Menon et al. (2013) regard wrinkles at the base of discoid fossils (Aspidella terranovica) and deformation of laminae in cut slabs as evidence for upward migration of a cnidarian polyp in marine shales, but their observations are better explained by a sessile-growing organism with growth rugae and downward-tapering rhizomorphs in shallow marine to intertidal environments. The observations of Menon et al. are not evidence of mid-Ediacaran (560 Ma) animals, and neither are the supposed trails of Liu et al. (2010), which are more likely tilting traces with characteristic overlapping loops and intermittent sliding (Retallack, 2010).
Several of the observations of Menon et al. falsify their interpretation of upward migration, notably the deformation of laminae both upward and downward, and marked narrowing of the deformed zone downward (Menon et al., 2013, their figure 3). These features are also seen in other thin sections of A. terranovica from Ferryland, Newfoundland (Fig. 1A), here shown with laminae deformed both upward and downward around sandstone casts of Aspidella, and below that, tubular rhizomorphic structures (Fig. 1C). These observations are incompatible with the broad base of cnidarian polyps or any consistent direction of movement. Crescentic wrinkles interpreted by Menon et al. as trail backfills can instead be interpreted as growth rugae (Fig. 1D), as in slightly exhumed but firmly attached organisms (Fig. 1D). Downward-deflected laminae can be explained by rhizomorph growth, and upward-deflected laminae as onlapping or bowed up by growth increments.
The strongly tapering bases of “equilibration trails” of Menon et al. were more likely rhizomorphs at the base of A. terranovica. Comparable tubular features attached to presumably uprooted Aspidella were illustrated by Gehling et al. (2000, and their figure 14), and are also figured here (Figs. 1A and 1B). My observations of gray color, dominantly clayey grain size, pyritic nodules, flaser bedding, and isolated ripple trains at two sites at Ferryland support the interpretation that the Aspidella facies was deposited in a very shallow marine bay or lagoon protected from wave action. The surfaces colonized densely by rhizomorphic Aspidella may have been an Ediacaran equivalent of pyritic intertidal paleosols (Sulfaquents of Soil Survey Staff, 2010), like those of modern salt marshes (Nelson et al., 1996). Menon et al. are correct that the Aspidella facies bears little resemblance to sequences of red, formerly well-drained, calcic and gypsic paleosols supporting more complex Ediacaran fossils (Retallack, 2013), but that does not rule out waterlogged intertidal paleosols for Aspidella and interpretation as a rhizomorphic button lichen with growth rugae (Fig. 1D).
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