Benthic macroalgae are important components of modern and past ecosystems. They are more efficient primary producers than phytoplankton (Bunt, 1975) and provide food and habitats for many other organisms (e.g., Steneck et al., 2002).

The evolution of benthic macroalgae experienced a prolonged stasis since its first appearance (Du et al., 1986; Dong et al., 2008; Xiao, 2013) in the Mesoproterozoic to early Neoproterozoic, and a sudden expansion in morphospace in the Ediacaran (Xiao and Dong, 2006; Bykova et al., 2020). The cause of the increase in morphological disparity might be the drop of pCO₂ in the atmosphere and seawater after the Cryogenian Snowball Earth glaciations (Xiao and Dong, 2006).

The morphology of the holdfast of benthic macroalgae also diversified with the expansion in morphospace. Holdfasts attach macroalgae to substrates, and their morphologies are greatly influenced by the property of substrates and reflect the attachment strategy adopted by the macroalgae. As a result, the morphology of holdfasts may serve as a proxy, to some extent, for the substrate conditions. For instance, Dornbos et al. (2005) and Lei et al. (2014) have investigated the evolution of attachment strategies of benthic metazoans as a response to the changing substrate conditions in the Cambrian.

The agronomic revolution (AR) or the Cambrian substrate revolution (CSR) is one of the greatest shifts in substrate conditions, transforming from Proterozoic-style substrates to Phanerozoic mixed layers. Typical Proterozoic-style substrates were firm and well-layered, with low water content and ubiquitous microbial mats; in contrast, typical Phanerozoic-style soft substrates were soupy, water-rich mixed layers (Bottjer et al., 2000). The AR was driven by increasing bioturbation that incorporated water into the sediment (Kloss et al., 2015), but its exact timing and extent have not been yet clearly defined. In the early Cambrian, the AR has already had an effect (Dornbos et al., 2005), and had almost been finished in the late Cambrian (Bottjer et al., 2000). Considering the relatively diverse and abundant trace fossils in the late Neoproterozoic (e.g., Chen et al., 2018; Chen et al., 2019; Xiao et al., 2019), the AR is supposed to have a root back in the Precambrian, but the scarcity of Precambrian biotas restrained the insight to the evolution of substrate conditions during this period. Recently, a large number of specimens of fossil macroalgae with holdfasts preserved intact yielded from the Lantian biota (Yuan and Cao, 1999; Yuan et al., 2011; Bin et al., 2013) and the Miaohe biota (Xiao et al., 2002; Ye et al., 2019a) offered a chance to examine the evolution of substrate conditions in the Ediacaran.

This study focuses on the evolution of macroalgal holdfasts, attachment strategies and substrate conditions during the Ediacaran Period by examining Lagerstätten in South China: the early Ediacaran Lantian biota and the late Ediacaran Miaohe biota. Both biotas are preserved in black shales, and they also share comparable geological settings and depositional environments. We described holdfast morphologies of benthic macroalgae and analyzed their attachment strategies to understand the dynamic of substrate conditions in the early and late Ediacaran.

The Lantian biota is preserved in the Member II black shale of the Ediacaran Lantian Formation in the Anhui province (Figures 1A,B). Underlain by the Cryogenian Leigongwu diamictite and topped by the latest Ediacaran Piyuancun Formation, the Lantian Formation holds the fossil record of life between the end of the Snowball Earth to the eve of the Cambrian Explosion. It consists of four lithological members at the Lantian section (Figure 1B), i.e., in ascending order, Members I–IV. The lowest Member I is the cap carbonate (4 m thick); the Member II consists of siltstone (23 m thick), mudstone (8.6 m thick) and fossiliferous black shale (34 m thick); the Member III consists of dolomite interbedded with black shale (26 m thick) and limestone (40 m thick); the uppermost Member IV consists of black mudstone, 20 m in thickness (Guan et al., 2014; Wan et al., 2014; Wan et al., 2016). The Lantian Formation is the lithostratigraphic equivalent to the Doushantuo Formation in the Hubei Province (Yuan et al., 2011), which has been constrained to be between 635 Ma to 551 Ma (Condon et al., 2005), and therefore the age of the Lantian biota is considered between 635 and 590 Ma (Yuan et al., 2011; Wan et al., 2016).

The sedimentary environment of the Lantian Formation was a restricted shelf basin (Zhu et al., 2007) at low to middle paleolatitudes (Li et al., 2008; Zhang et al., 2015). The in situ preserved benthic macroalgae and fine parallel laminations suggest that the deposition probably took place in a quiet environment below the storm wave base but within the photic zone (Yuan et al., 2011). Nevertheless, occasionally alignments of fossils in parallel and/or perpendicular directions indicate relatively weak current or wave movements.

The Miaohe biota refers to the fossil assemblage preserved in a siliceous black shale unit known as the Miaohe Member of the Ediacaran Doushantuo Formation (Figures 1C,D) which crops out in the Hubei and Guizhou provinces. Traditionally, the Miaohe Member is considered the equivalent of the uppermost black shale member of the Ediacaran Doushantuo Formation (Jiang et al., 2011; Xiao et al., 2017; Zhou et al., 2017). The Doushantuo Formation, as mentioned above, is a lithostratigraphic equivalent of the Lantian Formation. In the Miaohe area of the Hubei province and Wenghui area of the Guizhou province, the Doushantuo Formation overlain the Cryogenian Nantuo Formation and is overlaid by the latest Ediacaran Dengying and Liuchapo formations respectively. The Doushantuo Formation is 255 m thick at Miaohe and 79 m thick at Wenghui. Within the Doushantuo Formation, four members are discerned, in ascending order: a cap carbonate unit at the bottom (Member I, 5 m thick at Miaohe and 7 m thick at Wenghui), black shale interbedded with dolomite (Member II, 150 m thick at Miaohe and 10 m thick at Wenghui), laminated dolomite (Member III, 80 m thick at Miaohe and 17 m thick at Wenghui), and black shale and organic-rich mudstone at the top (Member IV, 20 m thick at Miaohe and 45 m thick at Wenghui). Nevertheless, An et al. (2015) proposed an alternative correlation that regards the Miaohe Member as an equivalent of the lower Shibantan Member of the Dengying Formation. The Miaohe Member yields diverse macroscopic fossils in the Miaohe area and several other localities in Hubei (Xiao et al., 2002; An et al., 2015; Ye et al., 2019a), and also in the Wenghui area in Guizhou (Zhao et al., 2004; Wang et al., 2014; Wang et al., 2016). Its age is constrained to 551 Ma (TIMS U-Pb dating, Condon et al., 2005). This black shale member is interpreted as transgressive sediments deposited during sea-level rise (Jiang et al., 2007; Jiang et al., 2011), which may represent a restricted intra-shelf basin setting in the Yangtze Gorges area (Zhu et al., 2013) at low paleolatitudes (Li et al., 2008), and probably represent a continental slope environment in the Wenghui area (Jiang et al., 2011). The surface sediment of the Wenghui area was considered to be soupy (Wang and Wang, 2008).

Specimens from the Lantian and Miaohe biotas are deposited in the Nanjing Institute of Geology and Palaeontology, except for the Gesinella and Globusphyton specimens which are deposited in Guizhou University.

In order to determine attachment strategies of benthic macroalgae in the Lantian and Miaohe biotas, a detailed examination of macroalgal holdfasts was conducted. Observations and measurements were carried out directly on fossil specimens or pictures of fossils from these biotas. Based on functional morphology, the holdfasts were classified into forms that were either adapted to firm and coherent substrates which dominated the Precambrian seafloor, or soft and soupy substrates that are more common in the Phanerozoic.

As a part of interpreting the adaptive morphology of globular holdfasts, a bivariate Spearman rank-order correlation (Spearman, 1904) was performed to determine if there is a correlation between the size of the thallus and the holdfast of Flabellophyton, which is the most representative and abundant taxon in the Lantian biota. A relatively small holdfast compared to the thallus may reflect minimal or no bioturbation (Loduca et al., 2017), hence the correlation (or lack thereof) would indicate if substrate conditions were relevant to the survival of benthic macroalgae or not. We focused on holdfasts instead of the amorphous organic mass here, because the organic mass may represent adhesive polypeptides (e.g., Levi and Friedlander, 2004) exuded from the holdfast rather than an anatomical structure. Two variables used here are the thallus area and the cross-section area of the globular holdfast. The thallus of Flabellophyton was an inverted hollow cone originally, and its fan-like fossil represents a projection of a cone on a two-dimensional surface (Wan et al., 2020). The projection is also equal to the frontal area facing into the flow direction, making it an important parameter for benthic macroalgae to withstand moving water. The cross-section area of the globular holdfast is calculated from the diameter which could be directly measured.

Fossil specimens were photographed under a Nikon DSLR D850, a Zeiss stereo microscope Axio Zoom V16. Measurements were made from photos of specimens using ImageJ, version 1.52a (Rasband, 2018). Statistical analysis was carried out by using SPSS, version 23 (IBM Corp, 2015).

Adaptive morphological analysis indicates there are 6 holdfast forms (tapered base, globose holdfast, composite globose holdfast, discoidal holdfast, filamentous rhizoid, and horizontal rhizome) for benthic macroalgae from the early Ediacaran Lantian biota and late Ediacaran Miaohe biota in South China. The morphological variations between holdfasts may reflect various attachment strategies and further imply general conditions of substrates. In the Lantian biota, all the benthic macroalgae with attachment structures are adapted to firm substrates, while in the Miaohe biota, a significant number (41%) of species are adapted to relatively soft substrates. The great diversification in holdfast morphology and attachment strategies which was marked by the appearance of complex rhizoidal holdfasts was a response to the diversification of macroalgae and changing substrate conditions during this period, i.e., the decline of microbial mat ecosystem and the founding of a benthic algae-dominated ecosystem. The results also provide evidence that the onset of the agronomic revolution (the Cambrian substrate revolution) likely predates the Cambrian and had a protracted effect on paleoecology and evolution onwards.