Silicified Horodyskia and Palaeopascichnus From Upper Ediacaran Cherts in South China
By Dong, Lin Xiao, Shuhai; Shen, Bing; Zhou, Chuanming
Abstract: Horodyskia is one of the earliest known macroscopic life forms, with a fossil record dating from c. 1.4 Ga. Palaeopascichnus represents a key Ediacaran element with world-wide distribution. However, their body constructions and affinities are poorly understood, partly because previously described species are mostly preserved as casts and moulds in siliciclastic rocks. Silicified specimens from the upper Ediacaran Liuchapo Formation in eastern Guizhou, South China, are described as Horodyskia minor sp. nov. and Palaeopascichnus jiumenensis sp. nov. Their taxonomic assignments are based on their uniserial arrangement of spheroidal or discoidal units, which are connected by a filament and surrounded by a quartz halo. They are unlikely to be brown algae, animal traces, faecal pellets, colonial metazoans, or giant sulphide- oxidizing bacteria. Instead, we propose that Horodyskia and Palaeopascichnus may be phylogenetically related, and their collective morphologies allow tentative comparison with agglutinated foraminifers: the segments can be compared with cytoplasm-filled chambers, connecting filament with small passage between chambers, and quartz haloes with agglutinated tests. However, their ontogeny appears to be distinct from that of modern foraminifers. The occurrences of Horodyskia fossils in Mesoproterozoic and Ediacaran rocks indicate an extremely long range (c. 900 Ma) and echoes the proposition of extended evolutionary stasis in the Proterozoic.
Horodyskia and Palaeopascichnus are two enigmatic Proterozoic fossils sharing a body construction characterized by unisenally arranged similar-shaped units (spheroidal in Horodyskia and discoidal or spheroidal in Palaeopascichnus). Horodyskia, colloquially known as ‘string of beads’, has been reported from the >1443 +- 7 Ma Appekunny Formation of the Belt Supergroup in Montana (Horodyski 1982; Fedonkin et al. 1994; Evans et al. 2000; Yochelson & Fedonkin 2000; Fedonkin & Yochelson 2002) and the 1211-1070Ma Manganese Group of the Bangemall Supergroup in Western Australia (Grey & Williams 1990; Martin & Thome 2001; Grey et al. 2002; Martin 2004), as well as in Ediacaran successions of Lesser Himalaya and North China (Mathur & Srivastava 2004; Shen et al. 2007). Palaeopascichnus, previously interpreted as a meandering trace fossil (Fedonkin 1985), has been recently reinterpreted as a body fossil with uncertain phylogenetic affinity or possible affinity with xenophyophore foraminifers (Seilacher et al. 2003, 2005; Jensen et al. 2006; Shen et al. 2007). It is characterized by uniserially arranged discoidal or saucer-shaped units (‘stack of dishes’) and represents one of the most widely distributed Ediacaran fossils that is known from Russia, Ukraine, Australia, Newfoundland, and North China (Urbanek & Rozanov 1983; Fedonkin 1985; Narbonne et al. 1987; Jenkins 1995; Gehling et al. 2000; Shen et al. 2007).
Although both taxa are represented by a large number of specimens from multiple geographical regions, their body constructions and phylogenetic affinities are poorly understood. For example, Horodyskia was interpreted as a macroalga (Grey & Williams 1990) or a tissue-grade colonial eukaryote with linearly arranged beads connected by an underground stolon (Yochelson & Fedonkin 2000; Fedonkin & Yochelson 2002). Likewise, there is no consensus about the affinity of Palaeopascichnus. It has been variously interpreted as a trace fossil (Urbanek & Rozanov 1983; Fedonkin 1985), an alga (Haines 2000), or a giant rhizopodan protist similar to modern xenophyophores (Seilacher et al. 2003). The divergent opinions about their phylogenetic affinities largely result from the poor resolution of their cast and mould preservation in relatively coarse- grained siliciclastic rocks. Here we report the occurrence of Horodyskia and Palaeopascichnus permineralized in fine-grained chert of the upper Ediacaran Liuchapo Formation in eastern Guizhou, South China. The new data shed additional light on the morphological reconstruction and phylogenetic interpretation of Horodyskia and Palaeopascichnus, and, given the occurrence of Horodyskia in Mesoproterozoic rocks, have evolutionary implications for the history of early heterotrophic eukaryotes.
The Liuchapo Formation in eastern Guizhou Province is underlain by the lower Ediacaran Doushantuo Formation and overlain by the lower Cambrian Jiumenchong Formation (Figs 1 and 2) (Bureau of Geology & Mineral Resources of Guizhou Province 1987). The 20 m thick Doushantuo Formation in this area predominantly consists of siltstone and shale with minor chert and phosphorite. The upper Doushantuo Formation in eastern Guizhou contains carbonaceous compression fossils that are taxonomically similar to those from the Miaohe Member of the uppermost Doushantuo Formation in the Yangtse Gorges area (Xiao et al. 2002; Zhao et al. 2004), where an ash bed has yielded a U-Pb age of 551.1 +- 0.7 Ma (Condon et al. 2005). The 40 m thick Liuchapo Formation consists of dark grey, thinto medium- bedded chert. It is traditionally regarded as a slope facies equivalent of the platform facies, carbonate-dominated Dengying Formation; in other words, it was deposited between the platform edge and deep basin. The uppermost 0.25 m of the Liuchapo Formation, described as the Gezhongwu Member, contains abundant and diverse small shelly fossils indicating a basal Cambrian Meishucunian age (Qian & Yin 1984; Wang et al. 1984). The overlying 200 m thick Jiumenchong Formation consists of carbonaceous black shale with hexactinellid sponges and eodiscinid trilobites (Bureau of Geology & Mineral Resources of Guizhou Province 1987), and it is correlated with the lower Cambrian Niutitang Formation.
Fossils described in this paper were collected from the middle Liuchapo Formation at Jiumen village, Danzhai county, eastern Guizhou Province (Fig. 1). They are restricted to a 3 m interval about 30 m below the Liuchapo-Jiumenchong boundary (Fig. 2). The fossiliferous horizon is regarded as late Ediacaran in age (551 – 542 Ma) because it underlies Meishucunian small shelly fossils in the Gezhongwu Member and overlies strata equivalent to the 551 Ma Miaohe Member of the uppermost Doushantuo Formation. Previous work indicated that the Liuchapo Formation was deposited below the fair- weather wave base (Bureau of Geology & Mineral Resources of Guizhou Province 1987).
The fossils were studied in thin sections and polished slabs using transmitted and reflected light microscopy. Selected slabs were serially ground to reconstruct their 3D morphology. To characterize the compositional difference between the fossils and surrounding matrix, a few highly polished slabs were analysed using a CAMECA SX-50 electron microprobe.
Morphological description and systematic palaeontology
Genus Horodyskia Yochelson & Fedonkin 2000
Type species. Horodyskia moniliformis Yochelson & Fedonkin 2000.
Original diagnosis. ‘Presumed colonial organisms of small, vertically oriented, short wide cones, hemispherical on the upper surface, growing from a horizontal tube’ (Yochelson & Fedonkin 2000).
Horodyskia minor sp. nov.
Diagnosis. A species of Horodyskia characterized by small beads, 0.1-0.7 mm in diameter, commonly surrounded by a halo of light- coloured microcrystalline quartz, and connected with each other by an organic filament (Fig. 3).
Description. Series (‘string’) consists of uniserially arranged spherical segments (‘beads’) that are separated by a gap (‘spacing’). Strings are nearly straight to strongly curved. They are typically oriented parallel to the bedding plane, with rare occurrences of vertically or obliquely oriented strings (Fig. 3f). Serial grinding shows that the beads are spheres, rather than wide cones with a hemispherical top as described by Fedonkin & Yochelson (2002). Several specimens show evidence for an organic filament that connects neighbouring beads (Fig. 3c). As a result of sediment compaction, beads can be vertically compressed to become oblate; this is best seen in thin sections perpendicular to the bedding plane (Fig. 3e and f). Beads are dark-coloured in thin sections, probably because of concentration of carbonaceous material and/or pyrite. They are separated by a gap that is comparable in size with the bead diameter. Beads in the same series are of uniform size, but can vary between 0.1 and 0.7 mm among strings. String is surrounded by a halo that consists of light-coloured, organic-poor, microcrystalline quartz.
Measurements. Strings 0.5-5.5 mm in length (mean 3.3 mm, SD 1.3 mm, n = 16), each with 3-30 beads. Beads 0.1-0.7 mm in diameter (mean 0.32 mm, SD 0.15 mm, n = 20). Spacing between beads 0.02-1 mm (mean 0.29 mm, SD 0.29 mm, n = 20). Bead diameter, spacing, and halo thickness are positively correlated (Fig. 4).
Etymology. Latin, minor, with reference to the small bead size compared with the type species, Horodyskia moniliformis.
Material. We examined 73 specimens in 12 slides and polished slabs (Table 1).
Type specimen. The arrowed specimen in Figure 3a is designated as the holotype (slide number 06JM-4; coordinates 143.8 x 15.8; museum catalogue number VPIGM-4606), deposited at Virginia Polytechnic Institute Geosciences Museum. Discussion. The small bead size of Horodyskia minor (Fig. 5) distinguishes it from H. moniliformis from the Belt Supergroup (Fedonkin & Yochelson 2002), Horodyskia fossils from the Bangemall Supergroup (Grey & Williams 1990), and possible Horodyskia specimens from the upper Ediacaran Kauriyala Formation (Krol D) in Lesser Himalaya (Mathur & Srivastava 2004). The bead size of H, minor overlaps with that of specimens described as H. moniliformis! from the upper Ediacaran Zhengmuguan Formation of North China (Shen et al. 2007; Fig. 6a). Thus, it is possible that the Zhengmuguan specimens that were described under an open nomenclature should be placed in H. minor. However, we refrain from making a formal synonymization, because a statistical test indicates that their bead size is statistically different (two- tailed t-test assuming equal variances, P [much less than]0.05; Fig. 5). In addition, more work is needed to confirm whether the absence of certain features (e.g. haloes, connecting filaments, oblique or vertical orientation of strings relative to bedding plane) in the Zhengmuguan specimens is of biological or (more likely) taphonomic significance.
Despite its smaller beads (Fig. 5), Horodyskia minor is similar to the type species H. moniliformis in the ‘string of beads’ morphology and positive correlation between bead size and spacing (Fig. 4). Several specimens of H. minor (Fig. 3c) also show vestige of an organic filament that connects neighbouring beads; this organic filament can be compared with the connecting stolon of Horodyskia fossils from the Belt and Bangemall supergroups (Grey & Williams 1990; Yochelson & Fedonkin 2000; Fedonkin & Yochelson 2002; Martin 2004). H. moniliformis has not been studied in thin sections and it is uncertain whether it has a quartz halo, although there is a hematite halo surrounding the beads of H. moniliformis from the Belt Supergroup (Horodyski 1982; Fedonkin & Yochelson 2002).
Genus Palaeopascichnus Palij 1976, emend. Shen et al. 2007
Type species. Palaeopascichnus delicatus Palij 1976.
Palaeopascichnus jiumenensis sp. nov.
Diagnosis. A species of Palaeopascichnus characterized by a spherical terminal segment and unisenally arranged, saucershaped segments (typically <0.7 mm in width) (Fig. 7). The concave side of discoidal segments points to the same direction, with the spherical terminal segment located at the concave end of the series. Segments are surrounded by a halo of light-coloured microcrystalline quartz and connected with each other by an organic filament.
Description. Most specimens are preserved parallel or nearly parallel to the bedding plane, although some are obliquely oriented (Fig. 7g and h). A light-coloured, carbonaceous-poor halo of microcrystalline quartz material surrounds the series (Fig. 7). Series length ranges from 0.9 to 5.0 mm, and width from 0.3 to 1.0 mm. Most series are sinuous or curved; straight strings are uncommon. They never branch, and typically maintain a more or less constant width. Among observed specimens, there are 4-30 segments per series.
Observed in thin sections, the saucer-shaped segments are crescent-shaped and the spherical terminal segment is more or less circular. They are typically dark coloured, probably because of the presence of carbonaceous material and/or pyrite. Crescent-shaped segments in the same series have uniform size and shape, and they are oriented in the same direction, with their concave side pointing to the spherical terminal segment (Fig. 7). The spherical segment is slightly smaller than crescent-shaped segments, which have very sharp lateral edges. Segments are spaced evenly. In axial sections, a thin carbonaceous filament that connects adjacent segments can be seen (Fig. 7a-c, e and f).
The 3D morphology of the segments is best appreciated by combining thin sections cut perpendicular and parallel to bedding plane, as well as continuous grinding of specimens (Fig. 7i-l). The crescent-shaped segments are curved discoids in three dimensions, uniserially arranged to form a ‘stack of dishes’. The spherical morphology of the terminal segment is confirmed by continuous grinding. Both types of segments have well-defined boundaries, consistent morphologies, and consistent occurrence (i.e. terminal location of spherical segments). Thus, their morphologies are unlikely to be taphonomic in origin, although some segments can be vertically compressed as a result of sediment compaction, as can be seen in thin sections cut perpendicular to the bedding plane (Fig. 7g and h).
Measurements. Crescent-shaped segments 0.07-0.23 mm in thickness (mean 0.12 mm, SD 0.04 mm, n = 14), 0.23-0.65 mm in width (mean 0.34 mm, SD 0.13 mm, n = 14), with a spacing of 0.03-0.35 mm (mean 0.12 mm, SD 0.09 mm, n = 14). Spherical terminal segment are slightly smaller, 0.14-0.55 mm in maximum diameter (mean 0.27 mm, SD 0.1 mm, n = 13). Segment width and spacing are positively correlated (Fig. 8).
Etymology. The species epithet is derived from the fossil locality, Jiumen village of eastern Guizhou Province in South China.
Material. We examined 70 specimens in 12 thin sections and polished slabs (Table 1).
Type specimen. The specimen illustrated in Figure 7f is designated as the holotype (slide number 06JM-3; coordinates 147.5 x 23.7; museum catalogue number VPIGM-4611), deposited at Virginia Polytechnic Institute Geosciences Museum.
Discussion. The Liuchapo specimens share the basic body construction of serially arranged segments with other Palaeopascichnus species. There are four previously described species, P. delicatus Palij 1976, P. sinousos Fedonkin 1985, P. minimus Shen et al. 2007 (Fig. 6b), and P. meniscatus Shen et al. 2007, from Ediacaran successions in Russia, Ukraine, Newfoundland, and North China (Palij 1976; Fedonkin 1985; Gehling et al. 2000; Shen et al. 2007). In addition, unnamed Palaeopascichnus-like fossils have been reported from Ediacaran successions in Newfoundland (Narbonne & Hofmann 1987) and South Australia (Haines 2000). P. jiumenensis from the Liuchapo Formation can be distinguished from other Palaeopascichnus species by its relatively small segment size (Fig. 9), the lack of branched series, spherical terminal segments, connecting filament, and halo. Admittedly, some of the morphological differences (e.g. the lack of connecting filaments) may be taphonomic, because they may not be preservable through casting and moulding by coarsegrained sediments.
Previously described Palaeopascichnus species have a wide range of segment shapes, including straight, curved, subcircular to elliptical, and crescent-shaped segments when viewed on a bedding plane. To some degree, this variation reflects how the originally discoidal segments were compacted (Shen et al. 2007). Palaeopascichnus jiumenensis has curved discoidal segments and a terminal spheroidal segment. It is possible that its discoidal segments, when compressed on a bedding plane, would mimic those of P. minimus. However, the presence of a terminal spheroidal segment in multiple specimens of P jiumenensis from the Liuchapo Formation but in none of P. minimus from the Zhengmuguan Formation (Shen et al. 2007) suggests that they represent two species, despite their similar segment size. Thus, we choose to keep them as separate species.
Most Liuchapo beads and dishes are preserved in series, although some are disarticulated or fragmented (e.g. isolated beads in Fig. 7i-1 that do not appear to be chained into a string by connecting filaments). With a few exceptions of perpendicularly or obliquely oriented specimens (Figs 3f and 7g), most series lie parallel to the bedding plane and their random orientation indicates that they were not transported or preferentially orientated by water currents (e.g. Figs 3a and 7i-l).
What distinguishes the Liuchapo fossils from previously described Horodyskia and Palaeopascichnus species is their unique preservation by early diagenetic silicification. This is in sharp contrast to previously known examples of these two genera, which are mostly preserved as casts and moulds in sandstone and siltstone, including positive hyporeliefs (Urbanek & Rozanov 1983; Fedonkin 1985; Narbonne et al. 1987; Gehling et al. 2000), negative hyporeliefs (Grey & Williams 1990; Fedonkin & Yochelson 2002; Grey et al. 2002; Martin 2004), positive epireliefs (Fedonkin & Yochelson 2002), negative epireliefs (Grey & Williams 1990; Fedonkin & Yochelson 2002), or highly compressed casts (Haines 2000; Shen et al. 2007). The Liuchapo Formation represents a novel taphonomic pathway for Horodyskia and Palaeopascichnus preservation and provides complementary information about their taphonomy, morphology, and ecology. The discovery of silicified Horodyskia and Palaeopascichnus in Ediacaran cherts accentuates the recent observation that many macroscopic Ediacaran fossils can be preserved in multiple taphonomic windows, including casting and moulding in sandstone and siltstones (Gehling 1999), carbonaceous compression in shales (Xiao et al. 2002), pyritization in siliciclastic rocks (Chistyakov et al. 1984; Ding et al. 1992), preservation in carbonates (Xiao et al. 2005), and now permineralization in cherts. Only when we combine the information from different taphonomic windows can we understand the complete morphological and ecological spectrum of these Ediacaran organisms.
Petrographic observations (Figs 3 and 7) indicate that the segments and connecting filaments of Horodyskia minor and Palaeopascichnus jiumenensis are defined by dark grey material, probably enriched in organic matter and/or pyrite, whereas the haloes are free of carbonaceous material and consist of microcrystalline (< 10 [mu]m) quartz. Elemental maps show that, compared with surrounding matrix, the segments of Palaeopascichnus jiumenensis have elevated Al and Mg concentrations but subdued Si concentration (Fig. 10). We interpret the elemental data as evidence for a moderate concentration of clay minerals in the segments. Thus, like the compressed casts of Palaeopascichnus from the Ediacaran Zhengmuguan Formation in North China (Shen et al. 2007) and Horodyskia from the Belt Supergroup (Horodyski 1982), the segments of Palaeopascichnus jiumenensis are characterized by elevated clay concentrations. The taphonomic significance of the clay minerals is still uncertain, but given the recent interest in the role of clay minerals in exceptional preservation (Butterfield 1995; Orr et al. 1998), it is important to note the preferential occurrence of clay minerals in Palaeopascichnus segments. Morphological and ecological reconstruction
The 3D preservation of silicified Horodyskia minor and Palaeopascichnus jiumenensis in upper Ediacaran chert of the Liuchapo Formation provides critical information for their morphological reconstruction. By integrating observations of thin sections cut perpendicular and parallel to the bedding plane, as well as serial thin sections, we are able to reconstruct H minor and P jiumenensis as uniserially arranged spheroidal and discoidal segments that are connected by a thin organic filament and enveloped in a quartz halo (Fig. 11). In P. jiumenensis, the discoidal segments are slightly curved, with their concave side pointing to a spherical terminal segment. The reconstructed morphologies of H. minor and P jiumenensis are in broad agreement with those of other species of these two genera (Fedonkin & Yochelson 2002; Shen et al. 2007), including the existence of a connecting ‘stolon’ (Grey & Williams 1990; Fedonkin & Yochelson 2002; Martin 2004) and a halo in H moniliformis (Horodyski 1982; Fedonkin & Yochelson 2002, fig. 3).
The palaeoecology of Horodyskia minor and Palaeopascichnus jiumenensis is unresolved, because their simple morphologies do not have any unequivocal implications for functions. None the less, the rare perpendicular and oblique orientation indicates that they were probably not planktonic organisms. Instead, they may have been benthic organisms, either erect or procumbent benthos, living near the water-sediment interface.
The phylogenetic affinity of Horodyskia and Palaeopascichnus has been controversial. Horodyskia was interpreted as inorganic sedimentary structures (Horodyski 1982, 1983), dubiofossils (Fedonkin & Runnegar 1992), or pseudofossils (Hofmann 1992). Grey & Williams (1990) proposed its probable biological origin with affinity to the Phaeophyceae. Recently, Horodyskia was reinterpreted as a tissue-grade colonial eukaryote (Fedonkin & Yochelson 2002). Similarly, Palaeopascichnus was once regarded as a trace fossil (Urbanek & Rozanov 1983; Fedonkin 1985), but this interpretation has been questioned by several workers (Jensen 2003; Seilacher et al. 2003, 2005; Jensen et al. 2006; Shen et al. 2007). Instead, Palaeopascichnus has been recently reinterpreted as a giant rhizopodan protist similar to modern deep-sea xenophyophores (Seilacher et al. 2003), a group of agglutinated foraminifers (Tendal 1972; Pawlowski et al. 2003b).
Horodyskia minor and Palaeopascichnus jiumenensis share the uniserial arrangement of repetitive segments with other described species of the two genera (Shen et al. 2007). Unless this rather unusual body construction evolved independently in multiple taxa, the morphological similarities mean that these two genera may be phylogenetically related and that the Liuchapo material can place constraints on their phylogenetic interpretation.
Grey & Williams (1990) compared Horodyskia fossils from the Bangemall Supergroup with many sedimentary, diagenetic, and biogenic structures (e.g. scours, flutes, gutter casts, prods, skip marks, crystal moulds, metazoans, trace fossils, cyanobacteria, macroalgae). They concluded that the Bangemall fossils probably represent impressions of macroalgae. Specifically, they argued that gas bladders of the modern brown macroalgae Scaberia agardhii and Hormosira banksii resemble the Bangemall beads. To strengthen their interpretation, Grey & Williams (1990) pressed Scaberia agardhii thalli on wet sand to make markings similar to the Bangemall beads. However, the macroalga interpretation does not account for the organic-poor haloes that enclose the Belt and Liuchapo beads. Also, unequivocal non-calcareous algal fossils preserved in younger sediment (e.g. Xiao et al. 2002), including Cystoseirites ornata, a Miocene laminarialean brown alga with serially arranged gas bladders (Parker & Dawson 1965), have little relief. Furthermore, a morphological comparison with Hormosira-like brown algae is discouraging if Horodyskia is phylogenetically related to Palaeopascichnus, whose dish-like segments are morphologically different (and by inference functionally distinct) from the spherical floats of brown algae.
A competing hypothesis was proposed by Fedonkin & Yochelson (2002), who interpreted Horodyskia moniliformis as a tissuegrade colonial eukaryote. In several other studies (e.g. Grey et al. 2002; Martin 2004), this tissue-grade colonial eukaryote is understood as a colonial animal. Fedonkin & Yochelson (2002) interpreted the Horodyskia beads as colonial units that had a conical shape with a hemispherical top and were connected to a horizontal, somewhat subterraneous ‘stolon’. However, continuous grinding of three- dimensionally silicified Horodyskia specimens from the Liuchapo Formation showed that their beads are more or less spherical, with a certain degree of vertical compression because of sediment compaction. To account for the positive relationship between bead size and spacing, Fedonkin & Yochelson (2002) proposed a rather unusual ontogenetic model in which alternating beads donate their biomass to their neighbours during growth, thus increasing spacing. As Fedonkin & Yochelson (2002) pointed out, there are no modern analogues to this growth model. Furthermore, the vertical or oblique orientation of some Liuchapo strings (Figs 3f and 7g), if preserved in life position, is inconsistent with the reconstruction and interpretation of Fedonkin & Yochelson (2002).
Palaeopascichnus was originally described and interpreted as a trace fossil (Palij 1976; Urbanek & Rozanov 1983; Fedonkin 1985). Superficially, Palaeopascichnus is similar to meandering or grazing traces. However, there are no turning points at the lateral edge of Palaeopascichnus segments (Jensen 2003). Thus, the segments in Palaeopascichnus are unlikely meandering or grazing traces. Alternatively, the segments could represent backfilling sediments along a burrow that is parallel to the main axis (rather than the segments) of Palaeopascichnus. Indeed, axial cross-sections of some Palaeopascichnus specimens from the Liuchapo Formation (Fig. 7) superficially resemble vertical cross-sections of Taenidium (Hantzschel 1975), Muensteria, or Zoophycos spreiten (e.g. Savrda 2003, fig. 7). However, this alternative interpretation is inconsistent with the wide spacing (e.g. Fig. 7f), connecting filaments (Fig. 7e and f), and spherical terminal segment of Palaeopascichnus jiumenensis. Importantly, the terminal segment of P jiumenensis has been shown, through continuous grinding, to be spherical in shape, rather than a cylindrical outlet to the sediment surface.
Both Horodyskia and Palaeopascichnus have been compared with faecal pellets, similar to Hormosiroidea and Neonereites, respectively (Grey & Williams 1990; Fedonkin & Yochelson 2002; Jensen 2003). Although the enrichment of clay and carbonaceous material in the segments of the Liuchapo fossils is consistent with a faecal pellet interpretation, the segment shape (spherical in Horodyskia minor and crescent-shaped in Palaeopascichnus jiumenensis) is distinct from the typically ellipsoidal and ovoidal morphology of faecal pellets (Haberyan 1985; Robbins et al. 1985; Brodie & Kemp 1995). Further, faecal pellets are not expected to be widely spaced and connected by a thin filament. The presence of a spherical terminal segment followed by crescent-shaped ones in P. jiumenensis finds no comparison among faecal pellet strings. Thus, we concur with other workers that Horodyskia and Palaeopascichnus are unlikely meandering traces, burrows or faecal pellets (Grey & Williams 1990; Fedonkin & Yochelson 2002; Jensen 2003; Seilacher et al. 2003, 2005; Jensen et al. 2006).
It has been suggested that Horodyskia may represent chained bacterial cells (Grey et al. 2002). The discovery of chained bacterial cells in the giant sulphide-oxidizing bacterium Thiomargarita namibiensis (Schulz et al. 1999) makes this interpretation attractive. T. namibiensis is described as a ‘string of pearls’ because its chained cells are replete with refractive sulphur globules and visible to the naked eye (Schulz et al. 1999). It occurs abundantly in nutrient-rich sediments underlying the oxygen minimum zone. The cells are spherical, with a diameter of 100- 300 urn, but can be up to 750 urn in diameter. These features make T. namibiensis a possible modern analogue to Horodyskia. However, T. namibiensis cells are held together by a mucous sheath, rather than a connecting filament. Its cells are more closely spaced (with a space less than one-tenth of cell diameter) than Horodyskia beads (with a spacing one-quarter to three times bead diameter; Figs 3 and 4). The light-coloured, carbonaceous-free halo in Horodyskia is inconsistent with an organic-rich mucous sheath. In addition, Horodyskia beads from other Proterozoic successions (Fig. 5) significantly exceed the cell size of Thiomargarita. Furthermore, it is inconceivable that Thiomargarita cells would be able to leave 3D casts and moulds in coarse-grained sandstones. More importantly, if Horodyskia and Palaeopascichnus represent a phylogenetically related group, a comparison between Palaeopascichnus segments and Thiomargarita cells seems difficult. As an alternative, we consider an interpretation that compares Horodyskia and Palaeopascichnus with uniseriate agglutinated foraminifers. Some modern uniseriate agglutinated foraminifers (Loeblich & Tappan 1988), such as Cylindroclavulina bradyi (Loeblich & Tappan 1988; plate 201, fig. 13), are remarkably similar to H. minor and P. jiumenensis. Indeed, modern uniserial foraminifers such as Cylindroclavulina bradyiao have spherical or crescent-shaped chambers connected by a narrow passage. Moreover, the spheroidal terminal segment in Palaeopascichnus is intriguingly similar to the proloculus of some uniseriate foraminifers. It is thus tempting to consider the beads and dishes in Horodyskia minor and Palaeopascichnus jiumenensis as cytoplasm-filled chambers that were preserved by early diagenetic silicification. This interpretation is consistent with the enrichment of organic matter in the segments, as it has been shown that cellular content preserved in early diagenetic cherts and phosphates can have elevated concentration of organic material (Zhang et al. 1998).
It is also tempting to interpret the connecting filament as a narrow passage between chambers and the light-coloured, carbonaceous- free, microcrystalline halo as an agglutinated test. Such an agglutinated test would have been silicified and recrystallized during early diagenesis, masking its detrital nature. Some Horodyskia specimens from the Belt Supergroup are surrounded by haloes of iron oxide (Fedonkin & Yochelson 2002), which may also represent agglutinated material. The lack of organic matter and the diffuse boundary of the halo indicate that it was unlikely to be organic or calcareous.
It is interesting to note that uniseriate arrangement is a common feature among many modern foraminifer groups, including organic- walled allogromids such as Resigella moniliformis (Loeblich & Tappan 1988, plate 9, fig. 11), agglutinated textularids such as Cylindroclavulina bradyi (Loeblich & Tappan 1988, plate 201, fig. 13), and calcareous foraminifers such as Eonodosaria evlanensis (Loeblich & Tappan 1988, plate 222, fig. 4). Also, agglutinated tests occur in a wide range of foraminifers, both unilocular and multilocular (Pawlowski & Holzmann 2002; Pawlowski et al. 2003a), and they are among some of the earliest known foraminifers in the Cambrian (Cope & Mcllroy 1998; Sen Gupta 1999; Mcllroy et al. 2001; Streng et al. 2005).
Some morphological aspects of Horodyskia and Palaeopascichnus make a foraminifer interpretation less attractive. Modern uniseriate foraminifers grow unidirectionally, with ontogenetically later and typically larger chambers added adaperturally. Although segments of some Palaeopascichnus series do enlarge toward one end of the series (Haines 2000) and some Horodyskia specimens do show progressive increase in bead size toward one end of the series (Fedonkin & Yochelson 2002, fig. 14c), the case for a unipolar increase in bead size remains to be substantiated. In fact, for both Horodyskia and Palaeopascichnus, it is uncertain whether the segments in the same series were appended sequentially during ontogeny (i.e. foraminifer- style growth) or were formed simultaneously in early ontogeny followed by subsequent growth in size (e.g. Fedonkin & Yochelson 2002). In addition, the spacing of some Horodyskia beads (Fedonkin & Yochelson 2002; Shen et al. 2007) is wider than that for typical uniseriate foraminifers. Finally, a foraminifer interpretation implies the existence of an aperture in one of the terminal segments, a feature that remains to be unambiguously demonstrated in Horodyskia and Palaeopascichnus.
It should be mentioned that Seilacher and colleagues have already argued that Palaeopascichnus is a xenophyophore foraminifer (Seilacher et al. 2003). Modern xenophyophores are agglutinated foramiriifers characterized by several unique features, including stercomata derived from faecal pellets, granellae, and barite precipitates in protoplasm (Tendal 1972; Hopwood et al. 1997). Whereas our analysis broadly supports Seilacher et al.’s interpretation, elemental mapping of H. minor and P. jiumenensis does not reveal elevated barium concentration (Fig. 10). Thus, although Horodyskia and Palaeopascichnus have some general morphological similarities to foraminifers, their phylogenetic affinity with foraminifers or xenophyophores remains speculative.
Recent molecular phylogenetic analyses (Stechmann & Cavalier- Smith 2002; Nikolaev et al. 2004; Burki & Pawlowski 2006) have recognized six eukaryote clades: opisthokonts (animals and fungi), amoebozoans (lobose amoebae and slime moulds), rhizarians (foraminifers, radiolarians, and cercozoans), excavates (archaezoans, discicristates, and loukozoans), chromalveolates (chromists and alveolates), and plants (rhodophytes, glaucophytes, and green plants). The exact phylogenetic relationships among the six clades are controversial, but there is some evidence suggesting that the deepest eukaryotic divergence occurred between the unikonts (opisthokonts and amoebozoans) and bikonts (the remaining four clades) (Stechmann & Cavalier-Smith 2002; Nikolaev et al. 2004).
If Horodyskia and Palaeopascichnus can be interpreted as agglutinated foraminifers (Seilacher et al. 2003; this paper), they provide significant insight into the Proterozoic evolution of heterotrophic eukaryotes. Previously, the earliest fossil record of rhizarians was represented by the 742-770 Ma vase-shaped microfossil Melicerion poikilon, which is interpreted as a filose testate amoeba, a member of cercozoan rhizarians (Porter et al. 2003). The earliest unquestionable foraminifer fossils are represented by the basal Cambrian agglutinated foraminifers Platysolenites and Spirosolenites (Mcllroy et al. 2001). li Horodyskia is an agglutinated foraminifer, its occurrence in the Belt and Bangemall supergroups extends the total-group rhizarian and foraminifer fossil record to the Mesoproterozoic, and would imply that the divergence of bikonts and rhizarians from their respective sister groups must have occurred before 1.4 Ga. If so, Horodyskia joins a short list of Mesoproterozoic bikont fossils, which currently include the red alga Bangiomorpha from the 1204 +- 22 Ma Hunting Formation in arctic Canada (Butterfield 2000) and the chromalveolate Palaeovaucheria from the >1005+-4Ma Lakhanda Group in southeastern Siberia (Rainbird et al. 1998; Woods et al. 1998).
Despite the uncertain phylogenetic affinity of Horodyskia and Palaeopascichnus, the new fossils together with Mesoproterozoic occurrences of Horodyskia provide important insight into the evolutionary stasis seen in Proterozoic fossils (Butterfield 2007). The Proterozoic features a number of extremely long-ranging taxa, including Tawuia, Chuaria, Valeria, and Tappania. These genera show taxonomic and morphological stasis over 10^sup 7^-10^sup 8^ years. This extended stasis is interpreted as a macroevolutionary consequence related to the very simple ecological interactions in the absence of metazoans (Butterfield 2007). The occurrences of Horodyskia in the 1.4 Ga Appekuny Formation, 1211-1070Ma Manganese Group, upper Ediacaran Kauriyala (Krol D), Zhengmuguan and Liuchapo formations indicate a stratigraphie range of 900 Ma, accentuating the hypobradytelic evolution of an ecosystem without animal predators and grazers (Schopf 1994; Butterfield 2007).
Silicified Horodyskia and Palaeopascichnus from the upper Ediacaran Liuchapo Formation allow us to better characterize their morphologies and to explore their phylogenetic affinities. Both genera are characterized by uniserially arranged spheroidal and discoidal segments that are connected by a thin organic filament (Fig. 11). The silicified specimens also preserve evidence for an agglutinated test. The fundamental similarities in the body construction of Horodyskia and Palaeopascichnus imply that the two genera may be phylogenetically related. The body construction of Horodyskia and Palaeopascichnus also encourages a morphological comparison with agglutinated foraminifers, although their ontogeny seems to be distinct from that of modern foraminifers. The occurrence of Horodyskia in Mesoproterozoic and Ediacaran rocks indicates an extremely slow pace of Proterozoic morphological and taxonomic evolution in the absence of metazoans.
We acknowledge funding from the Petroleum Research Fund (42231- AC8), National Natural Science Foundation of China (40628002), Chinese Ministry of Science and Technology (2006CB806400), and National Science Foundation (EAR-0545135). We thank Xunlai Yuan for support, Yangeng Wang for field assistance, R. Tracy and C. Loehn for help in electron microprobe, and A. Ekdale and two anonymous reviewers for constructive comments.
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Received 3 May 2007; revised typescript accepted 31 August 2007.
Scientific editing by Howard Falcon-Lang
LIN DONG1, SHUHAI XIAO1, BING SHEN1 & CHUANMING ZHOU2
1 Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
2 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology,
Chinese Academy of Sciences, Nanjing 210008, China
Copyright Geological Society Publishing House Jan 2008
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