Doushantuo Phosphorite

Abstract

Introduction

Discovery and General Features

Discovery and general features goes here

Locality

Geological Setting

geol setting text goes here – can probably get something useful from Zhang et al.?

Age

In general, age constraints on the Doushantuo Formation are rather weak. Chemostratigraphic profiles suggest that Doushantuo fossils predate the last strongly positive carbon isotope excursion of the Proterozoic, dated as 549 ± 1 Ma in Namibia (Grotzinger et al. 1995). Similarly, Doushantuo microfossils provide biostratigraphic evidence that this formation predates the 555 ± 3 Ma sandstones of the Redkino Series, northern Russia, which contain diverse Ediacaran body and trace fossils. Bio- and chemostratigraphic correlations further suggest that Doushantuo fossils are older than diverse Ediacaran assemblages found in Australia, Ukraine, and northern Siberia. However, in the absence of direct radiometric constraints, it is uncertain whether Doushantuo fossils predate frondose Ediacaran remains from Newfoundland, dated at 565 ± 3 Ma, although the age of 570 Ma for the Doushantuo fossils (proposed in Martin et al. 2000 and adopted here) places them some 5 Ma earlier. Knoll 2003, p. 141, suggests a range of 600 to 590 Ma. Whichever is correct, the deposit seems certain to post-date the Varanger ice age.

In the Yangtze Gorges area, the type locality of the Doushantuo Formation, tuffs in the Cap Carbonate of the basal Doushantuo Formation have yielded a U–Pb age of 635.23 ± 0.57 Ma (Condon et al. 2005, Zhu et al. 2007), and a tuff bed near the top of the Doushantuo Formation has yielded a U–Pb age of 551.07 ± 0.61 Ma (Condon et al. 2005). Comparison of carbon isotope chemostratigraphy and sequence stratigraphy between the Weng’an and the Three Gorges areas suggests that the upper phosphorite unit at Weng’an postdates the Gaskiers glaciation (ca. 580 Ma; Condon et al. 2005)” (Du et al. 2017).

Fossil Content

Soft-tissue fossils preserving cellular structures, notably including the earliest record of sponges, occur in the Doushantuo phosphates exposed near Weng’an. The reported biota now includes a diverse, well-preserved floral assemblage of multicellular thallophytes, acritarchs, and cyanophytes. The specimens include tissues from a form of seaweed that has many of the cellular characteristics of modern marine plant life. Xiao et al. 1998 describes algae with cellular structure preserved in three-dimensional detail.

The associated faunal assemblage was not at first recognised: The abundant globular objects now thought to be preserved cnidarian and bilaterian embryos, were initially interpreted as phytoplanktonic organisms. Some appear to be metazoan embryos preserved in early cleavage stages indicating “that the divergence of lineages leading to bilaterians may have occurred well before their macroscopic traces or body fossils appear in the geological record” (Xiao et al. 1998, p. 553).

Earliest Sponges?

Sponges are widely recognised as the most primitive of living metazoans, occupying a basal position in metazoan phylogeny, as a sister group to all other Metazoa (e.g. Nielsen 2001, p. 30, 506-507, but see Dunn et al. 2008, Schierwater et al. 2009, and Ryan et al. 2013 for alternative possibilities). Thus their first occurrence in the fossil record is a datum of particular interest. Although sponge biomarkers and possible spicules have been reported from other Late Proterozoic rocks (Zhao et al. 1985, 1988; McCaffrey et al. 1994; Moldowan et al. 1994; Brasier et al. 1997; Xiao & Knoll 2000), only rare occurrences of Precambrian sponge body fossils have been reported. The earliest record is of presumed sponge remains is from the Doushantuo phosphates, ~590 to 570 Ma (Tang et al. 1978; Ding et al. 1988; Li et al. 1998; but see Steiner et al. 1993 and Zhang, Yuan & Yin 1998 for contrary views).

However, sponges could have occurred earlier and not been recognised; spicules are not necessarily diagnostic, even in living sponges (Dr. Allen Collins, Smithsonian Institution, pers. comm.)

Fossil Embryos

Reports of fossilized eggs of marine invertebrates are rare. This may, however, largely be due to the difficulties of recognising them. There is an abundance of small globular structures in the fossil record, including that of the Cambrian. Bengtson & Zhao 1997 reports basal Cambrian embryos from the Dengying Formation, Shaanxi, China. Zhang & Pratt 1994 reports Middle Cambrian spherical fossils, 0.3 mm in diameter, that under a smooth membrane preserved a polygonal pattern which the authors interpreted as remains of blastomeres belonging to 64- and 128-cell stages of arthropod embryos. In some other cases, at least a general resemblance to eggs has been noted. Fig. 2 illustrates an example of the Dengying material for comparison with the Doushantuo fossils.

Interpretation of the roughly 500 μm (micrometer) spheroidal bodies as metazoan embryos containing one, two, four, eight, and more cells is supported by the observation that the fossils are the same size no matter how many cells they contain. Modern early-stage embryos behave similarly as their constant volume is divided and redivided. In contrast, algal cells tend to be similar in size, so the diameter of a clump of algal cells would depend on the number of cells in the cluster.

Some of the four-cell embryos exhibit a bilaterian tetrahedral cleavage pattern. If correctly interpreted, this finding provides further evidence that relatively modern characteristics such as bilaterality evolved before the great radiation of the Cambrian explosion.

“We want to stress that we make no claim that organisms we would recognize as polychaetes or echinoderms existed at the time the Doushantuo sediments were deposited. The comparisons with modern forms ... are intended only to show that the morphological characters of the fossil gastrulae are paralleled in detail by those of modern bilaterian gastrulae. Furthermore, to accommodate the diversity of the fossil gastrulae both deuterostome and spiralian models appear to be required” (Chen et al. 2000, p. 4459).

These assignments are not unquestioned, however. Xue et al. 1999 expresses doubts about the metazoan interpretations of some Doushantuo fossils, noting in particular that the great abundance of Megasphaera in the Doushantuo phosphorites precluded interpretation as eggs or embryos. Additionally, the absence of both gastrula and adult fossils requires some explanation. Xiao & Knoll 2000 (p. 784) notes the well-known strategy of extremely abundant egg production, practiced by many modern invertebrates to counter high mortality rates, and further suggests that various sedimentary factors such as a low sedimentation rate, winnowing and redeposition may have acted to concentrate the fossils. The absence of later developmental stages can also be explained by a number of biological and sedimentary processes. Moreover, it is possible the fossil Praviglobus? dentisuturalis Xue et al. 1995 may represent the later (post-embryonic) developmental stage of some indeterminate metazoan.

Even so, diagenetic effects are sometimes difficult to distinguish from genuine biological structures, and much of the evidence from this source, though widely accepted, remains equivocal.

Stratigraphy

Draw on the overview material from Zhang et al.; redraw columns A and B (skip C)

In central Guizhou, near Weng’an, late Proterozoic sedimentary rocks are represented by the Nantuo, Doushantuo, and Dengying formations. The lowermost unit is the Nantuo Formation (~620 to 590 Ma) which uncomformably overlies the middle Proterozoic Banxi metamorphic complex and is composed principally of tillite associated with the last Varanger glaciation. The overlying early Vendian [CHANGE OR AT LEAST TRANSLATE] Doushantuo Formation (~590 Ma at its base to ~565 Ma at its top) is represented by a phosphate-dolostone sequence at Weng’an, where it is 33 to 55 m thick and consists mainly of dark phosphate, cherty phosphate, chert, and gray dolomite. The overlying Dengying Formation, of Ediacaran (~565 Ma to 544 Ma) age and containing rare Ediacaran body fossils in the lower part and basal Cambrian shelly fossils (including Cloudina) near the top, is a 180 m thick dolomite sequence.

“The Doushantuo Formation typically overlies the Marinoan Nantuo Tillite (Zhou et al. 2004) and underlies Dengying Formation dolostones, which contain Cambrian small shelly fossils near their tops in some areas (Wang et al. 1984). Commonly interpreted as having been deposited in a shallow-marine environment above storm wave base (e.g., Zhang et al. 1998), the Doushantuo at Weng’an is ~40 m thick and contains two phosphorite intervals, both representing shallowing-upward sequences separated by a laterally continuous karstic, subaerial exposure surface (e.g., Xiao & Knoll 1999). Recent work has shown that the basal dolostone of the Doushantuo Formation, below the subaerial exposure surface, is a post-Snowball Earth cap carbonate on top of the Marinoan Nantuo Tillite (Zhou et al. 2004)” (Dornbos et al. 2006, p. 4).

“The Weng’an Phosphorite Member is exposed in the centre of Guizhou Province, southern China .... In the Weng’an area, the Doushantuo Formation is exposed along the axis of the Mt. Beidou anticline ... and consists mainly of two phosphorite units separated by a dolomite bed overlying a glaciogenic deposit known as the Nantuo Formation, which was deposited during the Marinoan glaciation (Xiao et al. 1998, Dornbos et al. 2006). Therefore, the Doushantuo Formation was deposited after the Cryogenian Snowball Earth interval (Condon et al. 2005). The Doushantuo Formation is overlain by the Dengying Formation, which consists of a thick succession of carbonate rocks containing sparse Ediacaran fossils and basal Cambrian, small shelly fossils in its uppermost part (Jiang et al. 2011). We collected fossils from the lower black bituminous facies of the upper phosphorite unit, Weng’an area ....” (Du et al. 2017).

“Results indicate that there are two genetically related phosphatic lithofacies within the animal and animal-embryo-bearingWeng’an PhosphoriteMember: a lower black facies and an upper gray facies. Within each of these facies, phosphogenesis and phosphatization took place under different environmental conditions. The black facies is a pyrite-rich bituminous phosphorite that lacks dolomite in its lowermost two meters and contains evidence for lower levels of reworking than the gray facies. Deposited on top of a karstic sequence boundary, the black facies is interpreted as a condensed, sediment-starved deposit that shallows upward into the higher-energy gray facies. Abundant matrix forming dolomite and greater levels of reworking characterize the gray facies. Both facies were deposited in shallow, nearshore-marine environments” (Dornbos et al. 2006).

“In Hubei and Guizhou provinces of China, the latest Varanger glaciation event is represented by the Nantuo tillites. The tillites are believed to have an age of 610-590 Ma, and a U-Pb radiometric date suggests a greater age for the underlying formation. The Doushantuo Formation lies immediately above the Nantuo Formation, representing transgressive deposits that occurred as the result of a rise in sea level because of the melting of continental glaciers. The time gap between the end of the glaciation and the beginning of transgressive deposition is of unknown length, except that it is certainly younger than the 610- to 590-Ma-old Nantuo tillites. The age of the Doushantuo Fm could be as old as 580 Ma, and pending direct measurement, its age must fall within the range 570 ± 20 Ma [Saylor et al. (1998) argue that it is toward the younger end of this range]. It is important to stress, however, that whatever the absolute time horizon represented by the Doushantuo Fm, it is likely to precede the lowest strata with which bilaterian remains have so far been associated” (Chen et al. 2000, p. 4457).

About 15 km west of Weng’an, the Doushantuo Formation consists of three units: the Lower Phosphate unit (20 m thick), the Middle Dolostone unit (3.7 m) and the Upper phosphate unit (15 m). A dark phosphate bed at the top of Lower Phosphate unit has yielded some of the richest finds reported so far.

Biomarkers extracted from the phosphorite samples indicate that the main sources of the organic component are non-vascular plants, protists, bacteria and archaebacteria. Some of the biomarkers indicate a strongly reducing (oxygen-poor) depositional environment characterised by high salinity and low terrigenous input (Yin et al. 1999, p. 509).

“In summary, Doushantuo phosphorites are widespread on the Yangtze Platform and are distributed in relatively shallow environments along a Neoproterozoic, east-facing passive margin. Phosphogenesis is incompletely understood, but models calling for upwelling onto a broad shallow platform … may be applicable …” (Xiao & Knoll 2000, pp. 769-771).

Lithology

“The Doushantuo Fm is a marine deposit containing phosphate-dolomite sequences. In Beidoushan, in the Weng’an district of central Guizhou Province, the phosphate deposit is divided by an erosive surface into two units. The fossils described here are from the base of the upper phosphate unit, 0.2-6 m in thickness. This high-resolution fossil bed is about 30% phosphate, present as the mineral fluorapatite [Ca₅(PO₄)₃F]. Phosphatic beds within this deposit are grainstones composed of 1- to 5-mm phosphoclasts. These derive from a phosphatic surface that formed on the sea floor, in the process recrystallizing existing surface sediments. In addition to replacing carbonate sediments, soft tissues of metazoan embryos, larvae, adults, and algae also appear to have been mineralized. The phosphatized sediment crust was then broken into small fragments by heavy current activity and then redeposited and mixed in with adjacent lime muds” (Chen et al. 2000, p. 4457).

The fossil embryo-bearing upper Weng’an phosphorite unit consists of two facies: a lower black facies ... and an upper grey facies (Dornbos et al. 2005). A petrographic study has shown that the black facies is more organic rich and contains less reworked material. The best-preserved fossils with exquisite anatomical structures have been recovered from this black facies (Dornbos et al. 2006).

Microfossils in the Weng’an Phosphorite Member were variably resistant to decay during diagenesis of the phosphorite (Xiao & Knoll 1999), some showing relatively little, but others significant decay before phosphatization (Dornbos et al. 2005). In general, primary biological structures were relatively well preserved during phosphatization, but phosphatic encrustations were also added later (Xiao & Knoll 1999)” (Du et al. 2017).

Taphonomy

Analysis of the rocks and preservation of the Weng’an fossils suggests that the fossil organisms were buried alive by catastrophic sediment incursions. Phosphate alternation preserved the morphological details of the soft tissues so finely that it is possible to study the fossils at the cellular level.

Just how phosphate manages to freeze even the tiniest details of soft tissues is unclear. When the Doushantuo formed, large amounts of dissolved phosphate were apparently delivered to a shallow sea floor covered by low-oxygen waters, a place where small creatures could be preserved prior to substantial degradation.

For further discussion of the fossilization process, see Zhang et al. 1998.

“The Weng’an Phosphorite Member is 9 to 10m thick in this region.... All animal fossils—including animal eggs and embryos—preserved in the Doushantuo Formation are from this interval. Considering that fossils in the upper Doushantuo Formation are separated from the lower Doushantuo Formation by a subaerial exposure surface, they probably were not influenced by any unusual post-Snowball paleoceanographic conditions. Although the subaerial exposure surface below it could represent a post-Marinoan glacial regression, there currently is no evidence of characteristic cap-carbonate features in the upper Doushantuo Formation” (Dornbos et al. 2006, p. 4).

Inferred Paleoenvironment

paleoenvironment goes here

“Carbonates of the Doushantuo Member III in this study are characterized by extremely low d¹³C values (lowest to -10‰), which can be correlated to the Shuram negative d¹³C excursion (or DOUNCE) (McFadden et al., 2008; Lu et al., 2013). This widespread d¹³C excursion has been proposed to reflect either a primary marine signal or the product of secondary alteration, both of which suggest global environmental extremes during this period (e.g., Grotzinger et al. 2011). eNd(t) values recorded in these carbonates from South China exhibit a dramatic negative excursion ... in step with the d¹³C variation” (Wei et al. 2019).

Significance

“The Neoproterozoic Doushantuo Formation of southwest China has yielded the earliest known unambiguous fossil evidence for animals in the form of phosphatized animal eggs and embryos (Xiao et al. 1998; Li et al. 1998; Xiao & Knoll 2000; Knoll & Xiao 2001; Xiao 2002), as well as candidates for the earliest fossil bilaterians (Chen et al. 2004a; see Bengtson & Budd 2004, Chen et al. 2004b, for discussion). While the importance of these animals and animal embryos is undeniable, an assortment of other potential phosphatized fossils also has been described recently from the Doushantuo phosphorites, including putative gastrulae, larvae, and microscopic adult sponges and cnidarians (Li et al. 1998; Chen et al. 2000; Xiao et al. 2000; Chen et al. 2002). While these recent fossil discoveries rightfully have attracted much attention, the Doushantuo Formation actually has yielded significant fossils for the last two decades. These fossils include cyanobacteria (e.g., Zhang 1981 [probably Zhang 1986]; Awramik et al. 1985), acritarchs (e.g., Yin & Li 1978), and red algae (e.g., Zhang 1989) preserved in the cherts and phosphorites of the Doushantuo” (Dornbos et al. 2006).

“With its animal-fossil-bearing interval recently dated at around 580 Mya (Barfod et al. 2002; Condon et al. 2005), the Doushantuo Formation provides a critical window into early animal evolution. Despite the spectacular nature and obvious significance of this Lagerstätte, complete with well-preserved individual cells (e.g., Butterfield 2003), attempts to place the Doushantuo fossils in a detailed paleoenvironmental context are only just beginning (e.g., Zhang et al. 1998; Xiao & Knoll 1999)” (Dornbos et al. 2006).

“Microfossils in the Weng’an Phosphorite Member were variably resistant to decay during diagenesis of the phosphorite (Xiao & Knoll 1999), some showing relatively little, but others significant decay before phosphatization (Dornbos et al. 2005). In general, primary biological structures were relatively well preserved during phosphatization, but phosphatic encrustations were also added later (Xiao & Knoll 1999)” (Du et al. 2017).

Taxa

Kingdom Plantae

Phylum “Thallophyta” (thallose seaweeds)

Earliest reports from the Doushantuo Formation include Xiao et al. 1998, which recorded “abundant thalli with cellular structure preserved in three-dimensional detail show that latest-Proterozoic algae already possessed many of the anatomical and reproductive features seen in the modern marine flora” (Xiao et al. 1998, p. 553).

Kingdom Metazoa

Phylum Unknown

Chen et al. 2000 fig. 3A-G

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Fig. 3. Putative fossil embryos that resemble bilaterian gastrulae. (A–G) Fossils resembling deuterostome embryos; (H) Modern example (gastrulae of the sea urchin Mespilia globulus, ref. 49) In A, C, and E, the archenteron is bent to one side, and in A and C displays bilobed outpocketings; (A) The nearer ectodermal layer is thicker compared with the opposite one (possible oral and aboral ectoderms, respectively; compare H). (C) A section in the plane indicated by the small arrowheads in A.(B and D) Polarized light microscope images, showing that the cells comprising the outpocketings are differently oriented, as they appear in different colors from those constituting the walls of the gut. In A, part of the outer wall is deformed (arrow) by a crystal grain visible in B (light pink). (G), Another specimen displaying invaginating archenteron at early midgastrula stage. (H) Modern sea urchin gastrulae (49). (I and J), Fossils resembling modern spiralian gastrulae; (K) Modern polychaete embryos in which the dashed lines indicate yolky endoderm cells and dots represent mesoderm cells (Eupomatus, left; Scoloplos, right, redrawn from Anderson, ref. 50). In the fossils I and J, the archenteron is thick-walled (cf. cross section in C), and in J all of the cells in the embryo, including the ectodermal wall, are conspicuously larger relative to the size of the embryo. Note also the column of cells along the archenteron in J. (Scale bars represent 50 μm.)

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Chen et al. (2000) stress that they make no claim for recognisable modern taxa in Doushantuo-aged sediments. Their comparisons with modern forms are “intended only to show that the morphological characters of the fossil gastrulae are paralleled in detail by

those of modern bilaterian gastrulae [and that], to accommodate the diversity of the fossil gastrulae, both deuterostome and spiralian models appear to be required” (Chen et al. 2000, p. 4459).

+ Critique by Xiao et al. 2000

Phylum Porifera

Class Demospongiae

Discussion: The occurence of sponges in the Doushantuo Formation was reported by Li et al. (1998) who claim to have identified 35 individual sponges in thin sections; mostly globular although a few tubular, and ranging in size from 150 to 750 μ m. Their evidence comprises structures strongly resembling thin monaxonal spicules that are randomly dispersed in the mesohyl. The spicules survived chemical treatment by hydrochloric acid, indicating that they are siliceous. Most of the spicules are 0.5 to 1.0 μm in diameter and 10 to 60 μm long. The largest is 4 μm in diameter and 100 μm long.

Further claims, to have observed cellular-level characteristic structures of living sponges – including the epidermis, porocytes, amoebocytes, sclerocytes, and spongocoel – are far less certain.

Interpretation: Li et al. 1998 proposes that the Doushantuo sponges are monaxonid Demospongiae because their skeleton consists exclusively of siliceous and monaxonal spicules. However, it is generally interpreted that Hexactinellida evolved before both Calcarea and Demospongiae, and the recently discovered Ediacarian sponges from Mongolia (Brasier et al. 1997) are also referred to Hexactinellida.

The Weng’an sponge remains of Doushantuo age (Early Vendian), therefore may require revision of phylogenetic relations among the four major classes of phylum Porifera. Sponges are a monophyletic metazoan group and comprise the sister groups demosponges, hexactinellids, archaeocyaths, and calcareous sponges. Our data imply that the ancestral form in sponges lies among the demosponges.

Sponges are a major component in Lower Cambrian Chengjiang fauna. There, the skeletons in most species are represented exclusively by diactins, which form a regular, reticulate skeletal framework, and these fossils are classified as demosponges. The data from Chengjiang fauna demonstrate that main clade of early sponges, the monaxonid Demospongiae was diverse in the Lower Cambrian. Li et al. 1998 concludes that diactins evolved before other types of spicules.

Analysis of the associated rocks suggests that in the Late Proterozoic, silica biomineralization in the sponges happened in an eutrophic environment. Calcareous biomineralization in sponges (for example, archaeocyathids) is first seen in the Tommotian (~530 Ma), postdating the silica biomineralization by more then 50 million years. Calcareous biomineralization is mainly seen in oligotrophic settings. Archaeocyathids, which are possible representatives of coralline sponges, have a secondary calcareous skeleton of high Mg-calcite and are possibly derived from demosponges.

(A) Li et al. 1998 fig. 1A (37584 bytes) (B) Li et al. 1998 fig. 1F (18311 bytes)

(C) Li et al. 1998 fig. 2D (21009 bytes)

Fig. 4: Many of the flat polygonal epidermal cells have their cytoplasmic contents and nuclei preserved. Dense granules surrounding the nucleus are probably cytoplasmic organelles. Scattered among the epidermal cells are porocytes that form incurrent pores. In the mesohyl, amoebocytes with a variety of shapes were seen. In one of the best preserved specimens, six spicules were closely associated with sclerocytes. The plasma membrane of the sclerocyte is still attached to one end of the spicule. The darkly stained spongocoel has many amorphous inclusions, representing undigested debris.

(A) Reproduction of fig. 1A from Li et al. 1998; a possible longitudinal section of a tubular phosphatized sponge, showing the randomly dispersed monaxonal spicules (s) in the mesohyl (m). Two spicules firmly associated with spicule-producing cells, the scleocytes, are encircled. Scale bar is 100 μ m.

(B) Reproduction of fig. 1F from Li et al. 1998; two sclerocytes (sc) with their developing spicules (s) and plasma membrane (pm). Scale bar is 10 μm.

(C) Reproduction of fig. 2D from Li et al. 1998; a proposed embryo at the blastula stage. Scale bar is 50 μm.

Phylum Cnidaria

Class Hydrozoa

Chen et al. embryo paper figs 2C, 2D

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Fig. 2. Putative cnidarian embryos and larvae. (A) Oblique section of a possible fossil anthozoan planula. (B) Schematic view of a transverse section of the late planula of the anthozoan Euphyllia rugosa. The larval stage represented in A and B is constituted of an outer monocellular layer, the ectoderm, within which is an inner endodermal layer with various mesenteric folds and immature septa. This complicated bilayered structure is typical of anthozoan late planula larvae. Note the individual cells visible in the ectodermal layer at lower left in A, where it has separated from the endodermal layer. (Scale bar, 100 mm.) (C and D) Putative fossil gastrula of hydrozoan medusa; (C) Bright field; (D) Polarized light. Under polarized light (D), both layers show the same crystal orientation at arrows, as indicated bythesamecolors. The modern hydrozoan embryo shown in E is Liriope mucronata. B is from Chevalier (47); E from Campbell(48). (Scale bar in C is 50 μm.)

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Class Anthozoa

Chen et al. embryo paper fig 2A

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Fig. 2. Putative cnidarian embryos and larvae. (A) Oblique section of a possible fossil anthozoan planula. (B) Schematic view of a transverse section of the late planula of the anthozoan Euphyllia rugosa. The larval stage represented in A and B is constituted of an outer monocellular layer, the ectoderm, within which is an inner endodermal layer with various mesenteric folds and immature septa. This complicated bilayered structure is typical of anthozoan late planula larvae. Note the individual cells visible in the ectodermal layer at lower left in A, where it has separated from the endodermal layer. (Scale bar, 100 mm.) (C and D) Putative fossil gastrula of hydrozoan medusa; (C) Bright field; (D) Polarized light. Under polarized light (D), both layers show the same crystal orientation at arrows, as indicated bythesamecolors. The modern hydrozoan embryo shown in E is Liriope mucronata. B is from Chevalier (47); E from Campbell(48). (Scale bar in C is 50 μm.)

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Order ?Tabulata

Tabulate corals in the strict sense are confined to the Paleozoic, originating in the Ordovician or possibly Cambrian, and becoming extinct in the late Permian. The Doushantuo forms attributed to the Tabulata are millimeter-scale tubes that display tabulation and apical budding characteristic of the group.

Sinocyclocyclicus guizhouensis Xue et al. 1992

Description: Tubes are circular in cross section, with a diameter of 0.1-0.3 mm; the diameter can vary markedly along the length of a single individual. No demonstrably complete tubes are known, but incomplete specimens can be more than 1 mm long. Specimens occur both as gregarious clusters with individuals oriented subperpendicular to bedding and as dispersed specimens without preferred orientation. Some tubes have thick (ca. 10-15 μ m), even multilamellate outer walls (fig. 5F), likely formed or modified by diagenetic phosphatization. Others are preserved as internal molds without enveloping walls (fig. 5A). Bent specimens show folds on the compressional but not the extensional side, indicating that walls were originally flexible.

Cross-walls oriented perpendicular to the main axis divide tubes into a more or less regular series of chambers 6-12 μ m thick. Most cross-walls are complete, but some tabulae extend only part of the way across the tube; they may intersect with adjacent cross-walls to form wedge-like chambers. Limited observations suggest that cross-walls are not perforated by through-going internal structures such as siphuncles.

Well-preserved specimens show that tabulae may curve slightly where they intersect with the tube wall. Indeed, well-preserved walls show that the point of insertion is thickened and wedge-like in cross section, manifested in internal molds as distinct eaves at tablet boundaries. In a few tubes, cross-walls are absent or only vaguely visible; without exception, such specimens are poorly preserved, with interiors filled by secondary silica, dolomite, or phosphatic filaments and spherules.

Terminal ends are poorly known; however, a few specimens taper to a blunt conical termination. One tube contains what appears to be a distinct terminal chamber, defined by a complete but strongly concave cross-wall. Abutting tabulations are incomplete and curved at their intersection with the chamber floor.

A few specimens show a distinctive pattern of dichotomous branching (fig. 5A); tubes expand gradually along one axis to the point of dichotomy and then split into two branches, the combined circular cross sections of which are equal in area to the elliptical section below the branch point. Finally, a thin (1-μ m) ridge has been observed running along the concave side of a single curved internal mold (fig. 5F); its structural or systematic importance is unclear.

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