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A rare disc-like holdfast of the Ediacaran macroalga from South China

Published online by Cambridge University Press:  18 September 2017

Ye Wang ,
Yue Wang  and
Wei Du
Ye Wang
Affiliation:
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China 〈gboywangye@126.com〉
Yue Wang*
Affiliation:
School of Resources and Environments, Guizhou University, Guiyang 550025, China 〈gzyuewang@126.com〉
Wei Du
Affiliation:
Department of Earth Science and Astronomy, The University of Tokyo, Tokyo 153-8902, Japan 〈duwei@ea.c.u-tokyo.ac.jp〉
*
*Corresponding author: Yue Wang 〈gzyuewang@126.com〉
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Abstract

The fixing organ of the Precambrian macroalga was briefly described by most researchers as a holdfast or rhizoid, suggesting a fixation structure and/or tissue differentiation. An Ediacaran macroscopic alga, Discusphyton whenghuiensis n. gen. n. sp., with a complex disc-like holdfast and an unbranching thallus, has been collected, together with abundant and diverse macrofossils (i.e., the Wenghui biota) in black shales of the upper Doushantuo Formation (~560–551 Ma) in northeastern Guizhou, South China. The Wenghui biota lived in a relatively low-energy marine environment and was preserved in situ or nearby their growth position. Morphologically, the macroalgal thallus, including the compressed lamina and cylindrical stipe, might have been suspended in the water column for photosynthesis. Its holdfast, a rare fixing form, is complex in structure and construction, consisting of a globular rhizome and a discoidal rhizoid. The large-sized discoidal rhizoid is regarded as a flat-bottomed and dome-shaped organ to attach the macroalga on the water-rich muddy seafloor. The globular rhizome, expanded by a thallus on the substrate, was originally harder and spherical nature within the dome-shaped rhizoid. It may have been an important organ as a steering knuckle to connect between the stipe and the rhizoid. The macroscopic metaphyte D. whenghuiensis n. gen. n. sp. shows the appearance of complex holdfast in morphology and bio-functions. However, not enough is known, in the absence of more information, to decipher the phylogenetic affinity of D. whenghuiensis n. gen. n. sp. and the origin of a discoidal rhizoid.

Information

Type
Articles
Information
Journal of Paleontology , Volume 91 , Issue 6 , November 2017 , pp. 1091 - 1101
Copyright
Copyright © 2017, The Paleontological Society 

Introduction

Abundant and diverse carbonaceous compressions (the Wenghui biota), including macroscopic macroalgae, metazoans, and ichnofossils, are found in the Ediacaran black shales of the upper Doushantuo Formation in northeast Guizhou, South China (Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005, Reference Wang, Wang and Huang2007, Reference Wang, Wang and Huang2008, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Lei, Chen and Hou2010, Reference Wang, Chen, Wang and Huang2011, Reference Wang, Huang, Chen, Hou, Yang and Du2012, Reference Wang, Wang, Du and Wang2014, Reference Wang, Du, Komiya, Wang and Wang2015a, Reference Wang, Wang, Du and Wang2016a; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Yin, Stefan, Liu, Wang and Gao2008b, Reference Tang, Yin, Liu, Gao and Wang2009; Wang and Wang, Reference Wang and Wang2008, Reference Wang and Wang2011; Zhu et al., Reference Zhu, Gehling, Xiao, Zhao and Droser2008). A disc-like compression, with an unbranching carbonaceous stipe growing out from its center, is one of the abundant taxa in the Wenghui biota. Nevertheless, the Ediacaran disc-like compression, including system description and name, has remained poorly understood in terms of classification and functional morphology. Wang et al. (Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005) first reported a single image of the disc-like compression, and regarded it as a holdfast of macroscopic alga, enabling its owner to live on water-rich clay sediments. Wang and Wang (Reference Wang and Wang2006) and Wang et al. (Reference Wang, Wang and Huang2007) considered the unbranching thallus with a disc-like holdfast as a significant embodiment of organic differentiations of macroalga. Wang et al. (Reference Wang, Wang and Huang2007) combined and described the various holdfasts preserved together with the algal thallus as “unnamed holdfast forms” of macroalgae, including discoidal, spherical, asteroidal, cone-shaped, and filamentous holdfasts. Many researchers also interpreted the disc-like compression as a holdfast of macroscopic alga (Tang et al., 2008, Reference Tang, Yin, Liu, Gao and Wang2009; Wang et al., Reference Wang, Lei, Chen and Hou2010, Reference Wang, Wang, Du and Wang2014, Reference Wang, Wang, Du and Wang2015b; Cheng et al., Reference Cheng, Wang, Chen, Wang and Zhong2013). In addition, a half-circle holdfast with an unbranching thallus was assigned by Tang et al. (Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Jin, Wang, Ding, Zhao and Gao2014) and Wang et al. (Reference Wang, Zhao, Yang and Wang2009) to the macroalga Gesinella Steiner et al., Reference Steiner, Erdtmann and Chen1992 and Baculiphyca Yuan et al., Reference Yuan, Li and Chen1995, respectively.

More recently, numerous specimens of the disc-like compression have been collected in black shales of the upper Doushantuo Formation at Wenghui Village, Jiangkou County, Guizhou Province, South China (Fig. 1). These well-preserved compressions provide new information on its affinity and biological function.

Figure 1

Ediacaran Doushantuo Formation stratigaphic section at Wenghui, Jiangkou, northeastern Guizhou, South China. Disc-like compressions with carbonaceous stipes described in this paper are found in upper Doushantuo black shales at Wenghui.

Stratigraphical and environmental settings

In the Wenghui section (27°50'07''N, 109°01'20''E), the Ediacaran Doushantuo Formation (>71 m thick) is divided into four members. The lowest Member I is composed of dolostones (cap carbonates), overlying tillites of the Nantuo Formation. The overlying Member II consists of black shales and dolostones. Member III is characterized by dolostones and muddy dolostones, with black shales. The uppermost Member IV consists of fossiliferous black shales, underlying bedded cherts of the Liuchapo Formation (Fig. 1). Apart from the disc-like compression, abundant and diverse macroscopic fossils (i.e., the Wenghui biota) have been collected in black shales of the Member IV (e.g., Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005, Reference Wang, Wang and Huang2007, Reference Wang, Wang and Huang2008, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Lei, Chen and Hou2010, Reference Wang, Chen, Wang and Huang2011, Reference Wang, Wang, Du and Wang2014, Reference Wang, Du, Komiya, Wang and Wang2015a, Reference Wang, Wang, Du and Wang2016a; Wang and Wang, Reference Wang and Wang2006, Reference Wang and Wang2008, Reference Wang and Wang2011; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Yin, Stefan, Liu, Wang and Gao2008b, Reference Tang, Yin, Liu, Gao and Wang2009; Zhu et al., Reference Zhu, Gehling, Xiao, Zhao and Droser2008; Cheng et al., Reference Cheng, Wang, Chen, Wang and Zhong2013).

Another Ediacaran marcrobiota, the Miaohe biota, was also found in black shales of the Doushantuo Member IV at Miaohe, Yangtze Gorges area, South China (see Zhu and Chen, Reference Zhu and Chen1984; Chen and Xiao, Reference Chen and Xiao1991, Reference Chen and Xiao1992; Chen et al., Reference Chen, Xiao and Yuan1994a, Reference Chen, Chen and Xiao2000; Ding et al., Reference Ding, Li, Hu, Xiao, Su and Huang1996; Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002, Reference Xiao, Droser, Gehling, Hughes, Wan, Chen and Yuan2013; Yuan et al., Reference Yuan, Xiao, Yin, Knoll, Zhou and Mu2002). Both the Miaohe and Wenghui biotas can be paleontologically correlated (e.g., Baculiphyca taeniata Yuan, Li, and Chen, Reference Yuan, Li and Chen1995, emend. Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Beltanelliformis brunsae Menner in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974; Enteromorphites siniansis Zhu and Chen, Reference Zhu and Chen1984, emend. Wang, Wang, and Huang, Reference Wang, Wang and Huang2007; Eoandromeda octobrachiata Tang et al., Reference Tang, Yin, Stefan, Liu, Wang and Gao2008b; Liulingjitaenia alloplecta Chen and Xiao,Reference Chen and Xiao1992, emend. Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Longifuniculum dissolutum Steiner, Erdtmann, and Chen, Reference Steiner, Erdtmann and Chen1992; Miaohephyton bifurcatum Steiner, Reference Steiner1994, emend. Xiao, Knoll, and Yuan, Reference Xiao, Knoll and Yuan1998; Protoconites minor Chen, Xiao, and Yuan, Reference Chen, Xiao and Yuan1994a, emend. Wang, Wang, and Huang, Reference Wang, Wang and Huang2007; and Zhongbaodaophyton crassa Chen, Xiao, and Yuan, Reference Chen, Xiao and Yuan1994a, emend. Wang, Wang, and Huang, Reference Wang, Wang and Huang2007). Lithologically, the Doushantuo successions can be correlated in both the Miaohe and Wenghui sections (Qin et al., Reference Qin, Zhu, Xie, Chen and Wang1984; Wang et al., Reference Wang, Qin, Zhu and Chen1987; Liu and Xu, Reference Liu and Xu1994; Zhu et al., Reference Zhu, Zhang and Yang2007; Jiang et al., Reference Jiang, Shi, Zhang, Wang and Xiao2011; Wang et al., Reference Wang, Huang, Chen, Hou, Yang and Du2012). In the Yangtze Gorges area, a zircon U-Pb thermal ionization mass spectrometry age from the top of the Doushantuo Formation is 551.1±0.7 Ma (Condon et al., Reference Condon, Zhu, Bowring, Wang, Yang and Jin2005) and a Re-Os age of the base black shale of the Doushantuo Member IV is 591±5.3 Ma (Zhu et al., Reference Zhu, Becker, Jiang, Pi, Fischer-Gödde and Yang2013). However, Kendall et al. (Reference Kendall, Komiya, Lyons, Bates, Gordon, Romaniello, Jiang, Creaser, Xiao, McFadden, Sawaki, Tahata, Shu, Han, Li, Chu and Anbar2015) suggested that the 591±5.3 Ma Re-Os age may reflect post-depositional alteration and estimated the age of Member IV, pointing to extensively oxygenated oceans, as ~560–551 Myr.

In the Wenghui biota, not only macroscopic fossils but also filamentous rhizoids of macroalgae are well preserved, therefore previous researchers generally considered that this biota lived in a relatively low-energy environment and was preserved in situ or nearby their growth position (Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005, Reference Wang, Wang and Huang2007, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Chen, Wang and Huang2011, Reference Wang, Wang, Du and Wang2014, Reference Wang, Du, Komiya, Wang and Wang2015a, Reference Wang, Wang, Du and Wang2016a; Wang and Wang, Reference Wang and Wang2006, Reference Wang and Wang2011; Cheng et al., Reference Cheng, Wang, Chen, Wang and Zhong2013).

Materials and methods

Repository and institutional abbreviation

All studied specimens from Member IV of the Ediacaran Doushantuo Formation in the Wenghui section are preserved as carbonized compressions. These specimens are reposited in Guizhou University, China.

Systematic paleontology

Genus Discusphyton new genus

Type species

Discusphyton wenghuiensis n. gen. n. sp.

Etymology

From the Latin discus and phyton, with reference to the dis-like holdfast with an unbranching thallus.

Diagnosis

Unbranching clavate thallus stalked on the center of a disc-like or half-circle holdfast. Stipe (lower part of thallus) is cylindrical, upper part compressed to a lamina. The thallus substrate expands to a globular structure (globular rhizome), embedded into the smooth discoidal rhizoid. The disc-like holdfast comprises a globular rhizome and a discoidal rhizoid (Fig. 2).

Figure 2

Drawing of the principal structures of Discusphyton wenghuiensis n. gen. n. sp.

Discussion

The fixing organ of the Precambrian macroalga was briefly described by most researchers as holdfast or rhizoid (e.g., Du, Reference Du1982; Chen and Xiao, Reference Chen and Xiao1991, Reference Chen and Xiao1992; Steiner et al., Reference Steiner, Erdtmann and Chen1992; Steiner, Reference Steiner1994; Chen et al., Reference Chen, Xiao and Yuan1994a, Reference Chen, Chen and Xiao2000; Zhu and Chen, Reference Zhu and Chen1995; Yuan et al., Reference Yuan, Li and Chen1995, Reference Yuan, Li and Cao1999, Reference Yuan, Xiao, Yin, Knoll, Zhou and Mu2002, Reference Yuan, Chen, Xiao, Zhou and Hua2011; Ding et al., Reference Ding, Li, Hu, Xiao, Su and Huang1996; Chen and Wang, Reference Chen and Wang2002; Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Wang et al., Reference Wang, Wang and Huang2007, Reference Wang, Wang, Du and Wang2016a; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Yin, Liu, Gao and Wang2009; Singh et al., Reference Singh, Babu and Shukla2009). The Ediacaran macroalgal holdfast was divided by Wang and Wang (Reference Wang and Wang2006b) into stem-holdfast (upper part of the expanded thallus substrate), pedicel-holdfast (lower part of the expanded thallus substrate), and filament-rhizoid (grow on the expanded thallus substrate). Wang et al. (Reference Wang, Wang, Du and Wang2015b) considered that the holdfast of the Ediacaran macroalga Zhongbaodaophyton consists of the tuberous rhizome expanded thallus substrate and the filamentous rhizoids growing on the rhizome. The disc-like holdfast of Discusphyton n. gen. obviously consisted of a globular rhizome and a discoidal rhizoid.

The compression is characterized by an unbranching thallus stalked on the center of a smooth disc-like holdfast, without filamentous rhizoids, which differs from other Ediacaran macroscopic algae. It also differs from the Pre-Ediacaran macroalgae with disco-like holdfast in its large-sized discoidal rhizoid.

In the Ediacaran period, many discoidal metazoans were reported in many areas of the world (e.g., Sprigg, Reference Sprigg1947; Wade, Reference Wade1972; Duan and Lin, Reference Duan and Lin1980; Fedonkin, Reference Fedonkin1982; Glaessner, Reference Glaessner1984; Sun, Reference Sun1986; Seilacher, Reference Seilacher1992, Reference Seilacher1999; Farmer et al., Reference Farmer, Vidal, Moczydlowska, Strauss, Ahlberg and Siedlecka1992; De, Reference De2003; McCall, Reference McCall2006; Zhang et al., Reference Zhang, Hua and Reitner2006; Serezhnikova, Reference Serezhnikova2007; Tang et al., Reference Tang, Yin, Stefan, Liu, Wang and Gao2008b; Wang et al., Reference Wang, Wang and Huang2008, Reference Wang, Chen, Wang and Huang2011, 2012a, Reference Wang, Wang, Du and Wang2016a). Tang et al. (Reference Tang, Jin, Wang, Ding, Zhao and Gao2014) considered that the macroalgal holdfasts, that had fallen off the thallus and been preserved independently, were similar to or possibly mistaken as the disc-like metazoan. Discusphyton n. gen. differs from these Ediacaran metazoans in its discoid-like compression with smooth face and unbranching thallus, without concentric or radiating structure.

Discusphyton wenghuiensis new genus new species

Figures 2, 3.1–3.11, 4.1–4.9

Figure 3

Discusphyton wenghuiensis n. gen. n. sp. from the upper Doushantuo Formation, Wenghui, Jiangkou, Guizhou, China. (1) Holotype JK50-1460; (2) paratype WH-P-50014; (3) WH-P-42146; (4) JK45-0717; (5) TY42-147; (6) JK42-1064; (7, 8) JK42-0146, with magnified view of the holdfast; arrow marks detail of folds; (9, 10) JK42-0147, with magnified view of the holdfast; (11) WH-P-04120.

Figure 4

Discusphyton wenghuiensis n. gen. n. sp. fallen from thallus from the upper Doushantuo Formation, Wenghui, Jiangkou, Guizhou, China. (1, 2) Paratype JK48-0031, with magnified view of the central part; (3, 4) WH-P-01060, with magnified view of the central part; (5) TY42-0014; (6) TY50-0082; (7) WH-P-01087; (8) WH-P-01051; (9) WH-P-01022. Arrows in 2, 5, and 6 mark the ring-shaped depression.

2005 Macroalgal holdfast [gen. et sp. indet.] Reference Wang, He, Yu, Zhao, Peng, Yang and ZhangWang et al., fig. 4.10.

2006 Macroalgal holdfast [gen. et sp. indet.] Reference Wang and WangWang et al., pl. 2, fig. j.

2007 Unnamed holdfast forms [gen. et sp. indet.] Reference Wang, Wang and HuangWang et al., p. 836, pl. 2, fig. 23.

2008a Macroalgal holdfast [gen. et sp. indet.] Reference Tang, Yin, Liu, Duan and QaoTang et al., pl. 1, fig. 12.

2008a Gesinella hunanensis Steiner et al., Reference Steiner, Erdtmann and Chen1992; Reference Tang, Yin, Liu, Duan and QaoTang et al., pl. 2, fig. 19.

2009 Baculiphyca sp. Reference Wang, Zhao, Yang and WangWang et al., fig. 2.2.

2009 Macrooscopic alga fossil [gen. et sp. indet.] Reference Tang, Yin, Liu, Gao and WangTang et al., fig. 2g.

2013 Discoidal rhizoid of macroalga [gen. et sp. indet.] Reference Cheng, Wang, Chen, Wang and ZhongCheng et al., fig. 1.3.

2014 Macroalgal holdfast [gen. et sp. indet.] Reference Wang, Wang, Du and WangWang et al., fig. 4p.

2014 Gesinella [sp. indet.] Steiner et al., Reference Steiner, Erdtmann and Chen1992; Tang et al., fig. 2.2.

2014 Discoidal holdfast [gen. et sp. indet.] Reference Wang, Wang, Du and WangWang et al., figs. 5A, B.

Etymology

After the village of Wenghui in Jiangkou County, northeastern Guizhou Province, South China, in where abundant macroscopic fossils are collected.

Material

57 specimens from the Wenghui section in northeastern Guizhou, South China.

Types

Holotype, WH50-1460 (Fig. 3.1); Paratypes, WH-P-5014 (Fig. 3.2) and JK48-0031 (Fig. 4.1, 4.2).

Diagnosis

As for genus.

Description

Carbonaceous compression having a disc-like holdfast and an unbranching algal thallus (Fig. 3). The disc-like holdfast consists of a discoidal rhizoid and a globular rhizome. The discoidal rhizoid, with smooth surface and edge, typically is preserved in the bedding planes as a circular carbonaceous film or mass that commonly has nonuniform density, gradually lighting in color from the center to the edge (Figs. 3.1–3.3, 3.5, 3.11, 4.1, 4.3, 4.5–4.9). In a small number of specimens, it is preserved as a half-circle (Fig. 3.6–3.10) in which two arc-shaped edges are observed (Figs. 3.7, 4.8), which appear to be folded discoidal rhizoids. The globular rhizome, a globular structure within the discoidal rhizoid, is expanded by the thallus substrate and bears a smooth surface (Fig. 3.1, 3.2, 3.6). The carbonaceous density of the globular rhizome is thicker than that of the discoidal rhizoid, forming an obvious border between them (Fig. 3.1, 3.2, 3.6, 3.11). On the surface of the discoidal rhizoid center, it generally has a more or less rounded hump (Figs. 3.3–3.5, 3.7–3.11, 4.2, 4.6–4.8), possibly formed by the globular rhizome. The unbranching clavate thallus is incompletely preserved. The lower part of thallus, a terete stipe, is three-dimensionally preserved (Fig. 3.1–3.5, 3.7–3.10), with smallest diameter at the primal stipe (connection between stipe and rhizome) and increasing gradually upwards in diameter (Fig. 3). Connecting with the stipe, the compressed lamina (upper part of thallus) is a rapidly expanded portion (Fig. 3.6, 3.7, 3.9), but not observed the completely preserved lamina. In some specimens that thallus has fallen off, a circular cross section of the stipe remains on the center of the discoidal rhizoid (Fig. 4). A shallow, ring-shaped depression surrounding the primal stipe is present on the surface of the discoidal rhizoid (Fig. 4.1, 4.2, 4.5–4.9). The discoidal rhizoid is 3.0–14.9 mm in diameter. The longest preserved thallus is 14.6 mm. The globular rhizome is 0.6–2.9 mm in diameter. The stipe is 3.4–9.7 mm in length and 1.7 mm in the preserved maximum diameter. The lamina is 17.4 mm and 4.1 mm in the preserved maximum length and maximum width, respectively. The growth rates of the stipe width and the lamina width per unit length (=[difference between maximum and minimum widths]/[length between two measuring points]×100%) are 0.9–9.2% and 12.9–25.4%, respectively.

Discussion

Discusphyton wenghuiensis n. gen. n. sp. is similar to Gesinella hunanensis (Steiner et al., Reference Steiner, Erdtmann and Chen1992) in the unbranching thallus, but the later has a cone-shaped rhizome with many filamentous rhizoids (see Steiner et al., Reference Steiner, Erdtmann and Chen1992; Steiner, Reference Steiner1994; Chen and Wang, Reference Chen and Wang2002; Wang et al., Reference Wang, Wang and Huang2007, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Wang, Du and Wang2014; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a). Similarly, D. wenghuiensis n. gen. n. sp. is different from the other unbranching macroalga Baculiphyca taeniata Yuan et al., Reference Yuan, Li and Chen1995, emend. Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002 by its disc-like holdfast without filamentous rhizoid. With a disc-like holdfast, Sitaulia minor Singh, Babu, and Shukla, Reference Singh, Babu and Shukla2009 from the Sirbu Shale Formation in India consist of an oval-shaped thallus and an indistinct disc-like holdfast (see Singh et al., Reference Singh, Babu and Shukla2009, p. 14, pl. 1i) that differs from D. whenghuiensis n. gen. n. sp. In addition, the identifiable disc-like holdfast of D. wenghuiensis n. gen. n. sp. in this paper is >3.0 mm in diameter. In summary, the small-sized holdfast of D. wenghuiensis n. gen. n. sp. is similar to other small-sized circular macroalgae (e.g., Chuaria Walcott, Reference Walcott1899 and Beltanelliformia Menner in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974), but the disc-like holdfast of D. wenghuiensis n. gen. n. sp. bears a smooth surface and a circular cross section of the stipe on its center.

Occurrence

Upper Doushantuo black shales of the Ediacaran at Wenghui, Jiangkou, Guizhou, China.

Measurements

The morphometric parameters of Discusphyton wenghuiensis n. gen. n. sp., including the diameters of both the discoidal rhizoid and globular rhizome, width and length of the stipe, and preserved width and length of the lamina, were measured using an electronic vernier micrometer accurate to 0.01 mm. Because of preservation, the measurement values of the rounded hump formed by the globular rhizome and the circular cross section of the stipe on the discoidal rhizoid were considered, in this paper, as proxies for the diameter of the globular rhizome and the primal stipe.

Based on measurements of 57 specimens of D. wenghuiensis n. gen. n. sp., the diameters of the discoidal rhizoid, the globular rhizome, and the primal stipe are closely related with each other (Fig. 5); the diameters of the globular rhizome and the primal stipe, however, are not significantly related to the growth rates of the stipe and lamina widths (Fig. 6).

Figure 5

Cross-plots of the diameters of the discoidal rhizoid, the globular rhizome, and the primal stipe (1–3): (1) discoidal rhizoid versus globular rhizome; (2) discoidal rhizoid versus primal stipe; (3) globular rhizome versus primal stipe. (4) Bar graph of rhizoid diameter versus number of specimens.

Figure 6

Cross-plots of growth rates of the stipe and the lamina. (1) Rhizoid diameter versus growth rate of the lamina; (2) rhizoid diameter versus growth rate of the stipe; (3) rhizome diameter versus growth rate of the lamina; (4) rhizome diameter versus growth rate of the stipe.

Morphological features and their interpreted functions

Discusphyton wenghuiensis n. gen. n. sp., which is found in back shales of the Ediacaran Doushantuo Formation, is characterized by a disc-like compression with an unbranching thallus in its center. It is easily understood and regarded as a macroscopic alga in morphology (Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005, Reference Wang, Wang and Huang2007, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Lei, Chen and Hou2010, Reference Wang, Wang, Du and Wang2014, Reference Wang, Du, Komiya, Wang and Wang2015a, Reference Wang, Wang, Du and Wang2016a; Wang and Wang, Reference Wang and Wang2006; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Yin, Liu, Gao and Wang2009, Reference Tang, Jin, Wang, Ding, Zhao and Gao2014; Cheng et al., Reference Cheng, Wang, Chen, Wang and Zhong2013).

The three-dimensionally preserved stipe of D. wenghuiensis n. gen. n. sp. (Fig. 3.1–3.5, 3.7–3.10) confirms its originally cylindrical nature. The cylindrical stipe had a relatively stiff property to support the lamina (no completely and three-dimensionally preserved specimen), suspending it in the water column for photosynthesis. Connecting the stipe to the discoidal rhizoid, the globular rhizome is three-dimensionally preserved as a carbonated mass that is thicker than the discoidal rhizoid to form a distinct border with the discoidal rhizoid (Fig. 3.1, 3.2, 3.6). Conversely, a polar of the globular rhizome connecting with the stipe has no obvious boundary (Fig. 3.2, 3.6). One possible interpretation is that the globular rhizome and the cylindrical stipe are different from the discoidal rhizoid in organic matter or tissue density. Moreover, the rounded hump on the surface of the discoidal rhizoid center (Figs. 3.3–3.5, 3.7–3.11, 4.2, 4.6–4.8) implies that the globular rhizome had an originally harder and spherical nature within the discoidal rhizoid. Thus, the spherical rhizome with smooth surface possibly is embedded in the discoidal rhizoid (Fig. 2).

The non-uniform carbonaceous film or mass of the discoidal rhizoid, thinning from the center to the edge (Figs. 3.1–3.3, 3.5, 3.11, 4.1, 4.3, 4.5–4.9), indicates that the original nature of the discoidal rhizoid is thicker in the center than in the edge. In addition, the semicircularly preserved holdfast is visible as two arc-shaped edges (Fig. 3.7, 3.8), opposite to the thallus, and an obvious rounded hump (Fig. 3.6–3.10), so that it can be interpreted as a folded discoidal rhizoid that was, under certain hydrodynamic conditions, folded towards its bottom. Thus, we believe that D. wenghuiensis n. gen. n. sp. has a flat-bottomed and dome-shaped rhizoid, in which a harder and spherical rhizome is embedded (Fig. 2).

In many specimens (Fig. 4.1, 4.2, 4.5–4.9), there is, on the surface of the discoidal rhizoid, a ring-shaped depression surrounding the primal stipe, which can be interpreted either as original in nature or a compression structure. Many researchers considered that the Ediacaran Wenghui biota lived in a relatively low-energy marine environment (e.g., Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005, Reference Wang, Wang and Huang2007, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Chen, Wang and Huang2011, Reference Wang, Wang, Du and Wang2014; Wang and Wang, Reference Wang and Wang2006; Cheng et al., Reference Cheng, Wang, Chen, Wang and Zhong2013). Using microscopic characteristics of the upper Doushantou back shales and features of macroalgal holdfasts, Wang and Wang (Reference Wang and Wang2006) estimated that the upper Duoshantuo back shales contained ~78% water during their deposition. The macroalgal disc-like holdfast was regarded as providing an anchor on the water-rich muddy seafloor (Wang et al., Reference Wang, He, Yu, Zhao, Peng, Yang and Zhang2005; Wang and Wang, Reference Wang and Wang2006). In the macroagla D. wenghuiensis n. gen. n. sp., the spherical rhizome is composed of denser organic matter or tissues than the discoidal rhizoid, and provided an important organ connecting the cylindrical stipe to the dome-shaped rhizoid. As the thallus of D. wenghuiensis n. gen. n. sp. swung in the lower-energy seawater, the harder and spherical rhizome, with smooth border, might have rolled to ensure that the dome-shaped rhizoid remained stably anchored on the water-rich muddy seafloor. Thus, the ring-shaped depression on the surface of the discoidal rhizoid is regarded as an organic structure for the swing stipe (Fig. 2), rather than and abiotic structure.

Based on measurement results of D. wenghuiensis n. gen. n. sp., the positive correlations of the diameters of the discoidal rhizoid, the globular rhizome, and the primal stipe show that there is a significant relationship among the three of them (Fig. 5). Apparently, the discoidal rhizoid and the globular rhizome grew up together, during growth of D. wenghuiensis n. gen. n. sp., to serve for attaching and stabilizing functions. The primal stipe apparently grew up together with the discoidal rhizoid and the globular rhizome, which may have served for stronger connection to the thallus. However, the growth rates of the stipe width and the lamina width show no apparent relation to the diameters of the globular rhizome and the primal stipe (Fig. 6), implying that the growth of the thallus may have been more easily influenced by photosynthesis.

Problematic affinity of Discusphyton

In the Ediacaran carbonaceous fossils, the ultrastructural and biochemical characters are generally unavailable, so that the classification of such remains is usually based on its morphology to assess the taxonomic diversity and systematic affinities (Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002). The morphologies of the disc-like compressions in the Wenghui biota have been considerably modified by taphonomic processes. Moreover, its anatomical details are lacking owing to the homogenized carbonaceous compressions. Therefore, taxonomic assignment of the macroscopic compression is generally based on their characters in populations.

Taxonomic assignment

Generally, tissue differentiation for serving various bio-functions is considered as a key trait of eukaryotic alga or metaphytes (e.g., Du and Tian, Reference Du and Tian1985a, Reference Du and Tianb; Zhu and Chen, Reference Zhu and Chen1995; Yuan et al., Reference Yuan, Li and Chen1995, Reference Yuan, Xiao, Yin, Knoll, Zhou and Mu2002, Reference Yuan, Chen, Xiao, Zhou and Hua2011; Ding et al., Reference Ding, Li, Hu, Xiao, Su and Huang1996; Chen et al., Reference Chen, Chen and Xiao2000; Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Wang and Wang, Reference Wang and Wang2006; Wang et al., Reference Wang, Wang, Du and Wang2015b). Discusphyton wenghuiensis n. gen. n. sp. in the Ediacaran Wenghui biota is a centimeter-scale macroscopic fossil that had differentiated into the stipe and the lamina main for photosynthesis, the discoidal rhizoid for attaching on the seafloor, and the globular rhizome for connecting the stipe to the rhizoid. The large-sized discoidal rhizoid and the three-dimensionally preserved globular rhizome and stipe indicate that D. wenghuiensis n. gen. n. sp. had a holdfast organ to stably attach its body on seafloor and a stronger thallus to enhance its competitiveness for sunlight. Nevertheless, it is a pity that the homogenized carbonaceous compressions lacked microstructural details, so that the phylogenetic affinity of the macroscopic metaphyte D. wenghuiensis n. gen. n. sp. cannot be resolved in this paper.

Temporal distribution of disc-like holdfast features

Holdfast forms in previous publications have been reported in the Precambrian, mainly for indicating the fixation effect and/or tissue differentiation, but little significant attention has been paid to more detailed description and discussion.

The ribbon-like, coiled Grypania Walter, Oehler, and Oehler, Reference Walter, Oehler and Oehler1976, emend. Walter et al., Reference Walter, Du and Horodyski1990 was commonly regarded as an eukaryotic macroalga (e.g., Walter et al., Reference Walter, Oehler and Oehler1976, Reference Walter, Du and Horodyski1990; Runnegar, Reference Runnegar1991; Hofmann, Reference Hofmann1992; Han and Runnegar, Reference Han and Runnegar1992; Kumar, Reference Kumar2001; Knoll et al., Reference Knoll, Javaux, Hewitt and Cohen2006; Sharma and Shukla, Reference Sharma and Shukla2009; Xiao, Reference Xiao2013; Wang et al., Reference Wang, Wang and Du2016b). The earliest known occurrence of Grypania was reported from the Palaeoproterozoic Negaumee Iron Formation in Michigan, USA (Han and Runnegar, Reference Han and Runnegar1992), dated to 1870 Ma (re-dated by Schneider et al., Reference Schneider, Bickford, Cannon, Schulz and Hamilton2002). Walter et al. (Reference Walter, Du and Horodyski1990) considered that the unbranching G. spiralis with a cylindrical body was fixed to the sediment–water interface by an unknown attachment. A rounded terminus at the innermost end of the coiled G. spiralis was interpreted as an expression of the end of its body anchored or nestled into sediments (Wang et al., Reference Wang, Wang and Du2016b). We can understand G. spiralis as a benthic macroalga that used its end to serve the fixing function (Fig. 7.1). With holdfasts that were briefly described by Yan and Liu (Reference Yan and Liu1997) as the acuminate base (Fig. 7.2) and by Zhu and Chen (Reference Zhu and Chen1995) as the rhizoidal holdfast (=rhizoid) (Fig. 7.3) and doubtful disc-like holdfast (=?rhizome) (Fig. 7.4), some macroscopic algae were reported in the late Palaeoproterozoic Tuanshanzi Formation (1700 Ma) in North China. In the early Mesoproterozoic Gaoyuzhuang Formation (1560 Ma) in North China, a rod-like holdfast (=rhizome) (Fig. 7.5) and a possible spheroidal holdfast (=?rhizome) (Fig. 7.6) were reported by Zhu et al. (Reference Zhu, Zhu, Knoll, Yin, Zhao, Sun, Qu, Shi and Liu2016), but no detailed description of the two specimens was provided. Previously studied macroalgae from the Mesoproterozoic Vindhyan Supergroup (1000 Ma), Central India, by Kumar (Reference Kumar2001) considered that a millimeter-scale alga constituted three parts: the holdfast preserved as Tilsoia (Kumar, Reference Kumar2001) and Suketea (Kumar, Reference Kumar2001), the thallus preserved as Tawuia (Hofmann and Aitken, Reference Hofmann and Aitken1979), and Chuaria (Walcott, Reference Walcott1899), which represented a compressed cyst-like body; that is, the millimeter-scale alga has a holdfast consisting of a discoidal rhizoid (Tilsoia and Suketea) and a rhizoid (the substrate of Tawuia) (Fig. 7.7). Another macroalga, Longfengshania (Du, Reference Du1982) (Fig. 7.8), which was first reported in the middle Neoproterozoic Changlongshan Formation (900–860 Myr) in North China (Du, Reference Du1982), was regarded to have various (tuberous, rhizoidal, and disc-like rhizoid) holdfast forms (Du and Tian, Reference Du and Tian1985a, Reference Du and Tianb; Liu and Du, Reference Liu and Du1991) that, however, were not seen in systematic descriptions.

Figure 7

Various forms and possible evolution of the Precambrian macroalgal holdfasts. (1) End part nested into sediments (=?original rhizome) of Grypania, modified from Wang et al. (2016, fig. 3); (2) acuminate base (=?rhizome) of Tuanshanzia, after Yan and Liu (Reference Yan and Liu1997, fig. 3); (3) rhizoidal holdfast (=rhizome with filamentous rhizoids) of Zhu and Chen (Reference Zhu and Chen1995, fig. 3I); (4) doubtful disc-like holdfast (=?rhizome) of Zhu and Chen (Reference Zhu and Chen1995, fig. 3D); (5) rod-like holdfast (=rhizome) of Zhu et al. (Reference Zhu, Zhu, Knoll, Yin, Zhao, Sun, Qu, Shi and Liu2016, fig. 3d); (6) possible spheroidal holdfast (=?rhizome) of Zhu et al. (Reference Zhu, Zhu, Knoll, Yin, Zhao, Sun, Qu, Shi and Liu2016, fig. 3e); (7) holdfast represented by Tilsoia and Suketea, after Kumar (Reference Kumar2001, fig. 16); (8) holdfast of Longfengshania, after Du (1985a, fig. 2); (9) globular or tuberous holdfast (=rhizome) of Flabellophyton, after Yuan et al. (Reference Yuan, Li and Cao1999, fig. 2F); (10) disc-like holdfast (=?rhizome) of Baculiphyca sp., after Yuan et al. (Reference Yuan, Li and Cao1999, fig. 2B); (11) globular rhizome with filamentous rhizoids of B. taeniata, after Xiao et al. (Reference Xiao, Yuan, Steiner and Knoll2002, fig. 3.2); (12) cuneate rhizome with filamentous rhizoids of Gesinella, after Steiner et al. (Reference Steiner, Erdtmann and Chen1992, pl. 1, fig. 8); (13) tuberous rhizome with filamentous rhizoids of Zhongbaodaophyton, after Wang et al. (Reference Wang, Wang, Du and Wang2015b, fig. 2); (14) globular rhizome and discoidal rhizoid of Discusphyton new genus. Arrows mark the possible evolutionary trend of the Precambrian macroalgal holdfasts.

In the early Ediacaran, specimens of branched thallus were reported in the Lantian flora in South China, and the macroalgal holdfasts were also described as globular or tuberous holdfasts (=rhizome) (Fig. 7.9) and disc-like holdfasts (=?rhizome) (Fig. 7.10) (e.g., Chen et al., Reference Chen, Lu and Xiao1994b; Yuan et al., Reference Yuan, Li and Chen1995, Reference Yuan, Li and Cao1999, Reference Yuan, Xiao, Yin, Knoll, Zhou and Mu2002, Reference Yuan, Chen, Xiao, Zhou and Hua2011; Tang et al., Reference Tang, Yin and Gao1997, Reference Tang, Yin, Liu, Gao and Wang2009). Abundant and diverse macroscopic algae were reported in the upper Doushantuo black shales of the mid–late Ediacaran in South China, and most of their holdfasts commonly consist of a more or less expanded thallus substrate (including globular, tuberous, and cuneate rhizomes) and/or many filamentous rhizoids growing on the expanded rhizome (Fig. 7.11, 7.12) (e.g., Chen and Xiao, Reference Chen and Xiao1991, Reference Chen and Xiao1992; Chen et al., Reference Chen, Lu and Xiao1994b, Reference Chen, Chen and Xiao2000; Ding et al., Reference Ding, Li, Hu, Xiao, Su and Huang1996; Hu, Reference Hu1997; Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Wang and Wang, Reference Wang and Wang2006; Wang et al., Reference Wang, Wang and Huang2007, Reference Wang, Zhao, Yang and Wang2009, Reference Wang, Chen, Wang and Huang2011, Reference Wang, Wang, Du and Wang2014, Reference Wang, Du, Komiya, Wang and Wang2015a, Reference Wang, Wang, Du and Wang2016a; Tang et al., Reference Tang, Yin, Liu, Duan and Qao2008a, Reference Tang, Jin, Wang, Ding, Zhao and Gao2014). Remarkably, there is a rarely known disc-like holdfast of Discusphyton wenghuiensis n. gen. n. sp., consisting of a globular rhizome and a large-sized discoidal rhizoid, but no filamentous rhizoid (Figs. 2, 7.16).

For these fossil records, we consider that the rhizome may have originated as the thallus substrate nestled into sediments and the rhizoid may be the secondary growth of the rhizome. From the brief and/or incomplete descriptions in previous publications, we infer that the variation tendency of the macroalgal holdfast may be from small to large size in its own share of the thalli and from simple to complex in form and structure (Fig. 7). However, it is difficult, at present, to decipher if the discoidal rhizoid originated from the bonded filamentous rhizoid, or an individual of the secondary rhizome, or others.

Conclusions

With a large-sized disc-like holdfast and an unbranching thallus, Discusphyton whenghuiensis n. gen. n. sp. was collected in the upper Doushantuo black shales (~560–551 Ma) of the Ediacaran in northeastern Guizhou, South China. The benthic macroalga lived in a relatively low-energy marine environment; it not only had an unbranching thallus, but also recorded the appearance of a complex holdfast. The unbranching thallus, consisting of a compressed lamina (upper part of thallus) and cylindrical stipe (lower part of thallus), mainly served for photosynthesis. The disc-like holdfast is composed of a discoidal rhizoid and a globular rhizome. The discoidal rhizoid is regarded as an originally flat-bottomed and dome-shaped organ that served to anchor its macroalgal body on the water-rich muddy seafloor. Connecting the cylindrical stipe to the dome-shaped rhizoid, the globular rhizome was an originally harder and spherical feature that is composed of denser tissues than the dome-shaped rhizoid. The harder and spherical rhizome with smooth surface is embedded in the dome-shaped rhizoid and may have been an important organ as a steering knuckle to ensure the dome-shaped rhizoid stably anchored on the seafloor. However, it is difficult, at present, to decipher the phylogenetic affinity of the macroscopic metaphyte D. wenghuiensis n. gen. n. sp. and the origin of discoidal rhizoids.

Acknowledgments

We thank villagers of Wenghui, Jiangkou, Guizhou, for assistance in the field. This study was supported by the National Science Foundation of China (grant No. 41762001, No. 41663005, and No. 41572024) and CAGS Research Fund of China (grant YYWF201602).

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Figure 0

Figure 1 Ediacaran Doushantuo Formation stratigaphic section at Wenghui, Jiangkou, northeastern Guizhou, South China. Disc-like compressions with carbonaceous stipes described in this paper are found in upper Doushantuo black shales at Wenghui.

Figure 1

Figure 2 Drawing of the principal structures of Discusphyton wenghuiensis n. gen. n. sp.

Figure 2

Figure 3 Discusphyton wenghuiensis n. gen. n. sp. from the upper Doushantuo Formation, Wenghui, Jiangkou, Guizhou, China. (1) Holotype JK50-1460; (2) paratype WH-P-50014; (3) WH-P-42146; (4) JK45-0717; (5) TY42-147; (6) JK42-1064; (7, 8) JK42-0146, with magnified view of the holdfast; arrow marks detail of folds; (9, 10) JK42-0147, with magnified view of the holdfast; (11) WH-P-04120.

Figure 3

Figure 4 Discusphyton wenghuiensis n. gen. n. sp. fallen from thallus from the upper Doushantuo Formation, Wenghui, Jiangkou, Guizhou, China. (1, 2) Paratype JK48-0031, with magnified view of the central part; (3, 4) WH-P-01060, with magnified view of the central part; (5) TY42-0014; (6) TY50-0082; (7) WH-P-01087; (8) WH-P-01051; (9) WH-P-01022. Arrows in 2, 5, and 6 mark the ring-shaped depression.

Figure 4

Figure 5 Cross-plots of the diameters of the discoidal rhizoid, the globular rhizome, and the primal stipe (1–3): (1) discoidal rhizoid versus globular rhizome; (2) discoidal rhizoid versus primal stipe; (3) globular rhizome versus primal stipe. (4) Bar graph of rhizoid diameter versus number of specimens.

Figure 5

Figure 6 Cross-plots of growth rates of the stipe and the lamina. (1) Rhizoid diameter versus growth rate of the lamina; (2) rhizoid diameter versus growth rate of the stipe; (3) rhizome diameter versus growth rate of the lamina; (4) rhizome diameter versus growth rate of the stipe.

Figure 6

Figure 7 Various forms and possible evolution of the Precambrian macroalgal holdfasts. (1) End part nested into sediments (=?original rhizome) of Grypania, modified from Wang et al. (2016, fig. 3); (2) acuminate base (=?rhizome) of Tuanshanzia, after Yan and Liu (1997, fig. 3); (3) rhizoidal holdfast (=rhizome with filamentous rhizoids) of Zhu and Chen (1995, fig. 3I); (4) doubtful disc-like holdfast (=?rhizome) of Zhu and Chen (1995, fig. 3D); (5) rod-like holdfast (=rhizome) of Zhu et al. (2016, fig. 3d); (6) possible spheroidal holdfast (=?rhizome) of Zhu et al. (2016, fig. 3e); (7) holdfast represented by Tilsoia and Suketea, after Kumar (2001, fig. 16); (8) holdfast of Longfengshania, after Du (1985a, fig. 2); (9) globular or tuberous holdfast (=rhizome) of Flabellophyton, after Yuan et al. (1999, fig. 2F); (10) disc-like holdfast (=?rhizome) of Baculiphyca sp., after Yuan et al. (1999, fig. 2B); (11) globular rhizome with filamentous rhizoids of B. taeniata, after Xiao et al. (2002, fig. 3.2); (12) cuneate rhizome with filamentous rhizoids of Gesinella, after Steiner et al. (1992, pl. 1, fig. 8); (13) tuberous rhizome with filamentous rhizoids of Zhongbaodaophyton, after Wang et al. (2015b, fig. 2); (14) globular rhizome and discoidal rhizoid of Discusphyton new genus. Arrows mark the possible evolutionary trend of the Precambrian macroalgal holdfasts.