Lifestyle of the Octoradiate Eoandromeda in the Ediacaran

Authors: Wang, Ye, Wang, Yue, Tang, Feng, Zhao, Mingsheng, and Liu, Pei Source: Paleontological Research, 24(1) : 1-13 Published By: The Palaeontological Society of Japan URL: https://doi.org/10.2517/2019PR001

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Downloaded From: https://bioone.org/journals/Paleontological-Research on 15 Feb 2020 Terms of Use: https://bioone.org/terms-of-use Access provided by Bing Search Engine Paleontological Research, vol. 24, no. 1, pp. 1–13, JanuaryEoandromeda 1, 2020 ’s Lifestyle 1 © by the Palaeontological Society of Japan doi:10.2517/2019PR001

Lifestyle of the Octoradiate Eoandromeda in the Ediacaran

YE WANG1, YUE WANG1, FENG TANG2, MINGSHENG ZHAO3 and PEI LIU1

1Resource and Environmental Engineering College, Guizhou University, Huanx 550025, Guiyang, Guizhou, China (e-mail: [email protected]) 2Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Street 100037, Beijing, China 3College of Paleontology, Shenyang Normal Univeristy, 253 Huanhe N Street 110034, Shengyang, Liaoning, China

Received May 14, 2018; Revised manuscript accepted January 26, 2019

Abstract. The octoradiate Eoandromeda Tang et al. from the Ediacaran of South China and South Australia is poorly understood and there are different interpretations of its morphology and paleoecology. The carbona- ceous compression known as Eoandromeda, which is collected in black shales of the upper Doushantuo Forma- tion in northeastern Guizhou, South China, shows various patterns and complex structures. Eoandromeda is interpreted as an umbrella-shaped metazoan, with a dome-shaped polar structure on the top of its body, eight dextrally spiraling arms and tapering skirts. The spiral arms are differentiated into a main segment consisting of rigid and thick masses and a distal segment consisting of flexible and thin masses. Thus, we consider that the spiral arms may have consisted of gelatinous masses and primarily grew in their distal segments (approximately the tapering skirts). The numerous feather-like structures on the platy spiral arms are regularly arranged into two longitudinal rows. We believe that Eoandromeda lived in a primitive “Seabed Grassland” with an abun- dance and diversity of macroalgae and was capable of swimming in the water column by flapping its feather-like structures. Based on measurements of the maximum diameter of the disk-shaped compression and the maximum width of the spiral arms, Eoandromeda can be divided into a juvenile stage (<10 mm diameter) that has not been found, an adult stage (10–30 mm diameter) with a high growth rate in the width of the spiral arms, and a senes- cent stage (>30 mm diameter) with a slow growth rate in the width of the spiral arms. Fully grown Eoandrom- eda, with thick and rigid spiral arms, may have mostly stayed on the sediment surface, temporarily swimming to seek new habitat or prey. The juveniles, better swimmers, may have had a soft body with soft and thin arms, unlikely to be preserved, and may have been easily transported by water currents.

Key words: metazoan, paleoecology, South China, swimming ability, Wenghui biota

Introduction nofossil-kind” of Li et al. (1996) and Ding et al. (1996) instead of an ichnogenus, but assigned no ichnospecies, Carbonaceous compressions are abundant and diverse no type ichnospecies and no holotype (see Ding et al., in black shales of the Ediacaran Doushantuo Formation 1996, p. 126–127). The discoidal compression reported from western Hubei (i.e., the Miaohe biota; see Chen and by Wang et al. (2005) in the Wenghui biota was still Xiao, 1991; Chen et al., 1994, 2000; Steiner, 1994; Ding referred to by the ichnofossil-name Eilscaptichnus (Wang et al., 1996; Xiao et al., 2002; Ye et al., 2017), and north- et al., 2005, 2007). However, Wang et al. (2007) consid- eastern Guizhou (i.e., the Wenghui biota; see Wang et al., ered it as a discoidal metazoan, not an ichnofossil. Based 2007, 2008, 2014, 2015, 2016; Wang and Wang, 2008, on the International Code of Zoological Nomenclature 2011; Tang et al., 2008a, b, 2009a), South China (Fig- rules, Tang et al. (2008a) proposed a new genus Eoan- ure 1). Li and Ding (in Ding et al., 1996) first found two dromeda (containing only one species E. octobrachiata) specimens (publishing one of them) of a discoidal com- to replace Eilscaptichnus (nomen nudum) and interpreted pression with octoradiate pattern in the Miaohe biota, and it as an unknown diploblastic metazoan with eight dex- regarded them as an ichnofossil formed by a worm dig- trally spiraling arms. Tang et al. (2008b) suggested that ging spirally on the sediment surface. Based on the code Eoandromeda may have been a coelenterate. Based on the of ichnofossil nomenclature of Li et al. (1996) and Ding central area on the discoidal compressions and the numer- et al. (1996), they named the discoidal compression as ous feather-like structures on the spiral arms, Wang et al. Eilscaptichnus Li and Ding in Ding et al., 1996, an “ich- (2008) proposed that Eoandromeda, with its octoradiate

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Figure 1. Locations and horizons of Eoandromeda Tang et al. from black shales of the upper Doushantuo Formation of the Ediacaran, South China. A, Ediacaran palaeogeographical configuration of the Yangtze region (modified from Liu and Xu, 1994 and Jiang et al., 2011), showing the comparable paleogeographical settings of both Wenghui and Miaohe biotas; B, location of the measured section at Wenghui, Jiangkou, Guizhou, China; C, D, columnar sections of the Doushantuo Formation exposed at Wenghui, Jiangkou, northeastern Guizhou (C) and Miaohe, Zigui, western Hubei (D), showing horizons bearing Eoandromeda.

symmetry, may have been a ctenophore. Coincidentally, morphies of the crown group, such as tentacles, statoliths, another preservation type (a cast and mold) of Eoandrom- polar fields, and biradial symmetry, and interpreted it as eda was reported in the Ediacara Member in the lower part an early stem-group ctenophore. At present, the metazoan of the Rawnsley Quartzite in South Australia (Zhu et al., interpretation is accepted by multiple researchers (e.g. 2008). Based on the Chinese and Australian specimens, Wang et al., 2009, 2011, 2014, 2015, 2016; Tang et al., Zhu et al. (2008) suggested that Eoandromeda may have 2009a, 2011a, b, 2014; Xiao et al., 2013; MacGabhann, been a diplobastic-grade animal, sharing some features 2014), though its systematic classification is still contro- with ctenophores and cnidarians. Tang et al. (2011a, b) versial. On the other hand, few paleoecological studies considered that Eoandromeda apparently lacks synapo- on Eoandromeda have addressed the problem of whether

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it was benthonic (Tang et al., 2008a, b, 2009a; Zhu et reposited in the Institute of Geology, Chinese Academy al., 2008) or planktonic or swimming (Wang et al., 2008, of Geological Science. 2009, 2010, 2011, 2015; Tang et al., 2011a, b, 2014). Examination of all collected specimens in black shales Morphological features and biological of the upper Doushantuo Formation at the Wenghui sec- interpretations tion of Jiangkou County, Guizhou Province, South China (Figure 1B) shows that these carbonaceous compressions Body appearance exhibit various patterns and possess complex structures. All specimens of Eoandromeda octobrachiata from Here we attempt to infer the lifestyle of Eoandromeda the Wenghui section are separately preserved on the bed- from its morphological characters. ding planes of the Doushantuo black shales. Most of them show circular disk-shaped (Figures 2A–D, 3A, C, G, I, Geological settings 4C, E, H) and symmetrically or asymmetrically oval disk- shaped (Figures 2E–H, 3E, 4A, G, I) compressions, which At the Wenghui section (27°50′07″N , 109°01′20″E), led to Eoandromeda being regarded as a discoidal meta- the Ediacaran Doushantuo Formation (>71 m thick), zoan in early publications (see Wang et al., 2007, 2008; overlying the diamictites of the Nantuo Formation and Tang et al., 2008a, b, 2009a, b). The Australian specimens underlying the bedded cherts of the Liuchapo Formation, preserved as cast and mold in siliciclastic sedimentary can be subdivided into four members (Figure 1C). The rocks were interpreted by Zhu et al. (2008) as a dome- lowest Member I is a 4.1-m-thick dolostone (cap carbon- shaped organism. However, the bell-shaped outline of E. ates). The Member II is composed of muddy dolostones octobrachiata (Figure 2I–K), which was reported in the and black shales. The overlying Member III consists of Wenghui biota and interpreted as a lateral compression dolostones and muddy dolostones, with interbedded black (Wang et al., 2009), indicates that Eoandromeda may shales. The uppermost Member IV is more than 31 m have had an umbrella-shaped body (Wang et al., 2009, thick and characterized by fossiliferous black shales, con- 2010, 2011, 2015; Tang et al., 2011a, b) because it was taining the Wenghui biota. The lithology of the Doushan- possibly preserved as the aforementioned compression tuo Formation at the Wenghui section is remarkably simi- patterns. lar to that at the Miaohe section (Figure 1C, D; Wang et One of the most significant diagnostic characters is al., 1987; Liu and Xu, 1994; Zhu et al., 2007; Jiang et that Eoandromeda has eight spiral arms (see Tang et al., al., 2011; Wang et al., 2012). In addition, the Wenghui 2008a, p. 33). Each arm, with a round inner end and a biota can be paleontologically correlated with the Miaohe tapering outer end, is always dextrally spiral on the upper biota, sharing many of the same species (see Wang et al., surface of the bedding plane and extends to the margin 2007, 2012, 2016; Tang et al., 2009a; Ye et al., 2017). from the center or near center of the compression (Figures In South China, Eoandromeda in both the Wenghui and 2–4). In addition, the arms of the Australian specimens Miaohe biotas has been treated as one of the significant spiral in a dextral direction (see Zhu et al., 2008). Thus, fossils in the divisions of assemblages and biozones in the we suggest that the dextrally spiraling arms are a genu- middle–late Ediacaran (Tang et al., 2009a, 2015; Wang ine feature of the Eoandromeda organism. In most of the et al., 2011, 2014, 2015, 2016). The macrofossil-bearing specimens, the spiral arm is tapering in its distal segment Member IV black shales of the Doushantuo Formation and slightly extends outside the margin of the compres- in both the Wenghui and Miaohe areas date to ca. 560 to sions (Figures 2G–I, K, 3C, E, 4D, G, I), meaning that the 551 Ma within the episode of extensive ocean oxygen- margin of Eoandromeda’s body was not smooth (Tang et ation (see Kendall et al., 2015; Li et al., 2015; Wang et al., 2011a, b), but had eight tapering projections (skirts). al., 2017; Wang and Wang, 2018). Moreover, a small open structure, which is at or near the center of the disk-shaped compressions (Figures Materials and methods 2A–G, 3A, C, G, I, 4C–E, G–I) and commonly covered by a thin carbonaceous film (light colour), is surrounded All studied specimens were collected from the Mem- by a carbonaceous ring in some specimens (Figures ber IV black shales of the Doushantuo Formation at the 2B–D, F, 3A, E, G, 4C, D, G, H). The connection of the Wenghui section and are preserved as carbonaceous com- carbonaceous ring to the inner ends of the arms through a pressions. They were obtained by mechanically cleaving narrow neck (Figures 2B, C, 3A, G, 4C, D, H) was specu- the bedding planes, not processed in other ways. Speci- lated by Tang et al. (2011a, b) to perhaps be related to a mens with the prefix MH-, WH- and JK-A- are repos- nervous system. In the lateral compressions, a crater-like ited in School of Resources and Environment, Guizhou structure with a convex or concave surface is topmost on University, China. Specimens with the prefix JK- are the bell-shaped compressions (Figure 3I–L), positionally

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Figure 2. The various forms of Eoandromeda octobrachiata Tang et al. from the upper Doushantuo Formation in northeastern Guizhou, China. A, JK05011 (paratype); B, JK5808; C, MH-A-4251; D, JK5809; E, WH-A-02002; F, WH-A-04291; G, JK08014; H, WH-A-04272; I, MH-42-0086; J, JK08016; K, L, JK-A-54-0089; K, complete specimen; L, magnified view of K, showing detail of dome-shaped structure.

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corresponding to the carbonaceous ring surrounding the Zhu et al., 2008, fig. 2C, E, J, K; Xiaoet al., 2013, fig. 1B; small open structure in the disk-shaped compressions. MacGabhann, 2014, fig. 1k), while its main segment con- In the Australian specimens, a central concavity on sole sists of relatively rigid and thick masses (Figure 4B–G; impressions (see Zhu et al., 2008, fig. 2A–E, I; Xiao et see Zhu et al., 2008, fig. 2C, F, I, J; Xiao et al., 2013, fig. al., 2013, fig. 1B; MacGabhann, 2014, fig. 1k) or a central 1B; MacGabhann, 2014, fig. 1k). Thus, the spiral arm of convexity on bed sole impressions (see Zhu et al., 2008, Eoandromeda may have consisted of gelatinous masses, fig. 2F, J, K) can be interpreted as a cast or mold of the not a stiff material as hypothesized by Tanget al. (2008a, open structure. Therefore, we believe that Eoandromeda b, 2009a, b). In addition, two ends of every feather-like had a small dome-shaped structure, a polar structure on structure have one tapering trait in common on the edge the top of its umbrella-shaped body. In this case, the car- and the center of a spiral arm (Figures 3B–F, H–M, 4J, bonaceous ring, in which may be covered nervous tissues K), meaning that the spiral arms have had little defor- (Tang et al., 2011a, b), may be interpreted as a vertically mation in their cross sections. In other words, the spiral or obliquely overlapped edge of the dome-shaped polar arm of Eoandromeda may be a platy structure rather than structure. the tubular structure of Zhu et al. (2008) and Tang et al. In general shape, Eoandromeda octobrachiata was an (2011a, b), to support the attachment of many lamellar umbrella-shaped metazoan, with a dome-shaped polar soft tissues. Based on 48 measurable specimens of E. structure, eight dextral arms and tapering skirts. octobrachiata, the maximum width of the spiral arm is closely related to the maximum diameter of the compres- Structural characteristics sion (Figure 5A; see below), implying that the growth of On the spiral arms of Eoandromeda octobrachiata, the Eoandromeda may influence the development of spiral numerous feather-like structures formed by carbonaceous arms. Therefore, we deduce that in the course of growth of masses are distinctly darker and thicker than the spiral Eoandromeda, the spiral arms primarily grew in length in arms (Figures 2G, 3A–P, 4G, J, K). They were consid- their distal segments (approximately the tapering skirts), ered to be regularly arranged into one row (Wang et al., and their thickness and width subsequently increased. 2008, 2011, 2015; Tang et al., 2011a, b). After careful Between adjacent spiral arms, a spiral furrow is visibly examination of the studied specimens from northeastern thinner than the spiral arms in carbonaceous density. Its Guizhou, we now believe that the feather-like structures, width is relatively constant in the same position of the parallel to each other, are arranged regularly into two same individual (Figures 2B–E, G–I, 3A, C, E, G–I, 4A, longitudinal rows, alternately interlacing about half the C–E, G, H, J), except the deformed segments of spiral length (Figures 3A–M, 4J, K). Their widths and lengths arms (Figures 2H, 3C, I, 4A–J). The constant width of are gradually diminishing toward both ends of the arm, the spiral furrow indicates that the spiral arms were a respectively (Figures 3A–K, 4J). In addition, the three- main growth area for the surface of the umbrella-shaped dimensional preservation of the feather-like structures body. Tang et al. (2008a, 2009b) interpreted that the spiral can be observed in some specimens (Figure 3B, E–H, furrow may have been formed by a flattened membrane M) and in thin sections (Figure 3N–P), confirming that and suggested that the spiral arms were enveloped by the feather-like structures were the original tissues on the the flattened membrane. However, the three-dimension- spiral arms. Two tapering ends of the feather-like struc- ally preserved spiral arms (Figures 2A, D, 3A, B, E–H, ture, in the same specimen and on the same arm, were M, 4A–D, I) and feather-like structures (Figure 3B, F, preserved in different orientations (Figures 3B–D, I–M, E–H, M–P) confirm that nothing covered them. We sug- 4J, K), meaning that the feather-like structure was a flex- gest that the spiral furrow under the spiral arms may be ible and soft tissue and its base attached on the arm. Cor- the flattened membrane inferred by Tang et al. (2008a, respondingly, the furrows on the arms can be observed 2009b), to function as a protective membrane for its body, in the Australian specimens (see Zhu et al., 2008, figs. although no evidence confirms how it was connected to 2A, B, G, H). Therefore, we support the interpretations the spiral arms. of Wang et al. (2008) and Tang et al. (2011a, b) that the feather-like structure possibly was a flexible lamella of Measurements soft tissue, tightly attached on the spiral arm. Several morphometric parameters of Eoandromeda The spiral arms of Eoandromeda gradually become, octobrachiata in the Wenghui biota, including maximum toward the outer ends, thinner in thickness and lighter in diameter of a compression, maximum and minimum radi- color (Figures 2B, E, G–I, 3A–J, M, 4D–I). In the Chinese uses of a compression (from the center of the polar struc- and Australian specimens, the spiral arm has a significant ture to the edge of the compression), and maximum width difference in its distal segment, as comparatively flexible of the spiral arm, were measured using a Vernier microm- and thin masses (Figures 2A, H, K, 3C, E 4A, D, G–J; see eter accurate to 0.1 mm.

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Figure 3. The feather-like structure on the spiral arm of Eoandromeda octobrachiata Tang et al. from the Doushantuo Formation in Guizhou, South China. A, B, MH-A-04178; A, complete specimen; B, showing the numerous feather-like structures; C, D, JK-A-55-0025; C, complete specimen; D, detail of the feather-like structures; E, F, JK10914; E, complete specimen; F, showing the feather-like structures; G, H, JK10903; G, complete specimen; H, detail of the feather-like structures; I, J, JK-10909; I, complete specimen; J, detail of the feather-like structures; K, L, MH-A-04209; K, complete specimen; L, detail of the feather-like structures; M–P, counterpart of JK05006; M, complete specimen; N–P, showing the feather-like structure in thin sections.

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Figure 4. The spiral arms of Eoandromeda octobrachiata from black shales of the Doushantuo Formation in Guizhou, China. A, B, JK05006 (holotype); A, complete specimen; B, magnified view of A, showing detail of spiral arms; C, WH-A-04124; D, JK10329; E, MH-50-0313; F, MH-50-0379; G, JK5741; H, JK-A-54-0089; I, JK2452; J, K, WH-A-04023; J, complete specimen; K, magnified view of J, showing detail of the feather-like structures.

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Figure 5. Cross-plots of the size of Eoandromeda octobrachiata. A, maximum diameter of the compression vs maximum width of the arm; B, maximum diameter of the compression vs number of specimens; C, maximum diameter of the compression vs central deflection rate; D, central deflection rate vs number of specimens. Central deflection rate is equal to the ratio between minimum and maximum radiuses of the compression.

The maximum diameter of Eoandromeda is directly (>30 mm diameter), with a slow growth rate in spiral arm proportional to the maximum width of the arm (Figure width. 5A), meaning that the width of the spiral arm increased Here we propose to use a central deflection rate of with the growth of the Eoandromeda’s body. In a num- Eoandromeda, the ratio between minimum and maximum ber of specimens, it appears to be shaped as a bell curve, radiuses of a compression, to roughly evaluate the tilting peaking in diameter at 25–30 mm and ranging in diameter degree of the carbonaceous compression formed by the from 10.7 mm to 53.0 mm (Figure 5B). Corresponding to compressed umbrella-shaped body. The results show that the peak in diameter, an inflection point occurs at a diam- the central deflection rates between 1.0 and 0.6, 0.6 and eter of ca 30 mm in the cross-plots of the maximum width 0, and less than 0, respectively, are 64.6% for obverse of the arm (Figure 5A). The measurements imply that or nearly obverse compressions, 29.2% for oblique com- ontogeny of the Eoandromeda organism can be divided pressions, and 6.3% for lateral compressions (Figure 5D). into three life stages (Figure 5A): the juvenile stage (<10 The dominance of the obverse or nearly obverse com- mm diameter), of which no specimen has been found (see pressions means that most of the Eoandromeda’s body below); the adult stage (10–30 mm diameter), with a high was preserved in life orientation before its death, with growth rate in spiral arm width; and the senescent stage the upturned polar structure. However, there is no sig-

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nificant correlation between the diameter and the central maximum 50 mm long (see Wang et al., 2007); Zhongba- deflection rate (Figure 5C), indicating that the compres- odaophyton Chen et al., 1994, emend. Wang et al., 2015 sion patterns of Eoandromeda are related to their death branches up to nine times in unequal dichotomies and is states and/or taphonomic characteristics rather than their 36 mm in maximum length of thallus (see Wang et al., life stages. Thus, we consider that most Eoandromeda, 2015). That is, Eoandromeda lived in a primitive “Sea- regardless of being adults or elders, experienced a quiet bed Grassland” that was composed mainly of macroalgae death and eventual burial by sedimentation, manifested in (up to tens of centimeters long), of which about half were a dominant preservation style of the upward convex side branching macroalgae with multiple branches. of Eoandromeda with dextral spiral. Lifestyle Life and death The radiosymmetric pattern is common in ancient and living animals; however, animals with helical radiosym- Living environment metry were reported in some fossils apart from Eoandrom- At present more than 32 genera and 34 species, includ- eda, for example the Ediacaran Trilobozoa (Fedonkin, ing macroscopic algae, metazoans and a few ichnofos- 1990) Tribrachidium Glaessner, 1959 and the Cambrian sils, have been described in the Wenghui biota (see Wang Helicoplacoidea (Echinodermata) Helicoplacus Durham et al., 2016, 2017). In fossiliferous black shales of the and Caster, 1963, Westgardella Durham, 1967, and Poly- Doushantuo Member IV at the Wenghui section, the mac- placus Durham, 1967. The helicoplacoids, which are roscopic fossils and the filamentous rhizoids of macroal- preserved as globularly three-dimensional with many gae are well preserved; and some directionally preserved ambulacral and interambulacral plates, were considered macrofossils and numerous fossil fragments are found. to have lived on the seafloor as suspension feeders (see Thus, previous researchers (e.g. Wang et al., 2005, 2011, Durham and Caster, 1966; Durham, 1963; Dornbos and 2014, 2015, 2017; Wang and Wang, 2006, 2011, 2018) Bottjer, 2001; Wilbur, 2006; Smith et al., 2013). With a suggested that the Wenghui biota was preserved in situ disk-shaped outline and convex side uppermost, the tri- or near their life position and lived in a relatively low- radiate metazoans Tribrachidium were considered as energy environment, with occasional water currents. benthic animals firmly attached on a leathery biomat Based on counting the macroalgae in large-area speci- (Seilacher, 1999; Seilacher et al., 2003; McCall, 2006). mens, the densities of macroalgae on the bedding planes, Unlike the bare arms of Tribrachidium and the relatively which had been collected together with Eoandromeda, stable forms of the ambulacral and interambulacral plates are at 11.8–32.2 per square decimeter (32.0–51.0% of of helicoplacoids, the numerous feather-like structures the branching thallus and 49.0–52.0% of the unbranch- of the octoradiate Eoandromeda have variable patterns ing thallus), averaging over 25 macroalgae per square arranged regularly on the spiral arms. In addition, the decimeter (43.9% of the branching thallus and 56.1% of symmetrically spiral arms of Eoandromeda cover most the unbranching thallus) (Figure 6C). The counting result of the umbrella-shaped body and taper distally to form generally indicates that numerous macroalgae lived in the the skirts, meaning that the organism may have lacked an Wenghui area during the Ediacaran, though the result is effective pattern of horizontal movement on the seafloor. only based on samples that were selected because they Based on the features of the feather-like structures on the contain macroscopic fossils. Most of the macroalgae spiral arms, Wang et al. (2008, 2011, 2015) and Tang et al. among the Wenghui and Miaohe biotas possess a hold- (2011a, b) proposed that the Eoandromeda organism was fast, so that they are considered to have been fixed on the able to swim and live in the water column as the numer- seafloor (e.g. Chen and Xiao, 1991; Steiner, 1994; Ding ous feather-like structures could be flapped to produce et al., 1996; Chen et al., 2000; Xiao et al., 2002; Wang a propelling force, like the ciliary rows of ctenophores. et al., 2007, 2015, 2016, 2017; Wang and Wang, 2018). The feather-like structure, which can be interpreted as a Paleoecologically, the thallus and holdfast of macroalga lamellar soft structure attached tightly on the platy arms, may have been suspended in the water column for photo- could actively flap in their upper portions (see above). synthesis and nestled into sediments to anchor its owner The flexible lamellae could not only beat rhythmically in on the seafloor, respectively (Chen et al., 1994; Ding the direction of the arm length to generate a propelling et al., 1996; Wang et al., 2005, 2015, 2017; Wang and spiral flow (Wang et al., 2008; Tang et al., 2011a, b), but Wang, 2006, 2018). In other words, there was a primitive also selectively shake along the lamella length to change “Seabed Grassland” (Chen et al., 1994, 2000; Wang et the orientation of the Eoandromeda’s body. Meanwhile, al., 2005, 2007, 2011) in the Ediacaran Yangtze Sea. In the continuous flapping of the flexible lamellae also pro- the Wenghui biota, Gesinella Steiner et al., 1992, with duced a spiral flow on the opposite side of the polar struc- an unbranching thallus, is the longest macroalga, up to a ture, maybe the oral side, helping the animal to prey. The

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Figure 6. Paleoecological and taphonomical reconstructions of Eoandromeda. A, showing a relatively low-energy environment; B, showing an occasional stronger water current; arrow shows the direction of flow; C, the density of macroalgae in every layer together with Eoandromeda.

rhythmical beatings of the lamellae mean that Eoandrom- we speculate that the earlier suspension-feeding animals eda may, like ctenophores, have a neural system covered with radiosymmetry may have selected the spiral pattern with the dome-shaped polar structure for a synchroniza- to increase length, thereby enhancing the efficiency of tion mechanism. In addition, a helicoplacoid with the the developed tissues and/or organs, because their tissues spiral-radiate pattern was regarded as an earliest fossil and/or organs may have been imperfectly developed. Bio- echinoderm (Smith et al., 2013) and the primary stage in functionally, the umbrella-shaped body can have been an the phylogeny of echinoderms (Nichols, 1969; Sumrall, adaptation to motile life in the water column; the eight 1997). Taking up most of the surface of Eoandromeda, the tapering skirts and spiral furrows also can actively keep a spiraling arm, which can grow in width and length with constant body orientation in the water column. growth of its body (see above), may provide a platform However, the eight spiral arms of Eoandromeda, espe- to increase the number and robustness of the soft lamel- cially their thick and rigid main segments, decreased and lae for producing spiral flow more effectively. Therefore, reduced its ability to move vertically in the water column.

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On the other hand, the abundance of macroalgae, includ- and rigid arms reduced its ability to swim; and the primi- ing nearly half of the branching macroalgae, impeded and tive “Seabed Grassland”, including abundant and diverse obstructed the movement of its radially symmetric body macroalgae, may have impeded the movement of its in the water column. In addition, the dextrally spiraling radially symmetric body. The obverse or nearly obverse arms are selectively preserved in an upward orientation compressions indicate a preservation in life orientation. on the bedding planes, meaning that the corpses of Eoan- Thus, Eoandromeda may not have been a good swimmer, dromeda had not been overturned after they died. The mostly staying on the sediment surface and temporarily result of the central deflection rate roughly indicates that swimming to seek new habitat or prey. Having not found most specimens of Eoandromeda are preserved in life ori- small-sized (<10 mm diameter) specimens, we assume entation (see above). Therefore, Eoandromeda may have that the juveniles of Eoandromeda may have been better been a poor swimmer rather than a benthonic metazoan, swimmers than the adults, and had a softer body with the mostly staying on the sediment surface in or near a primi- growing arms, unlikely to be preserved and easily trans- tive “Seabed Grassland” and temporarily swimming to ported by water currents. seek new habitats or prey (Figure 6A). Unfortunately, no small-sized (<10 mm diameter) spec- Acknowledgements imens of Eoandromeda have been found in northeastern Guizhou or reported in other areas (see Ding et al., 1996; We are grateful to the reviewers for their constructive Zhu et al., 2008; Xiao et al., 2013; MacGabhann, 2014). comments that improved this manuscript greatly and to It can be interpreted that the juveniles of Eoandromeda the Wenghui villagers for their help during our work in were unlikely to be preserved or they were not preserved the field. This research was supported by the National Sci- in their growth position. The distal segment of the spiral ence Foundation of China (No. 41762001, No. 41663005, arm, a flexible and thin mass, may be a germinative zone and No. 41763006), Guizhou Science and Technology (see above). 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