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Journal of Paleontology 91(6):1091–1101
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., 2005; Wang and Wang, 2006). 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., 2002). The morphologies of the disc-like compressions in the Wenghui biota have been considerably modified by tapho- nomic processes. Moreover, its anatomical details are lacking owing to the homogenized carbonaceous compressions. There- fore, 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, 1985a, b; Zhu and Chen, 1995; Yuan et al., 1995, 2002, 2011; Ding et al., 1996; Chen et al., 2000; Xiao et al., 2002; Wang and Wang, 2006; Wang et al., 2015b). 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 con- necting 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 metaphyteD. 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, 1976, emend. Walter et al., 1990 was commonly regarded as an eukaryotic macroalga (e.g., Walter et al., 1976, 1990; Runnegar, 1991; Hofmann, 1992; Han and Runnegar, 1992; Kumar, 2001; Knoll et al., 2006; Sharma and Shukla, 2009; Xiao, 2013; Wang et al., 2016b). The earliest known occurrence of Grypania was reported from the Palaeoproter- ozoic Negaumee Iron Formation in Michigan, USA (Han and Runnegar, 1992), dated to 1870 Ma (re-dated by Schneider et al., 2002). Walter et al. (1990) 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., 2016b). 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 (1997) as the acuminate base (Fig. 7.2) and by Zhu and Chen (1995) 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. (2016), but no detailed description of the two specimens was provided. Previously studied macroalgae from the Mesoproterozoic Vindhyan Supergroup (1000 Ma), Central India, by Kumar (2001) considered that a millimeter-scale alga constituted three parts: the holdfast preserved as Tilsoia (Kumar, 2001) and Suketea (Kumar, 2001), the thallus preserved as Tawuia (Hofmann and Aitken, 1979), and Chuaria (Walcott, 1899), 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, 1982) (Fig. 7.8), which was first reported in the middle Neoproterozoic Changlongshan Formation (900–860 Myr) in North China (Du, 1982), was regarded to have various (tuberous, rhizoidal, and disc-like rhizoid) holdfast forms (Du and Tian, 1985a, b; Liu and Du, 1991) that, however, were not seen in systematic descriptions. 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., 1994b; Yuan et al., 1995, 1999, 2002, 2011; Tang et al., 1997, 2009). Abundant and diverse macroscopic algae were reported in the upper Doushantuo black
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