The claim by Retallack (2010) that the ancient Mistaken Point trace fossil assemblage (Liu et al., 2010) might turn out to be a species of tool mark known only from shallow water, while interesting, is redundant because all our evidence points to a depositional environment that was unequivocally deep marine. Earlier publications did indeed favor a shallow marine interpretation (Misra, 1971). However, all subsequent sedimentologically focused studies of the Mistaken Point Formation have inferred deep-water turbidite or contourite depositional mechanisms (e.g., Narbonne et al., 2001; Wood et al., 2003; Ichaso et al., 2007), conclusions with which we firmly concur.
Sedimentological evidence for the presence of subtidal depths includes the dominance of partial Bouma sequences in beds of silt-grade material, the prevalence of paleocurrents showing co-axial paleoflow directions parallel to the inferred basin margin (Wood et al., 2003), as well as a conspicuous absence of shallow-water indicators such as the oscillation ripples seen at much higher stratigraphic levels.
Intriguingly, the only study to have inferred a supratidal origin for the Mistaken Point biota is Retallack (1994). The fact that the more than 4000 m of succession in the Conception Group in southeast Newfoundland is mudstone-rich, but devoid of fluvial channel facies, effectively renders the non-marine interpretation of the Mistaken Point biota as dubious, to say the least. The fully marine interpretation of the succession is actually supported by Canfield et al. (2007), who clearly explain the non-uniformitarian nature of iron cycling in the Proterozoic.
The presence (not absence) of pyrite in association with Ediacaran macrofossils, for example with Fractofusus at Bristy Cove, is supported by many field observations. The microbial formation of pyrite by sulfate-reducing bacteria requires not only a source of sulfur (commonly seawater sulfate) but also an abundance of organic matter and iron as prerequisites for growth. Red staining of many Ediacaran beds in the area is likely due to the secondary oxidation of pyrite (Gehling and Narbonne, 2007). That said, even a paucity of pyrite does not unequivocally imply freshwater.
The carbonate concretions reported by Benus (1988) are nothing more than ferroan concretions consistent with deep-water methanogenesis (e.g., Curtis et al., 1986), while earlier accounts of oscillation ripples (Williams and King, 1979) are compressional features of tectonic origin (Benus, 1988; Wood et al., 2003). Higher in the stratigraphy, within the Signal Hill Group, the Ferryland Head Formation displays abundant wave-generated structures, wave-sorted sands, red beds with mudcracks, and rain-pits. These beds are markedly devoid of Ediacaran macrobiota.
We emphasize here that our paper (Liu et al., 2010) only claimed to report the earliest evidence for locomotion in the Ediacara biota, and not the first evidence for animal locomotion. Analysis of earlier claims will be published elsewhere. Comparisons made by Retallack between our trails and those of tilting marks ploughing through modern beach sands (Wetzel, 1999) can also be refuted. Firstly, the deep-water post-turbidite/contourite currents were too weak to be capable of transporting large clasts, even buoyant ones, leading merely to the deposition of clays. Skip marks, of the kind common in Ediacaran shallow-water sediments, are notably absent. Secondly, unlike tilting marks, we confirm that the trails include meandering forms with clear directional changes. Thirdly, where the trails end in circular impressions (Liu et al., 2010, our figure 2A), no hard objects are found.
The suggestion that our trails formed in shallow to supratidal conditions consistent with tilting marks (Retallack, 2010) seems to require not only special pleading but a form of circular reasoning. Such an interpretation is thus at odds with an increasing body of data obtained from sedimentology, palaeoenvironments, taphonomy, and actuo-palaeontology. As such, we stand by our original assertion that these structures provide clear evidence for locomotion in association with the Ediacara biota in the 565 Ma Mistaken Point Formation.