Abstract
The Weng'an Biota is a fossil Konservat-Lagerstätte in South China that is c. 570 – 609 myr old and provides an unparalleled snapshot of marine life during the interval in which molecular clocks estimate that animal clades were diversifying. It yields fossils that are three-dimensionally preserved in calcium phosphate with cellular and sometimes subcellular fidelity. The biota includes candidates for the oldest animals in the fossil record, including embryonic, larval and adult forms. We argue that, although the Weng'an Biota includes forms that could be animals, none can currently be assigned to this group with confidence. Nonetheless, the biota offers a rare and valuable window on the evolution of multicellular and soft-bodied organisms in the prelude to the Cambrian radiation.
The origin and evolutionary assembly of animal body plans comprises one of the most formative episodes in the history of life. Animals are ecosystem engineers and their appearance fundamentally changed our planet's ecology (Butterfield 2011a). Despite the importance of this evolutionary episode, many aspects of the timing and nature of the event remain poorly constrained. Molecular-clock analyses estimate that animals originated by the Cryogenian and diversified through the Ediacaran (Peterson & Butterfield 2005; Erwin et al. 2011; dos Reis et al. 2015), but fossil evidence of animals from before the Cambrian is controversial (Erwin et al. 2011; dos Reis et al. 2015; Cunningham et al. 2017). The Weng'an Biota is one of the few Lagerstätten from the critical interval in which early animals are expected according to molecular-clock studies. In this Ediacaran fossil assemblage, organisms are phosphatized in cellular and even subcellular detail, providing a rare glimpse of soft-bodied and multicellular life at this time. Early research appeared to fulfil expectations of the presence of metazoans with reports of embryonic (Xiao et al. 1998), larval (Chen et al. 2000, 2002) and adult (Xiao et al. 2000; Chen et al. 2002, 2004; Yin et al. 2015) animals from the Weng'an deposit. However, subsequent analyses have cast doubt on this view, and there is currently much disagreement over these interpretations (Bailey et al. 2007a; Huldtgren et al. 2011; Bengtson et al. 2012; L. Chen et al. 2014, Xiao et al. 2014a). Here, we review the stratigraphic position, geological age and environmental setting of the deposit, and present an overview of the biota and an assessment of the phylogenetic affinities of the various taxa.
Stratigraphy and age
The Weng'an Biota occurs within the Ediacaran Doushantuo Formation (551 – 635 Ma, Condon et al. 2005) of south China (Fig. 1). In addition to the phosphatized microfossils from Weng'an, this formation has yielded silicified microfossils (Yin et al. 2004) and macrofossils that are preserved as 2D carbonaceous compressions including macro-algae (Xiao et al. 2002) and the putative ctenophore Eoandromeda (Tang et al. 2008, 2011). The Weng'an Biota itself is known from localities in Weng'an County, Guizhou Province. The Doushantuo Formation overlies the Marinoan glacial tillites of the Cryogenian Nantuo Formation that can be dated to 635 Ma (Condon et al. 2005). It is overlain by the Ediacaran Dengying Formation, which contains fossils of the classical Ediacara macrobiota (Sun 1986; Xiao et al. 2005; Z. Chen et al. 2014). The base of the Dengying Formation can be dated to 551 Ma (Condon et al. 2005). In Weng'an, the Doushantuo Formation is composed of five units that have been described in detail by Xiao et al. (2014b) and Yin et al. (2015). The Weng'an Biota occurs mainly in Unit 4, the Upper Phosphorite Member, but also in Unit 5. Unit 4 is divided into 4A, a black phosphorite, and 4B, a grey dolomitic phosphorite (Dornbos et al. 2006; Xiao et al. 2014a,b; Yin et al. 2015).
The location and geological setting of the Weng'an Biota. (a) Map of China showing the location of Weng'an. (b) Geological map of the Weng'an area. (c) Stratigraphic column of the Beidoushan section in the Weng'an area indicating Units 1 – 4 of the Doushantuo Formation, the occurrence of the Weng'an Biota and the radiometric age constraints discussed in the text. Modified from Yin et al. (2015).
The age of the biota has been debated (Budd 2008; Erwin & Valentine 2013; Xiao et al. 2014a; Yin et al. 2015), with arguments focusing on the correlation of two karstic surfaces, one at the top of Unit 3 and the other within Unit 5 (for detailed discussion of Doushantuo correlation see Zhu et al. (2007), Zhu et al. (2013) and Yang et al. (2015)). If the lower surface is correlated to the c. 582 Ma Gaskiers glaciation (Condon et al. 2005) then the biota would be younger than 582 Ma. However, the lower surface may be older (Yin et al. 2015) and, if the upper karstic surface correlates to the Gaskiers glaciation (Xiao et al. 2014a), then the biota would be older than 582 Ma. Direct radiometric dates for Unit 4 at Weng'an have been inconclusive, giving Pb–Pb isochron ages of 572 ± 36 Ma for Unit 4A (Y. Chen et al. 2009) and 599 ± 4 Ma for Unit 4B (Barfod et al. 2002). However, a recent U–Pb date of 609 ± 5 Ma from a tuff immediately above Unit 4 at Zhancunping, in Hubei Province (Zhou et al. 2017), suggests that the Weng'an biota is probably older than 609 ± 5 Ma and probably older than the Gaskiers glaciation, which cannot be related to the karst surface at the top of Unit 3 if this date is correct. Acritarchs identical to those containing embryo-like fossils are found just above an ash band dated to 632.5 ± 0.5 Ma in Doushantuo sections from the Yangtze Gorges (Yin et al. 2007), suggesting that these organisms could have existed at this time. In summary, the biota is probably older than the classical but enigmatic Ediacaran biota and considerably predates the rich animal fossil record of the Cambrian. Putative animals from the assemblage are therefore candidates for the oldest animals in the fossil record.
Depositional environment, preservation and reworking
The Weng'an Biota is interpreted as having been deposited in an outer-shelf environment on a SE-facing passive margin (Jiang et al. 2011). Abundant wave ripples and cross-bedding features indicate deposition above fair-weather wave base (Xiao & Knoll 1999). The fossils were probably deposited in oxic conditions (Shields et al. 2004), although phosphatization may have occurred in anoxic sediments (Muscente et al. 2014; Schiffbauer et al. 2014). The soft-bodied organisms of the biota are three-dimensionally preserved in calcium phosphate and can be preserved at a subcellular level (Hagadorn et al. 2006; Huldtgren et al. 2011). However, even the best-preserved specimens are a complex amalgam of cements, making it challenging to determine which aspects represent preserved biology (Xiao et al. 2000; Bengtson 2003; Bengtson & Budd 2004; Cunningham et al. 2012a; Schiffbauer et al. 2012). The preservation of Weng'an fossils is discussed in Box 1. The fossils occur either as, or within, phosphatic grains that have been abraded and rounded, indicating transport from other parts of the basin after initial preservation (Xiao et al. 2007b). In unit 4A, a c. 5 m thick black phosphorite, the fossils occur in reworked phosphatic clasts. As a result, they cannot be released by acetic acid maceration and have generally been studied in petrographic thin sections (e.g. Chen et al. 2004; L. Chen et al. 2014; see Box 2 for a comparison of 2D and 3D analytical techniques). In unit 4B, a c. 10 m thick grey dolomitic phosphorite, the microfossils are abundant and in places are so concentrated that the layers resemble oolites. This is a grainstone composed largely of microfossils that have been phosphatized before being reworked, transported and winnowed (Xiao & Knoll 1999; Xiao et al. 2007b). The fossils can be extracted by dissolution of the carbonate constituents of rock samples in weak acetic acid and manual sorting of the resulting residues. Specimens can then be studied using scanning electron microscopy (e.g. Xiao et al. 1998) or tomographic techniques (e.g. Hagadorn et al. 2006) and can be re-embedded in resin and sectioned for further petrographic and geochemical analyses (reviewed by Cunningham et al. 2014).
Box 1: Preservation of Weng'an fossils
The Weng'an organisms, like all exceptionally preserved soft-bodied remains, were subjected to both post-mortem decay and later diagenetic and geological processes (Donoghue & Purnell 2009). These processes alter the morphology of the fossils in ways that can be unpredictable. Palaeontologists must take these factors into account rather than simply comparing the fossils with extant or freshly dead modern organisms. This is particularly true given the simple nature of the biological structures relevant to the interpretation of the Weng'an Biota. In the case of decay, carefully designed experiments can help constrain which features can feasibly be preserved, elucidate likely preservation pathways, and identify biases introduced by the decay process (Sansom 2014). Experiments have shown that animal embryos have a relatively high preservation potential, particularly when enclosed in a fertilization envelope, whereas primary larvae have an extremely low likelihood of being preserved (Raff et al. 2006; Gostling et al. 2008, 2009). In addition, these experiments have identified the likely mechanism for Doushantuo-type preservation: 3D replication of cells by robust bacterial pseudomorphs followed by phosphate mineralization (Raff et al. 2008). The diagenetic and subsequent geological processes that the fossil is subjected to can introduce artefacts (e.g. through crystal nucleation and growth) that have no connection to the original anatomy. These processes are difficult to simulate experimentally because of the timescales involved. An alternative approach is to characterize the mineral phases that preserve biological anatomy v. geological artefacts. This has been carried out in fossils where there is agreement regarding affinity and anatomy, and has allowed textural and chemical criteria to be established and then applied to more contentious Weng'an material (Cunningham et al. 2012a, 2014). In particular, geological artefacts tend to have larger, euhedral crystals with a preferred orientation, higher X-ray attenuation and characteristic void-filling textures. Geological artifacts typically also have high relative abundances of P, Ca and F, and low abundances of C and S. Failure to consider these processes and/or the 3D nature of the specimens can lead to misinterpretations of the fossils as discussed in Box 2.
Box 2: 2D versus 3D analysis
The Weng'an fossils have been subjected to a wide range of analytical techniques. These include scanning electron microscopy (which provides very high resolution images of the exterior of the specimen), analysis of petrographic thin sections (which affords a single cross-section but destroys the remainder of the fossil) and tomographic approaches (which allow the specimen to be imaged in three dimensions). Analyses of specimens in petrographic sections have yielded important insights, including features that have not been identified using other techniques, such as the cell clusters preserved within ‘Megaclonophycus’-stage embryoids (L. Chen et al. 2014). However, the 2D nature of the data can make it difficult to make inferences regarding the 3D anatomy of the fossil and caution must be exercised. Reports of gastrulae (Chen et al. 2000, 2002), cnidarian larvae (Chen et al. 2000, 2002) and adult bilaterians (Chen et al. 2004) probably result from misinterpretations of deformed cysts, which are abundant in the deposit, that have been studied only in two dimensions (Xiao et al. 2000; Bengtson 2003; Bengtson & Budd 2004; Bengtson et al. 2012).
Overview of the Weng'an Biota
Algae
A variety of algal taxa have been reported from Weng'an (Zhao 1986; Zhang 1989; Zhang & Yuan 1992; Xiao et al. 1998, 2004; Xiao 2004). Some simpler forms, such as Archaeophycus (Fig. 2j), show interesting similarities to extant bangialean red algae (Xiao et al. 1998), although other affinities cannot be ruled out (Xiao et al. 2014a). More complex forms such as Thallophyca and Paramecia share characters with floridophyte red algae including pseudoparenchymous construction, differentiated thalli and possible reproductive structures (Xiao et al. 2004). However, the putative algae have received relatively little attention because of the focus on the search for animals. Many specimens lack the overall form of a photosynthetic organism. For example, the blades typical of various seaweeds have not been recovered and it is hard to envisage how cells positioned centrally within globular masses would have functioned in a photosynthetic organism (Fig. 3a and b). Algae may have been used as a wastebasket taxon for assorted irregular forms and rejected animal candidates. These fossils merit further study and there is a prospect that they may include developmental stages of other organisms including the embryo-like fossils, especially given their typically larger size.
Scanning electron microscope images of fossils from the Weng'an biota. (a–f) Tianzhushania specimens at various stages of division from a single cell (a) to many hundreds of cells (f) Swedish Museum of Natural History (SMNH) X 6449–SMNH X 6454. (g) Helicoforamina SMNH X 6455. (h) Spiralicellula (from Tang et al. 2008). (i) Caveasphaera SMNH X 6456. (j) Archaeophycus, a putative red alga SMNH X 6457. (k) Mengeosphaera, an acritarch SMNH X 6458. (l) Eocyathispongia, a putative sponge, Nanjing Institute of Palaeontology and Geology (NIGPAS) 161760 (from Yin et al. 2015). Scale bar: (a) 320 µm, (b) 265 µm, (c) 265 µm, (d) 200 µm, (e) 245 µm, (f) 280 µm, (g) 395 µm, (h) 380 µm, (i) 250 µm, (j) 255 µm, (k) 130 µm, (l) 415 µm.
Synchrotron radiation X-ray tomographic microscopy (SRXTM; a–h) and light microscopy (i–k) images of Weng'an fossils. (a, b) a possible alga SMNH X 6459, comparable with Paramecia. (c, d) a peanut-shaped fossil SMNH X 6460. (e, f) Sinocyclocyclicus SMNH X 5322. (g, h) Ramitubus SMNH X 5326. (i–k) Light microscopy images of putative algae from Weng'an. Scale bar: (a, b) 270 µm, (c, d) 280 µm, (e, f) 180 µm, (g, h) 175 µm, (i, j) 140 µm, (k) 115 µm.
Acritarchs
The acritarchs (Fig. 2k) from Weng'an have been reviewed comprehensively by Xiao et al. (2014b) and form part of the Doushantuo–Pertatataka microbiota (Zhou et al. 2001, 2007; Liu et al. 2014). The phylogenetic affinities of acritarchs are unknown and they probably represent a polyphyletic assortment of eukaryotes (Huntley et al. 2006). Some Doushantuo acritarchs contain embryo-like fossils (Yin et al. 2007), leading to the suggestion that other Ediacaran acritarchs might be resting cysts of these organisms (Cohen et al. 2009). The interpretation of the Doushantuo–Pertatataka acritarchs is therefore at least partially linked to that of the embryo-like fossils, although they may well represent a polyphyletic assemblage. The affinities of taxa should be considered on a case-by-case basis.
Embryo-like fossils
Weng'an fossils that have been interpreted as the embryos of early animals (Xiao et al. 1998) have been the focus of most attention and debate (Fig. 2a–f). They have been interpreted as metazoans (Xiao et al. 1998) including bilaterians (Chen & Chi 2005; J. Chen et al. 2006, 2009a,b; Yin et al. 2013), as stem-group metazoans (Hagadorn et al. 2006; Schiffbauer et al. 2012; L. Chen et al. 2014) or as members of non-metazoan clades (Bailey et al. 2007a,b; Butterfield 2011b; Huldtgren et al. 2011, 2012; L. Chen et al. 2014; Zhang & Pratt 2014). We consider the various claims below.
Giant sulphur bacteria?
Bailey et al. (2007a,b) proposed an interesting hypothesis that the embryo-like fossils might be giant sulphur bacteria similar to the living Thiomargarita. These bacteria can be similar in size and shape to the embryo-like fossils from Weng'an and are capable of undergoing at least a few rounds of palintomic division. However, subsequent analyses have shown that these bacteria cannot account for key morphological aspects of the fossils such as the presence of ornamented envelopes, outer acritarch vesicles, and probable lipid vesicles and nuclei (Donoghue 2007; Xiao et al. 2007b; Huldtgren et al. 2011; Cunningham et al. 2012b). Moreover, evidence from experimental taphonomy showed that Thiomargarita cells are not replicated by biofilm-forming bacteria, meaning that they do not form the stable bacterial pseudomorphs that are thought to be the precursor to exceptional phosphatization (Cunningham et al. 2012b; see Box 1).
Bilaterians or eumetazoans?
Reports of embryonic bilaterians do not withstand scrutiny. Some are based on identifications of cell geometries argued to be unique to bilaterians. These include specimens purported to preserve endodermal cords (J. Chen et al. 2009b), polar lobes (J. Chen et al. 2006, 2009a; Yin et al. 2013), embryonic polarity (J. Chen et al. 2009a,b) and duet cleavage (J. Chen et al. 2009a). In each case, the specimens can be alternatively interpreted as examples of embryo-like fossils that have undergone taphonomic and diagenetic processes (Huldtgren et al. 2011; Cunningham et al. 2012a). Specimens interpreted as possible bilaterian or cnidarian gastrulae and larvae (Chen et al. 2000, 2002; Chen & Chi 2005) are more probably deformed cysts filled with phosphatic cements (Xiao et al. 2000). The presence of meroblastic embryos (Yin et al. 2016) would represent the long sought-after confirmation of the presence of animal embryos. A possible alternative interpretation of these specimens, which are associated with a large population known to undergo asynchronous division (Hagadorn et al. 2006), is that they have undergone unequal division. It is currently difficult to test between these possible interpretations.
Total group animals?
The embryo-like fossils probably represent one taxon dividing from a single cell to thousands of cells (Fig. 2a–f). This taxon has been named either Tianzhushania or Megasphaera according to different taxonomic interpretations (Yin et al. 2004; Xiao et al. 2014b). Tianzhushania is preferred here (see Box 3). These specimens were interpreted as animal embryos by Xiao et al. (1998) based on the similar size and the presence of palintomic cell division, Y-shaped junctions between cells and an ornate enclosing envelope. More recently, these have been considered as stem, rather than crown, animals because later stages lack evidence for epithelial organization, which is characteristic of modern embryos (Hagadorn et al. 2006). However, the placement of these fossils in the animal total-group has also been questioned (Bailey et al. 2007a; Butterfield 2011b; Huldtgren et al. 2011; L. Chen et al. 2014; Zhang & Pratt 2014).
Box 3: Taxonomy of the embryo-like fossils
The embryo-like fossils have been described under various genus and species names that are now considered to be different developmental stages or taphonomic variants of a single taxon (Huldtgren et al. 2011; Cunningham et al. 2012a; L. Chen et al. 2014; Xiao et al. 2014a,b). The names Megasphaera (single-celled specimens), Parapandorina (multiple polyhedral cells) and Megaclonophycus (large numbers of usually spheroidal cells) are now widely considered to be synonyms (e.g. Huldtgren et al. 2011; Cunningham et al. 2012a; Xiao et al. 2014a,b). However, because of different taxonomic interpretations, different researchers have referred to this taxon as either Megasphaera (e.g. L. Chen et al. 2014; Xiao et al. 2014a,b) or Tianzhushania (e.g. Yin et al. 2004; Huldtgren et al. 2011; Cunningham et al. 2012a). The genus Tianzhushania and its type species T. spinosa were described in 1978 for acanthomorphic acritarchs with cylindrical processes that were known from thin sections (Yin & Li 1978). Yin et al. (2001) subsequently described T. tuberifera based on specimens with both cylindrical processes and sculptured ornament. Megasphaera, with the type species M. inornata, was described in 1986 for smooth envelopes (Chen & Liu 1986) and later expanded by Xiao & Knoll (2000) to accommodate specimens with sculptured envelopes (M. ornata). Yin et al. (2004) showed that M. ornata specimens, when viewed in thin sections, could be surrounded by an outer wall identical to that of Tianzhushania. They therefore argued that T. tuberifera, which had been studied in thin sections, was the same species as M. ornata, which had mainly been studied in specimens isolated from acid residues. Yin et al. (2004) proposed Tianzhushania ornata as the valid name for this taxon. As the various embryo-like stages can be found inside these specimens, Huldtgren et al. (2011) argued that Tianzhushania is the senior synonym of Megasphaera, Parapandorina and Megaclonophycus. Xiao et al. (2014) noted that the diagnosis of Tianzhushania had never been formally emended to include specimens with sculptured envelopes. They therefore proposed to retain Megasphaera for smooth or sculptured specimens that lack processes and Tianzhushania for specimens that have smooth envelopes and processes. The new genus Yinitianzhushania (basionym T. tuberifera) was erected to accommodate those specimens that have sculptured envelopes and cylindrical processes. However, this classification differentiates Megasphaera from the other genera based on the absence of tubular processes, which probably results from taphonomic loss rather than a biological difference. We also find it unsatisfactory to place specimens from acid residues into Yinitianzhushania if they have a sculptured envelope and Tianzhushania if they do not. This would result in a specimen with a sculptured envelope and tubular processes being placed in one genus (Yinitianzhushania) if it had lost only its processes and another (Tianzhushania) if it had also lost its sculptured envelope. It is therefore preferable to place all of these taxa in a single genus, Tianzhushania, which we consider to be the senior synonym of Yinitianzhushania, as well as of Megasphaera, Parapandorina and Megaclonophycus, despite the fact that the diagnosis of Tianzhushania has not yet been formally revised to include specimens with sculptured envelopes.
None of the characters that have been used to justify an animal interpretation are exclusive to animals (see Fig. 4). Features such as palintomic cleavage, Y-shaped cell junctions and an ornate envelope are found in non-animal groups (Huldtgren et al. 2011, 2012). They are therefore consistent with an animal interpretation, but they are not diagnostic characters. They are insufficient, either in isolation or in combination, to justify an animal affinity.
Schematic representation of eukaryote phylogeny, modified after Rensing (2016), showing the distribution of characters relevant to the interpretation of the embryo-like fossil Tianzhushania. Here multicellularity includes both aggregative multicellularity (e.g. slime moulds) and clonal multicellularity (animals, plants, fungi, various algae), as well as both facultative (e.g. choanoflagellates) and obligate multicellularity.
It has also been suggested that Tianzhushania exhibits characters gained in the animal stem lineage. L. Chen et al. (2014) described discrete clusters of cells (‘matryoshkas’) within embryo-like fossils with hundreds of cells. They interpreted these as reproductive propagules and presented them as evidence for spatial cell differentiation, germ–soma separation and apoptosis. Based on these characters, along with functional cell adhesion, obligate multicellularity and the potential lack of a rigid cell wall, L. Chen et al. (2014) argued that Tianzhushania might be a stem-animal that had gained some, but not all, of the characters that are present in animals, but not choanoflagellates. The occurrence of dividing cells within the embryo-like fossils is merely an expectation of the existing observation that they exhibit asynchronous cell division (Hagadorn et al. 2006). Nevertheless, the interpretation of differentiation and germ–soma separation requires that the clusters are part of the embryo-like organism rather than an exogenous parasite. L. Chen et al. (2014) suggested that there is a developmental continuation from the typical cells of these specimens and the clusters, which rules out an exogenous origin. However, there is a discontinuity between monads, dyads and tetrads, which show evidence for palintomy, and larger ‘matryoshka’ clusters, which do not (Tang 2015; Cunningham et al. 2016). Although there must be a switch from palintomy at some point (Chen et al. 2017), the lack of intermediates weakens the evidence for an endogenous origin (Tang 2015; Cunningham et al. 2016).
The argument for apoptosis is that, because the enclosing envelope imposes a constant volume throughout ontogeny, the only way to create the space required for the putative reproductive structures to grow is for other cells to die off. However, the volume of Tianzhushania may well not be constrained in this way, given that early stages often do not occupy the full envelope, and putative later stages provide evidence for distension and then rupture of the envelope (Liu et al. 2009; Huldtgren et al. 2011). The evidence for differentiation, germ–soma separation and programmed cell death is therefore unconvincing. Moreover, there is also uncertainty regarding the other proposed animal characters. There is no evidence that the walls of Tianzhushania are any less rigid than those of non-animal groups such as non-metazoan holozoans (Marshall & Berbee 2011), amoebozoans (Olive 1975), ciliates (Park et al. 2005) or volvocalean algae (Hallmann 2006), which also have Y-shaped cell junctions. Nor is there evidence that cell adhesion is different from that seen in groups such as non-metazoan holozoans (volvocalean algae achieve adhesion through cytoplasmic bridges, which are absent in the fossils).
To summarize, Tianzhushania does not exhibit characters that are sufficient to identify it as an animal. Evidence for the presence of characters gained in the animal stem lineage is equivocal. There is therefore no justification for an interpretation of Tianzhushania as an animal, although a stem-animal affinity cannot be definitively rejected on the basis of current data (Huldtgren et al. 2011, 2012). Above all, the available evidence does not yet allow these fossils to be used to substantiate hypotheses on the timing of animal diversification.
Other possibilities
A number of alternative interpretations have been proposed, yet the affinities of these organisms remain uncertain. Comparisons with non-metazoan holozoans (Huldtgren et al. 2011), algae (Butterfield 2011b; L. Chen et al. 2014; Zhang & Pratt 2014) or ciliates are plausible but as yet unsubstantiated and require further investigation. The primary factor that has hindered interpretation of these fossils is poor understanding of the later stages of the organisms’ ontogeny. A number of candidates have been proposed, but none are widely accepted. Xiao et al. (2007a) suggested that helically coiled specimens (Fig. 2g), now named Helicoforamina, might be later stages, perhaps representing a coiled vermiform animal. However, this taxon is enigmatic and has also been argued to be an embryo of the ctenophore-like fossil Eoandromeda, which has eight spiral arms (Tang et al. 2008, 2011), or a single-celled stage of Spiralicellula (Fig. 2h), a form that resembles Tianzhushania, but differs in having coiled cells (Huldtgren et al. 2011). Peanut-shaped specimens with hundreds of thousands of cells (Fig. 3c and d) have also been interpreted as later developmental stages (Huldtgren et al. 2011). These have single cells and clusters of cells that are isolated from the main mass of cells and have been argued to be reproductive propagules. L. Chen et al. (2014) have also described specimens with putative reproductive propagules. If Tianzhushania does reproduce via propagules, then this indicates a lifecycle incompatible with at least crown animals. However, in both cases the interpretation as propagules has proven contentious (Xiao et al. 2012; Tang 2015) with a key issue being the incomplete knowledge of the lifecycle of Tianzhushania.
Sponge-like fossils
Structures from the Weng'an Biota have been interpreted as siliceous sponge spicules (Li et al. 1998). However, these have been shown by subsequent analysis not to be composed of silica and to lack convincing sponge characters (Zhang et al. 1998; Antcliffe et al. 2014; Muscente et al. 2015). More recently, Eocyathispongia (Fig. 3c) has been described as a possible sponge from Weng'an (Yin et al. 2015). This is known from a single specimen that is preserved at a cellular level. Eocyathispongia is considered to be one of the strongest candidates for a Precambrian sponge. However, although it could be a sponge, it has no convincing sponge apomorphies such as pores or spicules, just a generalized sponge gestalt. More detailed characterization of the anatomy of Eocyathispongia is required. For example, high-resolution tomography might reveal evidence for the presence or absence of sponge characters and help to constrain the affinity of this enigmatic organism.
Tubular microfossils
A group of tubular microfossils assigned to the genera Sinocyclocyclicus (Fig. 3e and f), Ramitubus (Fig. 3g and h), Crassitubus and Quadratitubus have been interpreted as cnidarian-like eumetazoans from the Weng'an Biota (Xiao et al. 2000; Chen et al. 2002; Liu et al. 2008). These genera have regularly spaced cross walls and have been compared with tabulate corals. In a coral-like body plan, the spaces between the cross walls represent the former living positions of the polyp and would be expected to be empty or filled with diagenetic cements. However, the fossils preserve biological structures in these spaces, which is incompatible with a cnidarian interpretation (Cunningham et al. 2015). There is no evidence to support a placement of these tubular fossils within eumetazoans or animals.
Vernanimalcula
Vernanimalcula has been described from thin sections as a miniature, adult bilaterian from the Weng'an Biota (Chen et al. 2004; Petryshyn et al. 2013). It is purported to preserve bilaterian characters such as a mouth, gut, anus and paired coelomic cavities. However, all of the putative bilaterian characters can be alternatively interpreted as artefacts resulting from abiological diagenetic apatite cements, which are ubiquitous in the deposit (Bengtson 2003; Bengtson & Budd 2004; Bengtson et al. 2012; Cunningham et al. 2012a). Moreover, 3D analyses show that Weng'an fossils such as acritarchs and fertilization envelopes that clearly lack bilateral symmetry can exhibit Vernanimalcula-like morphology when sectioned in particular orientations (Bengtson et al. 2012). There is no evidential basis for interpreting Vernanimalcula either as a bilaterian or as an animal of any kind.
Summary and prospects
Research into the Weng'an Biota is currently in a transitional phase. Early research involved many attempts to demonstrate the long-expected presence of animals, including bilaterians, in the Ediacaran. This has been followed by a spell of critical analysis of these claims that has shown that none of these fossils can so far be confidently identified as stem- or crown-group metazoans. The research is now entering a phase where more targeted analysis of the palaeobiology of each taxon is helping to constrain wide-ranging hypotheses of affinity more rigorously. Many fossils are known only from a few specimens and have received limited attention. One such example is the enigmatic fossil Caveasphaera (Fig. 2i), which has been tentatively compared with cnidarian embryos (Xiao & Knoll 2000) but requires further investigation. Such taxa betray a cryptic diversity that has been overlooked because of the focus on Tianzhushania.
The Ediacaran was a critical interval in the history of life (Butterfield 2007) and the Weng'an Biota offers a unique glimpse of microscopic, multicellular and soft-bodied organisms at this time. It can provide important insights into Ediacaran biology and the evolution of multicellular organisms at this time, possibly including animal-type multicellularity. Future work constraining the diversity, affinity and ontogeny of the Weng'an organisms is necessary before this potential is fully realized.
Acknowledgements
We thank S. Dornbos and an anonymous referee for constructive comments that helped improve the paper. Data statement: There are no data associated with this paper.
Funding
This work was funded by the Natural Environment Research Council (NE/J018325/1, NE/F00348X/1, NE/P013678/1), the National Natural Science Foundation of China (41672013), the Science Without Borders Program (CNPq, Brazil), the Danish National Research Foundation (DNRF53), the Swedish Research Council (2010-3929, 2013-4290), the Leverhulme Trust and the Royal Society (Wolfson Merit Award and Newton Advanced Fellowship).
- © 2017 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/)
References
The Weng'an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms
