We presented (Xiao et al., 2013) comparative paleobiological and functional morphological evidence that Ediacara fossils such as Eoandromeda are marine organisms, rather than terrestrial lichens or bacterial colonies that lived on or in paleosols (Retallack, 2013). Our conclusion was based on the following observations: (1) Eoandromeda and several other Ediacara forms are preserved in marine black shales (e.g., Miaohe Member or Member IV of the Doushantuo Formation, South China) that show no evidence of paleosol development, and they were not transported from a terrestrial environment; and (2) they show no morphological adaptations to a terrestrial lifestyle. In his Comment, Retallack (2014) argues that Eoandromeda represents colonial bacteria with chiral flagella, it lived in paleosols, and was washed into the sea to be preserved in the Miaohe Member.
Retallack claims that we did not offer an alternative to his interpretation. This claim is untrue, as we clearly stated that Eoandromeda is “a benthic diploblastic grade animal” (p. 1096), that Ediacara fossils discussed in our paper—including Eoandromeda—“were marine rather than terrestrial organisms” (p. 1098), and that the fossiliferous Ediacara and Miaohe members are both “marine successions” (p. 1096) (Xiao et al., 2013).
Retallack also states that we “are mistaken in considering interference-oscillation ripples [in the Ediacara Member] as evidence of marine conditions,” citing that such ripples can form in tidal flats, lagoons, lakes, and floodplains. That the rippled Ediacara sandstones are marine deposits is based on a suite of sedimentary structures, not just ripple marks. There is no need to repeat the extensive evidence for a marine origin of the Ediacara Member (Gehling, 2000; Antcliffe and Hancy, 2013; Callow et al., 2013; Gehling and Droser, 2013; Xiao and Knauth, 2013), but it is useful to point out that oscillatory ripple marks usually do not form in paleosols.
Retallack argues that Eoandromeda fossils in the Miaohe Member were uprooted from terrestrial paleosols and then washed into a marine basin, ignoring evidence to the contrary presented by us. Most Miaohe fossils are exceptionally preserved soft-bodied organisms, many of which are fully preserved algal thalli with fragile rhizoids (Xiao et al., 2002). Almost all Eoandromeda fossils from the Miaohe Member are completely preserved with all eight arms (Zhu et al., 2008; Tang, 2012). Eoandromeda has also been reported from slope and basinal facies (e.g., in Wenghui section) that were hundreds of kilometers from the land (Jiang et al., 2011). It is highly unlikely that Eoandromeda organisms, particularly if they were a colony of bacteria, would have remained intact if they had been uprooted and transported hundreds of kilometers.
In the Ediacara Member, Eoandromeda is preserved in situ, but is entirely confined to the relatively deep-water “sheet-flow sand” facies along the axis of a submarine canyon where there are no wave ripple marks, cross-beds, or any evidence of shallow-water environments, let alone intertidal or terrestrial environments. It has never been found in the relatively shallow “wavebase sand” facies where frondose and discoidal fossils are abundant (Gehling and Droser, 2013). This pattern of facies-dependent distribution is inconsistent with Retallack’s interpretation of Eoandromeda as a terrestrial organism.
Retallack’s interpretation is further contradicted by his own declaration that Chinese Eoandromeda and fronds “are more complete than Australian ones.” Shouldn’t uprooted and transported specimens in China be more fragmented than those preserved in situ in Australia? Retallack argues that Australian Eoandromeda specimens are preserved in greater relief than Chinese ones, and hence they were preserved in situ, whereas the latter were transported. We do not follow the logic of this argument. The different reliefs are related to the different taphonomic pathways: Australian specimens were replicated by early mineralization facilitated by extensive development of microbial mats in a sandstone facies (Gehling, 1999), whereas Chinese specimens are preserved as two-dimensional carbonaceous compressions in a black shale facies (Xiao et al., 2002).
Retallack alludes to the possibility that the Miaohe Member black shales could be “intertidal paleosols.” As evidence, he cites his personal observations of the Nantuo Formation, lower Doushantuo Formation, and Shibantan Member of the Denying Formation. The Nantuo and lower Doushantuo formations are 50–100 m.y. older than the Miaohe Member, and the Shibantan Member is millions of years younger. Thus, Retallack’s observations, regardless of their veracity, are irrelevant to the current debate.
- © 2014 Geological Society of America