two specimens intersect, their relationship appears to be overlapping (Fig. 4.4, 4.8) rather than self-overcrossing, as would be expected for intersecting grazing traces. Finally, the ribbons do not form well-organized meanderings as would be expected in animal grazers, particularly sophisticated grazers. These observations suggest that the trace fossil interpretation in general, and the grazing trace interpretation in particular, are questionable. We offer an alternative interpretation that Helanoichnus is a body fossil (see below).
FIGURE 3-1, Field photograph of the Quanjishan section. Black and white arrows point to the fossiliferous Zhoujieshan sandstone and the basal Zhoujieshan cap-carbonate atop the Hongtiegou diamictite, respectively. Geologist in ellipse for scale. 2, Dropstone in the Hongtiegou diamictite, Quanjishan section. Coin next to dropstone about 2 cm in diameter. 3, Dropstone in the Luoquan diamictite (equivalent to the lower Zhengmuguan diamictite), Luonan section, central Shaanxi Province, North China Block. Dropstone about 7 cm in maximum diameter. 4, Dropstone in the Zhengmuguan diamictite, Suyukou section, Ningxia Hui Autonomous Region. Marker pen about 15 cm in length.
Yang in Yang and Zheng, 1985, emended
Helanoichnus helanensis YANG in YANG AND ZHENG, 1985, p. 14, pl. I, figs. 3-5. [No holotype designated]
Parascalarituba ningxiaensis YANG in YANG AND ZHENG, 1985, p. 15, pl. I, fig. 9. [No holotype designated]
Helminthoida helanshanensis ZHANG in XING ET AL., 1985, p. 191, pl. 42, fig. 4. [No holotype designated]
Helminthopsis quanjishanensis ZHANG in XING ET AL., 1985, p. 192, pl. 42, fig. 6. [No holotype designated]
Helminthopsis quanjishanensis ZHANG in XING ET AL., 1985; ZHANG, 1986, p. 83, pl. IV, fig. 3.
Helanoichnus helanensis YANG in YANG AND ZHENG, 1985; YANG ET AL., 2004, p. 149, pl. 19, fig. 6.
Original diagnosis.-Same as original genus diagnosis by monotypy (Yang and Zheng, 1985).
Emended diagnosis.-Same as emended diagnosis by monotypy.
Description.-Fossils densely clustered on both lower and upper bedding surfaces with significant overlapping but no self- overcrossing (Fig. 4.3, 4.4, 4.8). Most specimens are preserved as curved or looped ribbons (Fig. 4.1, 4.5, 4.8). Ribbons do not branch. They range in length from few millimeters to ~10 cm, and in width from <1 mm to 2.5 mm (Fig. 5). Ribbon width can vary substantially along length (Figs. 4.2, 4.7, 5). No surface ornamentations, such as annulations, occur on ribbons. Some specimens extremely short (Fig. 4.6), perhaps because of incomplete preservation. In a few specimens where ribbon terminus can be observed, it can be pointed (Fig. 4.5, 4.7), blunt (Fig. 4.1, 4.2), or irregular (Fig. 4.5). Fossils distinguished from matrix by finer and lighter-colored sediments, and delineated from matrix by reddish or greenish boundary. No organic carbon can be observed on ribbon; this can be due to weathering or cast/mold preservation of the fossils. The Zhengmuguan fossils were collected in a quarry and stratigraphie orientation was not marked at collection, whereas the Quanjishan fossils were collected in situ with known orientations.
Material.-More than 100 specimens on 20 rock slabs.
Type.-The specimen illustrated in plate I, figure 5 of Yang and Zheng (1985) is here designated as a lectotype.
Occurrence.-The upper member of the Zhengmuguan Formation in the Helanshan area, Ningxia Hui Autonomous Region, North China, and the Zhoujieshan Formation, Quanji Group, in the Quanjishan area, Qinghai Province, Chaidam.
Discussion.-The only stated difference between Helanoichnus helanensis and Parascalarituba ningxiaensis, both of which occur in the upper Zhengmuguan Formation at Suyukou (Yang and Zheng, 1985), lies in the looser and more irregular coiling of the ribbons and somewhat discontinuous "back-fillings" in the latter species. The so- called "back-filling" structures were the reason why Parascalarituba ningxiaensis was interpreted as a trace fossil (Yang and Zheng, 1985). Based on our observation, the "backfilling" structures appear to be an artifact of the irregular outer margins of the ribbon. Therefore, Parascalarituba ningxiaensis is treated as a synonym of Helanoichnus helanensis.
FIGURE 4-Helanoichnus helanensis Yang in Yang and Zheng, 1985, from the Zhengmuguan (1-6) and Zhoujieshan Formations (7, 8), and Horodyskia moniliformis? Yochelson and Fedonkin, 2000, from the Zhengmuguan Formation (9-12). 1, ZMG-15 (museum number VPI-4554); 2, ZMG-15 (museum number VPI-4555). Note variation in the ribbon width; 3, ZMG-7 (museum number VPI-4556); 4, ZMG-IO (museum number VPI- 4557). Arrow points to the overlapping relationship between two specimens; 5, ZMG-18 (museum number VPI-4558). This coiled specimen has a tapering end (arrow); 6, ZMG-4 (museum number VPI-4559). A small fragment(?) with a tapering end; 7, ZJS-36 (museum number VPI- 4560); 8, ZJS-35 (museum number VPI-4561). The east-west oriented specimen overlaps the southeast-northwest oriented specimen (arrow). 9-12, Arrows point to examples of "beads" (presumably compressed spheres) that are uniserially arranged. 9, ZMG-1 (museum number VPI- 4562), 10, ZMG-1 (museum number VPI-4563), 11, ZMG-7 (museum number VPI-4564), 12, ZMG-7 (museum number VPI-4565). Scale bars represent 2 mm if not otherwise noted. In this and all other fossil illustrations, all photographs were taken under a reflected light microscope except otherwise noted. Illustrated fossils are reposited at the Virginia Polytechnic Institute and State University Geosciences Museum (VPIGM). The filed number (ZMG: Zhengmuguan; ZJS: Zhoujieshan) and museum number (VPI-) of each illustrated fossils are given. FIGURE 5-Measurements of 25 Helanoichnus helanensis specimens from the Zhengmuguan Formation. Multiple measurements of the width were randomly taken along the length of each specimen. Bars represent the 25th and 75th percentiles, caps represent the range, and transverse line in box represents the 50th percentile (medium) of the measurements.
FIGURE 6-Measurements of 5 Horodyskia moniliformis? specimens from the Zhengmuguan Formation. The diameter of each "bead" in the same series was measured and plotted. Bars represent the 25th and 75th percentiles, caps represent the range, and transverse line in the box represents the 50th percentile (medium).
Zhang in Xing et al. (1985) described Helminthoida helanshanensis from the Zhengmuguan Formation and Helminthopsis quanjishanensis from the Zhoujieshan Formation. After a systematic review and revision, Uchman (1995) transferred the type species of Helminthoida Schafhautl, 1851, H. irregulans Schafhautl, 1851, into the genus Nereites MacLeay 1839. The emended diagnosis of Nereites irregulans (Schafhautl, 1851) emphasizes the dense and multistory occurrence of this irregularly meandering trace (Uchman, 1995). Neither fossils from our collection nor Zhang's illustration (in Xing et al., 1985) show consistent widths or meandering patterns. The taxonomic assignment of the Zhoujieshan material to the genus Helminthopsis Heer, 1877, is also questionable. Helminthopsis has been diagnosed as a simple, unbranched, elongate, irregularly winding or meandering trace (Han and Pickerill, 1995; Wetzel and Bromley, 1996). However, Zhang's illustration (in Xing et al., 1985) of Helminthopsis quanjishanensis does not show the meandering features characteristic of Helminthopsis. Instead, Helminthopsis quanjishanensis is similar to Helanoichnus helanensis in its irregular external margins and lack of internal structures such as back-fillings.
Genus HORODYSKIA Yochelson and Fedonkin, 2000
Type species.-Horodyskia moniliformis Yochelson and Fedonkin, 2000.
Original diagnosis.-Presumed colonial organisms of small, vertically oriented, short wide cones, hemispherical on upper surface, growing from a horizontal tube (Yochelson and Fedonkin, 2000).
Discussion.-This genus was established on the basis of materials from the Mesoproterozoic Belt Supergroup (Horodyski, 1982; Yochelson and Fedonkin, 2000; Fedonkin and Yochelson, 2002). Similar fossils have been reported from the Mesoproterozoic Bangemall Supergroup in Western Australia (Grey and Williams, 1990; Martin, 2004). These fossils have been characterized as a "string of beads" (Grey and Williams, 1990; Martin, 2004), as they consist of a series of millimeter-sized spherical to subspherical objects that may be physically connected (Martin, 2004).
The Zhengmuguan material was originally described under the genus Neonereites Seilacher, 1960 and interpreted as strings of fecal pellets (Yang and Zheng, 1985). Neonereites is typically preserved as negative epireliefs consisting of serially arranged dimples that are closely spaced without obvious gaps (Hantzschel, 1975). It has been suggested that Neonereites, as defined by its type species, may represent a preservational variant of Nereites, which is diagnosed as meandering or spiraling, unbranched, predominantly horizontal trails consisting of a medial backfilled tunnel enveloped by an even to lobate zone of reworked sediment (Uchman, 1995; Mangano et al., 2000). Many Ediacaran fossils described as Neonereites have been explicitly excluded from this genus (Uchman, 1995). The Zhengmuguan specimens have millimetric gaps between adjacent "beads," and they are not likely to represent meandering trails. Thus the Zhengmuguan fossils do not belong to the genus Neonereites, and the genus Horodyskia may be their alternative taxonomic home.
HORODYSKIA MONILIFORMIS? Yochelson and Fedonkin, 2000
Neonereites uniserialis SEILACHER, 1960; YANG AND ZHENG, 1985, p. 13, pl. I, figs. 1, 2.
Description.-Uniserially arranged, millimetric spheres that form straight or curved sequences of centimetric length. Spheres flattened on both upper and lower bedding surfaces. No carbonaceous material present; "beads" composed of sediments finer-grained and lighter-colored than matrix. Number of "beads" in a series ranges from 7 to ~20. Flattened "beads" circular to subcircular in shape and on average 0.5-1.2 mm in diameter. "Beads" within same sequence almost identical in size (Fig. 6). Adjacent "beads" separated from each other by a gap of ~0.25 mm (Fig. 4.9, 4.10) to ~1 mm (Fig. 4.11, 4.12). Gap filled with material identical to matrix.
Material.-About 10 specimens on bedding surface of 3 slabs.
Occurrence.-The uppermost Zhengmuguan Formation in the Helanshan area, Ningxia Hui Autonomous Region, North China.
Discussion.-The "string of beads" from the upper Zhengmuguan Formation was originally described as Neonereites uniserialis and interpreted as fecal pellets (Yang and Zheng, 1985). As discussed above, the assignment to the genus Neonereites is not justified. It has been proposed that many Ediacaran Neonereiies-like fossils can be compared with palaeopascichnids (Seilacher et al., 2003, 2005; Jensen et al., 2006). In many cases, this is an appropriate evaluation. At the basic level, both Neonereites and Palaeopascichnus are characterized by serially arranged elements. In comparison to Neonereites, however, Palaeopascichnus consists of morphologically more variable elements that are more tightly arranged (see below). The widely spaced, presumably spherical elements of the Zhengmuguan population are more similar to those of Horodyskia. Thus, we tentatively place the Zhengmuguan population in Horodyskia moniliformis Yochelson and Fedonkin, 2000, the type and only species of Horodyskia.
Horodyskia moniliformis from the Mesoproterozoic Belt Supergroup and the Bangemall Supergroup has been interpreted as the oldest tissue-grade colonial eukaryote (Yochelson and Fedonkin, 2000; Fedonkin and Yochelson, 2002). Key evidence in support of this interpretation includes the observations that the sizes of the "beads" are positively correlated with the distances between them and that the "beads" appear to be connected by a stolon (Yochelson and Fedonkin, 2000; Fedonkin and Yochelson, 2002; Martin, 2004). However, neither such a correlation nor the connecting stolon has been observed in the Zhengmuguan specimens, although we cannot determine whether this is because of the relatively few and poorly preserved specimens from the Zhengmuguan Formation. Thus we place the Zhengmuguan material in Horodyskia moniliformis with a degree of uncertainty.
Genus PALAEOPASCICHNUS Palij, 1976, emended
Type species.-Palaeopascichnus delicatus Palij, 1976.
Original diagnosis.-"A series of small, parallel, in most cases arcuate, tightly packed furrows (negative epirelief) on siltstone surface. In positive hyporelief, there are respective ridges of similar structures. Ends of furrows indistinct, gradually passing into rock surface or blunt. Some furrows are transversely segmented by contractions. Furrows of one series number from 4 to 10 and over" (from Urbanek and Rozanov, 1983, p. 93).
Emended diagnosis.-Serially arranged segments or units that can be subcircular, elliptical, curved, or crescent in shape. Segments can be variable in shape and size even within a single series, but if they are curved, their convex sides tend to point in the same direction. Adjacent segments separated by narrow gap but not laterally connected to form meandering structures. Series straight or sinuous and sometimes branch, merge, or overgrow on each other. Series typically maintain a constant width, but they may expand in a convex direction (the direct in which segments are convex).
Discussion.-Palaeopascichnus is one of the most widespread Ediacaran genera, and it has been reported from Ediacaran rocks in Russia and Newfoundland (Urbanek and Rozanov, 1983; Narbonne et al., 1987; Sokolov and Iwanowski, 1990; Gehling et al., 2000). Similar fossils have been found in the Wonoka Formation and the Ediacara Member in the Flinders Ranges, South Australia (Jenkins, 1995; Haines, 2000). These fossils have dichotomously branching series, but they share the same basic morphologyserially arranged segments- with Palaeopascichnus from Russia and Newfoundland, and they likely belong to the same genus (Gehling, 2002). The diagnosis of Palaeopascichnus is here emended to accommodate the Australian material.
Palaeopascichnus was originally interpreted as an animal meandering trace (Urbanek and Rozanov, 1983), largely based on its superficial resemblance to meandering traces, or traces made by an organ moving transversely to the direction of animal movement (Fedonkin, 1981; Jensen, 2003). However, as pointed out by several Ediacaran workers (Gehling et al., 2000; Jensen, 2003; Seilacher et al., 2003, 2005), the elongated segments of Palaeopascichnus are not laterally connected. This observation is confirmed in the Zhengmuguan material and incorporated in the emended diagnosis of Palaeopascichnus. Published Palaeopascichnus or Palaeopascichnus- like fossils vary in their size, shape, and arrangement of the segments (see terminology in Fig. 7; Table 1). The segments can be straight (e.g., P. delicatus from Newfoundland; Narbonne et al., 1987), curved (e.g., P. delicatus from Russia and Newfoundland; Urbanek and Rozanov, 1983; Gehling et al., 2000; and Palaeopascichnus from South Australia, Haines, 2000), subcircular to elliptical (e.g., P. delicatus from Russia; Jensen, 2003), or crescent (e.g., Zhengmuguan material described in this paper). Typically, the segments are arranged tightly, with a very narrow gap in between (e.g., P. delicatus from Russia and Newfoundland, Urbanek and Rozanov, 1983; Sokolov and Iwanowski, 1990; Gehling et al., 2000). The series may be straight (P. delicatus, Urbanek and Rozanov, 1983; Sokolov and Iwanowski, 1990; Gehling et al., 2000) or sinuous (P. sinuosus Fedonkin, 1981; Sokolov and Iwanowski, 1990). The Wonoka material includes specimens with series that dichotomously branch, merge, or overgrow on each other (Haines, 2000). Typically, the width of the series remains more or less constant during growth, but it may expand to the convex direction (Haines, 2000). Thus, the diagnosis of Palaeopascichnus is here emended to incorporate these variations in recently described Palaeopascichnus fossils.
RGURE 7-Cartoon showing the descriptive terminology of Palaeopascichnus Palij, 1976. A Palaeopascichnus specimen consists of a series of segments. Dimensions of series are defined by series length and series width, whereas segments are described by segment width and segment thickness.
Several other Ediacaran genera share the general bodyplan- serially arranged units or segments-with Palaeopascichnus. These include Yelovichnus Fedonkin, 1985, Harlaniella Sokolov, 1972, Intrites Fedonkin, 1980, Gaojiashania Yang, Zhang, and Yin in Lin et al., 1986, and several forms identified as Neonereites. Jensen (2003) has shown that the segments in Yelovichnus gracilis Fedonkin, 1985, are closed, ovate-shaped loops, which probably represent the deflation and collapse of walled chambers. Other than the shape of the segments, Yelovichnus is broadly similar to Palaeopascichnus, and the two genera may be phylogenetically related (Jensen, 2003).
Harlaniella podolica Sokolov, 1972 is a twisted ropelike fossil, and has been reported from Russia and Newfoundland (Sokolov, 1972; Narbonne et al., 1987). The orientation of Harlaniella segments is oblique to the series axis. Harlaniella was originally interpreted as a spiral trace fossil similar to the Phanerozoic Helicolithus Azpeitia Mores, 1933 and Helicorhaphe Ksiazkiewicz, 1970. Jensen (2003) modeled the morphology of Harlaniella and found that it is unlikely to be a trace fossil. Thus, Harlaniella may also be phylogenetically related to Palaeopascichnus, sharing with it a basic bodyplan consisting of serially arranged segments.
Another form is Intrites punctatus Fedonkin, 1980, which is also found in Russia and Newfoundland (Narbonne et al., 1987; Sokolov and Iwanowski, 1990). Intrites punctatus consists of serially arranged segments as well, but the segments are circular to subcircular rings. It is possible that the segments of Intrites punctatus were originally tablet-shaped. Their circular to subcircular shape may have resulted from oblique to flat compression of disarticulated and deflated segments. Thus, lntrites punctatus may also be related to Palaeopascichnus.
TABLE 1-Comparison of Palaeopascichnus species and Palaeopascichnus-like fossils.
As discussed above, some Ediacaran fossils described as Neonereites are also characterized by serially arranged units. In particular, Neonereites renarius Fedonkin, 1985 (Sokolov and Iwanowski, 1990, pl. 26, fig. 2; pl. 27, fig. 4) from Russia has sub- circular to elliptical discs or rings that are similar to those in lntrites punctatus. Again, its basic bodyplan is very similar to that of Palaeopascichnus.
In addition to the Russian and Newfoundland forms discussed above, some Ediacaran species from China are also broadly similar to Palaeopascichnus. Ningxiaichnus Yang in Yang and Zheng, 1985, from the Zhengmuguan Formation at the Suyukou section of North China, was described as consisting of serially arranged, highly arched, U- shaped segments. The openings of the U-shaped segments are not oriented in the same direction. Instead, the segments are rather randomly oriented, possibly by taphonomic compression. The shape of the segments varies within the same specimen, ranging from U-shaped units to almost closed O-rings. Two species of Ningxiaichnus-N. suyukouensis Yang in Yang and Zheng, 1985 and N. minimum Yang in Yang and Zheng, 1985-were differentiated on the basis of segment size. Both species are morphologically similar, and probably phylogenetically related, to Palaeopascichnus.
Another form from China is Gaojiashania from the Ediacaran Dengying Formation at Ningqiang of South China (see Figs. 1 and 2 for location and stratocolumn). Gaojiashania consists of closely spaced (and probably articulated) circular rings (Lin et al., 1986; Zhang, 1986; Ding et al., 1992). However, Gaojiashania is significantly larger than most Palaeopascichnus, and Gaojiashania "series" do not branch or merge. In addition, Gaojiashania appears to be weakly biomineralized.
Palaeopascichnus was originally interpreted as an animal meandering trace (Urbanek and Rozanov, 1983). However, as pointed out by earlier workers (Gehling et al., 2000; Jensen, 2003; Seilacher et al., 2003, 2005), the repeating elements of Palaeopascichnus are not laterally connected to form meandering patterns. It appears that Palaeopascichnus is one of a number of Ediacaran body fossils that share a basic bodyplan of serially arranged segments.
PALAEOPASCICHNUS MINIMUS new species
Diagnosis.-Series, sometimes sinuous, consist of uniserially arranged, isomorphic segments that are <0.2 mm thick, <0.7 mm wide, and slightly curved. Segments separated from each other by gap of 0.1-0.2 mm. Series unbranched and maintain more or less constant width of <0.8 mm.
Description.-Fossils normally flattened on bedding surface and devoid of carbonaceous material. Series <10 mm long and may represent incompletely preserved fragments of larger specimens. Series consist of uniserially arranged, curved or crescent segments. Segments in same series identical in size and shape and parallel to one another (Fig. 8.1, 8.2, 8.4, 8.5). Each segment about 0.3-0.7 mm wide and <0.2 mm thick. Gaps between adjacent segments 0.1-0.2 wide, and filled with material identical to the matrix (Fig. 8.4, 8.5). Specimens slightly curved or rarely sinuous. In strongly bent specimens, segments fan out at convex side of series (Fig. 8.3, 8.4), suggesting series had some degree of flexibility.
Etymology.-Species epithet derived from the Latin minimus, referring to the small segments of this species.
Material.-More than 10 specimens.
Types.-The specimen illustrated in Figure 8.2 (field number: ZMG- 7; museum catalog number: VPI-4566) is here designated as the holotype, and the specimen illustrated in Figure 8.4 (field number: ZMG-7; museum catalog number: VPI-4568) as a paratype. The holotype and paratype are reposited at the Virginia Polytechnic Institute and State University Geosciences Museum (VPIGM).
Occurrence.-The upper part of the Zhengmuguan Formation at Suyukou in the Helanshan area, Ningxia Hui Autonomous Region, North China.
Discussion.-This species differs from other Palaeopascichnus species and Palaeopascichnus-Viks fossils by its much smaller size (Fig. 9, Table 1). The gaps between segments of P. minimus are greater than those of P. delicatus and P. sinuosus. Most specimens of P. minimus are only slightly curved, similar to P. delicatus. One specimen shows significant sinuosity with the segments diverging at the convex side (Fig. 8.3). We interpret the sinuosity as evidence for the flexibility of series. Thus, sinuous series may be a taphonomic feature.
PALAEOPASCICHNUS MENISCATUS new species
Figure 8.6, 8.7
Diagnosis.-Series strongly curved, consisting of uniserially arranged and closely spaced segments. Strongly crescent segments are in close contact axially, but their sharp, lateral edges are free and point in the same direction.
Description.-Strongly curved fossils flattened on both bedding surfaces and not associated with carbonaceous material. Series 1.5- 3 mm in width, and can be >100 mm in length. Segments 1 mm greatest thickness.
Etymology.-Species epithet derived from the Greek meniskos, referring to the crescent segments of this species.
Material.-About 10 specimens collected from the upper part of the Zhengmuguan Formation at Suyukou, Ningxia Hui Autonomous Region, North China.
Types.-The specimen illustrated in Figure 8.7 is designated as the holotype (field number: ZMG-16; museum catalog number: VPI- 4571). The holotype is reposited at the Virginia Polytechnic Institute and State University Geosciences Museum (VPIGM).
Occurrence.-The uppermost Zhengmuguan Formation at Suyukou, Helanshan area, Ningxia Hui Autonomous Region, North China.
Discussion.-The segments of Palaeopascichnus meniscatus are strongly crescent and closely spaced, distinct from those of other Palaeopascichnus species, which are either straight, slightly curved, or sub-circular (Table 1). Because the segments are flattened, it is uncertain whether they were originally funnel- shaped or crescent-shaped. Genus SHAANXILITHES Xing, Yue, and Zhang in Xing et al., 1984, emended
Type species.-Shaanxilithes ningqiangensis Xing, Yue, and Zhang in Xing et al., 1984.
Original diagnosis.-Ribbon-shaped impression with constant width. Specimens mostly fragmentary. Width ranges from 1 to 6 mm or more. Observed length ranges from 25 to 60 mm. Closely spaced transverse annulations visible on impressions. Ribbonshaped impressions do not have well-defined lateral boundaries. Thin annulations parallel to one another. Nineteen to thirty nine annuli per centimeter length of ribbon-shaped impressions (translated from Xing et al., 1984, p. 182).
Emended diagnosis.-Ribbon-shaped body fossils with millimetric width, centimetric length, clearly defined lateral margins, and numerous, closely spaced annulations. Ribbons may represent originally cylindrical tubes. No branching observed.
Discussion.-Annulated, ribbon-shaped fossils from the middle Dengying Formation in southern Shaanxi were first assigned to Sabellidites Yanichevsky, 1926 and interpreted as body fossils (M. Chen et al., 1975). Later, these fossils were redescribed as Shaanxilithes and reinterpreted as trace fossils (Xing et al., 1984). Annulated ribbon-shaped fossils collected from the Zhengmuguan and Zhoujieshan Formations are very similar to those from the middle Dengying Formation. The Zhengmuguan fossils were originally described as Taenioichnus Yang in Yang and Zheng, 1985, and interpreted as animal traces (Yang and Zheng, 1985). Subsequently, Taenioichnus was synonymized with Shaanxilithes, and both were interpreted as trace fossils (Li et al., 1997), although neither was listed in the most recent monographic study of Chinese trace fossils (Yang et al., 2004). The Zhoujieshan fossils were poorly illustrated in the literature as Sabellidites-like trace fossils (Y. Wang et al., 1980).
We agree with previous authors that Shaanxilithes is broadly similar to Sabellidites, which has been interpreted as a pogonophoran tube fossil (Sokolov, 1967, 1968). However, Sabellidites is characterized by annulated, organic tubes (Sokolov, 1967, 1968; Sun et al., 1986), whereas the tubular nature of Shaanxilithes remains to be confirmed in more specimens (e.g., twisted specimen of Taenioichnus; Yang and Zheng, 1985). Specimens of Shaanxilithes typically do not contain much carbonaceous material, let alone coherent organic tube walls. Moreover, it has been shown that Sabellidites tube walls consist of interwoven, submicrometer-sized filaments (Urbanek and Mierzejewska, 1977; Ivantsov, 1990; Moczydtowska, 2003), a feature that does not occur in Shaanxilithes.
Recently, Hua et al. (2004) presented further evidence that Shaanxilithes is a body fossil rather than an animal trace. Observations in support of a body fossil interpretation include twisted and folded specimens (Yang and Zheng, 1985, pl. 1, fig. 8), over-lapping rather than self-overcrossing relationships among densely packed specimens (Hua et al., 2004), and deformation of annulations arrangement caused by bending (i.e., annulations converge at concave side and diverge at convex side of bent specimens; Fig. 8.10, 8.11). Thus, the diagnosis of Shaanxilithes is here emended to accommodate these observations. Furthermore, the original diagnosis states that Shaanxilithes does not have welldefined lateral boundaries, whereas our own observation of material from the middle Dengying Formation at the type locality suggests that the fossils do have well-defined margins subtly distinguishable from the matrix.
FIGURE 8-Palaeopascichnus minimus n. sp. (1-5); Palaeopascichnus meniscatus n. sp. (6-7); and Shaanxilithes cf. ningqiangensis Xing, Yue, and Zhang in Xing et al., 1984 (8-12). 1, ZMG-T. Two specimens (museum numbers VPI-4566 and VPI-4567) of Palaeopascichnus minimus n. sp. located within a looped specimen of Helanoichnus helanensis; 2, ZMG-7 (museum number VPI-4566, holotype). A magnified view of the upper left part of 1; 3, ZMG-7 (museum number VPI-4568; paratype). A curved specimen of P. minimus. Arrow points to the displacement of segments due to bending; 4, ZMG-5 (museum number VPI-4569). Arrow points to the displacement of segments due to bending; 5, ZMG-5 (museum number VPI-4570). 6, ZMG-16 (museum number VPI-4571). Sinuous specimens of Palaeopascichnus meniscatus n. sp. from the Zhengmuguan Formation. 7, Magnified view of the lower left part of 6 (outlined), showing part of the holotype specimen (museum number VPI- 4571); 8, ZMG-15 (museum number VPI-4573), ribbons of Shaanxilithes cf. ningqiangensis with relatively constant width. Arrow points to an indentation; 9, ZMG-19 (museum number VPI-4574), poorly preserved specimens with irregular margins; 10, ZJS-44 (museum number VPI- 4575), specimens with relatively constant width. Arrow points to a false branching where two overlapping specimens are located at slightly different horizons; 11, ZJS-12 (museum number VPI-4576). Arrow denotes divergence of annulations due to bending; 12, ZJS-30 (museum number VPI-4577). Note the irregular margins of this specimen. Scale bars represent 1 mm in 2, 4, 5; 5 mm in 6; and 2 mm in all other figures.
FIGURE 9-Log plot, showing the correlation between segment width and segment thickness (see Fig. 7 for terminology) of Palaeopascichnus minimus n. sp., Palaeopascichnus meniscatus n. sp., and Palaeopascichnus specimens from Eastern Europe Platform (Sokolov and Iwanowski, 1990; Urbanek and Rozanov, 1983), Newfoundland (Narbonne et al., 1987; Gehling et al., 2000) and Australia (Glaessner, 1969; Haines, 1990).
SHAANXILITHES cf. NINGQIANGENSIS Xing, Yue, and Zhang in Xing et al., 1984
cf. Sabellidites YANICHEVSKY, 1926; M. CHEN, X. CHEN, AND LAO, 1975, p. 186, pl. 1, figs. 8, 9.
cf. Shaanxilithes ningqiangensis XING, YUE, AND ZHANO in XING ET AL., 1984, p. 182, pl. 28, figs. 19, 20.
Taenioichnus zhengmuguanensis YANG in YANG AND ZHENG 1985, p. 16, pl. I, fig. 8.
cf. Shaanxilithes erodus ZHANG, 1986, p. 83, pl. IV, figs. 11, 13b.
cf. Shaanxilithes sp. LI, YANG, AND L1, 1997, p. 73, pl. 5, fig. 2.
cf. Shaanxilithes sp. HUA, CHEN, AND ZHANG, 2004, pl. I, figs. 1- 6.
Original diagnosis.-Same as original genus diagnosis by monotypy (Xing et al., 1984).
Emended Diagnosis.-Same as emended diagnosis by monotypy.
Description.-Ribbons flattened as part and counterpart on both bedding surfaces. Fossils from Zhengmuguan Formation composed of light grey clay minerals with little contrast to grey matrix, whereas those from Zhoujieshan Formation weathered to a light grey color in sharp contrast to reddish to greenish matrix. Fossils normally less than 5 mm in width, up to several centimeters in length, and clearly delineated from the surrounding matrix by an outer boundary. Some ribbons maintain constant width along their length and have a smooth outer boundary (Fig. 8.8), whereas others show significant variations in width and have an irregular outer boundary (Fig. 8.9, 8.11, 8.12), possibly due to poor preservation. One to two annuli per millimeter of length (Fig. 8.9-8.11). Annulations poorly preserved in most specimens from Zhoujieshan Formation (Fig. 8.10-8.12) because of relatively coarse grain size. No branching observed.
Material.-More than 50 specimens collected from the Zhoujieshan Formation, Qinghai Province, and several poorly preserved specimens collected from the Zhengmuguan Formation at Suyukou, Ningxia Hui Autonomous Region. Yang and Zheng (1985) reported a very well- preserved specimen from the Zhengmuguan Formation in Ningxia Hui Autonomous Region (Taenioichnus zhengmuguanensis Yang in Yang and Zheng, 1985, pl. 1, fig. 8).
Occurrence.-The uppermost Zhengmuguan Formation in the Helanshan area, Ningxia Hui Autonomous Region, North China; the Zhoujieshan Formation, Quanji Group in the Quanjishan area, Qinghai Province; the Gaojiashan Member of the middle Dengying Formation in the Ningqiang area, Shaanxi Province; the Taozicong Formation in Qingzhen county, Guizhou Province; and the Jiucheng Member of the middle Dengying Formation (formerly lower Yuhucun Formation) in eastern Yunnan, South China (Hua et al., 2004).
Discussion.-Material from the Zhengmuguan and Zhoujieshan Formations is not well preserved. In particularly, the annulations are not as regularly disposed and the margin is not as smoothly delineated as in the type material from the middle Dengying Formation of southern Shaanxi. It is likely that the poor preservation is due to the relatively coarse grains of the host rocks, particularly in the Zhoujieshan Formation. Thus, we place the current material in an open nomenclature.
Given the poor preservation in the Zhengmuguan and Zhoujieshan Formations, it is also entirely possible that the lack of ornamentation or annulations in Helanoichnus may be due to poor preservation. As such, Helanoichnus helanensis and Shaanxilithes cf. ningqiangensis might be synonymous; they are similar other than the presence/absence of annulations. At present, however, these two species are kept separate pending further investigation.
Two species of Shaanxilithes have been established: S. ningqiangensis and 5. erodus. The difference between these two species lies in the seriated or irregular margin of the latter. As discussed above, the irregular margin is likely a preservation artifact. Thus S. erodus may be considered as a junior synonym of S. ningqiangensis.
The preservation styles of the Zhengmuguan and Zhoujieshan assemblages are quite similar. At both localities, fossils are typically preserved on both bedding surfaces as flattened clay-silt veneers; no significant amount of carbonaceous matter is currently present in these fossils. They are often densely packed, with a large number of specimens superposed on each other.
The clay-silt veneer is only about 100 [mu]m thick and composed of material much finer-grained than the matrix, as shown in thin sections perpendicular to the bedding plane (Fig. 10.1, 10.2). Electron microprobe analysis shows that the fossil material has greater Al/Si ratios than the matrix, indicating that aluminum-rich clay minerals are present in the fossils (Fig. 10.3). We are uncertain whether the lack of carbonaceous matter in both assemblages is primary or secondary. Because these fossils were collected from outcrops in relatively arid areas, it is possible that weathering oxidation on the long exposed surface has removed carbonaceous material that used to exist in these fossils. However, even the specimens collected from relatively fresh outcrops of the Zhengmuguan Formation at Suyukou show no trace of carbonaceous matter. If the lack of carbonaceous matter is primary, then the Zhengmuguan and Zhoujieshan fossils can only be interpreted as agglutinated tests or as casts and molds by aluminum-rich clay minerals, and their taphonomy may be understood using the Ediacara preservational models (Narbonne, 2005). This interpretation is worth considering particularly for understanding the taphonomy of Palaeopascichnus and Horodyskia; these two genera also occur in other Ediacaran successions where they are not known to be preserved with organic walls. For Shaanxilithes and possibly Helanoichnus, however, the lack of carbonaceous material is more likely due to secondary weathering. At least one specimen of Shaanxilithes cf. ningqiangensis (described as Taenioichnus zhengmuguanensis, Yang and Zheng, 1985, pl. 1, fig. 8) from the relatively fresh outcrop of the Zhengmuguan Formation at Suyukou shows twisted and folded tubular walls. Furthermore, specimens of Shaanxilithes ningqiangensis from the middle Dengying Formation at Ningqiang, southern Shaanxi, have a shining film reminiscent of Burgess Shale fossils. Thus, it is possible that clay minerals may have played a role in the preservation of Shaanxilithes ningqiangensis and the Burgess Shale taphonomic models (Butterfield, 1990; Orr et al., 1998) may be applicable.
FIGURE 10-Thin sections of Helanoichnus helanensis from the Zhoujieshan Formation (1) and Zhengmuguan Formation (2). Fossils were identified on bedding surface and were then embedded in epoxy for thin sectioning. sections were made perpendicular to the bedding plane, and cut along the length of the fossils. 1, Photomicrograph under reflected light. The fossil is represented by light-colored clay veneer at the top (arrows). 2, Photomicrograph under transmitted, nonpolarized light. The fossil is represented by the dark-colored clay layer at the top (arrows). 3, Al/Si versus Mg/Si plot (as determined by EDS) of a Helanoichnus helanensis fossil from the Zhoujieshan Formation and its surrounding matrix. Al/Si ratio of the fossil is greater than that of the matrix. Scale bars in J and 2 represent 0.25 and 0.5 mm, respectively. Thin section numbers: 1, 2JS30; 2, ZMG-ts.
The color contrast between the fossils and matrix is largely due to differences in grain size and composition. In addition, weathering may have also enhanced the color contrast. For example, the Zhengmuguan fossils, collected from relatively fresh outcrops, have a much more subtle contrast than the Zhoujieshan fossils collected from long exposed outcrops. Presumably, the reddish color in the Zhoujieshan sandstone is due to weathering and oxidation of iron-rich minerals in the matrix (but not in the fossils), thus enhancing the color contrast.
INTERPRETATIONS AND AFFINITIES
Helanoichnus helanensis.-This species is dominant in the Zhengmuguan assemblage and common in the Zhoujieshan assemblage. We reconstruct Helanoichnus helanensis as a cylindrical tubular organism that was flattened because of compression. Helanoichnus helanensis curves or coils in a way very similar to Grypania spiralis (Walcott, 1899) Walter et al., 1976, but it is rarely folded or twisted, indicating that it was probably a turgid cylindrical tube at the time of burial (Walter et al., 1990; Kumar, 1995).
The paleoecology and phylogenetic affinities of Helanoichnus helanensis are less clear. Given the lack of any holdfast structures, it is unlikely that Helanoichnus helanensis was an erect benthic organism. More likely, it was a flat-lying benthic organism, much like the Mesoproterozoic Grypania spiralis (Walter et al., 1990; Kumar, 1995). It does not have any features that would suggest a phylogenetic affinity with animal, fungi, algae, protist, or bacteria, although its simple morphology seems to be consistent with siphonous algae (e.g., the green alga Valonia Agardh, 1823), large protists, or colonial bacteria (e.g., cyanobacterial colonies of Nostoc Vaucher in Hornet and Flahault, 1888).
Horodyskia.-Horodyskia has been interpreted as a tissue-grade colonial eukaryote, with its serially arranged "beads" connected by horizontal stolons (Yochelson and Fedonkin, 2000; Fedonkin and Yochelson, 2002). The discovery of a strand connecting neighboring "beads" (Martin, 2004) lends further support to this morphological interpretation. Fedonkin and Yolchelson (2002) also speculate that Horodyskia was an epibenthic or shallow infaunal and a photosynthetic or chemosynthetic organism, which grew by amalgamation of smaller and closely spaced "beads" to make larger and widely spaced ones. Our specimens from the Zhengmuguan Formation at Suyukou are strongly flattened, and they do not allow an unambiguous test of Fedonkin and Yolchelson's morphological and paleoecological interpretations.
Palaeopascichnus.-This genus is characterized by serially arranged segments. The segments are morphologically variable, ranging from straight, curved, circular, elliptical, to crescent, although part of this variation may be due to taphonomy (e.g., a circular segment may be preserved in different shapes depending on whether it is compressed vertically, horizontally, or obliquely). It is likely that the segments were walled chambers that were articulated in life.
Palaeopascichnus was probably a benthic organism, and it appears to be a flat-lying benthic organism. It does not have a specialized holdfast, which would be expected for macroscopic, fanshaped organisms that were erect. Dichotomous branching (e.g., in the Wonoka material reported by Haines, 2000) is common but not unique to erect benthic organisms. Similarly, an overlapping relationship (Haines, 2000) is consistent with erect benthic organisms felled by water currents and procumbent benthic organisms overgrowing on each other. Merged series (e.g., Haines, 2000, fig. 6b) and mutual avoidance in densely packed specimens, however, are more consistent with a procumbent than erect lifestyle. Regardless, the consistent curvature of segments within the same series suggests directional growth through serial addition of new segments.
If all described Palaeopascichnus populations shared the same trophic strategy, then they were unlikely photosynthetic, because presumably in-situ preservation of Palaeopascichnus occurs in deepwater (below photic zone) Ediacaran successions in Newfoundland (Gehling et al., 2000). Zhuravlev (1993) and Seilacher et al. (2003) interpreted Palaeopascichnus as a xenophyophore protist. Modern xenophyophores are deep sea, epibenthic or infaunal, rhizopodial protists with a multinucleate plasmodium (granellare) within a branching tubular test system that consists of stercomare (fecal pellets material) and xenophyae (agglutinated material) (Tendal, 1972; Tendal et al., 1982; Levin, 1994). They typically have intracellular barite crystals (Gooday and Nott, 1982; Hopwood et al., 1997). Molecular phylogenetic analysis based on small subunit rRNA indicates that xenophyophores are probably foraminifers (Pawlowski et al., 2003). Whereas the general morphology of Palaeopascichnus is broadly similar to some epibenthic xenophyophores-for example, Stannophyllum Haeckel, 1889, some of the key xenophyophore features such as a branching tubular system and intracellular barite crystals have not been verified in Palaeopascichnus. Thus, a xenophyophore interpretation is possible, but the concentric zonations in Stannophyllum seem to be distinct from the loosely articulated segments of Palaeopascichnus.
However Palaeopascichnus is related to living eukaryotes, its fundamental bodyplan appears to be similar to that of Horodyskia; both genera being characterized by serially repeated segments or units. They differ only in the morphology and spacing of the segments. This observation may lead to a deeper understanding of serially modular Ediacaran organisms, as Horodyskia can be traced to the Mesoproterozoic successions (Horodyski, 1982; Grey and Williams, 1990; Yochelson and Fedonkin, 2000; Fedonkin and Yochelson, 2002; Martin, 2004). It is worth considering the possibility that the colonial, modular origin of Ediacaran organisms may have begun in the Mesoproterozoic.
Shaanxilithes.-Ribbons of Shaanxilithes are characterized by a well-defined peripheral boundary and closely spaced annulations. One specimen from the Zhengmuguan Formation shows evidence of twisting (Yang and Zheng, 1985, pi. 1, fig. 8), indicating that the organism was likely ribbon-shaped at burial, although it may have been cylindrical in life. There are a number of Precambrian and Cambrian annulated ribbon-shaped fossils, including Sabellidites, Sinosabellites W. Zheng, 1980, Protoarenicola G. Wang, 1982, Pararenicola G. Wang, 1982, and Sinospongia M. Chen in M. Chen and Xiao, 1992. These fossils have been interpreted as animals or possible animals (Sokolov, 1967; Sun et al., 1986; M. Chen and Xiao, 1992), although the animal interpretation for the latter four genera has been questioned (Qian et al., 2000; Xiao et al., 2002; Dong et al., in press). Shaanxilithes have been analogously interpreted as an animal (M. Chen et al., 1975) or animal trace (Xing et al., 1984), and these interpretations have also been challenged (Hua et al., 2004). Because of the lack of any diagnostic features, the phylogenetic affinity of Shaanxilithes remains problematic. IMPLICATIONS FOR REGIONAL CORRELATION
Due to the lack of direct radiometric dates, the depositional age of the Zhengmuguan and Zhoujieshan Formations is poorly constrained. The macroscopic fossils described here, although they may be phylogenetically problematic, can be used to facilitate regional and global correlation of these two formations. Helanoichnus helanensis and Shaanxilithes ningqiangensis also occur in Ediacaran successions in South China (Zhang, 1986; Hua et al., 2004). Both species occur in the middle Gaojiashan Member of the middle Dengying Formation in the Ningqiang area, southern Shaanxi. The Gaojiashan Member is correlated-on the basis of the common occurrence of Sinotubulites M. Chen et al., 1981-with the Shibantan Member of the middle Dengying Formation in the Yangtze Gorge area (Fig. 2) (Z. Chen, 1999; Hua et al., 2003, 2005). In the Yangtze Gorges area, the Dengying Formation is constrained between 551 and 542 Ma, based on direct dating and correlation with Ediacaran-Cambrian successions in Oman (Amthor et al., 2003; Condon et al., 2005). Thus it is likely that the fossiliferous upper Zhengmuguan Formation and the Zhoujieshan Formation are chronostratigraphically equivalent to the middle Dengying Formation. Consistent with this correlation is the occurrence of Palaeopascichnus in the upper Zhengmuguan Formation. Palaeopascichnus is widely known from middle-upper Ediacaran strata (e.g., the Ust Pinegia Formation in White Sea, Russian, Grazhdankin, 2004; Pound Quartzite and Wonoka Formation, South Australia, Glaessner, 1969 and Haines, 2000; Fermuse Formation, Newfoundland, Narbonne et al., 1987 and Gehling et al., 2000). Of course, the paleontological data from the Zhengmuguan and Zhoujieshan Formations indicate that Horodyskia has a long stratigraphic duration and is biostratigraphically less useful.
If our biostratigraphic correlation is correct, then the paleontological data from the upper Zhengmuguan Formation and the Zhoujieshan Formation also provide a minimum age for the underlying diamictites of the lower Zhengmuguan Formation and the Hongtiegou Formation. These diamictites have been variously considered to be Cryogenian, Ediacaran, or Cambrian. Based on our biostratigraphic correlation, they are unlikely of Cambrian age. They have to be either Cryogenian or Ediacaran in age, possibly correlating with the Nantuo (Condon et al., 2005), Gaskiers (Bowring et al., 2003), or Hankalchough glaciation (Xiao et al., 2004). More chronostratigraphic data are needed to distinguish these possibilities. Nonetheless, because the Zhengmuguan and Hongtiegou Formations (or their equivalents) represent the only Neoproterozoic diamictite interval in the North China and Chaidam blocks respectively, our data suggest that only one Neoproterozoic glaciation is recorded and preserved in these two blocks.
Five problematic fossil taxa-Helanoichnus helanensis Yang in Yang and Zheng, 1985, Horodyskia moniliformis? Yochelson and Fedonkin, 2000, Palaeopascichnus minimus n. sp., Palaeopascichnus meniscatus n. sp., and Shaanxilithes cf. ningqiangensis Xing, Yue, and Zhang in Xing et al., 1984-have been recognized from the slate of the upper Zhengmuguan Formation in the Helanshan area of the North China Block and the Zhoujieshan Formation of the Chaidam Block. All five taxa are interpreted as body fossils, not trace fossils. Three of the four described genera-Helanoichnus, Palaeopascichnus, and Shaanxilithes-appear to be restricted to the Ediacaran Period, and help to correlate the fossiliferous upper Zhengmuguan Formation and the Zhoujieshan Formation, in the North China and Chaidam blocks, respectively, with the middle Dengying Formation in South China, which has been dated between 551 and 542 Ma. This correlation indicates that the underlying diamictites in the lower Zhengmuguan Formation and the Hongtiegou Formation must be Cryogenian or Ediacaran in age.
The occurrence of Horodyskia in Ediacaran successions, if further confirmed by future investigation, suggests that at least some Ediacaran elements may have a deep root traceable into the Mesoproterozoic. Furthermore, the fundamental similarity in the bodyplans of Horodyskia and Palaeopascichnus, both of them macroscopic organisms with serially repeated segments, may be indicative of an underlying phylogenetic relationship. It is worth considering whether such modular organisms had any role in the origin of colonial eukaryotes or even the colonial origin of metazoans (Dewel, 2000). If so, then there may indeed have been a long fuse leading to the Cambrian explosion.
We thank Zhaochang Zhang of Ningxia Geological Survey for field assistance in the Helanshan area, and Yunshan Wang and Luyi Zhang for field assistance in the Quanjishan area. We would also like thank Luis Buatois, Whitey Hagadorn, Soren Jensen, and Gabriela Mangano for helpful discussion and constructive reviews. Financial support for this study was provided by National Science Foundation, Petroleum Research Fund, National Natural Science Foundation of China, and NASA Exobiology Program.
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